The Early History of Radioactivity (1896-1904) Thesis Presented For

Home , Edmond Becquerel, Henri Becquerel, Henri Moissan

The Early History of Radioactivity (1896-1904)

Thesis presented for the degree of Doctor of Philosophy in the Field of History of Science

Stephen Brian Sinclair

Department of History of Science and Technology Imperial College of Science and Technology University of London

May 1976 2

ABSTRACT

Both the beginning and end of this history are ostensibly well defined. Becquerel's quiet discovery of uranium rays came to fruition in the appearance of the first standard textbooks on radioactivity, with their claims for an independent subject area. One may see in the intervening period the progressive construction of a new bridge between physics and chemistry founded on a coherent theory of atomic transmutation and disintegration. My examination of the scene from the viewpoints of several interested parties reveals an alternative picture comprising complex linked series of discoveries, experiments and hypotheses of various levels. At the moving boundaries of research the results and conclusions of individuals were always subject to reinterpretation in their adoption by others. This study thus proceeds in the light of three main considerations. These are, firstly, parallel investigations in radioactivity by different workers; secondly, contemporary related areas of physical science such as X-rays, cathode rays, corpuscular theory; and thirdly, the relevant concepts developed earlier, during the nineteenth century. The introductory chapter concentrates upon the last of these, considering some long-standing hopes and unanswered questions concerning, for example, the unification of matter, ether, and electricity, and the relations between the chemical elements. This forms an essential part of the background to radioactivity. Chapter two describes the opening of the radiochemical field by the Curies, following Becquerel's original discovery, and discusses the blending of these results with Rutherford's earliest radiation studies. The third chapter deals with a confused phase where new observations 3

outran theory as 'emanations', 'induced' activities, spontaneous cathode-ray emissions, and dubious radiochemical claims combined with the ever-growing energy problem. The fourth and fifth chapters trace the emergence of rival theories, and the singular success of one of these at the expense of all others. 4

ACKNOWLEDGMENTS

I wish to thank for their assistance those who made manuscript materials available to me at the following institutions: Cambridge University Library, Bibliotheque Nationale, Academie des Sciences, Royal Institution of Great Britain, Wellcome Institute for the History of Medicine Library, Library of University College London, Library of the Royal Society, Bodleian Library, Science Museum Library, Imperial College Archives. To my research supervisor Dr. M.B. Hall of the Department of History of Science and Technology, Imperial College, I am greatly indebted for her patient advice throughout the project and for her careful reading of this thesis during its construction. TABLE OF CONTENTS

ABSTRACT 2

ACKNOWLEDGEMENTS 4

CHAPTER 1. NINETEENTH-CENTURY THREADS 8 1. Introduction 8 Scientific revolution - summary of threads. 2. Chemical atomic theories and the unity and complex ty of the chemical elements 12 Dalton's theory - evolution and compound nature of elements - Crookes - Stokes - Lockyer - J.J.Thomson. 3. Physical theories of matter, electricity, ether 24 Maxwell - various ethers - Larmor - Lorentz - Zeeman - Mme.Curie, Rutherford - Hertzian molecule. 4. Chemical physics. Physical chemistry 32 Hertzian atom - Stoney's electron theory - electrochemistry - vortex atom - J.J.Thomson - electricity and gases - corpuscular atom.

CHAPTER 2. THE DISCOVERY OF URANIUM RAYS AND RADIOACTIVITY 48 1. Becquerel's discovery of uranium rays (1896-7) 48 X-rays and phosphorescence.- Becquerel and uranium rays - nature of the rays - S.P.Thompson - other radiations - vapours and W.J.Russell's photographic work - electrical studies. 2. Rutherford, and the Cavendish Laboratory (1894-8) 70 Hertzian radiation and magnetism - X-rays and conductivity of gases - ionic theory - ultraviolet radiation - uranium rays. 3. Pierre Curie, Marie Curie and the new radioactive elements (1890.17)-- 92 P.Curie's researches - Marie Curie - thorium rays - G.C.Schmidt - polonium - radium - atomic property - energy source, speculations. 4. Theories and trends (1896-9) 110 Source of the energy - interest in uranium rays - development of theories - atomic change. 6

CHAPTER 3. EMANATIONS AND RADIATIONS 119 1. The ma netic deflection of the Becquerel rays 1 9-1900) 119 The rays from active substances and their magnetic deflection - magnetism and radioactivity - Becquerel's 'material rays'. 2. The discovery of induced radioactivity (1899) 129 The Curies' discovery, 'la radioactivite induite' - Rutherford's idea of a thorium emanation - properties of the emanation - the production of a radioactive deposit. 3. The source of radioactivity (1900) 147 The effect of temperature change and its implications - phosphorescence, Behrendsen, and the views of Becquerel - Marie Curie's speculations on atomic change and disintegration - Rutherford and the energy of radioactivity. 4. Emanations and the X-substances (1900-1) 162 Fitzgerald, and transmutations - A.Debierne and actinium - induced radioactivity: Giesel, Hofmann, and radiolead; P.Villard - Crookes and UrX - the Curies' views - E.Dorn and an emanation from radium - Rutherford's problems with the emanations - atmospheric emanation of Elster and Geitel.

CHAPTER 4. DISINTEGRATION, INDUCTION, TRANSFORMATION 179 1. The emergence of induction and disintegration gorfes (1901-1.) 179 Ionic and emanation hypotheses of Elster and Geitel - comparison of theories - induced radioactivity; the function of the gaseous medium - radioactive water; the induction theory of Curie and Debierne - Becquerel, uranium and auto-induction - Curies' criticism - Crookes and ultra-atomic diffusion - Martin and total disintegration - J.Stark and the genesis of atoms . 2. A quantitative theory of atomic transmutation T1902) 196 Introduction - F.Soddy - the first joint publication: an inert gas from thorium, a possible transmutation; ThX as the source of the emanation - Rutherford and the transmission of excited radioactivity - interpretations of Becquerel's views - the second publication: thorium and ThX, transmutation quantitatively observed - the new accompaniment version of the disintegration theory, and the question of induction. 7

CHAPTER 5. RECEPTIONS, GENERALISATIONS, SPECULATIONS 226 1. Reception of the disintegration theory (1902-3) 226 Introduction - changing views of the Caries; the heat from radium - F.Giesel - J.J.Thomson: wind, water, and atomic disintegration - Crookes and the mysteries of radium; a wider public - creation of helium; the Curies converted - the summit of fame - physicists and chemists: a campaign well fought. 2. The mechanism of radioactivity (1903-4) 252 Speculations of physical chemists: ether, energy, electrons - Soddy and the randomness of disintegration - physical models for radioactivity - Lodge's radiation-loss hypothesis - Kelvin's atomic theories - Nagaoka's saturnian atom - Thomson's corpuscular atom. 3. Conclusion 271 Rutherford and the succession of changes - cosmical, universal, and Proutian radioactivity.

NOTES FOR CHAPTERS 1 to 5 281

BIBLIOGRAPHY 331

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CHAPTER 1

NINETEENTH-CENTURY THREADS

1. Introduction The discovery of X-rays by ROntgen in 1895, which led directly to Becquerel's discovery of uranium rays, has been hailed as marking a watershed in physics, the beginning of a revolution. This interpretation generally invokes the further statement that from 1880 to the turn of the century physical science had attained a satisfactory state with few internal problems.'The latter idea, with its implication of some form of stagnation in research, seems the more dubious of the two. The words of Maxwell's 'Introductory Lecture on Experimental Physics' of 1871 have been used to support this view: the opinion seems to have got abroad, that in a few years all the great physical constants will have been approximately estimated, and that the only occupation which will then be left to men of science will be to carry on these measurements to another place of decimals.2 That this was not 'really the state of things to which we are approaching' is shown by his own continuation: But we have no right to think thus of the unsearchable riches of creation, or of the untried fertility of those fresh minds into which these riches will continue to be poured...3 And although the great chemist Marcelin Berthelot in introducing his book, Les Origines de L'Alchimie, of 1885, wrote that the worm was then 'without mystery',4 scientists in other areas took quite the opposite view. Oliver Lodge concluding his book Modern Views of Electricity, of 1889,. wrote: 'Conclusion' is an absurd word to write at the present time, when the whole subject is astir with life, and when every month seems to bring out some fresh aspect...5 And we will leave Lodge with a resounding word on this particular point, in his lecture on 'The Discharge of a 9 Leyden Jar' at the Royal Institution, in the same year; The present is an epoch of astounding activity in physical science. Progress is a thing of months and weeks, almost of days. The long line of isolated ripples of past discovery seem blending into a mighty wave, on the crest of which one begins to discern some oncoming magnificent generalisation. The suspense is becoming feverish, at times almost painful. One feels like a boy who has been long strumming on the silent key-board of a deserted organ, into the chest of which an unseen power begins to blow a vivifying breath. Astonished, he now finds that the touch of a finger elicits a responsive note, and he hesitates, half delighted, half affrighted, least(sic) he be deafened by the chords which it woara seem he can now summon forth almost at will.6 This happy view sprang from Hertz' recent effective experimental confirmation of Maxwell's electromagnetic theory of light. Taken at face value this opposes ideas of a static or stagnant science at the time. And although things were not considered to be so happy in related fields such as spectroscopy and molecular kinetic theory,7 which were beset with problems and contradictions in the last quarter of the century, they were always lively. As for the question of whether a date near the turn of the century should yet be considered as marking the beginning of a revolution in science, this is a difficult problem of interpretation. L.P.Williams, for example, considers the whole lifetime of Queen Victoria (1819-1901) as manifesting revolutionary changes in the viewpoint of physics, biology and chemistry.8 My purpose is not to attempt a full answer to the question, but to point out that the understanding of the bases of, and changes in, physical science at the time of the discovery of radioactivity can be aided by studies of the preceding period. These can throw light upon the science of radioactivity, whose phenomena were interpreted in current physical and chemical terms in its early years before it developed some concepts of its own. Thus an outline of chemical atomic theory and speculations on the unity, complexity and evolution of the chemical elements points to the Proutian and 10 evolutionary ideas which as will be seen, provided some of the first tentative explanations of the new phenomena. So also a thread of physical theories leads via elastic, fluid, and electromagnetic optical ethers to Hertz' experimental work on electric radiation. Similarly via optical electron theories of matter to Zeeman's discovery of the magnetic influence on sources emitting light. The former provided the subject of Rutherford's earliest publications, which contributed to later ideas on radioactive radiations. The latter played a part in the development of electron or corpuscular atomic theories. Finally to be considered is a theme in part uniting the above threads which are mainly but not entirely independent. This comes under the heading of chemical physics or physical chemistry, in which a rather thin line of physicists attempted to apply their science to chemistry; this was complemented by a few chemists thinking in terms of physical theory. Chemical thermodynamics does come under this heading, but is not discussed here, since it did not directly influence the early development of radioactivity. But the study of the electrical phenomena and properties exhibited by chemical substances, especially in the gaseous state, including cathode rays, provided a vital link between several of the areas mentioned above, as exemplified by the work of the physicist J.J.Thomson and the chemist William Crookes. A union of aspects of physics and chemistry was forged strongly in the science of radioactivity but owed much to the earlier unifying thread. This thread itself contributed independently to experimental atomic physics, for example in studies on rays of positive electricity in the early twentieth century. Other areas not to be discussed in this chapter will be mentioned briefly as the story of radioactivity unfolds. Kinetic theory of heat, and chemical thermodynamics, for example, enter this story but briefly, although forming part of the acceptable knowledge of most physical scientists at the time. Some techniques and developments relevant to radioactivity such as the work of the Becquerels and Stokes on phosphorescence; photographic techniques for the study of radiation; electroscopes and electrometers; various 11 technical traditions of chemical analysis; discovery of the inert gases, will also be mentioned or brought out in later discussion. It must again be noted that the areas covered in this chapter cannot be considered as completely independent at any time. That the opposing processes of fragmentation and unification appear to have occurred together during the progress of nineteenth and early twentieth century physical science, makes the interpretation of threads problematical. So also does the ready transfer of ideas among the scientists who formed something of a European and Colonial community within which published and private communication was abundant. Several influential scientists lived and worked through the whole of the period of some fifty years from the time of Faraday, the rise of chemical spectroscopy and the creation of the periodic table of the chemical elements, until radioactivity was established as a school of research based on the theory of atomic disintegration and transmutation. These points could lead one to place separate threads of development in the mind of one and the same scientist; but this does not seem impossible. The areas discussed under the three main headings thus point, in largely uncharted territory, towards the scientific ideas or knowledge used by those involved with radioactivity; the treatment is not intended to do more than this. It may also, however, serve to provide part of an answer to the question of how far developments of science related to the new experimental discoveries, around 1896, of X-rays, radioactivity, corpuscles or electrons, and the Zeeman effect can be considered as novel. 12 2. Chemical atomic theories and the unity and complexity of the chemical elements In reviewing7sr-Tudies on radioactivity after her first year's work on the subject, Mme.Curie put forward several speculations on the source of the continually radiated energy, not yet the problem that it was to be. One was that: L'6nergie utilisable des substances radioactives diminue constamment. On pourrait, par exemple, rattacher la radioactivit6 a la theorie de Crookes stir 11 6volution des 616ments, en attribuant la radioactivit6 aux 616ments a gros poids atomiques, qui se seraient form6s en dernier et dont l'6volution ne serait pas encore achevee. 10 Although this does not seem to be the hypothesis she favoured (nor was it accepted by Crookes himself at the time), that Crookes' idea of more than a decade earlier could be brought up in this way indicates its currency during the last years of the nineteenth century. It turned out to be nearer the mark than either had supposed. In a similar way, and perhaps more important for radioactivity, J.J.Thomson first announced his demonstration of subatomic corpuscles in a lecture at the Royal Institution on 'Cathode Rays' in 1897.11 Here he referred to the earlier suggestions by William Prout, unnamed chemists, and Norman Lockyer on the constitution of the chemical elements: The assumption of a state of matter more finely subdivided than the atom of an element is a somewhat startling one; but a hypothesis that would involve somewhat similar consequences - viz. that the so- called elements are compounds of some primordial element has been put forward by various chemists. Thus, Prout believed that the atoms of all the elements were built up of atoms of hydrogen, and Mr. Norman Lockyer has advanced weighty arguments founded on spectroscopic consideration, in favour of the composite nature of the elements.12 Such hypotheses were thus neither dead nor forgotten at the end of the nineteenth century, though their origin lies at its beginning or earlier, with ideas of the unity 13 of all matter predating the atomic theory of Dalton, and running parallel to its early development. Knight13 has shown how the alternative point-atomism of Boscovich was entertained by Davy and Faraday, and illustrates the debatable character of atomic hypotheses from their time to the 1870's. Dalton's atomic theory required that there be as many different types of true atom as undecomposable elements. Of these there were about 30 at the time, rising steadily to about 75 by the end of the century.14 Prout's hypothesis implied that each Daltonian atom of an element consisted of a number of combined atoms of hydrogen. And the nearness of atomic weights to multiples of H=1, i.e. to whole numbers, became clearer throughout the century. But as analytical results became more certain, so too did the discrepancies, with the atom of chlorine obstinately weighing about halfway between 35 and 36 hydrogen atoms. Prout's hypothesis itself, and the saving device of using 0.5, or smaller, as the basic unit provoked strong criticism. But as Farrar, for example, has shown,15 modified Proutian ideas developed in the second half of the century gaining much from analogies with related scientific areas. The existence and behaviour of organic radicals showed that compound units could behave in a quasi-atomic manner; and homologous series had Proutian implications which were soon taken up. Here two substances, analogous to C and H in organic chemistry, are required, rather than the usual single substance of Prout's hypothesis. Then, about 1860, independent generalisations were impressed upon the scientific public, and their combined influence was to be of considerable importance. The attainment of consistent atomic weights was dependent upon the acceptance of the remarkable suggestion by Avogadro, revived by Cannizzaro in 1860, that equal volumes of gas contain equal numbers of particles, regard- less of the mass or nature of these. Also essential was the related postulate of double atoms, like 02, in gaseous elements, which entailed the problem of the affinity of like atoms and its implications. Discussions of chemical philosophy, the different kinds of atomism, and the problems 14

of chemical affinity in the first half of the century have been provided by Levere.16 Only after 1860 could the patterns and families of chemical elements be put together to form the great generalisation of the periodic table. This, together with the evolutionary ideas expressed in Darwin's Origin of Species (1859) and the rise of spectrum analysis in the 1860's,17 provided the elements which appear in newer 'Proutian' speculations from the 1870's to the turn of the century. William Crookes, already well known for his work on the radiometer, high vacua, and inorganic analysis, among other things, combined the points just mentioned with others deriving from geology or mineralogy. In his long- remembered address to the Chemical Section of the British Association18 in 1886 he remarked that: The array of elements cannot fail to remind us of the organic world. In both cases we see certain groups well filled up, even crowded, with forms having among themselves but little specific difference. On the other hand, in both, other forms stand widely isolated. Both display species that are rare; both have groups that are widely distributed - it might be said cosmopolitan - and other groups of very restricted occurrence. Among animals I may mention as instances the Monotremata of Australia, and among the elements the metals of the so-called rare earths.19 Biological evolution was considered to be an unceasing process from the remote past and still actually occurring in the present; but Crookes denied this implication for the evolution of chemical elements: The analogy here suggested between elements and organisms is indeed not the closest and must not be pushed too far ... Nor would I for a moment suggest that any one of our present elements, however rare is ... in process of extinction, that any new element is in the course of formation, or that the properties of existing elements are gradually undergoing modification. All such changes must have been confined to that period so remote as not to be grasped by the imagination ... The epoch of elemental development is decidedly over...20 This could have been written as a reply to Mme.Curie's suggestion of the evolution of the heaviest atoms thorium 15 and uranium a decade later; it does indeed seem to express his opinion of any such suggestions for radioactivity for most of the period 1898-1904 during which Crookes worked and wrote on the subject. It is true that at the beginning of his speech he spoke favourably of Norman Lockyer's dissociation hypothesis, set out during the previous decade: Mr. Norman Lockyer has shown, I think on good evidence, that, in the heavenly bodies of the highest temperature, a large number of our reputed elements are dissociated, or as it would perhaps be better to say, have never been formed. Mr. Lockyer holds that 'the temperature of the sun and the electric arc is high enough to dissociate some of the so- called chemical elements, and give us a glimpse of the spectra of their bases'. But Crookes, apparently lining up with a majority of chemists, spoke against this view with the question: Is there, then,in the first place, any direct evidence of the transmutation of any supposed 'element' of our existing list into another, or of its resolution into anything simpler? To this question I am obliged to reply in the negative ... The highest temperatures and the most powerful electric currents at our disposal have been tried, and tried in vain.21 However, he considered the mineralogical association of like elements, and chemical periodicity, to be indirect but undeniable evidence of a former though now frozen Evolution. This had begun from an original 'protyle', possibly 'helium' and developed into an oscillating periodic table linking temperature of formation with atomic weight, atomicity, electrical, and magnetic properties; as temperature slowly decreased the lighter elements were first formed, and finally thorium then uranium. These ideas are best not considered as purely speculative, for they were closely linked to the experimental chemistry of the time via Crookes' work on the rare earths. With some modification these notions were repeated over the following few years,22 although not 23 highly regarded by other chemists. The delicate weighings required in chemical analysis had led Crookes 16 to the radiometer effect24 ten years earlier. This in turn led to the study of rarefied gases, cathode rays, and the 'fourth state of matter', and thence back to 'radiant matter spectroscopy' as a novel aid to chemical analysis. The experimental thread is here easier to follow than his ideas on the molecular or atomic structure of matter; by 1886 Crookes had formulated an unusual interpretation of the new spectroscopic properties of some rare earth elements.25 Whereas radiant matter spectroscopy indicated five fractionated components of yttria for example, the ordinary spark spectrum and the chemical properties of the five were identical, indicating but a single element. He speculated: that the structure of a chemical element is more complicated than has hitherto been supposed. Between the molecules we are accustomed to deal with in chemical reactions and ultimate atoms as first created, come smaller molecules or aggregates of physical atoms; these sub-molecules differ one from the other, according to the position they occupied in the yttrium edifice.26 The alternative required an element for each spectrum, five new elements for yttria alone. The apparent identity of the ordinary spark spectra of the new components occurred because in 'the intense heat of the electric spark, the little differences of molecular arrangement vanish'. His 'compound molecule explanation' was supposed to apply generally, for 'had we tests as delicate for the constituent molecular groups of calcium' this too might be resolved into simpler groupings.27 In this he seems to stand not far from the dissociation hypothesis of Lockyer; both have similarities to the theories of some physicists. Crookes' query 'whether there is an absolute uniformity in the mass of every ultimate atom of the same element. Probably our atomic weights represent a mean value ...'28 may or may not derive from Liveing's earlier admission that considering the thousands of iron spectrum lines he was 'almost driven to ascribe them to a mixture of differing molecules, though we have as yet no independent evidence of this'.29 The above statements indicate some of the difficulties of the terms used at that 17 time, with Crookes or his reporters employing 'ultimate atoms' in two different ways, also 'physical atoms' as synonymous with 'sub-molecules' and 'smaller molecules', which are different from chemical 'molecules'. Liveing, however, may have meant aggregates of what had been defined as chemical atoms. The proposition that the chemical elements or atoms might be complex seems to have been far less controversial than its extension to an actual dissociation of the elements; some evidence pointed only to the first statement, but spectroscopic observations were taken as pointing to both. For example, G.G.Stokes the Cambridge physicist early considered dissociation. His publications span the years 1840-1902, and he communicated with scientists ranging from Faraday at the beginning of this period to Crookes, S.P.Thompson and Henri Becquerel on radioactivity at its end; his rising reputation had been aided by work on mathematical optics and fluorescence.30 In 1854 he wrote to William Thomson (later Kelvin)31 concerning the 'enormous length' of the line-spectrum obtained from electric discharge between metal points, compared to the spectrum then of greatest range - that of the sun: I cannot help thinking that decompositions of a very high order may be going on in such an arc (the voltaic arc I mean) and that a careful examination of these lines may lead to remarkable inferences respecting the bodies at present regarded as elementary. There is nothing extravagant in this supposition: few chemists I imagine believe that the so-called elements are all really such. Now it is quite conceivable that chemically pure metals should agree with compounds of sodium in giving the bright line D. If this were made out I should say that perhaps these metals were compounds of sodium, but more probably they and sodium were compounds of some substance yet more elementary.32 Stokes was most cautious on such matters in publications; he later found fault with Lockyer, who according to his most recent biographer, was not.33 The inferences of Lockyer, first made public in 1873, and based on comparisons between stellar, solar, and laboratory34 spectra, developed into a comprehensive scheme of 18 dissociation of the elements. Stokes wrote in 1876 concerning Lockyer's Preliminary Note to the Royal Society on the 'Compound Nature of the Line-Spectra of Elementary Bodies' saying that the simplification of the calcium spectrum observed in the sun might well be due to variations with temperature of the relative intensities of the various molecular vibrations existing and observed in undissociated calcium: Hence, while I regard the facts you mention as evidence of the high temperature of the sun, I do not regard them as conclusive evidence of the dissociation of the molecule of calcium.35 The term 'molecule of calcium' used in this context indicates the continued acceptance of some kind of complexity of the element, but is otherwise not very clear. This is perhaps an intentional reserve: Stokes was familiar with chemical practice and nomenclature.36 The usage is comparable with the similar expression of a structured 'molecule of uranium' used twenty years later in Stokes' comments on the origin of the Becquerel rays.37 Writing to Lockyer in 1879, and again criticising his inferences, Stokes made it clear that he did accept the compound nature of the elements and believed the view to be generally favoured: the question observe is not, Are the elements compound bodies? But, has any satisfactory evidence been now obtained that they are compound bodies? You would, I imagine, find plenty of chemists, from Prout downwards, who would regard it as most probable that they were compounded. I may say that, in common I suppose with multitudes of others, I have long supposed for my part that they were.38 As for Lockyer's evidence the effect of impurities had not been ruled out; and Dumas' arguments on atomic weights, on which Lockyer had asked Stokes' opinion, were not strong. The importance of impurities here is that Lockyer's thesis of increasing dissociation from laboratory flame, arc, and spark to stellar temperatures relied upon the existence of common lines exhibited by different elements - these indicated common simpler components of the so-called elements. The actual presence of remaining traces of each element in a chemically separated pair would give such 19 common lines. Lockyer's development of new techniques for determining which lines were caused by impurities and which were truly common to different elements, and his controversies with some chemists over this, have recently been described by Brock,39 McGucken,40 and Meadows.41 Besides the difficulty with impurities which always proved a danger to interpretation there was the question whether common lines were not accidental i.e. meaningless, or simply very close. The latter was less likely to occur with greater dispersion of the spectra. Liveing and Dewar in collaboration conducted detailed comparisons of some of the relevant metallic laboratory spectra and having caused the majority of coincidences to vanish they published conclusions highly critical of Lockyer's hypothesis.42 By 1885 the opinions of chemists, physicists and astronomers tended towards the view that the dissociation hypothesis did not hold water; but Lockyer considered that it fitted astronomical observations such as those on the heights of element lines in the sun and on the differing metallic lines present in the spectra of stars of different temperatures. He continued to publish books and articles setting out or invoking the hypothesis, publicly and privately put up spirited defences, and in so doing perhaps incited others to technical advance. In later publications such as The Chemistry of the Sun, 1887; 'On the Chemistry of the Hottest Stars', 1897,47—;nd Inorganic Evolution,1900, he was able to drum up supporting opinions for his work from several chemists. But even some of these, for example Berthelot, Brodie and Crookes were, or had been, critics of the dissociation hypothesis. This seems to fit in both with a general rejection of his evidence and with Stokes' analysis of 187944 that 'multitudes' of chemists and others considered elements to be most probably compounded. One could say that a continuing belief that elements were complex or compounded and occasionally even dissociated in some way, remained in the last decades of the nineteenth century, despite Lockyer. The literature of this period shows a continuing 20 interest in periodic tables; discussion of Prout's hypothesis with regard to atomic weights was still alive and had always implied more than mere numerical juggling. Rayleigh (J.W.Strutt) in his Presidential Address to the Physics Section of the British Association in 1882, introducing his plans for the redetermination of gas densities, said: The other subject on which, though with diffidence, I should like to make a remark or two is Prout's Law according to which the atomic weightsEir the elements, or at least many of them, stand in simple relation to that of hydrogen. Some chemists think this speculative, but: Others, impressed more by the argument that the close approximation to simple numbers cannot be fortuitous, and more alive to the obvious imperfection of our measurements, consider that the experimental evidence against the simple numbers is of a very slender character.45 The gas density determinations which Rayleigh thought would settle the question led instead to the discovery by Ramsay and himself of an entire new group of elements, the inert gases. Those discoveries of 1895-8 perhaps owed something to the periodic table,46 caused in return several further modifications of periodic tables by Crookes, Stoney and others, and were vital for Rutherford's interpretation of radioactive emanations three or four years later. In 1901 R.J.Strutt (Rayleigh) then working on cathode and radioactive rays published 'On the tendency of the atomic weights to approximately whole numbers'47 restating his father's views of two decades earlier, and estimating a thousand to one probability against the randomness of the current atomic weights. His conclusion that there must be some law behind this indicates the continuing belief. More remarkable than this however are views expressed by the chemists who considered that they had effectively demolished Lockyer's dissociation hypothesis. Liveing believed that the simpler spectral patterns might occur 'like the overtones of a string',. 48 we have seen that he attributed more complicated spectra to the complexity of the chemical elements, which might be of an aggregate type. 21 This was shortly after his and Dewar's first hostile and effective criticisms of 1880-1 against Lockyer's ideas on dissociation. In spite of this, Liveing in his Address of 1882, providing further speculations on Prout's hypothesis at this meeting, asked 'Why may not the chemical elements be further broken up by still higher temperatures? A priori and from analogy such a supposition is extremely probable'.49 Thus both the complexity and dissociation of elements were considered feasible by Liveing. Dewar's views expressed six years later in a lecture on 'Phosphorescence and Ozone150 agree with this: In this experiment ozone is formed by the action of a high temperature owing to the dissociation of oxygen molecules and their partial recombination into the more complex molecules of ozone. We may conceive it not improbable that some of the elementary bodies might be formed somewhat like the ozone in the whole experiment, but at very high temperatures, by the collocation of certain dissociated constituents and with the simultaneous absorption of heat. This seems to mean that some elements had been formed by the dissociation then reassociation of others; it may be an exception -to the view51 that the only reasonable opinion at the time took the result of intense heat to be dissociation only. Some clarification of the ramifications of chemical atomism seems to have occurred by the end of the next decade as relevant studies on cathode rays and radioactivity were beginning to develop. However, Liveing's conclusions 'On the Flame-Spectrum of Mercury and its Bearing on the Distribution of Energy in Gases' of 189852 displays a stronger debt to spectroscopy and kinetic theory. The significance of mercury here stemmed from its status as an element whose vapour was definitely monatomic and not aggregated; this was well established by chemical experiments on combining weights and vapour density, and by gas kinetic theory. A difficult deter- mination53 of the velocity of sound in this vapour indicated the high ratio of specific heats expected for a monatomic gas which could store none of the absorbed heat energy internally. These corroborating results 22 were difficult to reconcile with spectroscopic observation and theory, which took the complex spark spectrum of mercury vapour to be caused by a multitude of internal vibrations. Liveing however regarded: the production of spectra by an electric discharge as essentially a different process from the production by heat ... a great many rays are given out by various elements in an electric discharge which have never been observed to result from mere heating.54 This point seems to distinguish Liveing's view from Stoney's electron theory of 189555 and from other electrical theories of spectra, to be discussed.56 Liveing's studies of the non-electrical excitation of spectrum lines in a high temperature flame, where chemical combination also could not occur, gave him a means of easing the problem of mercury. He considered that: heat ... is, in part, transformed into vibratory motion which affects the ether; and the true inference from the ratio of the specific heats appears to be, that, at the temperature at which this ratio was measured, the amount of heat converted into vibratory motion is very small...57 He identified the gaseous mercury 'molecules' of physical kinetic theory with 'chemical atoms', and remarked: It is possible that a chemically monatomic molecule may have, though it is not probable that it really has, a simpler constitution than a chemically complex molecule, and so may have not so many degrees of freedom as the latter, but still a plurality of degrees.58 Thus Liveing shows a clear usage of the terms of kinetic, chemical, and spectroscopic theories, and illustrates his current picture of gaseous molecules composed of one or more chemical atoms, which themselves have constituents capable of complex vibration. But evidently the principle of equipartition of energy among all degrees of freedom has been tacitly modified or sacrificed. This principle was one of Kelvin's 'clouds' over the dynamical theory of heat of 1901.59 In discussing the problems of 'practically monatomic' gases he attempted in a confused way to disperse the cloud by postulating 'satellites' of the atoms, with far smaller mass, which could be the 'ions' of J.J.Thomson. It is possible that Liveing had been similarly influenced 23 by the chemical atomic theory which Thomson founded in 1897 upon his new discovery of subatomic material particles. If so, then according to Thomson's later account60 he was one of very few at this time. Thus we see that a continuing theme involving the notions of unity and complexity of the chemical elements, .based on observational and experimental evidence, runs into the era of cathode rays and radioactivity. But questions concerning J.J.Thomson's choice of Lockyer for support in 1897 are interesting and difficult. If Thomson took note of Liveing's views, and they did meet at the Cambridge Philosophical Society, and from 1893 at the less 61 formal Cavendish Physical Society, one might have expected him to be aware of the past and present low reputation of the relevant parts of Lockyer's work. Thomson could indeed have quoted the words of Liveing himself on the complexity of the elements, as considered above. Stokes too, who had discussed cathode rays and X-rays with Thomson62 in 1896 or 1897, shared Liveing's 63 opinion. If only the most recent work of Lockyer were seized upon, ignoring as many did64 his earlier studies, there were again strong criticisms by the reputable Schuster.65 The latter suggested at the Royal Society discussion meeting of March 25th 1897 that the difference between stars emitting hydrogen and metal spectra for example was as well explained by differences in density and convection of layers of ordinary elements as by dissociation. For proof of dissociation, he stressed, traces of other elements from the electric discharge between iron poles must be found. But Lockyer had already blundered, along these lines,66 and surely no one seriously expected the challenge to be met. Thomson's first brief announcement of 'a state of matter more finely subdivided than the atom of an element' was made at a Royal Institution Lecture at the end of the next month. He later recalled in his autobiography67 that its reception was poor, and that it was perhaps not taken seriously. Such a response is understandable when one considers that he claimed quite clearly to have produced 24 by electric discharge not just the traces of other elements from iron, as recently demanded of Lockyer by Schuster, but a common material component of all chemical elements. The corpuscular atomic theory of Thomson was developed during the time of the young Rutherford's work and collaboration at the Cavendish Laboratory on closely related areas. This was to be essential to both men's understanding first of uranium rays and then of radioactivity. We shall see that Thomson's ideas of the complexity of the chemical elements were of long standing, and founded on physical theories of ether, electricity and matter. He was able to take up part of a long thread of chemical Proutian ideas and to weave it into his own physicists' theory of atomic structure. Others too who worked and thought upon radioactivity saw the significance of these ideas.

3. Physical theories of matter, electricity, ether When Hertz began his magnificent experiments on electric oscillations, there were many theories of electrical action. When he had finished them there was only one, Clerk Maxwell's. So said J.J.Thomson68 in 1894, shortly after the untimely death of Hertz. This was the year of Rutherford's first publication on magnetism and electric oscillations. In the 1860's Maxwell had developed a unified theory of optics and electromagnetism involving the explicit provision of a single ether through which both kinds of effect were considered to be transmitted. But this was slow to be accepted. As Schaffner has recently commented69 at the time there were not only competing theories of electromagnetic action but a number of non-electrical optical theories. While optical and electrical theories 25 came closer together in the minds of many scientists after the experiments of Hertz interactions with a third theme of electrical chemistry had already been increasing during the 1880's. The physical side will be discussed in this section in so far as it can be considered separately; both physicists and some chemists during this period were prone to look back to Faraday as an earlier master of chemistry, electrochemistry, and electromagnetism.70 Aspects of all these areas became more strongly united in the 1890's as spectroscopy, physical theories of the chemical atom and studies on radioactivity progressed. Maxwell in his Treatise on Electricity and Magnetism of 1873 went so far as to write 'before I began the study of electricity I resolved to read no mathematics on the subject till I had first read through Faraday's Experimental Researches on Electricity'.71 Maxwell considered that he had combined the methods of the German school of 'electricians and mathematicians', which involved 'action at a distance impressed on the electric fluids', with the more pictorial ideas of Faraday which involved 'real actions going on in the medium'. Faraday however had earlier expressed doubts as to the existence of such a medium72 and it was found to be a problematical concept, if a valuable one, even after Maxwell's work. The problems are indicated by the variety of ethers73 which in the nineteenth century were used in conjunction with physical principles to explain optical and electrical phenomena. R.T.Glazebrook, senior demonstrator at the Cavendish Laboratory, in his major 'Report on Optical Theories' to the British Association in 188574 discussed the major optical ethers and their relations with matter with regard to their explanations of reflexion, refraction, dispersion, diffraction, polarisation and other phenomena. None was without its flaws, but: The electro-magnetic theory, if we accept its fundamental hypothesis, is thus seen to be capable of explaining in a fairly satisfactory manner most of the known phenomena of optics. 26 The great difficulty is, as we have said, to account for the properties which the medium must have in order to sustain electrical stresses.75 Illustrative of the requirements of what may have been a mainly British viewpoint, Glazebrook saw a similar difficulty: of realising mechanically what electric displacement is, of forming for oneself a physical idea of a change of structure in some medium of unknown properties which shall obey the laws implied by the various equations satisfied by the components of electric displacement. 76 The earlier conflict between the rigidity required for transmission of light waves, and the fluidity necessary for free passage of the planets, had been eased by Stokes' view that these properties could be compatible in a medium both of small density and low rigidity. But to conceive of such a rigidity existing in an ether which was like an elastic solid, would not be sufficient to account for its capability of sustaining known electrical stresses. The solution might be to have a non-rigid, fluid ether, with a quasi-rigidity conferred upon it by filling it with vortices, in the form of filaments or rings. In Glazebrooke's opinion this could explain transmission of transverse waves and maintenance of electric stress, whilst electric and magnetic polarisation would then consist of definite arrangements of the filaments or rings. Similar views were held by some others at the time. Earlier in 1885 G.F.Fitzgerald had written77 from Dublin to J.J.Thomson with detailed suggestions of the 'infinite possibilities in a vortex-sponge' either of 'ring vortices i.e. molecular' or of filaments78 for explanations of electrostatic and electromagnetic phenomena. J.J.Thomson too in the previous few years had been developing an ether theory involving vortices, attempting to explain not only these physical phenomena, but also chemical matter and its properties, as will be seen in the following section. But the adherents to electromagnetic-optical ether theories were not great in number and were probably mostly British, until the 1890's. 27 Included in this school, Joseph Larmor, at Cambridge, developed a theory of optics and electrodynamics starting from a single ether and using a form of the Principle of Least Action expressed in terms of potential and kinetic energy. By 1893 he had developed 'A Dynamical Theory of the Electrical and Luminiferous Medium',79 and 'a method of evolving the dynamical properties of the aether from a single analytical basis': We shall show that an energy-function can be assigned for the aether which will give a complete account of what the aether has to do in order to satisfy the ordinary demands of Physical Optics; and it will then be our aim to examine how far the phenomena of electricity can be explained as non-vibrational manifestations of the activity of the same medium.80 Apparently for reasons concerning an explanation of the forces between permanent magnets, Larmor modified his view in a later addition entitled 'Introduction of Free Electrons'.81 He considered these electrons as 'electric centres' or 'nuclei of radial rotational strain', having adopted Stoney's expression for the electrolytic unit charge, and had a few words to say on the electrical nature of chemical energy and spectra. In another of the relatively few references to recent experimental evidence Larmor stated that J.J.Thomson had informed him of his determination of the velocity of the negative rays in vacuum tubes. This phenomenon Larmor saw as the projection of free electrons of purely electrical inertia. Our brief mention of studies of cathode rays is reserved for the next Section; these cannot however be entirely separated from mathematical physical theories, as Larmor's interesting interpretation shows. The ether electron theory of Larmor was shortly preceded by a differing and now much better known electromagnetic-optical theory of electrons - that of 82 83 Lorentz, published in 1892. Hirosige and McCormmach84 have described the structure and development of this theory in some detail. Lorentz here seems not to have unified the entities of matter, ether and electricity as strongly. as Larmor and others, but sought explanations in terms 28 of ponderable matter and a static ether connected via electrically charged particles, to which the usual principles of Newtonian dynamics and of energy are applied. The positive or negative charge was considered to be fundamental and was not explained further in terms of ether in the manner of some British physicists. The fundamental charged particles were supposed to be spherical, to possess mass and weight, to be contained within the ponderable molecules of which all matter consists, and to form the sole connection with the co-existing ether.85 One cannot say that the possibilities for chemical or electrochemical explanations were explored, though Lorentz acknowledged his debt to Helmholtz and Weber who had had some such interests in previous years. Lorentz' theory of 1892 and 189586 did however contribute indirectly to J.J.Thomson's first exposition of a corpuscular chemical atomic structure in 1897 via the discovery of Zeeman announced a few months earlier. Zeeman87 stated that he had used Lorentz' theory to give an actual estimate of the charge to mass ratio of the charged particles whose oscillations were taken to be the cause of the etherial vibrations constituting bright spectral lines. The discovery of the widening of the yellow sodium doublet under magnetic influence - the 'Zeeman effect' - and the quantitative use of Lorentz' theory gave a charge to mass ratio which Thomson noted88 as agreeing with that of his cathode ray corpuscles. Electronic or corpuscular atomic spectroscopy was developed rapidly after this time, but not to any great extent by Thomson nor by the workers on radioactivity, though its conclusions remained relevant. Lorentz saw his electron theory as deriving in part from Maxwell, Weber and Helmholtz. Larmor too gave credit to these scientists and to an Irish school of mathematical physicists including MacCullagh and Fitzgerald. Both of these later proponents of an electromagnetic electron theory shared the general view of the importance for their work of the striking experiments of Hertz a few years earlier. Hertz informed English readers in a collection 29 of his papers, Electric Waves being Researches on the Propagation of Electric Action with Finite VelocitTg9 that his experiments had been guided by Helmholtz' theory. This normally invoked Newtonian action-at-a-distance, but in a limiting case gave some results similar to Maxwell's with regard to the speed of propagation of electrical and magnetic quantities. Hertz devised simple apparatus for production and detection of electrical effects in the air, and demonstrated standing waves, reflection, refraction, polarisation, and a variety of measureable wavelengths, of the order of metres. His work of the late 1880's was accepted by physicists as an impressive demonstration of the validity of a unified optical-electromagnetic ether. This led to the fairly rapid publication not only of learned articles on these lines but of textbooks of various kinds. The use of these books by young scientists shortly to become involved in investigations of the radiation from uranium is of interest, and may be significant for interpretations of radioactivity. Paul Drude's Physik des Aethers auf elektromagnetischer Grundldge, published in 1894,90 was used by Marie Curie91 perhaps for her earliest researches on magnetic properties of tempered steeis.92 Its brief explanation of fluorescence and phosphorescence93 appears to be essentially a reinterpretation and development in electromagnetic terms of Stokes' ideas of some forty earlier.94 Drude represented the molecule of a body as a closed wire circuit whose natural electric vibrations are doubled in wavelength on changing to a linear form under intense excitation. This could also account for thermoluminescence, so-called by E.Wiedemann, where heating alone causes characteristic luminosity in some substances. Radioactivity was at first vaguely interpreted in terms of phosphorescence by Becquerel, then by Mme.Curie in terms of fluorescence, a rapid re-emission of received rays. Drude's book shows the electromagnetic view of optical phenomena generally accepted at the time; and he refers95 to recently published textbooks by Boltzmann96 and Poincare,97 which professed some differences in their 30 approach, and which were also available to the student. E.Wiedemann, who aided the work of G.C.Schmidt on fluorescence and the new uranium and thorium rays in 1895-9, shared Drude's electromagnetic view of fluorescence. In a letter98 to Stokes early in 1896 concerning his own and Dr.Schmidt's work on fluorescence of metal salt solutions and metal vapours he indicated his view that even with monatomic gases the illuminating mechanism was more complex than commonly supposed, that the emission from mercury vapour contradicted the kinetic theory of gases, and that the explanation might lie in supposing the molecule comparable to a type of oscillatory electrical circuit. Thus we see that the view of the 'molecule' behaving as an electrical system was developing soon after the work of Hertz of 1888. The identification of this electrical molecule with the chemical atom may be inherent in Wiedemann's letter; so also may the not uncommon idea • of complexity within the chemical atom; but these ideas were not explicitly discussed, nor are they suggested in Drude's book. Whether or not the idea of an electrically composed chemical atom occurred to Schmidt or Marie Curie before or after J.J.Thomson's corpuscular atomic theory, neither appears to have clearly used the idea for the early interpretation of radioactivity. The conception of an electrically constituted chemical atom appears more definitely in the textbooks of 0.Lodge and J.J.Thomson cited by the young Rutherford in his first published researches, of 1894.99 Thomson, having edited the third edition of Maxwell's Treatise on Electricity and Magnetism, 100published a considerable supplementary volume.101 Besides detailed treatments of electrical oscillations and Hertzian electromagnetic radiation to which Rutherford referred specifically102 there is a description of Thomson's own development of Faraday tubes of force for the understanding of chemical combination103 and the passage of electricity through gases.104 This approach dates from the early 1880's and contains clear allusions to the etherial-electrical 31 construction of chemical atoms.105 The Modern Views of Electricity of Oliver Lodge of 1889106 was one of the many Maxwellian books published soon after Hertz' experiments. Here Lodge gave non- mathematical accounts of electricity, magnetism and radiation. His explanations were based on a single fluid ether 'a continuous incompressible perfect fluid filling all space' possibly consisting of 'interlaced vortex filaments like a sponge' and were illustrated with one of the most thoroughly mechanical or machine-like depictions of its motion yet produced. Air and other dielectrics had interlocked wheels, with cogs upon them, to represent their etherial molecular structure, but 'in a metallic conductor the gearing is imperfect; it is a case of friction-gearing with more or less lubrication and slip, so that turning one wheel only starts the next gradually'.107 The oscillatory charge and discharge of a Leyden jar, much discussed after 1888, he illustrated by a device of weights and pulleys balanced by elastic strings, with sliding joints accounting for the residual charge effect. Despite the popular exposition in parts,108 Lodge had many points to make in this book and elsewhere which were found to be valuable. His discussion of rapidly varying magnetic fields, and the 'skin' effect of currents starting from the outside of a conductor, for example, were noted by Rutherford109 who may also have noticed some suggestive comments on phenomena explained by the electrical nature of chemical atoms.110 These then are some of the areas developed by physicists in the last decades of the nineteenth century, which form a background to the understanding of the younger physicists beginning their researches in the 1890's. 32 4. Chemical physics. Physical chemistry Aspects of chemistry and of physics in the 1880's and 1890's have so far been discussed. Perhaps even more important than either of these from our point of view were the attempts at applying physical theories to chemistry, together with the study of areas considered to be intermediate between the two, which became stronger during this period. Lodge's book of 1889 contains allusions to aspects of chemistry which are clear, if brief. He speculated as to whether the etherial 'whirls' or wheels might represent not simply electricity but 'atoms',111 and seems generally to have been prepared to use the word atom in its chemical sense. For example, in a chapter on the 'Mechanism of Electrical Radiation' the emission of light, understood as an electromagnetic phenomenon, is attributed to oscillation of the unit electrical charges whose existence he took electrolysis to have demonstrated: It can be calculated that the oscillation of an atomic charge would give rise to only ultra-violet rays. It is probably because these ultra-violet rays synchronize with the period of vibration of atomic charges that they have such extraordinarily powerful chemical effects...112 Lodge was one of the first to attribute the effect of heat in producing definite spectra of chemical substances to the thermal, hence mechanical, oscillation of the charged atoms in a molecule: Under the influence of heat the components of the molecule are set in vibration like the prongs of a tuning fork, the rate of vibration depending on and being characteristic of the constants of the particular molecule. The atoms being charged, however, their mechanical oscillation is necessarily accompanied by an electric oscillation, and so an electric radiation is emitted and propagated outwards...113 Lodge's suggestion as to the explanation of phosphorescence involved 'atoms receiving indirectly some of the ethereal disturbance, and so prolonging it by their inertia, instead of leaving it to the far less inertia of the ether alone'.114 He referred115 to the recent researches of Hertz, Ebert, Wiedemann and others on electrical effects of ultra-violet light, and attributed these to the same cause as the 33

chemical effects of light; this was to be clarified by J.J.Thomson and his associates C.T.R.Wilson, E.Rutherford and others at the Cavendish Laboratory in the next few years and by the German physicists J.Elster and H.Geitel, who were to develop strong interests in radioactivity. We recall that about five years after Lodge's account Drude, whose book of 1894 was consulted by Marie Curie, qualitatively compared the 'molecule' to an oscillatory electric circuit. Now although Lodge seems to have accepted that the mechanical oscillation of fixed atomic charges was the source of the luminosity caused by heating, he also suggested what appears to be an alternative idea that 'those short ethereal waves which are able to affect the retina, and which we are accustomed to call "light", may be really excited by electrical oscillations or surgings in circuits of atomic dimensions'.116 Rough estimates for a single loop of wire showed that this circuit would have to be of atomic dimensions to give frequencies of the right order. G.J.Stoney, however, in 1891 made a distinction between the two ideas, and specifically criticised not Lodge's version, but the suggestion that discharges between molecules could be the source of spectra: The lines of the spectrum of a gas are due to some events which occur within the molecules, and which are able to affect the ether. These events may be Hertzian discharges between molecules that are differently electrified, or they may be the moving about of those irremovable electric charges, the supposition of which offers the simplest explanation of Faraday's law of electrolysis ... Several considerations suggest that the source of the spectral lines is to be sought not in the Hertzian discharges, but in the carrying about of the fixed electric charges, which, for convenience, may be called electrons.117 This statement was reaffirmed and extended in 1894118 when he wrote that 'the motions going on within each molecule or chemical atom cause these electrons to be waved about in the luminiferous aether' and that 'the only other conceivable source of these spectra is excluded, viz., Hertzian undulations consequent upon electric discharges within and between the molecules'. This exclusion was 34 owing to Fitzgerald's estimate that the frequency would be higher than any known part of the spectra of gases. The proportionality of the wavelength emitted to the geometric mean of the capacity and self-inductance of a conducting circuit was the vital relationship for Lodge's calculation,119 but that accepted by Stoney and leading to such different conclusions is at present not clear. The cause of radiation adopted is of interest for its use of the electrolytic 'electron' entity, a term which was coined by Stoney himself at about this time.120 Considering its importance in the physical science of the next decade it is valuable to see briefly how this had arisen. A mathematical physicist with strong interests in kinetic theory, spectroscopy, and the chemical periodic table, G.J.Stoney had in 1871 provided a quantitative explanation of line spectra.121 The harmonics of a vibrating string were considered as the basis of a mathematical comparison, but the quantitative aspects were refuted by A.Schuster by the end of the next decade.122 During these years however, Stoney began to produce the basis of his electron theory which provided explanations in some areas of contact between physics and chemistry in the 1890's. A paper read to the British Association in 1874123 and published only in 1881, entitled 'On the Physical Units of Nature'124 combined chemical atomic theory with Faraday's Law to give the required Physical Unit of electricity: And, finally, Nature presents us, in the phenomenon of electrolysis, with a single definite quantity of electricity which is independent of the particular bodies acted on. To make this clear I shall express 'Faraday's Law' in the following terms, which, as I shall show, will give it precision, viz.:- For each chemical bond which is ruptured within an electrolyte a certain quantity of electricity traverses the electrolyte which is the same in all cases.125 Stoney here defined and discussed the chemical atom, gaseous molecule, and the 'hands or feelers which each atom has and which by grappling with the hands or feelers of other atoms, establish bonds between them'. He seems not to say that the bonds are purely electrical in nature, but stresses 35 that in electrolysis 'a definite quantity of electricity traverses the solution for each bond that in separated'. 126 Helmholtz' emphasis in a similar discussion in the same year was somewhat different as will be seen. The definite charges were employed by Stoney in a new explanation of spectral series of doublets, and triplets, about 1890. This involved mathematical analysis of waves emitted by charges following orbits within a molecule, this consisting of one or more chemical. atoms which may possibly be vortex atoms.127 What became of radiation produced by rotational and vibrational motion of molecules, which they possess according to kinetic theory, is not explained but this was a problem not only of Stoney's theory. By 1895 he had combined this electron theory with the kinetic theory of gases to account qualitatively for several phenomena of chemical physics such as phosphorescence, gas absorption spectra, luminosity in certain chemical reactions, and the ratio of specific heats of gases with complex molecules. His explanation of phosphorescence is of interest with regard to Becquerel's understanding of the newly discovered uranium rays in terms of this phenomenon six months later; the latter did not suggest details of mechanism, electrical, electronic or molecular. Stoney classified vibrations within molecules according to the ease with which they exchange energy with translational motion, upon collision. A motions are immediately affected, C motions are unaffected by collisions, and B motions are intermediate: Thus, when a phosphorescent body has been exposed to suitable light, t is an electron associated with Bb motions18 that is primarily acted on by the aether.129 When an electron is associated with more rapidly exchanging Ba motions or events, etherial vibrations received are transferred rapidly to translational motion, phosphorescence does not occur, the temperature rises and etherial undulations cease 'in other words, 130 the gas is one that has an absorption spectrum: 36 Extending this kind of explanation he noted that'the number of electrons within an atom may be greater than its place in Mendeleeff's table would seem to suggest' as shown by the chemical behaviour of potassium and bonds between molecules in crystals. The electron is now taken to be the principal factor in any chemical bond, and formulae such as H.CiC.H for acetylene, with one electron per bond, are given. He suggested that 'it is when excited by chemical reactions that electrons produce their most conspicuous luminous effects' which included the luminosity in electric discharge tubes.131 Though it is uncertain whether the electron theory of Stoney of 1895 and earlier was directly used by those studying radioactivity, the publications were readily available, and similar ideas were shared by others. Among these Arthur Schuster wrote not of the 'electrons within an atom' as had Stoney, but of electrons 'moving along the surface of an atom' about positions of equilibrium, to account for line spectra.132 Larmor's rotational ether electrons and their applications have been mentioned, and J.J.Thomson's ideas were akin to these at the time. In Germany similar developments were occurring; Ebert's paper on 'Heat of Dissociation 133 according to the Electrochemical Theory' of 1894 provided quantitative evidence that the energy of chemical bonds was purely electrostatic and not 'specially chemical'. He credited Helmholtz with the first statement, in 1881, of an electrolytic atomic charge. Stoney wrote134 to claim priority over Helmholtz on this point, and over Ebert for his auggestion135 that 'motions going on within each molecule or chemical atom cause these electrons to be waved about in the luminiferous ether' to produce spectra.136 The suggestion that the luminous radiation produced by friction, disruption of a crystal, and chemical reactions could be understood in terms of electrons which 'are started into activity'137 is of interest; for Rutherford struggled with this kind of idea, as did others, in studying the properties of radioactive emanations and their radiations some five years later. 37 It is to be noted that such a union of chemical and electrical science had developed comparatively recently; C.A.Russell has described the Faraday Lecture delivered by Helmholtz to the Chemical Society of London in 1881138 as marking the beginning of 'The Renaissance of Electro- chemistry', after the subject had suffered thirty years of disrepute.139 The claims of Stoney are ignored by Russell, and with justification, for although prior by some months, his statements of 1881140 seem not to have had the same influence nor to have gone as far as those of Helmholtz towards a purely electrical theory of chemical combination. Indeed at this time Stoney seems to have gone little beyond Maxwell's comments of 1873 that electrolytic phenomena necessitate the assumption of definite molecular charges,141 but that 'chemical combination is a process of a higher order than any purely electrical phenomenon'.142 On the other hand Helmholtz in 1881 while admitting 'other molecular forces' did stress that in all compounds 'the very mightiest among the chemical forces are of electric origin. The atoms cling to their electric charges, and opposite electric charges cling to each other...'143 That there was arising interest by the 1890's in the subject of electrochemistry, together with the study of colligative properties such as osmotic pressure, is shown by the publication of experimental and theoretical papers, some controversial, by many authors.144 Oliver Lodge provided a report 'On Electrolysis' for the British Association in 1885145 at the urgent request of H.E.Armstrong, President of the Chemical Section, who found Helmholtz' ideas of chemical affinity not proven. Lodge's words show a not entirely willing interest in the subject for 'though convinced of the immense importance' of its study, he considered it had 'the somewhat repulsive character attaching to any borderland branch of science - in this case not wholly physics nor wholly chemistry'.146 Other scientists saw this subject's importance but did not take Lodge's view of borderland branches. Some physicists, and chemists, though few in number found the investigation of such areas 38 rewarding. And it is true that most of the older scientists who had sufficient interest to publish on the new subject of radioactivity in the first few years of the twentieth century, were those studying such border areas during the last decades of the nineteenth. Among chemists were Armstrong, Crookes and Mendeleef; physicists include Becquerel, Kelvin, Lodge, Stoney, Schuster and J.J.Thomson.

The work of J.J.Thomson is of particular significance for radioactivity both with regard to his own interpretations of its problems, and to his influence upon Rutherford, before, during and after the latter's three years (1895-8) at the Cavendish Laboratory. From early in his scientific career Thomson developed strong interests in Maxwell's electrical theory and in the application of such physical theories to aspects of chemistry, especially with regard to the electrical properties of gases. By 1894, reporting to the British Association on 'The Connection between Chemical Combination and the Discharge of Electricity through Gases'147 he could conclude that his experiments: give hopes that the study of the passage of electricity through gases may be the means of throwing light on the mechanism of chemical combination. The work of chemists and physicists may be compared to that of two sets of engineers boring a tunnel from opposite ends—they have not met yet, but they have got so near together that they can hear the sounds of each other's works and appreciate the importance of each other's advances.148 These hopes, shared by others at the time, were soon to be fulfilled. However, Thomson's more ambitious ideas of the physical structure of chemical atoms date back at least as far as the early 1880's. These derived from the vortex atom theory of Sir William Thomson149 who had taken Helmholtz' mathematical treatment of vortices in a perfect fluid (1858) to represent chemical atoms composed of fluid 150 ether in motion; the former had been impressed by Tait's 39 smoke-ring demonstrations (1867). Permanence, indestructibility, the gas laws, and many possible modes of vibration for spectra, were all accounted for quantitatively or mainly qualitatively by single, linked, or knotted vortex rings. The explanation of gravitation and the inertia of matter was problematical as Maxwell's resurrection of Le Sage's theory indicates.151 Maxwell pointed out that this theory had the flaw of predicting a temperature rise of the material bodies involved due to the impact of the etherial corpuscles which were supposed to cause the net gravitational force. Kelvin in 1881152 said that he would not be satisfied with the vortex atom theory until chemical affinity, electricity, magnetism, gravitation, and inertia could be explained by it. He pointed to the insoluble contradiction between the isotropy of gravity and the anisotropy of crystals; he was evidently also unable to develop the chemical aspects of the vortex atom theory. And by about 1890 he seems to have rejected it in favour of Boscovichian explanations of physical and chemical properties; Kelvin now used action-at-a-distance force laws between atoms of matter - meaning chemical subatoms e.g. H = (h h) - and atoms of an electric fluid.153 Attempts to find unified explanations of the properties of matter, electricity and luminiferous ether were not so strong in Kelvin's thought as in that of some other British scientists, from the 1890's through the early period of radioactivity. J.J.Thomson was awarded the Adams prize of 1882 at Cambridge for 'A general investigation of the action upon each other of two closed vortices in a perfect incompressible fluid'. His mathematical treatment was published in the following year as A Treatise on the Motion of Vortex Rings154 with some significant additions on chemical applications. Referring to Kelvin, as he did on many points, Thomson introduced the study by pointing out that the vortex ring 'possesses many of the qualities essential to a molecule that has to be the basis of a dynamical theory of gases', it is indestructible and indivisible; 'the strength of the ring155 and the volume of liquid composing it remain for 40 ever unaltered', and rings 'will retain for ever the same kind of be-knottedness or linking'. It possesses kinetic energy by virtue of translational motion, and 'it can also vibrate about its circular form, and in this way possess internal energy', which was promising for explanations of heat and radiation. That the treatment was almost entirely kinematical, once having accepted Helmholtz' hydrodynamics, was taken to indicate its more fundamental nature compared with the ordinary 'solid particle' kinetic theory which required the assumption of repulsive forces.156 Thomson developed Kelvin's vortex atom version of gas kinetic theory as far as a derivation of Boyle's law and gave some suggestions for possible experiments to decide between this and the ordinary kinetic theory;157 it is not known whether these were tried. Applying these results to gaseous chemical compounds Thomson assumed that atoms combined in the manner of the association of two vortex rings of equal strength, when one overtakes the other. In this case, providing their dimensions are compatible, the hinder one passes through the one in front, they do not separate, but continue to circulate in and out of one another; a transverse section would show two separate circles rotating about a point midway between them. The disturbance of neighbouring rings would alter the radii and cause a brief separation resulting in continual change of partners as in the theory of Clausius and Williamson. The ratio of paired to free time is of importance with respect to whether combination occurs or not, and this factor might link the chemical strength with the dielectic strength of a gas. His attempts to extend the results for linked columnar and, mathematically similar, ring vortices to chemical bonding and atomic structure is of considerable interest. Firstly, the kind of linking considered was not in the manner of a chain, but like the strands of a twisted rope, thus: take a cylindrical rod and describe on its surface a screw with n threads ... bend the rod into a circle and join the ends, then each of the n threads 41 of the screw will represent the central line of the vortex core of one of the n equal linked vortices...158 The vortices could be independent twisted rings, or if not joined end to end there could result 'an endless thread with n loops'. He assumed that the atom of each element was composed in either of these similar ways, of a number of rings, or a single ring. It was demonstrated that six rings (or columns), or less would maintain a stable motion; the transverse section would show, for example, six separate circles on the points of a hexagon moving around the midpoint of the hexagon;159 this he compared with Mayer's experiments on the stable arrangements of thin vertical magnets floating in water. Thomson used the same analogy in 1897 when his corpuscular atomic theory was first set out.160 The picture of chemical combination is difficult to discern from Thomson's descriptions and is in need of clarification.161 The combination of two atoms each of a single ring is described above, and it is this kind of circulating and mutual overtaking motion which applies both to the linked rings within a single complex atom, and to the associated rings in a molecule of two or more atoms. Thus when two of the complex atoms have combined to form a molecule, the vortices form a unified system; should there be a total of more than six rings, they must group into 'primaries' consisting of six or fewer 'secondaries'. A transverse section of any molecule would show single circles, or groups of circles (primaries), arranged on each point of a polygon. Each primary group rotates about its own point, and the polygon itself rotates about its centre. An explanation of chemical valency, the main burden of the later additions to the essay, relied on the simplifying assumption that the strengths of all the rings composing an atom are equal. This led to the result that for atoms to form a stable molecule, the strengths of the primaries must be equal. This determines valency, and Thomson gives as an example an atom of two rings combining with an atom of one ring: 42 since for stability of connection, the strength of all the primaries which form the components of the compound system must be equal; the atom consisting of two links must unite with molecules containing two atoms of the one with one link.162 Thus the number of linked rings per atom is taken to be the fundamental valency. That HO is far less stable than H2O follows from the implicit supposition that the three rings of HO could not arrange themselves symmetrically in section. With the assumptions firstly that the linked rings of a single complex atom can break in various ways into two or more primary groups, and secondly that the single rings of monovalent atoms can join closely into a compound primary, again with a maximum of six rings per primary, apparent variable valency was explained. Water was probably not H-H-O, with three primaries and oxygen a monad, but more likely H2-0 with only two primaries and oxygen divalent. Hydrogen peroxide would then be H2-0-01 with three primaries, each of two secondaries. Thomson's further development of the theory in the following year, 1884,163 provoked a strong reaction from the physical chemist Ostwald,164 which was perhaps a marginally better reception than being ignored. During the following decade Thomson published no further descriptions of detailed atomic structure, and attempted to apply more general methods such as physical dynamics and the thermodynamics of Gibbs to aspects of chemistry.165 The most interesting aspect of his work from the point of view of our study was that on the relation of electricity with chemical combination in the gaseous state, which formed a major part of his researches. In his paper 'On a Theory of the Electric Discharge in Gases'166 published shortly before the Treatise of 1883 Thomson made an assumption strongly unifying matter and electricity - both were seen as manifestations of the same ether: Let us now suppose that we have a quantity of gas in an electric field. We shall suppose, as the most general assumption we can make, that the electric field consists of a distribution of velocity in the medium whose vortex-motion constitutes the atoms of the gas.167 The attempt was then described, to relate the electric strength, chemical stability, temperature, and pressure of 43 a gas to one another, together with some quantitative calculations and suggestions for experiments. He stressed the intimate connection which he saw between electrical conduction and chemical action: Thus, according to the view we are now discussing, chemical decomposition is not to be considered merely as an accidental attendant on the electrical discharge, but as an essential feature of the discharge, without which it would not occur.168 This was a view which Thomson maintained, extended, and studied experimentally for the next decade, and although he did not develop the vortex atom theory further during this period, in which its original proponent Kelvin came to reject the idea totally,169 his explanation of the electric field in terms of fluid motion became more detailed. Thomson's description of the field in terms of 'tubes of force' of 1891,170 the theory of electrical oscillations, and descriptions of the experimental and theoretical work on the discharge of electricity through gases, were treated in his Recent Researches in Electricity and Magnetism, of 1893;171 these were cited by Rutherford in his first papers, on magnetism and electromagnetic radiation, during the next two years. Each tube of electrostatic force is of unit strength, starting on a unit electrolytic positive charge and ending on a negative one, or else forming a closed ring.172 They consist of vortex columns or filaments in the ether whose kinetic energy constitutes the potential energy of the electrostatic field; magnetic effects are produced by their lateral motion, which also constitutes the propagation of light in a quasi-corpuscular fashion.173 Thomson's theory of chemical combination of 1893, with its hints of subatomic structure, was purely electrical - an extension of his views of 1883 and a modification of the theory of Helmholtz of 1881.174 All unclosed tubes join pairs of atoms, which are considered to be chemically combined if the tube is of molecular dimensions, but chemically free if the tube is long: 'when a tube falls on an atom it may modify the internal motion of the atom and thus affect its energy',175 which accounted for the differing affinity of atoms for electricity postulated by Helmholtz. 44 'Now the laws of Electrolysis show that the number of Faraday tubes which can fall on an atom is limited; thus only one can fall on an atom of a monad element, two on that of a dyad and so on'.176 The atoms in chemically saturated compounds can receive no more tubes so that each end of an unclosed tube always falls on a free atom. The existence of free electricity in electrolytes, gases, and metals too, therefore always requires free atoms and hence chemical decomposition. Thomson gave diagrams of the arrangement and movement of the tubes between atoms in a gas,177 but did not explain how the vortex tubes of force were linked to the 'internal motion of an atom', nor what form he now conceived this motion to take. However, in his paper on 'The Relation between the Atom and the Charge of Electricity carried by it',178 published in 1895 at about the time of the arrival of J.A.McClelland, E.Rutherford and J.S.Townsend to study at the Cavendish, Thomson gave something of a picture of an electro-chemical vortex atom theory. The atom seems to be differently constituted from that of 1882-3 and appears incidentally to suggest an explanation of the directional nature of valency, which the earlier theory did not: Now let us consider the atoms on which these tubes end. Let us suppose that these atoms have a structure possessing similar properties to those which the atoms would possess if they contained a number of gyrostats all spinning in one way round the outwardly drawn normals to their surface.179 Within two years Thomson had constructed the first corpuscular atomic model. His support for this theory in some way involved Lockyer's dissociation hypothesis, Lorentz' electron theory and the newly discovered Zeeman effect, as we have seen. But the attainment of such a theory depended not only upon the discovery of the charged material 'corpuscle' by means of improved experiments on the cathode rays, but on remnants of the vortex theory of the chemical atom. This is illustrated by others who had developed electrical and vacuum techniques sufficiently to obtain results similar and perhaps prior to those of Thomson, but did not make such significant use of them at the time. 45 180 181 The publications of W.Kaufmann and E.Wiechert in 1897 are notable in this respect. From his experimental results Kaufmann drew only the limited conclusion that if the cathode rays were particles then their charge to mass ratio was 107, which was unexpectedly large. Wiechert obtained a similar e/m value; he went further than Kaufmann by assuming that the rays were in fact particles, and that they possessed the electrolytic charge. He thus estimated their mass to be of the order of 1/1000 of the lightest atom. Wiechert continued to concentrate on mathematical electrodynamics and did not develop the chemical implications.182 It is generally stated that Thomson's first conception of a subatomic corpuscle dates from about the beginning of 1897, perhaps after knowing of Zeeman's new experimental results.183 This was closely linked with his experiments showing that the magnetic deflection of cathode rays was 184 independent of the gas in which they were produced, as well as mean free path, velocity, and other considerations. However, Thomson's continuing interest in chemical atomic structure throughout the experimental developments has previously not been clearly brought out. We have already noted his theory of electrical conduction of all kinds published in 1895, which involved vortical tubes of force linking atoms containing gyrostats; another aspect of Thomson's chemical ideas soon became evident. In April 1896, reviewing experimental advances on the recently 185 discovered Ontgen and Becquerel rays, he several times slightly misstated Rontgen's result that for a series of metals the absorption of the X-rays is in the same order 186 as their density. For example, he mentioned 'Rbntgen's discovery of the close connection between the absorption of these rays and the atomic weight of the absorber'.187 Either version fitted the experimental results but Thomson's statement may be significant for he also suggested that the absorption of these rays, considered to be of very short wavelength, was caused by Proutian primordial atoms. The expression 'atomic weight' was indeed correctly replaced by 188 'density' in the report of the Rede Lecture given some weeks later, but in this we read: 46 There seems no simple relation between the density of a body and its transparency to visible radiation or electrical vibration; in the case of the Wintgen rays, however, it seems the greater the density the greater the opacity. This appears to favour Prout's idea that the different elements are compounds of some primordial element, and that the density of a substance is proportional to the number of primordial atoms; for if each of these primordial atoms did its share in stopping the Wintgen rays, we should have that intimate connection between density and opacity which is so marked a feature for these rays.189 That Prout's hypothesis could be brought up on such evidence indicates a predilection for the idea in 1896. The statement was made with reference only to X-rays, seen as radiation in terms of tubes of force,190 and not to cathode rays, seen as particles with etherial effects. However, in his well-known paper of 1897191 Thomson laid stress upon the distances penetrated by the cathode ray particles without mentioning X-rays: 'Now Lenard found that this distance depends solely upon the density of the medium and not upon its chemical nature or physical state'.192 By this time Thomson considered the cathode ray corpuscles to consist of 'a substance from which all the chemical elements are built up'193 and in imagining this building194 he considered the possibilities of Mayer's magnets more deeply than with the vortex atom of 1882-3. In the atom the material corpuscular components, whose charge might be much greater than the electrolytic charge (Stoney's electron),195 were held together by tubes of force, many for each corpuscle. Only a single stray unit tube would bind for example a hydrogen to a chlorine atom, giving HC1. He did not discuss the existence of vortices within a corpuscle to which the tubes are connected nor, later, the possible gyrostatic nature of the unit-charge corpuscles which he soon accepted. Thomson's stress, and defence, of the material nature of the corpuscles seems compatible with some kind of etherial vortex explanation of matter, electricity and light. But Thomson had said that such a theory 'cannot be said to explain what matter is, since it postulates the existence of a fluid possessing inertia'.196 A purely electromagnetic explanation of matter, 47 in which studies of the radioactive rays played a vital part, was to develop by 1901.197 As Thomson struggled with experiments and Proutian ideas involving X-rays and cathode rays in 1896, and was taking an interest in the Becquerel rays, Rutherford wrote home from Cambridge: I am working very hard in the Lab. and have got on what seems to me a very promising line - very original needless to say. I have some very big ideas which I hope to try and these, if successful, would be the making of me. Don't be surprised if you see a cable some morning that yours truly has discovered half-a-dozen new elements, for such is the direction my work is taking. The possibility is considerable but the probability rather remote.198 Rutherford's known laboratory notes199 and his published papers on the newly adopted subject of the electric properties of gases exposed to radiation - X-rays, uranium rays, and ultra-violet light - give us obscure clues as to the nature of these new ideas. We have, however, noted the long-standing interest of his Professor in chemical atomic theory. We shall see that Rutherford shared this interest and how what may have been no more than a light-hearted prediction became fulfilled and exceeded through radioactivity. But the existence of the first new chemical element arising from these studies was proposed in France within two years of Rutherford's boast - and then he did not believe it. 48 CHAPTER 2

THE DISCOVERY OF URANIUM RAYS AND RADIOACTIVITY

1. Becquerel's discovery of uranium rays (1896-7) From ROntgen's famous experimental discovery of X-rays stemmed Henri Becquerel's discovery of uranium rays. Within days of ROntgen's paper 'On a new kind of radiation'1 many scientists were using the readily available Hittorf, Lenard or Crookes vacuum tubes to verify, and advance studies on the new phenomenon. The most spectacular aspect of the rays was their capability of penetrating certain solid materials to produce fluorescence or photographic action on an adjacent screen or plate, giving images for example of bones within the living body. Most of the thousand articles and fifty books published on the subject during the year of 1896 were concerned solely with such medical possibilities.2 However, there were several points for physicists to consider, the question of the source of the rays being an important one. Their discoverer pointed out that they appeared to come from the most brightly fluorescing part of the glass of the vacuum tube,3 and it was this statement, though not at first hand, which came to set Henri Becquerel on a somewhat novel line of research. It is my intention firstly to describe and discuss the experiments and hypotheses which constitute Becquerel's discovery of uranium rays and to indicate some of the neglected background to those studies. This is followed by an account of other varied and novel researches emerging during the period 1896-7 which may provide an improved interpretation of the contemporary attitude to the Becquerel rays. Unlike the events leading to the discovery of X-rays, which remain obscure,4 the beginnings of the experimental work with which we are now concerned were clearly described by Becquerel, some seven years later,5 leaving only a few questions in doubt: Dans la seance de l'A.cadgmie des Sciences du 20 janvier, au moment ou M.H.Poincarg venait de montrer les premieres radiographies envoy6es par 49 M.ROntgen, je demandai a mon confrere si l'on avait determing quel etait, dans l'ampoule vide productrice des rayons X, le lieu d'emission de ces rayons. Il me fut repondu que l'origine du rayonnement etait la tache lumineuse de la paroi qui recevait le flux cathodique. Je pensai aussit8t a rechercher si l'emission nouvelle n'etait pas une manifestation du mouvement vibratoire qui donnait naissance a la phosphorescence, et si tout corps phosphorescent n'emettait pas de semblables rayons. Je fis part de cette idee et de ce projet a M.Poincare...6 The weekly report of the Academy for 20th January in fact contains only a brief note of the exhibition of some of the earliest X-ray photographs; H.Poincare,Professor of mathematical physics at the Sorbonne, first published the idea ten days later.7 The experimental connection between fluorescence and X-ray emission was eroded throughout the year, as it was shown that the point of incidence of the cathode rays upon solid objects was the primary source, that rays were not emitted from other brightly glowing parts of the glass tube, and that metals could emit X-rays unaccompanied by visible fluorescence.8 However, such a connection was under active consideration at the beginning of 1896, and whilst J.J.Thomson at Cambridge reported failure to obtain penetrating radiation from fluorescing glass, gases, and luminous paint,9 others, in France, announced success. Henri Becquerel was the third of four successive members of the family to be Professor of Physics at the Natural History Museum in Paris; his father Edmond and himself were well known for their work on various aspects of physics, particularly phosphorescence.10 G.G.Stokes had coined the term 'fluorescence' for emission of light only during the time of irradiation, and considered 'phosphorescence' which continued after irradiation had ceased, to be of a different nature. Edmond Becquerel, however, insisted on an experimental continuity between the two, using his rotating 'phosphoroscope' of 1858 for timing short durations. Whilst he always used the term 'phosphorescence',11 some maintained Stokes' distinction, and others continued to use the terms indiscriminately. 12 During the last quarter of the nineteenth century 50 the subjects of phosphorescence and fluorescence had provided opportunities for a considerable effort in experimentation upon a large number of inorganic and, in the 1880's, organic substances, as solids, solutions or gases. Studies included examinations of the relations between the wavelengths and intensities of absorbed and emitted light, the rise and decay of intensities with time, and the marked effect of traces of metal compounds in solids. Following Stokes' discovery of fluorescence, and his explanation of the phenomenon in terms of vibrations of molecules, in 1852,13 E.Lommel then others in Germany developed mathematical theories of oscillations of particles, natural, forced and damped, which partly agreed with the observed phenomena.14 A clear picture of the part played by studies of luminescence in the development of physical science has yet to be created; this does not concern us here, but we have noted that several scientists who were linked with radioactivity entertained qualitative electrical atomic-molecular views of phosphorescence in the 1890'8.15 E.Becquerel who is said to have dominated the experimental field16 until about 1880 put forward only 17 qualitative explanations of his results; his son Henri continued the researches in a similar way. Some of the latter's comments concerning uranium salts are of interest in throwing some light on the way in which he understood the phenomena of phosphorescence, which came to be so closely linked with the new uranium rays in 1896. In discussing the 'Relations entre l'absorption de la lumiere et 1'6mission de la phosphorescence dans les compos6s 18 d'uranium' in a paper of 1885 he concluded that the compounds of uranium are in 'un etat moleculaire' such that they exert a selective absorption of harmonically related wavelengths of light, and that some of these compounds in the uranic series also emit harmonically related bands of a longer wavelength; some bands were common both to absorption and emission spectra, which suggested that the property 'de vibrer a l'unisson' might be the actual cause of absorption. He noted that the broad green band of the emission spectrum of incandescent 51 uranium vapour was very close to an absorption band characteristic of uranous salts. Perhaps he considered that the emission of light, or of certain bands by uranium vapour might be due to its molecular state, but Becquerel gave no detail. Six years later, in 1891, his views 'Sur les lois de l'intensite de la lumiere 6mise par les corps phosphorescents119 were that the laws of rise and decay of luminous intensity could be related to an equation of simple harmonic vibration, modified by taking into account the loss of energy of the oscillating molecules via the 'ether intermoleculaire', which loss was taken as proportional to the square of a velocity term. And in a note of the same year 'Sur les differentes manifestations de la phosphorescence des mineraux sous l'influence de la lumiere ou de la chaleur'20 Becquerel speculated on the mechanism by which the 'conservation indefinie dans les corps' of the energy of phosphorescence-by-heat (later termed thermoluminescence) might be achieved. Was the substance in a state comparable with magnetism, or: La deperdition d'energie est-elle continuellement compensee? Ce sont des questions que l'on ne saurait decider actuellement et sur lesquelles les etudes ulterieures apporteront peut-kre quelque lumiere.21 He did not publish further on this point himself; his rival E.Wiedemann in conjunction with G.C.Schmidt had provided the beginnings of a chemical explanation by 1895.22 Becquerel's studies of the connection between the new X-rays and the phenomenon of phosphorescence led him to his great discovery of uranium rays and to questions of energy which were even more difficult to answer. Henri Becquerel's first publication for some three years was again on the subject of phosphorescence. In his note of 24th February 1896 'Sur les radiations emises par phosphorescence'23 he cited not Poincare but Charles Henry, of the gcole Pratique des Hautes Etudes, Paris, with whom there were to be controversies later that year.24 Henry in his paper on 'Augmentation du rendement photographique des rayons ROntgen par le sulfure de zinc phosphorescent'25 showed that phosphorescent zinc sulphide placed in the path of the rays from a Crookes tube intensified the photographic 52 effect of rays penetrating aluminium. Ho quoted Poincare's query: Ne peut-on alors se demander si tous les corps dont la fluorescence est suffisamment intense n'emettent pas, outre les rayons lumineux, des rayons.X de Röntgen, quelle que soit la cause de leur fluorescence...26 and considered that he could answer it in the affirmative. This statement of Poincare, repeated by Henry, does contain 27 the point claimed as his own by Becquerel in 1903, but one cannot thereby deny this claim. Becquerel also mentioned 28 Niewenglowski's announcement of the previous week that the sulphide of calcium as well as zinc produced penetrating rays during irradiation merely by sunlight. Becquerel extended these observations to some unnamed phosphorescent substances and in particular to uranium salts whose brilliant phosphorescence of short duration was well known. The techniques which were involved, simple though fraught with pitfalls, are illustrated by Becquerel's description of some of his first experiments.29 He wrapped a Lumiere bromide-in-gelatine photographic plate in two sheets of dense black paper, light-proof for a day's exposure to sunlight. Upon this was placed a crystalline lamella of potassium uranium sulphate, with a glass slip interposed to avoid the chemical effects of vapours. Exposure to sunlight for several hours to produce the required penetrating phosphorescence, followed by development of the plate, revealed a blackened area. Becquerel reported his success in obtaining images of metal objects by placing these between the phosphorescent source of the penetrating rays, and the sensitive plate. This had previously been achieved only by means of X-rays. The fortunate experiments made by Becquerel during the following week and announced at the next meeting of the Academie on 2nd March,30 were immediately seen by himself as important, and can now be interpreted as a significant marker in the initial stages of Becquerel's research. He firstly described experiments showing that photographic action occurred after excitation by reflected, refracted, or diffused sunlight, and through aluminium and copper foil as well as black paper, and then stressed his 53 view of the importance and the unusual nature of some new observations. Combinations of a lamella of the double sulphate of uranium and potassium, K(UO)SO4.H20, aluminium sheet, and photographic plate prepared for irradiation on Wednesday 26th and Thursday 27th February were never thus treated, owing to the intermittent character of the sunlight during these days. They were placed in a drawer, and then developed on Sunday 1st March.31 Expecting weak images, instead he found very intense ones, and immediately concluded that the action must have been occurring in darkness, 'l'action avait da continuer a 1'obscurit6'. He found experimental confirmation that day. Three lamellae, one placed directly on a photographic plate, the others with glass or aluminium sheets interposed, were left for five hours in a cardboard box inside another cardboard box, in a drawer. The whole operation was performed in the dark-room. Becquerel found images, weaker with the aluminium, exactly as when the crystals were irradiated by sunlight. His interesting, but tentative, hypothesis was that the effects might be due to invisible radiations 'emises par phosphorescence' but of a duration 'infiniment plus grande' than that of visible phosphorescence, known to be but 1/100 sec. for the salt used. He considered that he had found 'un nouvel ordre de phenomenes'. Difficulties were seen when Becquerel repeated his conclusions at the Physical Society meeting, later that week, adding that certain phosphorescent crystals did not give the rays. For if the effects were due to radiations of shorter wavelength than those from the sun as Becquerel's comparison with Lenard and Röntgen rays was taken to imply, Stokes' law of phosphorescence would be contradicted; it was therefore suggested that the new penetrating rays might instead be of a longer wavelength.32 Experimental investigation and comparison of his new rays with X-rays and an examination of the relationship with visible phosphorescence was Becquerel's way forward and away from the heavy criticisms levelled at the work of G.Le Bon on Ilumi6re noire'P His remarkable results, announced within days34 and standing 54 unchallenged for two years, appeared to provide a nearly complete answer to the question of the nature, if not the source, of the new rays, and furthermore to clarify the understanding of X-rays. Placing the salt within the vessel of a gold-leaf electroscope which was shielded from electric radiations35 by a metal screen, and against ultra-violet rays by yellow glass, Becquerel showed that the new rays caused the dissipation of positive and negative charges at equal rates. Thus applying this electrical and more quantitative technique he found that the time of collapse of the leaves was proportional to the thickness of an interposed aluminium screen. These were important properties, which X-rays were known to possess by this time, so that the new rays were evidently similar to X-rays. By means of a steel plane mirror, and a concave hemisphere of tin, diffuse photographic images were obtained. Becquerel took these results to indicate a definite, if diffuse, reflection of the rays, in conjunction with more striking experiments. Glass tubes filled with a powdered phosphorescent substance were attached perpendicular to a glass sheet which was placed upon the photographic plate. The resulting image, which was a clear circle with a black disc within, surrounded by a blackened area, proved that the invisible rays were both reflected and refracted in the same way as light. These experiments were later found to be repeatable,36 but Becquerel's interpretation was to be rejected in a little over two years37 when the images were attributed to 'secondary' radiation. These results were soon to be joined by some which are dubious or inexplicable and by others which stand to this day. Becquerel's survey of phosphorescent minerals showed that several uranium salts and two calcium sulphide specimens gave rays capable of penetrating 2mm.of aluminium but that other phosphorescent substances failed to give penetrating rays. Perhaps, he thought, the phenomenon could be likened to visible 'thermoluminescence'; the latter involved earlier excitation, then an emission of visible radiation upon gentle heating at a later time. 55 The calcium sulphide specimens had vanished from Becquerel's list of emitting substances by the time of his fourth report delivered two weeks later on 23rd March. How they had come to be included at all is not clear. He remarked that no form of thermal or electrical excitation could re-excite the emission of invisible radiation from these still highly phosphorescent substances and that Troost, Professor of General Chemistry at the University of Paris, in following Becquerel's first experiments, had 38 experienced a similar effect. Thus only uranium salts were now left.39 Their emission of penetrating rays, still continuing after fifteen days, was reported by Becquerel to be strongly intensified after illumination by electric spark or arc, slightly intensified by daylight, but imperceptibly by the light of burning magnesium, as determined photographically; this latter method did however cause excitation as determined electroscopically. His conclusion was that the emission of invisible rays was a kind of phosphorescence, excited by certain radiations, but not closely connected with ordinary visible phosphorescence nor fluorescence. From the modern point of view, there should have been no change in the intensity of the radiation by these means. Perhaps he was misled by a belief that the new rays ought to be capable of excitation in the manner of ordinary phosphorescence, combined with the flexibility of interpretation which his qualitative experimental methods allowed. It was to become apparent during the following year that such an increase in intensity could not be produced; this was to place the phenomenon further beyond accepted explanations. But even as they were announced the problems of reconciling his conclusions with Carnot's principle, as well as Stokes' law, were recognised and discussed for example at the meeting of the Physical Society which Becquerel addressed at this time.40 In his fourth note to the Academy41 Becquerel was able to confirm the distinction between the invisible and visible phosphorescence by the remarkable procedure of melting a crystal of uranium nitrate in its own water of crystallisation, when it ceased to exhibit phosphorescence 56 or fluorescence, and then allowing recrystallisation by cooling, while excluding all luminous excitation. There could now be no emission of visible radiation, but the photographic effect of the invisible rays was as strong as ever. Becquerel was now turning towards a study of the nature of the radiations; his final attempts at finding their source were highly, if not completely, successful, 42 and are described in his next two papers. On 30th March he noted that 'un nouvel exemple d'ind6pendance entre les deux ph6nomenes d'6mission' was provided by the invisible rays detectable from the non-fluorescent solution of uranium nitrate; and further, that visibly phosphorescent sulphides could not be induced to emit invisible penetrating rays, upon excitation with X-rays. After a seven-week silence, awaiting Moissan's latest preparations of pure metallic uranium,43 the purpose of Becquerel's sixth note to the Academy was to announce the 'Emission de radiations nouvelles par l'uranium metallique'.44 The inorganic chemist Henri Moissan had turned from his successful researches on fluorine towards high temperature studies of boron, carbon and diamond, and silicon, in the 1890's; he had applied his newly developed electric furnace of 189245 to the problem of preparing pure samples of . refractory metals such as zirconium, chromium, manganese, 46 tungsten, molybdenum, vanadium, titanium and uranium, whose melting points lie above about 2000 degrees C. He achieved the first fusions of some of these, and also examined the carbides and carbon solutions of these and other metals, including uranium.47 Thus Becquerel found that it was not only the salts of uranium, but the element uranium itself from which the rays were emitted. Uranium carbide, commercial uranium powder and crystallised and cast uranium metal gave stronger photographic and electrical effects than potassium uranium sulphate; the charged leaves of an electroscope collapsed about four times faster with uranium metal, at rates of the order of 1 to 8 degrees per minute of time. Salts stored in a double lead box shielded from all 57 exciting radiation continued to emit radiation at a very slowly decreasing intensity. The excitation of the emission above this level upon irradiation by the sun or more effectively by electric arc or spark was, strangely, confirmed; the level of emission declined to the normal within hours. These results involving decay or excitation were not corroborated by other workers, and were soon forgotten, but at the time they agreed with Becquerel's conclusions concerning the nature of the new phenomenon. Without describing a possible mechanism he stated that this was the first example of a metal exhibiting invisible phosphorescence. Hence 'radiations uraniques' and 'rayons uraniques' he named the rays in his next communication some months later.48 Their intensity, he now cautiously admitted, seemed hardly to have changed in eight months, which fact was completely outside ordinary phosphorescent phenomena: 'on n'a pu reconnaitre encore ou l'uranium emprunte l'energie quill emet avec une si longue persist- ance'.49 Becquerel had not solved this problem by the time of his last paper of the series, delivered six months 50 later in April 1897; the problem of the energy source was to worsen during the next seven years, to reach crisis proportions in the view of some scientists. But by 1897 Becquerel was concentrating more upon the nature and properties of the invisible rays; these studies formed an important part of his discovery in the opinion of fellow physicists. We have seen that by 9th March 1896 Becquerel's experiments indicated that the new rays possessed the properties of penetration, and discharge of electrified bodies, in common with X-rays, but were reflected and refracted like visible light. On 23rd March51 he reported qualitative confirmation of refraction, using a glass prism and a linear source of the rays consisting of a glass tube of lmm. diameter filled with crystalline uranium nitrate. A further comparison between X-rays and the new rays was the proportion absorbed by the same screen. Becquerel found that X-rays were weakened four times as much as the new rays, and took this to indicate that the two kinds of radiation differed in wavelength. As for the 58 cause of the dissipation of electric charges by both radiations, Becquerel professed ignorance, but noted that the gas appeared to become conducting. Others with previous experience in this area were working somewhat more successfully on this problem at the time, as will be seen. Becquerel's next note to the Academy was specifically 'Sur les proprietes differentes des radiations invisibles emises par les sels d'uranium, et du rayonnement de la paroi anticathodique d'un tube de Crookes'.52 He confirmed unequal absorption of these radiations by different substances, both photographically and electroscopically, the new rays being generally the more penetrating. Also, that the new rays were non-homogeneous, as others had found for X-rays, by interposing screens between source and electroscopic detector. The most strikingly successful of Becquerel's experiments, so it seemed, was his attempt to detect polarization of the rays, using crossed and parallel tourmalines, with which Riintgen had obtained negative results for X-rays. Using the double sulphate of uranium and potassium as the source of the rays, Becquerel found that plates developed after sixty hours showed clearly stronger intensities for parallel tourmaline crystals. Becquerel took this to show that the invisible rays suffered double refraction and polarisation of the two refracted rays, followed by unequal absorption of the differently polarised rays by the second tourmaline. Although never repeatable and still inexplicable, these results were eagerly accepted without public question until 1899. A reviewer of 1898 commented that Becquerel's demonstrations of the reflection, refraction, double refraction, and polarisation of uranium rays showed that 'there can be no reasonable doubt that they are short transverse ether waves';53 their possesion of all the properties of X-rays indicated that the latter were similar. Indeed, the view that these studies of uranium rays were valuable in throwing light upon the nature of X-rays had been expressed early in 1896. Some three weeks after Becquerel's announcement of the polarisation of uranium rays, J.J.Thomson's comment appeared in print: 59 The radiation from the uranium salts is thus intermediate in properties between ordinary light and Riintgen rays; and as there can be no question but that this radiation consists of transverse vibrations, inasmuch as it can be polarised, it affords presumptive evidence that the RCintgen rays are also due to transverse vibrations.54 J. Perrin made similar points,55 so too did G.G.Stokes whose transversal irregular ether pulse theory of X-rays was accepted by many physicists.56 As the most crucial of Becquerel's results on the properties of the uranium rays, and the conclusions drawn from them were shown to be false, X-rays too began to be seriously considered in a different light;57 but that is another story.

Despite the unreliability which was soon shown to exist in some of his experimental work, Becquerel's achievement in arriving at a demonstration of the existence of uranium rays was of a remarkable nature. This is high- lighted by the parallel studies by some of his contemporaries, which were seen as being related to Becquerel's rays. Some of these studies proved to be very short-lived, others were more reliable and came to be distinguished from those on uranium rays, with varying degrees of difficulty and rapidity. We have already noted that Becquerel and Troost had obtained photographic effects which they interpreted as being caused by rays penetrating black paper and aluminium after issuing from phosphorescent sulphides; these results stood only for a few weeks, though they led to the discovery of uranium rays. It is notable that S.P.Thompson, in London, followed a closely similar path; like Becquerel he cited C.Henry on the augmentation of X-ray photographs by phosphorescent zinc sulphide 58and found that phosphorescent substances emitted such rays on their own account. The merits of his claim59 to an independent discovery of the rays from uranium salts have been discussed;60 his explanation of the effect was the same as Becquerel's; he coined the name 'hyperphosphorescence' but this was little used. Like Becquerel he 60 had found that 'a phosphorescent substance such as sulphide of Barium' during and after illumination, emitted, besides the visible radiation of phosphorescence, rays which were like X-rays in being invisible, penetrating aluminium sheets and producing a photographic effect. On communicating this to Stokes,61 the authority on phosphorescence, he was advised first not to delay publication then, a few days later, that he had 'already been anticipated'62 by Becquerel in papers earlier that month. Thompson63 specified uranium salts only after he had seen these papers and had been notified by Stokes that the French scientist had attributed the radiation to metallic uranium6 4 He asked Crookes for a specimen of uranium metal; the latter was examining the efficacy of metals as radiators of X-rays under the impact of cathode rays, and replied: I have my metallic uranium in a vacuum tube at present, testing it against platinum as a radiator of the unknown X. So far it is decidedly the better of the two. I have another small piece which will be disengaged tomorrow, and then you shall have it.65 Uranium in metallic form was not readily available, though H.Moissan provided some specimens; Crookes reported that he had to discontinue these experiments owing to lack of the metal;66 there was no difficulty in obtaining compounds. No doubt some of Becquerel's experiments of 1896 were repeated by Stokes or others at Cambridge, Crookes or S.P.Thompson, G.Le Bon in France,67 or by German physicists. But no-one published a denial of the vital proof of double refraction and polarization during that year nor the next. The results of S.P.Thompson, Becquerel and Troost, who had found that penetrating rays were emitted from phosphorescent sulphides, were obtained by the photographic method of detection; indeed the purpose of these experiments, and those of C.Henry, was to obtain photographic images. Becquerel from the first realised the danger of chemical action from vapours and took precautions against this; others did not. He knew that substances opaque to visible light could be transparent to Hertzian waves, infra-red and ultra-violet light, which might produce photographic or electrical effects, and took measures to 61 avoid the possibility of confusion with uranium rays. Others, in the rush to publish on X-rays and similar phenomena during 1896-7, may have neglected any of these points, and produced photographic results with conclusions which were soon proved false. A few scientists thus took up Becquerel's work with misguided enthusiasm finding Becquerel rays where we see none now. H.Muraoka of the Physical Institute, Kyoto, Japan, in his paper on the light from glow-worms,68 published towards the end of 1896, claimed that in addition to visible light these worms emitted rays similar to those of Becquerel in being capable of penetrating metals. The photographic plate was blackened only in proximity to the cardboard of the container, and the author explained this by a concentration effect of the cardboard upon the glow- worm rays. A partial denial of these results, attributing some of the effects to vapours, in the following year is to his credit.69 In similar fashion, W.Arnold70 attributed the action of certain substances such as metallic sulphides, uranium salts, and retene on photographic plates to 'Becquerelstrahlen'. And A.F.McKissick reported from the Alabama Polytechnic Institute, U.S.A., a similar success in his search for phosphorescent substances which emitted the Becquerel rays. He examined uranium salts but found sugar to be the best emitter.71 P.Spies72 and F.Maack73 reported intensification of the photographic effect of the rays from uranium by the interposition of certain substances; we may suppose that secondary radiations or chemical vapours from these substances could have produced such an effect. These results, essentially extensions of Becquerel's work, were not developed further. But Gustave Le Bon considered that his own researches on radiation constituted a branch of study conceived independently of and prior to that of Becquerel and that Becquerel's work was encompassed within his own.74 He was supported in this by P.de Heen, Professor of Physics at the University, and Director of the Institute of Physics, Li6ge, Belgium.75 Le Bon had previously published on psychology,76 and he is considered by social scientists as a serious contributor 62 to that field.77 However, from the time of his first announcement concerning an unknown type of 'lumiere noire'78 he was heavily criticised by G.H.Niewenglowski,79 81 82 A. and L.Lumiere,80 Perrigot and Becquerel at the Academy; these scientists were either unable to repeat the experiments or attributed the photographic effects of the new penetrating rays, which Le Bon supposed were emitted by solid substances after irradiation, to red or infra-red rays. Le Bon did not accept most of the criticisms and continued to publish.83 He extended his studies from the photographic to the electric actions, and concluded that since all bodies when acted upon by light produce effects similar to but smaller than those of uranium, this was but one instance of a general phenomenon of penetrating radiation.84 After reopening and extending discussions of 'lumiere noire' when radioactivity had become more important 85 in 1900 he was again criticised, this time by P.Curie, who pointed out that all the characteristics of 'lumiere noire' could be accounted for by the well-known properties of 'rayons calorifiques infra-rouges'. In his writings on the universal dissociation of matter and emission of material particles86 Le Bon seems to have expressed views similar to the idea of a general radioactivity of all matter which, as will be seen, some workers on radioactivity entertained during and after 1903. Although Le Bon's books were popular from 1905, no reputable student of radioactivity regarded his work as significant. However, the criticisms of Le Bon's early photographic work were not quite clear-cut. For in 1905 Rutherford stated not 87 that the effects were due to infra-red or red radiations but 'that there seems to be little doubt that the effects 88 are due to short ultra-violet light waves'. Other possible causes of photographic effects were made clear during 1896, and should perhaps have been seen by Arnold, Muraoka, McKissick and others previously mentioned, as a warning. For R.Colson of the Conservatoire des Arts et Metiers, in 1896-7 pointed to a decomposition of the salts in the plate by such causes as mechanical pressure, chemical actions especially when damp, warmth . when damp, intense infra-red radiation, very feeble light 63 acting for a long period, as well as visible and ultra- violet light, and X-rays.89 Freshly cleaned zinc surfaces blackened the sensitive plate in air or in vacuo, as did magnesium, but aluminium did not. After attempting experimentally to determine whether effects were due to 'une radiation ou une emanation' Colson pointed to E.Demar?ay's detection of the vapour of metallic zinc at temperatures as low as 184 degrees C. and attributed the effect to metal vapours which could penetrate some materials.90 These findings may explain some of the dubious experimental results of 1896-7; they parallel to some extent the work of Dr. W.J.Russell, F.R.S., Lecturer in Chemistry at St.Bartholomew's Hospital, 'On the Action exerted by certain Metals and other Substances on a Photographic Plate'.91 This line of research is particularly significant since it was thought to be related to uranium radiation both before and after the discovery of the radioactive 'emanation' from the metal thorium. Rutherford took pains to make the distinction clear92 but was not immediately successful in persuading all scientists of this. Russell began his paper by explaining that being in possession of uranium compounds used for spectroscopic examination some years before, he had repeated 'some of the very important experiments which Becquerel has made with these compounds%93 Russell referred only to the photographic work, ascertained that no luminous excitation was necessary over seven months, and noted that specimens kept in the dark seemed if anything slightly more effective in their action. In addition to this he found that a perforated zinc screen, intended to show up the effect of a card painted with yellow oxide of uranium, gave instead an image which was the reverse of that expected: the greatest action occurred beneath the zinc. He was able easily to repeat this, with variations: so that the only explanation of the action was that the zinc itself must be able to effect a change of the same kind as the uranium, at all events to act on a photographic plate.94 Russell went further than Colson, in showing that a variety of substances, though not all, produced an effect through 64 many bodies, but not through glass, even the thinnest. As for metals and alloys, which had to have bright surfaces: 'The following is a rough list of active metallic bodies approximately in the order of their activity: mercury, magnesium, cadmium, zinc, nickel, aluminium, pewter, fusible metal, lead, bismuth, tin, cobalt, antimony'.95 Zinc salts, and some metals were not active'; a vague correlation with the 'electrical series' was suggested. Russell's findings appeared to have extended Becquerel's results; although he did not make the point, these may have tended to cast some doubt upon the latter's original experiments. For Becquerel's descriptions of the arrangements which produced photographic effects through metal screens, had not explicitly excluded the effects which the screens themselves were now shown to have. Furthermore, the strawboard pill-boxes, used as containers for the uranium salts being examined photographically, were found to be more active than the contents; woods and varnishes were also more active than uranium. Photographic plates laid face upwards in a cardboard box for a week were 'very appreciably affected', but were protected by a glass screen. Perhaps Russell had in mind the fact that Becquerel placed his arrangement of salt-screen-plate in a cardboard box within a wooden drawer for long periods. For copal varnish the cause was definitely attributable to a vapour, but this seemed unlikely for strawboard: Still more interest attaches to the action of the metals; do they emit a vapour so delicate in constitution and in such a quantity that it can readily permeate celluloid, gelatine &c., and produce a picture of the surface from whence it came, or is it a form of energy (possibly what has been called dark light) that these bodies emit? Zinc kept and polished in the dark loses none of its activity.96 However, Russell did state that the action through glass. 'proves that there is at least a marked difference between the action exerted by metallic uranium and that by zinc and other metals'.97 The close link between Russell's and Becquerel's findings was evidently still felt by W.Crookes over a year 65 later, in September 1898: It now appears that some bodies, even without special stimulation, are capable of giving out rays closely allied, if not in some cases identical, with those of Professor RUntgen. Uranium and thorium compounds are of this character, and it would almost seem from the important researches of Dr.Russell, that this ray-emitting power may be a general property of matter, for he has shown that nearly every substance is capable of affecting the photographic plate if exposed in darkness for a sufficient time.98 But by this time the connection had loosened considerably. C.T.R.Wilson, pursuing his experiments at Cambridge on condensation nuclei produced in gases by radiation, showed that a uranium salt strongly influenced the condensation of water vapour in a glass chamber, from within its stoppered glass container wrapped for hours in tinfoil. This proved that tinfoil was transparent to the agent influencing condensation, and that the uranium salt 'continues to be active when kept in the dark'. Thus, a few months after the publication of Russell's paper, Wilson supposed 'There can be little doubt therefore that the effects on the condensation are really due to the radiation studied by Becquerel'.99 A further clarification from the Cavendish Laboratory came with J.J.Thomson's note 'On the effect of zinc and 100 other metals on a photographic plate'. In this he credited Stokes with the suggestion, at an earlier meeting of the Cavendish Physical Society on Russell's paper, that a blast of air between photographic plate and source of action might distinguish clearly between radiation and vapour. The distorted images obtained in this way showed that vapours were the cause. Russell was invited to deliver the Royal Society's Bakerian Lecture of 1898, which he did, on the 101 subject of the photographic actions, now attributing the actions to vapour of some kind; at the time of Crookes' Presidential Address of 1898, Russell considered that the effects of metals were due to the surface formation of 102 hydrogen peroxide vapour. He continued these studies, which became largely separated from radioactivity, 103 producing interesting pictures of substances in the dark. 66 A later chemical author agrees with Russell's explanation of the effects of metals, and describes his research as a 'classic work on the subject'.104 Perhaps Crookes was not quite up to date in his linking of Becquerel rays with Russell's findings, towards the end of 1898, but his was not the last statement of this kind as will be seen. The clearest distinction was pointed out by G.C.Schmidt who had concluded that thorium emitted a radiation with some properties in common with uranium rays.105 Equally significant was his statement that the other substances, mentioned by Arnold, Pellat, Colson, Russell, Muraoka and Henry with regard to their photographic effects, were not analogous. Schmidt considered the most important property of the rays to be the electrical effect rather than the photographic, and supposed that 'Diese beiden Eigenschaften gehen also nicht Hand in Hand'.106 The electrical properties of emitting substances were henceforth always to occupy a more important place in investigations than the photographic. This was due not only to Schmidt - others too showed an interest in this aspect of the new rays, in 1897. Becquerel himself had discovered the effect and moved very much in the direction of electrical studies in his last three Notes, before temporarily leaving the subject.107 C.T.R.Wilson's work on uranium rays was related to studies of the electrical properties of gases at Cambridge, and his fellow research student E.Rutherford was also interested in the electrical properties of uranium rays in 1897.108 Certainly the discharge of electricity produced by the rays gave quantitative measures which could be correlated with their intensity. The photometric estimation of relative blackening of photographic plates was less reliable, and far more time- consuming, though used for several years by W.Crookes, for example. Once Becquerel began to use the electroscope as a means of studying the rays, he obtained clearer indications of their properties, if not their cause. Towards the end of 1896 Becquerel in employing the electroscope found that the rays were not homogeneous and were absorbed to 67 different extents by different materials. He was able to extend to uranium rays J.J.Thomson's demonstration that temporarily conducting gas could be drawn off after irradiation with X-rays.109 And in 1897 Becquerel reported that the speed of collapse of the electroscope leaves under the influence of uranium rays was proportional to the square root of the density of the surrounding gas whose pressure was varied:110 J.J.Thomson had explained the conductivity of gases in electrolytic-ionic terms but Becquerel gave no theoretical account. Confirmation of Becquerel's results on the penetration, duration, photographic and electrical properties of the rays was provided by J.Elster and H.Geitel in their paper 'Veber Byperphosphorescenz'.111 They, too, noted that the source of the energy was still completely obscure,'noch vollsthndig dunkeln'. The main aim of this paper was, however, to investigate whether the charge loss of the photoelectric effect, studied for several years by the authors, might be attributed to conductivity produced in the surrounding air by a hyperphosphorescent emission of invisible rays. But photoelectrically sensitive substances were found to emit no electrically detectable invisible rays and furthermore highly hyperphosphorescent uranium salts were not photoelectrically active. Hence they concluded that the photoelectric phenomenon could not be explained by an emission of rays of the kind exhibited by metallic uranium and its salts. Similarly, without providing enlightenment as to the processes involved, E.Villari112 confirmed some of Becquerel's results including the diminution of electrical conductivity of gases at lower pressures, as well as Kelvin's work on the electrical equilibrium found to exist between uranium and any metal in an adjacent position. Kelvin's researches on uranium formed a part of the Glasgow experimental studies on the electrification of air which began in 1889 and were themselves a revival of earlier investigations of atmospheric electricity.113 Descriptions of experiments on the electrification of air and other gases by flames, by bubbling through liquids, 68 and by electrified needles, and on the diselectrification of air by metal gauze 'filters', using the quadrant electrometer, were published under the names of Kelvin, M.Maclean, A.Galt and others during 1894-5. These Glasgow physicists took up studies of the temporary conductivity produced in air by X-rays as found by a number of authors early in 1896,114 extended these to the effects of ultra-violet light and of uranium, and published their results in the first half of 1897. The experiments specifically on uranium 'Electric Equilibrium between Uranium and an Insulated Metal in its Neighbourhood'1115 'Experiments on Electric Properties of Uranium',116 and 'On, the Electrification of Air by Uranium and its Compounds'117 were performed with a disc of the metal 5 cm. in diameter and fr cm. thick obtained from Moissan by about February 1897. The authors firstly confirmed Becquerel's results on the diselectrification of electrified bodies and showed that the affected air acted like water in allowing a pair of dissimilar metals to develop an e.m.f. between themselves. They showed empirically, with even less theoretical discussion than in Becquerel's publications during the same few months, that aluminium was 'transparent to the uranium influence'. This 'influence' produced saturation currents i.e., not increasing with increased voltage, in a variety of gases at different pressures. The leakage or current at higher pressures was approximately proportional to the pressure and at the lower ones to the square root of the pressure. This latter result, together with the existence of saturation currents, had been important factors in the development of J.J.Thomson's electrolytic theory of conduction in gases during 1896. Kelvin and his associates had followed the literature118 but were very reserved in their attitude to this theory. This may be due in part to their being on the opposite side of the contact electricity debate from J.J.Thomson; much of Kelvin's work on conduction in gases seems to have been aimed at resolving the disagreement.119 Kelvin had three years earlier indicated his acceptance of the idea that a molecule of a gas could be 69 charged with electricity,120 which implied a criticism of Thomson's electrolytic-ionic theory of 1893,121 and it is not clear if Kelvin was still thinking in terms of charged molecules in April 1897. But by May he does seem to have inclined towards an electrolytic gas-conduction theory. This, however, was not linked to the considerable quantitative results obtained; neither he nor his associates developed this theory beyond the brief speculations of a Royal Institution Lecture.122 Kelvin closely linked his comments on conduction in gases with the electrical phenomena exhibited by uranium. The source of the energy for the 'quasi electrolytic phenomena, induced by uranium in air' was a problem: We may conjecture evaporation of metals; we have but little confidence in the probability of the idea. Or does it depend on metallic carbides mixed among the metallic uranium? I venture on no hypothesis.123 For by the time of this lecture he had accepted both Becquerel's proof of the emission from uranium of a radiation 'of the same species as light' and his comparison of the phenomenon with phosphorescence. Kelvin's newly adopted view of normal electrolysis was published within days of the lecture; it involved a one-fluid electrical modification of his Boscovichian chemical atomic theory. A single chemical atom was equivalent to one of Kelvin's 'ponderable atoms', unlike the theory adopted in 1896.124. In the new theory each atom is of a definite radius, and contains a few detachable point atoms of pure electricity called 'electrions'; force laws between atoms and electrions apply. J.C.Beattie continued experiments on 'Leakage of Electricity from Charged Bodies at Moderate Temperaturesli25 investigating 'what becomes of the electricity which leaks away from an insulated body in certain conditions',126 without clearly distinguishing the effect of uranium salts from that of white phosphorus and various heated salts; he made no explicit use of the electrolytic theory of conduction through gases which was becoming accepted by 1899. Kelvin seems to have renewed his interest in uranium rays only from 1903 when the subject of radioactivity was exciting widespread interest; his explanations 70 were expressed in terms of the electrion-atom theory of 1897. Ernest Rutherford and the Curies, leaders in that field by 1903, had in 1897 each begun its study with quantitative electrical investigations of the problematical phenomenon of uranium radiation. An examination of Rutherford's earlier researches may help us to understand the background to his particular success.

2. Rutherford, and the Cavendish Laboratory (1894-8) That Ernest Rutherford successfully pursued experimental researches in three branches of physics during the period 1893-8 is well known.12 7 Although these three branches required somewhat different experimental techniques, it should be pointed out that there were underlying connections between the physicist's understanding of magnetism and Hertzian radiation, the behaviour of gaseous matter under the influence of various radiations, and uranium rays. We have seen how relevant aspects of chemical and physical theories developed in the preceding period of the nineteenth century128 and we shall now illustrate some of the problems and ideas which the young Rutherford considered as he moved towards his major studies in radioactivity. His first publication, of research performed at Canterbury College, Christchurch, of the University of New Zealand, was on the 'Magnetisation of Iron by High- frequency Discharges,129 on which subject there were but a few conflicting comments in the literature, on the effects of introducing iron components into electrical circuits. Rutherford's highly competent, and sometimes ingenious experimental techniques enabled him to show that 71 iron was indeed strongly magnetic for frequencies greater than 100 million per second. He found the magnetic effect of the leyden jar discharge upon steel wires to be proportional to their diameters, not their areas, which confirmed that their magnetism was confined to a thin skin. The chemical means used to examine the depth and nature of the magnetised skin appears to be quite original. The method developed was one of magnetometer measurement during controlled dissolution of the surface by nitric acid, after calibration by dissolving a uniformly magnetised wire. Rutherford's results showed that on moving inwards from the surface, the magnetometer deflection decreased to zero, changed direction, rose to a maximum then returned to zero. The depth of penetration, of order 1/100 in. was proportional to the maximum current passed, and the magnetisation always consisted of an outer layer, and an inner thicker layer magnetised in the opposite direction. Rutherford considered that these layers represented the first two half-oscillations of the exponentially decaying sine curve of the leyden jar discharge. The effect of the leyden jar and Hertz' dumb-bell discharge in lowering the saturated magnetisation of iron whatever the direction of the discharge, which Rutherford discovered in the course of these early studies, was soon to serve as the basis of one of the earliest magnetic detectors of Hertzian waves. This effect also gave a clear experimental demonstration that the discharge was oscillatory in nature as theory predicted; further experiments indicated a very rapid decay of intensity. He considered that: The subject of the decay of the amplitude of the vibrations of a leyden-jar discharge is of considerable interest, especially in connection with the resistance of spark gaps and the radiation of energy into space.130 For terms representing each of these entered into the complete discharge equation. We recall Oliver Lodge's idea of a Hertzian-oscillator chemical atom131 in a work to which Rutherford makes reference in this paper132 of 1894. It is uncertain whether Rutherford had this particular idea in mind during the course of these studies, but 72 he does seem to have been thinking in chemical and molecular terms. His method of chemical removal of the surface magnetisation is itself remarkable; it also bears a striking similarity to some of his crucial experiments on 133 radioactivity, about five years later. The experiments described above led him to the conclusion that: iron may be shown to be strongly magnetic for the highest frequencies yet obtained. If the molecules of iron can follow the changes of magnetic force, which is reversed 1,000,000,000 times per second, there can be very little magnetic viscosity, and the molecules must move as freely as when under the influence of an alternating current of 100 per second.l34 'Magnetic Viscosity! was the subject of Rutherford's second publication;135 for this he devised an ingenious falling-weight timing apparatus for obtaining series of definite small time intervals of less than 1/100,000 second. Measurements of the rise and decay of induced magnetic forces in iron and steel were compared for rapid and slow cycles. The considerably differing curves obtained for rapid and for slow cycles indicated 'quite appreciable magnetic viscosity' for iron and steel at frequencies of 1,000 per second. This does seem to have contradicted the conclusions of his previous paper where the use of frequencies of 100,000,000 had shown that 'the molecule of iron can swing completely round in less than a hundred-millionth part of a second'. He found 'the interpretation of the results very difficult' and attributed the discrepancy to a possible variation with frequency of the force required to cause this rapid swing.136 That this was not the only way of understanding magnetism at this time is shown by the considerable and perhaps better known researches of P.Curie, who viewed molecular magnetic theories with disfavour, preferring a kind of 'phase' or state-of-matter explanation, as will be seen.137 But Rutherford, as well as those whom he cited in these first two papers, mainly 0.Lodge and J.J.Thomson,138 show no signs of this. The work of the latter contains 139 interesting depictions of the way in which the rotations 73 of a molecule, composed of atoms arranged in a particular fashion, could produce permanent magnetism by continually disturbing the tubes of force surrounding the molecule; a 'shearing' of positive and negative tubes past one another would give no electrical effects, only magnetic. The former, Lodge, as shown above,140 explained magnetism in terms of etherial cogged wheels. Thomson also gave explanations of radiation, chemical combination, electrical conduction through gases, and other phenomena in terms of chemical atoms, molecules, and the etherial vortical tubes of force where possible. Rutherford may well have read Thomson's earlier works, which were much involved with unified physical and chemical explanations, as has been seen. Indeed, Rutherford later told Rayleigh that while still in New Zealand he had read everything that J.J. had written.141 This was a considerable amount; by 1895 Thomson had published some fifty papers and four books. It was perhaps because he was familiar with and impressed by this quantity of material that Rutherford 142 apparently expected to find him somewhat 'fossilized' at their first meeting; Rutherford was aged twenty-five and Thomson only thirty-eight at the time. Perhaps he had classed Thomson with Kelvin (1824-1907) and Rayleigh (1842- 1919) who were older, and whose works he had also consulted.143 On the basis of his experimental work on magnetism . Rutherford was awarded an 1851 Exhibition Science Scholar- ship as a graduate of the University of New Zealand. A change in the Cambridge University regulations144 enabled him to become the first non-Cambridge graduate to start work for a postgraduate degree at the Cavendish Laboratory. It is not clear, however, whether the new two-year degree played any part in his decision to work with Thomson.145 During his first term at Cambridge, beginning in October 1895, Rutherford continued his work on magnetism, extending it particularly in developing a sensitive magnetic detector of electromagnetic radiation. Marconi was developing the more successful 'coherer' type of detector for long-distance signalling at about this time. By means of its demagnetising 74 effect on a bundle of magnetically saturated iron needles, Rutherford was able to detect the radiation, through brick walls, at distances up to half a mile,146 and succeeded in impressing the scientific and wider circles at Cambridge with his demonstrations. This work is summarised in his paper on 'A Magnetic Detector of Electrical Waves and some of its Applications' published in 1897147 by the Royal Society with the usual delay. This included an account of the studies on surface magnetism performed at Canterbury College which provided the basis of the detector; but the studies on magnetic viscosity of difficult interpretation, as well as speculations on the nature of the molecular motions involved in magnetism, were omitted. By the time this paper was presented to the Royal Society, in June 1896, Rutherford's attentions were already concentrated upon phenomena connected with the newly discovered X-rays; their discovery was some four months old when he wrote in April: I am working with the Professor this term on Rantgen Rays. I am a little full up of my old subject and am glad of a change. I expect it will be a good thing for me to work with the Professor for a time.148 He thus exchanged the study of one form of penetrating radiation for another, and there was a further interesting connection between Rutherford's 'old subject' and his new one. For in mid-1896 J.J.Thomson saw the conductivity produced in gases by X-rays in terms of a magnetic molecular analogy. His work with Rutherford in the next few months was to change this; to understand the significance of this change one must look to Thomson's views of the nature of the electrical conductivity of gases and the light thrown upon the subject by X-rays during 1896. In doing so we shall see that the term 'ion' was commonly used by Thomson in his earlier electrolytic theory of electrical conduction through gases, and that some quantitative aspects of this theory were under active consideration before 1896, although the 'ionisation' theory of Thomson and Rutherford developed during this year was an advance on all previous work. In the Recent Researches of 1893, which contains a lengthy 75 chapter on the 'Passage of Electricity through Gases'149 Thomson repeated his view of 1883 that: chemical decomposition is not to be considered as an accidental attendant on the electrical discharge, but as an essential feature of the discharge without which it could not occur.150 And he was able on this basis to give a 'working hypothesis' of the 'very complex and very extensive phenomena' of the discharge tube,151 upon which we can only touch here. At this time his view of the cathode rays, identified as bluish lines causing phosphorescence of glass, was that owing to their magnetic deviability and other properties they must be charged particles and not the purely etherial phenomena as suggested by some German physicists. However, these were not 'molecules' as W.Crookes had called them, but the free, and necessarily charged atoms from dissociated molecules; negative atoms in the neighbourhood of 152 the cathode were strongly repelled. However, these negative rays or cathode rays 'play but a small part in carrying the current through the gas', deduced partly from the fact determined by Thomson using a rotating mirror method that the luminosity in the tube travels in the opposite direction and 'with an enormously greater velocity 153 than we can assign to these particles'. Instead, he took the bulk of the current passing through a gas to be carried electrolytically, somewhat in the manner of the theory of Grotthus for conducting solutions, which involved complete chains of associated molecules bridging the two 155 electrodes.154 Rough calculations of the electrostatic force between the hydrogen atoms in a molecule showed that 'the separation of the atoms cannot be effected by the 156 direct action of the electric field upon them'. But the existence of chains of polarised molecules, broken up into short lengths by collisions, would ease the separation of an atom from the molecule at the end of a chain. The high velocity of the luminous discharge was explicable by the jumping of the ends of the successive unit tubes of force along a chain of molecules, at a far greater velocity 157 than a moving charged atom. Also explained in a similar way was the stratification, whose non-luminous portions 76 were seen as parallel Grotthus chains; the luminous areas were at their ends, where atoms were being detached. This view of electric discharge through gases, within the fairly narrow limits of pressure involved here, was repeated at the British Association meeting of 1894158 and extended as far as a quantitative estimate of the 'very small number of charged ions' necessary to make rarefied gases the 'exceedingly good conductors of electricity' which they were observed to be. By the conductivity 'we could easily detect the presence of free ions though they only amount to one part in 7000 of the total gas'.159 The effect of water in facilitating chemical combination and, as Thomson found, the electrical discharge, suggested that its presence might facilitate the formation of the aggregates of molecules thought necessary for the discharge 'by supplying nuclei round which they may condense'.160 C.T.R.Wilson took up a related experimental study, touched upon by Thomson,161 of the effect of various nuclei on the condensation of water vapour, as the latter's last comments on electrical conduction by gases, before the discovery of X-rays, were published. In an article 'On the Electrolysis of Gases'162 Thomson described his use of the spectroscope to detect qualitatively 'the movement of the ions in opposite directions along the discharge tube', and the resulting decomposition of hydrogen chloride and other gases; he drew the interesting conclusion that positively and negatively charged hydrogen atoms exhibited different spectra. At the end of 1895, shortly before the discovery of X-rays, Thomson's discussion of 'The Relation between the Atom and the Charge of Electricity carried by it'163 shows the strength of his continuing interest in the electrical properties of gases. He supposed that the ions of gaseous electrolysis do not have the same persistency of sign as in the electrolysis of solutions; evidently the two kinds of electrolysis were different and such an assumption helped to explain this; E.Wiedemann and G.C.Schmidt performed gaseous electrolyses, and discussed these differences in atomic-molecular terms.164 Thomson alone invoked vortex explanations: he supposed that the 77 attraction of atoms for electricity was related to the arrangement of the etherial vortices within atoms, and the emergent unit tubes of force.165 His speculation as to the mechanism of chemical combination of hydrogen and chlorine, considering the necessity of a third substance for this and many reactions, again involved his idea of the association of molecules, which always remained valuable and flexible. In this case, for example, association occurred to facilitate interchange of electrical charges, but not to 'free' an atom; the gas did not become conducting during the course of the chemical combination, showing that ions, easily detectable by this property, were absent. During the first months of 1896, the new X-ray photography was being tried at the Cavendish Laboratory, and probably at every other physical laboratory in Europe. By the end of February Rutherford wrote home that he was already tired of it;166 but there was more in X-ray studies than this, for by then Thomson and J.A.McClelland were engaged in the investigation of one of the few properties of the rays to which Rdntgen had not at first laid claim167 By 29th January Thomson168 had found that the rays caused a dissipation of electrostatic charges of either sign. Conductivity produced in the surrounding air, or in any solid dielectric, was pronounced as the cause within days169 This was different from the effect of ultra-violet light, which caused the dissipation of negative charges on clean metal surfaces, as Elster and Geitel had shown.170 Thomson stressed his conclusion that 'all substances when transmitting these rays are conductors of electricity' and repeated his view that such conductivity in any substance occurred 'by a splitting up of its molecules'. This was a novel link between radiation and the electrical conductivity of gases which led to advances in experiment and theory. It was not necessary for the gas to be rarefied, heated or electrically stressed, and investigations could now be made over far wider ranges of conditions than in the discharge tube. Continuing studies during 1896-7 provided a more quantitative understanding both of radiation and of 78 the molecular electrical structure of gases. And the views of Thomson were to undergo some modification as experimental studies developed. By March 1896 he was able to give an account of considerable progress in a joint paper with McClelland 'On the Leakage of Electricity through Dielectrics traversed by Rontgen Rays'.171 An important experimental result was the proportionality of the leak or current to the square root of the pressure of an irradiated gas. Now from the standard kinetic theory of the dissociation of gases 'the number of ions is proportional to the square root of the pressure'; hence the conductivity of the gas was itself proportional to the number of ions present but independent of their mean free path and velocity. The most significant results emerged from investigations of the dependence of the current upon the voltage applied: unlike electrolytic solutions,and irradiated solid dielectrics, Ohm's law was not obeyed for irradiated conducting gases. Instead a maximum current was always obtained at a low voltage; this current could not be exceeded even for large increases of the voltage, 'a very remarkable and characteristic property of the conductivity produced by these rays in a gas'.172 This agreed with the pressure-current relationship in showing that the conductivity depended only upon the number of ions present and not upon their velocity when the maximum current flowed. Although the explanation of the mechanism involved was not expressed very clearly by the authors,173 if one takes into account Thomson's ideas on conduction through gases during previous years, discussed above,174 an interpretation is possible. The authors supposed that the effect of the X-rays was continually to produce 'chains of molecules or aggregations of some kind'. As we have seen, the assumption was that these could readily release the free atoms essential for conduction. The aggregates produced by X-rays were supposed to be of such a kind that 'the component atoms with their electrical charges could rearrange themselves with facility; the time T required for this rearrangement being independent of the intensity of the electric field'. This, Thomson seems to imply, 79 would happen spontaneously to each chain, once formed, in the absence or presence of any electrie field. Each rearrangement would effectively transfer a definite quantity of electricity from one end of the chain to the other; but in the absence of an electric field the net effect of many randomly orientated transfers would be zero. When a weak field is applied a proportion of the aggregates become 'polarised' in a manner 'analogous to that of the molecular magnets in a piece of soft iron under an external magnetic field'. With some chains similarly orientated, the spontaneous transfers of electricity would produce a current. As the voltage increases, more chains become aligned and the current rises, but 'as soon as all the chains get pulled into one direction the current will reach a maximum value and be independent of the electro- motive force'.175 This electrolytic mechanism is seen to differ from that for ordinary electrolysis generally accepted now, and probably by many then; in electrolysis of solutions, conductivity depends largely upon the (measurable) velocities176 of the aggregates of ion plus neutral molecules. Thomson's next major paper on the subject, published after a period of more than six months and now in conjunction with Rutherford, contained modified views of conduction through gases, which were more closely allied to the accepted mechanism of the electrolysis of dilute solutions. And the continuation of this research by Rutherford enabled Thomson by the end of 1896 to publish for the first time an account of a mechanism by which X-rays could produce the particles assumed to be responsible for electrical conductivity in the normally insulating gases. It is to be noted that the use of the electrical conductivity produced in a gas by radiations as 17 'a very sensitive and convenient measure of the intensity' 7 did not at first depend on any particular ionisation theory. However, all later explanations, both scientific and historical, from 1898 to the present, of this means of determining the intensity of X-rays and radioactive radiations are given in terms of the theory of Thomson 80 and Rutherford of September 1896.178 There are some interesting clues as to the path of the development of this theory from March to September.179 In April 1896, as Rutherford left his study of Hertzian radiation, and magnetism, and began the experimental work on X-rays and electrical conductivity, Thomson wrote to Nature on 'The RUntgen Rays'.180 His brief comment on the 'saturation' conductivity produced by X-rays in gases, was that: The relation between the rate of leak and the potential difference thus exhibits the same general features as that between the magnetisation of a piece of soft iron and the magnetising force.181 In his Rede lecture in June Thomson indicated that he still supposed that the conductivity was caused by the transmission (not absorption) of the X-rays;182 he again used the magnetic analogy for the more detailed voltage-current curves obtained by Rutherford and himself: 'When the rays are strong, the curve is like that of soft iron; when the rays are weak, it is like steel'.183 A major objective of the work was clearly the attainment of an understanding of the detailed mechanism of the conduction process; it may or may not have been a deliberate attempt to ascertain the dimensions of the aggregates or chains of molecules assumed to be involved, which led to the observation, again not agreeing with Ohm's law, that: In some experiments recently made by Mr. Rutherford and myself, we found that using a constant potential difference the rate of leak was smaller across a very thin plate of air than across a thicker one; it thus appears that the process of conduction through a gas is one that requires a considerable amount of room.l84 Perhaps the researchers entertained the idea that the dimensions of the aggregates or chains of molecules involved might be those found in the very different conditions of the striations of the discharge tube. These were of the order of one millimetre in width and were thus composed of millions, or many thousands, of molecules. It was not made clear how these are produced by radiation, nor how a confined space restricts the transfer of electricity. A week after Thomson's Rede lecture of June 1896 81 Rutherford wrote of his struggle with this research: 'My scientific work is progressing fairly well but it is rather a difficult subject I am on at present'.185 But within a further three months many of the problems of this area had been eased into a modified theory. The explanation expounded in Thomson's and Rutherford's joint paper 'On the Passage of Electricity through Gases Exposed to Rontgen Rays'186 remained one of electrolytic conduction occurring by means of aggregates produced in a gas by the radiation; but series of experiments had now clarified the nature of these aggregates. A progressive step in the experimental work had been the piping of the gas from the point of irradiation into a separate vessel for examination of its conductivity. Removal of the conductivity by certain filters187 showed the 'coarse character' of the conducting entity within the gas, which we can see fitted with Thomson's thinking over the previous few years. Most important in understanding the nature of this entity was the 'very suggestive result' that the conductivity produced by irradiation could be greatly diminished or entirely destroyed upon application of an electric field, of a few volts potential difference, across the gas as it passed along the tube before reaching the vessel in which the leakage was tested. The electric field was applied by inserting a central wire within a metal tube inside the tube along which the gas passed. On placing a glass tube over the central wire, thus maintaining the field but preventing the current, they were able to conclude that: the peculiar state into which a gas is thrown by the ROntgen rays is destroyed when a current of electricity passes through it. It is the current which destroys this state, not the electric field.188 This gave a simple explanation of 'saturation': the maximum current will be the current which destroys the conductivity at the same rate as this property is produced by the RUntgen rays.l89 This could still have agreed with the earlier theory in which the polarization and orientation of aggregates or chains was assumed. What was new, however, was the 82 assumption that what they called 'conducting particles' were actually electrically charged', and that the velocity of translation played a vital part in conductivity. This implies a different mechanism for conduction from that envisaged previously for the chains or aggregates had themselves been supposed to form the electrolytically conducting path. But this was not mentioned in the paper published in November; the authors now explicitly placed their explanation of conductivity in line with that accepted for the electrolysis of dilute solutions: We shall find that the analogy between a dilute solution of an electrolyte and gas exposed to the ROntgen rays holds through a wide range of phenomena, and we have found it of great use in explaining many of the characteristic properties of conduction through gases.190 191 Although the ability to impart a charge to a gas, and 192 the absence of polarization seem to be properties outside the analogy, the qualitative and quantitative explanations it provided were clearly 'of great use'. The authors were able to derive an important quantitative relationship equating the rate of increase of the number of charged particles with the difference between the rate of production by the X-rays, and the rate of destruction 2 both by recombination (proportional to n ) and by the passage of current. As in ordinary electrolytic theory, this current was expressed in terms of the 'sum of the velocities of the positively and negatively electrified 193 particles'. These equations accounted for the higher resistance of thinner layers of gas, but without quantitative agreement. Close correlations between the equations and experimental results were however very successfully attained for the voltage(E)/current(i) curves for various differently irradiated gases, which rose to a limiting value of i according to an equation of the form A - i = B.i2/E2 . The value of the limiting current gave easily an estimate of the proportion of the gas electrolysed as 1/(3x1012):194 compare Thomson's similar calculations on gaseous 'free ions' in discharge tubes, of 1894.195 Using the curves Thomson and Rutherford were 83 able roughly to estimate the time of spontaneous diminution of the number of particles to one half after the rays had ceased, at about 1/10 sec., a precursor of half-life estimates for radioactive gases. And using this they arrived at a first estimate of 0.33 cm./sec. per volt/cm. for the sum of the velocities of the oppositely charged particles, upon which the current depended. This was 'very large compared with the velocity of ions through an electrolyte' but small compared to the 50 cm./sec. for 'an atom of a gas carrying an atomic charge' which implied, as had been assumed, 'that the charged particles in the gas exposed to the ROntgen rays are the centres of aggregation of a considerable number of molecules'.196 It seems possible that a distinction was already being made between 'charged particle' and 'aggregation' implying a certain kind of mechanism, but this was made more explicit as Rutherford continued the research. It was soon after the reading of this paper to the British Association that he wrote home of the possibility of discovering new chemical elements.197 Thomson in a note appended to Rutherford's next publication, dated December 1896, 'On the Electrification of Gases exposed to ROntgen Rays, and the Absorption of Rontgen Radiation by Gases and Vapours1198 gave the first description of the way in which the radiation might cause the conductivity of a gas. Thomson supposed that the moving tubes of force comprising the radiation produced charged particles by 'dissociatiOn of one molecule, or production of one positive and one negative ion'; this implies that aggregation is subsequent to dissociation. Rutherford's delicate experiments on the absorption of the rays had shown that gases which were good conductors of electricity under irradiation were also good absorbers of the radiation. This suggested for the first time, although it was still not stated explicitly other than in Thomson's note, that it was not the transmission but the absorption of the rays which resulted in conductivity. Rutherford's bare statement that 'Experimentally it was found that the rate of leak of a gas is proportional to the intensity of 84 radiation at any point'199 is enigmatic. For until this time the expressions he italicised had been taken as . practically synonymous; the rate of leak was the only measure of intensity. It may be that his experiments showing that the rays 'appeared to emanate in all directions from the anode' 200led him to suppose that the intensity diminished according to an inverse square law in the manner of light from a point source. Once this was assumed - and the tentative opinion was that X-rays were pulsations or vibrations of a similar kind to light - an experimental demonstration that the rate of leak was proportional to the inverse square of the distance from the source could have given rise to the conclusion as stated. Whether or not this is so the statement itself marks an important . clarification in his understanding of the phenomenon. The process was now seen as an absorption of the radiated energy by gas or vapour, with the resulting production of a small number of pairs of oppositely charged ions, around which aggregation occurred. J.J.Thomson added that one Faraday tube would be removed from the radiation for each molecule dissociated; but Rutherford never expressed himself in terms of these tubes in publications. Thus outlined, the theory was developed experimentally during the next few years by several research students at Cambridge. Rutherford concentrated on this field for his remaining two years here, at first working on the separation and examination of the oppositely charged ions existing in conducting gases. Preliminary experiments201 showed that a gas in the conducting state could be made to acquire a. net charge, and this was attributed to an excess of ions of one sign over the other. At this time Kelvin and his associates were interested in such points and may have attributed this to the acquisition of electrical charge by the molecules; J.Perrin in Paris accepted an ionisation hypothesis,202 but the Cavendish school appears to have been far ahead in experiment and theory. In Rutherford's next publication on 'The Velocity and Rate of Recombination of the Ions of Gases exposed to Rlintgen Radiationc203 dated July 1897, he described an 85 ingenious method of timing the passage of ions of one sign: only half of the gas, between plates 16 cm. apart, was irradiated the rest was screened so that ions of one sign would have to travel through 8 cm. of non-conducting gas before arriving at the oppositely charged plate to produce a rapid deflection of the connected electrometer. What was perhaps at first unexpected was the result that the velocity of positive and negative ion always appeared to be equal, not only in elementary gases such as hydrogen, but in compounds with asymmetric molecules such as hydrogen chloride. Furthermore the hydrogen ion velocity was different in different gaseous hydrogen compounds. John Zeleny, however, in his discussion 'On the ratio of the velocities of the two ions produced in gases by ROntgen radiation; etc.'204 soon afterwards demonstrated experimentally that negative ions generally possessed a slightly higher velocity; these researches show the ingenuity with which the ions were manipulated and the reality with which these researchers endowed them. The equality assumed by Rutherford of the velocities of gaseous ions certainly did not apply in the electrolysis of solutions where each ion possessed an individual mobility. That the observed velocities depended more upon the gas used than the nature of the dissociated ion gave rise to the hypothesis that the size of the cluster of molecules, formed around the central charged particle, was determined only by an equilibrium between intensity of bombardment by surrounding gas molecules and the magnitude of attraction provided by the central charge of the ion. A comparison of the observed velocity of the ion, for example 10.4 cm./sec. for hydrogen gas, with that of 340 cm./sec. calculated for a molecule of hydrogen carrying an atomic charge, gave estimates of the sizes of the carriers involved. Rutherford found more moderate sizes than formerly may have been supposed; in the present example 5.5 molecular diameters for the hydrogen ion in hydrogen gas. 205The understanding of the electrical structure of gases was to be an essential feature of Rutherford's future studies of radioactivity, both experimentally and 86 theoretically. For example, his experimental investigations on the 'decay' of after-conductivity, by 'blowing' and static methods, confirmed the relationship dn/dt = a.n2 for the disappearance of the ions. His observations agreed with the theoretical curve relating the declining number of ions to the time, and with the theoretically calculated time T taken 'for the number of conducting particles to fall to half their number', given by the equation T = 1/N.a ; N is the maximum number which depends on the intensity of the radiation, a is a different constant for each gas, T was found to be of the order i sec. The superficial similarities with studies of the decay of radioactivity on which Rutherford was to work some two years later in 1899 are striking. But the deeper and more complex links between his studies of radiations, the magnetic and electrical properties of matter, and uranium rays, will become more evident. Rutherford indicated in his paper on ions in gases exposed to X-rays, dated July 1897 and now under discussion, that experiments on uranium radiation had already begun, and that the ions produced in gases by this means were the same as with X-rays.206 We can see that the subject of uranium rays was considered to be of note at the Cavendish Laboratory; for in 1896 its Professor stated that he found Becquerel's discovery 'exceedingly interesting'207 and that he had obtained photographic effects by means of uranium salts.208 G.G.Stokes also showed an interest in this new form of 209 phosphorescence during 1896-7. In July 1897 Rutherford stated his interest in uranium rays and announced preliminary results, as has been stated; in October C.T.R.Wilson announced his confirmation of these by the cloud- condensation method; then Stokes at the Cavendish Physical Society, and J.J.Thomson at the Cambridge Philosophical Society in November discussed the implications of W.J.Russell's article.210 But it was early in 1899 before. Rutherford's promise of further results made in 1897211 was fulfilled. In the intervening period he published one paper on 87 the problematical subject of 'The Discharge of Electri- fication by Ultra-violet Light'.212 This was previously understood, and confirmed in this paper, as an effect produced mainly at metallic surfaces and not within the volume of the gas; the effect of X-rays was the opposite of this. Ultra-violet light discharged negatively electrified metals and caused zinc and some other metals to acquire a positive charge. Rutherford cited213 some of the literature on the subject from the ten years of its history into which we cannot go deeply here. He mentioned only the theory of surface disintegration,214 and not Lodge's supposition that the effects probably 'depend on some synchronised disturbance set up in the air ... in contact with the substance, a disturbance resulting in some kind of chemical action'.215 Rutherford's main concern in this paper was to investigate the nature of the carrier of the current. Using the kinds of experimental technique developed at the Cavendish Laboratory during the previous two years, he found that the current was carried by free gaseous ions, of negative charge only, and not by particles of metal. The use of a variety of different metals, from lead to sodium amalgam, showed that 'the velocity of the carrier is independent of the metal on which the light falls%216 This indicated that the carrier was produced not from the metal itself but from the gas near its surface. To provide a mechanism for the phenomenon was no doubt one desired object of this research, but none was published at this time. For the origin of uranium rays, an equally difficult subject, Rutherford did suggest the outline of a mechanism. A study of this radiation was the subject of his most substantial paper then published: 'Uranium Radiation and 217 the Electrical Conduction Produced by It' was dated September 1898, the month in which the young scientist left Cambridge to replace Callendar as Macdonald Professor of Physics at McGill University, Montreal. The suggestion 218 adopted by Rutherford was not his own, though it may have relied to some extent on his results. It had been put forward by J.J.Thomson at the beginning of 1898 in a 88 note 'On the Diffuse Reflection of ROntgen Rays'.219 Thomson pointed out that these diffusely reflected rays were, like uranium rays, similar to X-rays but less penetrating. He mentioned the experiments of Sagnac on secondary rays emitted by metallic surfaces, as did Marie Curie a few months later in relation to her own speculation as to the origin of uranium rays.220 Thomson supposed that secondary rays were produced during the ionisation of the molecules of the material, solid, liquid or possibly gas, by the incident X-rays. He provided a diagram showing how the tube of force joining the atoms in a molecule could be broken by the influence of an incident radiated tube, and concluded that owing to the rapid movement of tubes during the course of dissociation: Ionization (if sudden) may thus be expected to give rise to rays having properties similar to those of the secondary Ontgen rays. 221 As for uranium rays: It seems not impossible that in the case of a complicated structure like the uranium atom regrouping of the constituents of the atom may give rise to electrical effects similar to those which occur in ionization and might possibly be the origin of the uranium radiation.222 We recall that Thomson had made his first announcement of the subatomic cathode particle in April 1897 and had fully set out his theory of the corpuscular chemical atom in a publication of October 1897. The suggestion concerning regrouping of the corpuscles constituting the uranium atom can be considered as the first published statement that the emission of uranium radiation is a property of the atom; it precedes those of Marie Curie and G.C.Schmidt made later in 1898.223 But this kind of idea was not new: as the previous Chapter shows, some physicists and chemists attributed the emission of characteristic radiations, but of longer wavelengths or different wave forms, to internal vibrations of chemical atoms. However, the energy required for the emission of atomic spectra was known to be positively provided, in obvious ways. Becquerel had clearly stated the problem of the source of the energy of the uranium rays 89 and Rutherford, who cited eight of Becquerel's nine papers as well as later French authors on the subject, saw this as a question to which there were now some answers. An interesting point which emerged from Rutherford's investigations of 1898 was that the rays from uranium consisted of two distinct portions. He considered that these beta and alpha rays were comparable with X-rays and secondary X-rays respectively, and speculated that the alpha rays might thus be produced at the surface of the active substance by the supposed primary beta radiation.224 Rutherford quoted Thomson's idea that a rearrangement of the constituents of the uranium atom could give rays similar to those produced by the sudden ionisation of a gas.225 But he tacitly modified this as required by his experimental results by insinuating that such rays from gases were similar to soft primary X-rays rather than to the less penetrating secondary X-rays which Thomson had suggested. The existence of two kinds of uranium X-ray could thus be understood, but only in part. For in his opinion 'The cause and origin of the radiation continuously emitted by uranium and its salts still remain a mystery'.226 Rutherford was able to ease this with the comment that on account of the smallness of its energy 'the radiation could continue for long intervals of time without much diminution of internal energy of the uranium',227 but such relief was only temporary. The question of the possible diminution of this radiation proved to be an important point in the struggle to understand the subject . as the mystery deepened and widened during the next few years. Rutherford's stated aim of 1898 was a study of 'Uranium Radiation and the Electrical Conduction Produced by It'.228 His repetition of Becquerel's experiments, but with entirely negative results, led him to the. firm conclusion that this radiation was neither refracted nor polarised;229 Becquerel had to agree.230 And by successive interposition of metal foils Rutherford came to his conclusion that the radiation was 'complex' consisting of at least two distinct types each approximately homogeneous, one readily absorbed (alpha) and one more penetrative 90 (beta),231 each of about the same coefficient of absorption in gases as X-rays.232 He may have owed something both to Becquerel,233 who had indicated the complexity of the uranium rays, and to J.J.Thomson and J.A.McClelland234 who had used metal foils in demonstrating the considerable variety and complexity of X-rays from different bulbs. Significantly, however, Rutherford found that different compounds of uranium gave rays of the same composition, as indicated by the foil method and by his own more sensitive method of absorption in gases. Despite the speculation that the alpha rays were secondary to the beta, which Thomson may never have accepted,235 this result seems to be equivalent to a demonstration of the emission of a definite spectrum consisting of two main components. It may possibly have been taken by the Cavendish researchers as a further indication that the property of emission belonged to the uranium atom. This latter view, also suggested by Marie Curie and G.C.Schmidt in 1898, had been put forward by each in connection with their independent pronouncements that thorium was the only other element giving similar spontaneous radiation. Rutherford's brief examination236 of the thorium rays showed, in spite of some capricious but interesting variability in readings, that they were of a different penetrative composition from uranium rays; these were the problems he took to Canada. With regard to the discovery of thorium rays he mentioned237 only Schmidt, who had in fact been slightly earlier than Marie Curie in publishing the discovery. One paper of Marie and Pierre Curie was cited by Rutherford238 but only to criticise seriously their conclusions. Owing to the ready absorption of the alpha rays by any material, including that of the emitting substance itself, Rutherford noted that 'the rate of leak due to any uranium compound depends largely on its amount of surface'. Thus the state of division of the layers of powdered salts used made it 'difficult to compare the quantity of radiation given out by equal amounts of different salts'.239 Now such a comparison had been the very means by which the Curies had , firstly come to suspect, and secondly to begin to isolate 91 chemically a new element; its characteristic, they said, was its great power of radiation compared with uranium. Rutherford dissolved a crystal of uranium nitrate in water and allowed it to evaporate so as to deposit a very thin layer of the salt.. This simple exercise gave a higher than normal leakage due mainly to alpha radiation, which had the greatest electrical effect. 'It is possible' he wrote: that the apparently very powerful radiation obtained from pitchblende by Curie may be partly due to the very fine state of division of the substance rather than to the presence of a new and powerful radiating substance.240 Although the Curies had noted241 the effect of the thickness of the layer of salt for uranium and thorium rays, they had published no analysis of the composition of the rays, nor had they developed studies on their electrical effects in gases. With his deep experience in these areas at the Cavendish Laboratory, surely Rutherford could not have erred on this important point: but wrong he was. His success came with thorium; the Curies found that studies of their new elements polonium and radium went from strength to strength. 92 3. Pierre Curie, Marie Curie and the new radioactive elements (1890-8) Towards the end of 1898 Rutherford disagreed with the Curies over the existence of a new radiating element; he had lost this point even before his paper appeared in print early in 1899. But these parties were to disagree on more complex issues concerning radioactivity from 1901 to 1904 and their approaches appear to have differed well before this time. One could say that they were near to being adherents of different schools of scientific thought. The main point of distinction was the attitude towards the various mechanical, molecular and etherial models. These as we have seen were applied in considerable variety for explanations in physical science. But a tradition which can be called 'positivist'242 cast doubt upon the validity and even the utility of such models, in the last decades of the nineteenth century. While all scientists saw the attainment of general laws as a vital part of scientific progress some were sceptical of the models which others made their goal and considered the seeking of general relationships to be the sole objective of science. There was however a variety of opinion between these views; Maxwell for example in considering his attempt 'to imagine a working model' explaining the rotational character of magnetic and optical phenomena wrote that: The problem of determining the mechanism required to establish a given species of connections between the motions of the parts of a system always admits of an infinite number of solutions. Of these, some may be more clumsy or more complex than others, but all must satisfy the conditions of mechanism in genera1.243 And W.Ostwald appears to have changed from an atomistic understanding of chemistry and physical chemistry to a completely anti-atomistic and purely energetic or thermo- dynamical approach, in about 1890.244 A fuller discussion would lead us into areas of the philosophies of nineteenth century scientists but I wish only to indicate that P. Curie tended to be critical of atomic or molecular models and sought explanations rather in terms of general laws; he was able in this way to provide a lasting 93 contribution to aspects of experimental science in providing such laws apparently without recourse to models. In his earliest series of researches, published jointly with his older brother Jacques during 1880-2 on piezo-electricity of crystals,245 it is difficult to discern any leaning towards either of the two approaches; both were clearly valuable in this case. The research was performed at the Mineralogy Laboratory of the Sorbonne whilst both brothers were 'preparateurs'; Friedel was director here,246 and had himself worked on pyroelectricity - the production of electrical polarity in crystals upon change of temperature.247 Crystals possessing axes with dissimilar extremities, i.e. hemihedral with inclined faces, exhibited this polarity at these extremities. The Curie brothers claimed the discovery of a new, but related, way of producing electrical polarity - by varying the mechanical pressure, applied along these axes; this was later named piezo-electricity. Some indications of the production of electrical effects by mechanical treatment of crystals had been known for many years248 but the experimental work of the Curie brothers was a considerable advance. They clarified the quantitative nature, symmetry and reversi- bility of the effect using tourmaline and quartz crystals. When the collaboration of the brothers ended in 1883, with Jacques taking up the post of 'Maitre de Conf6rences, at the University of Montpellier249 and Pierre becoming 'Chef des travaux de Physique' at the new Ecole municipale de Physique et de Chimie industrielles in Paris, each retained an interest in the physical and geometric properties of crystals. During the decade 1883-93 Pierre Curie performed little experimental research but continued studies 250 of aspects of piezo-electricity. These led to the development of the kind of electrometric apparatus by means of which Pierre and Marie Curie were able to measure -11 the currents of order 10 amps produced by uranium, thorium, and the new radioactive elements which they were later to discover. It was during the period 1883-93 that Pierre Curie seems to have moved away from molecular explanations. In 94 1881, the discussion 'Sur les phgnomenes electriques de la tourmaline et des cristaux h6miedres a faces inclinees1251 was conducted in molecular terms. One cannot say whether the views expressed belonged to Jacques or Pierre Curie or to C.Friedel; perhaps they were common to all. The authors attributed pyroelectricity and piezo-electricity to the same cause - a contraction or dilation along a particular axis. Discussing a deeper structural origin they expressed disagreement with a view which likened the rows of molecules in a pyroelectric crystal to a thermo-electric pile. Here, a set of successive cones of copper and bismuth, for example, exhibited the required momentary polarity on change of temperature. They considered that a better explanation of the separation of charge by pressure, and of the particular symmetry of the phenomenon, was provided by the hypothesis of the permanent polarisation of the molecules in a crystal, with the end faces normally maintained in the neutral state by a layer of electricity 'condensee sur la surface': '1'idee que les molecules sont polarisees est en parfait accord avec ce fait que l'electricite ne se montre libre sur les bases'.252 They concluded the discussion by commenting that the 'extremite aigue' of each molecule in these crystals was permanently negatively charged with respect to its base, and that 'la forme de la molecule paralt avoir l'influence preponderante'. Pierre Curie seems never again to have published favourable comments on any molecular mechanism put forward to explain physical phenomena. His interest in piezo- electricity remained,253 but as others entered the field in the 1880's and 1890's he inclined more towards a study of the geometrical symmetry involved.254 Other authors had considered symmetry in physical science in a vague manner255 but Curie developed these studies in a systematic and original way in applying the criterion of symmetry, as used for the classification of crystals in mineralogy, to 256 the phenomena of physical science. In his paper 'Sur la symetrie dans les phenomenes physiques. Symetrie d'un 257 champ electrique et d'un champ magnetique' Curie applied 95

considerations of symmetry to a variety of electrical, magnetic, optical and thermal phenomena without the use of mechanical or etherial models. He classified phenomena by their symmetry, and used the proposition that the existence of a characteristic dissymmetry was a necessary requirement for the production of an effect by a cause. This indicated which phenomena could and which could not exist. As Curie himself pointed out, thermodynamics provided a different and more quantitative indication of • possibility.258 And we note that other scientists on occasion used dynamical principles, without mechanical hypotheses, to develop equations whose terms they supposed might correspond to some phenomenon existing or un- discovered.259 It seems that the consideration of symmetry was probably the least important of these three general methods of physical science in the nineteenth century; 26;) but this is perhaps not so later, in the twentieth. While developing his theoretical ideas on symmetry, Pierre Curie also performed experimental research. His paper on 'Propriet6s magnetiques des corps a diverses temperatures'261 appeared in 1895, the year after his publication on symmetry considerations in physics. Although there was no specific application of the principle to magnetism, the generality of Curie's approach to the interpretation of his results contrasted with the usual molecular view of magnetism. We have seen how Rutherford understood hysteresis in terms of molecular magnets and how he sought to follow the rapid motion of these in an oscillating field. Curie, on the contrary, looked to functions of physical state as an analogy by which to explain his experimental results and touched but briefly upon molecular theories. Faraday had found that all bodies exhibited magnetism, distinguished the three varieties of this property, and had noted that iron, strongly magnetic (ferromagnetic) at normal temperatures, became weakly magnetic (paramagnetic) at high temperatures. Curie now showed that diamagnetism, which was possessed by most or perhaps all substances, did not vary with temperature. This result 96 pointed to the independence of diamagnetism from the other kinds of magnetism; on the other hand he was able to demonstrate by several series of measurements at different temperatures that ferromagnetism and paramagnetism were closely related. All ferromagnetic bodies were progressively transformed, on heating, into paramagnetic bodies. Both the inverse relationship between absolute temperature and intensity of magnetisation of paramagnetic substances which Curie demonstrated experimentally,262 and the curves of transformation of ferromagnetic substances, were compared in some detail with gas-liquid phase phenomena. For paramagnetism, likened to the gaseous state, the corresponding equations I = A.H/T and 263 D = (l/R).P/T possessed interesting similarities. The analogy was further supported by the I/T curves for paramagnetic-ferromagnetic transformations, which were very similar to the continuous D/T curves for gas-liquid transformations near the critical temperature, as determined for carbon dioxide by Amagat.264 In short, the functions f(T,H,T) = 0 and f(D,P,T) = 0 possessed strong though not complete similarities.265 This comparison led Curie to his single comment on a molecular analogy between magnetism and the condensation of fluids; the rapid augmentation of magnetic intensity in weaker fields as the temperature falls may occur 'quand l'intensite d'aimantation des particules magngtiques est asset forte pour qu'elles puissent rgagir les unes sur lee autres'.266 His analyses of the results on magnetism of 1895 show just the kind of minimal mechanical or structural explanation which Curie was to give for radioactivity. Marie and Pierre Curie were in 1898 among the first to describe uranium radiation as an atomic property yet they avoided any deeper public discussion of the mechanism of this atomic radiation; nor did they describe, as did others, the kind of structure an atom might have in order to possess such a capability. As will be seen Pierre Curie considered radioactivity in terms of a general analogy with the transmission of heat in its various forms. 97 Manya Sklodowska had arrived in Paris in 1891 at the age of 24 and became one of few women science students at the Sorbonne; she followed her elder sister Bronya who had come to Paris from Poland to study medicine, about five years earlier.267 Manya, or Marie, was awarded the degree of Licence es Sciences Physiques in 1893 and the same in mathematics in 1894.268 Then, after her marriage to Pierre Curie in 1895, she passed the examination requirements to become 'Agregee de 1'Enseignement Secondaires des Jeunes Filles' in mathematics in 1896.269 Her first research was of a partly industrial nature, performed for the Societe d'Encouragement pour l'Industrie Nationale de France, on the magnetic properties of tempered steels of different types from various steelworks in France.270 The magnetic measurements involved were somewhat similar to those of her husband's thesis on magnetic properties at different temperatures published in 1895;271 his knowledge may have been helpful for her work. Although chemistry does not figure among her academic qualifications she had apparently followed courses in chemical analysis whilst in Poland.272 These may have aided her in determinations of the composition of the steels as well as in her new subject of research uranium radiation, which quickly developed into the wider field of radioactivity. It is largely upon her work with radioactive substances and the discovery of the element radium that Marie Curie's present fame rests. The beginnings of her popular renown came in 1903 when spectacular developments in radioactivity were very much in the public eye; public honours accumulated considerably during 1903-4.273 The tragic accidental death of Pierre Curie in 1906 aroused great public sympathy for the widow; she was then appointed Assistant Professor at the Sorbonne taking the place of her husband in the Chair of Physics created for him in 1904. This was an unprecedented appointment for a woman as was her full Professorship in 1908.274 Her second Nobel Prize, on this occasion for Chemistry and not shared, and her involvement in the 275 matrimonial separation of P.Langevin, both in 1911, afforded continuing public interest. Work with the first 98 267 mobile medical X-ray machines in the War of 1914-18 and the inseparable association of her name and that of her daughter277 with the ever more important radium and radioactive elements seal Marie Curie's lasting fame.278 Nevertheless, it must be pointed out that the theoretical explanation of radioactivity to which the Curies adhered during the period now under consideration, did not agree with that developed by Rutherford and which is now accepted. Her claim279 of priority for the transformation- disintegration theory, made in 1906, is open to doubt: Cette hypothese se trouve parmi celles qui ont ete indiquees par M.Curie et moi d6s le debut de nos recherches sur la radioactivite. Mais elle a ote surtout procisee et developpee par Rutherford et Soddy, auxquels elle est, pour cette raison, generalement attribuee.280 For although some aspects of the theory of 1906 were present among the several speculations put forward in 281 her earlier publication of January 1899 to which she referred this claim, it will be shown that these were shared by others. And she omitted to mention the Curies' strong opposition to Rutherford's and Soddy's theory, during the important intervening period from 1901 to 1903. But we hope to look more deeply into such points in later discussions.282 Our interests in the remainder of this Section are to follow the work of the Curies into their studies on radioactivity, to provide a much-needed discussion of their earliest theories and speculations, and to examine their conclusion that the emission of radiation by uranium is an 'atomic property'. Marie Curie tells us that her, and Pierre's, first interest in uranium rays dates from the second half of 1897, at the time when her magnetic experiments were complete, and when she was seeking a subject on which to 283 begin research for a doctoral thesis. Becquerel's last paper on the subject for some time was read to the Academie in April of that year284 but there is no indication of direct communication between Becquerel and the Curies until later, in 1898. Besides one possibility, that Marie Curie 285 came upon the subject simply by reading about it there 99 are several possible links with contemporary scientists interested in the subject. That some part was played by Pierre Curie in her decision to investigate this area was later indicated by Marie.286 The Curies, particularly Pierre, attended meetings of the French Physical Society287 and may have heard Becquerel's reports of his researches. J.Perrin and G.Sagnac each reviewed Becquerel's work in 1896,288 and were acquainted with the Curies, possibly as early as 1897; there was also the long-standing friendship 289 between Pierre Curie and Ch.Ed.Guillaume. The latter's interest in uranium rays is shown by his discussion of the energy problem at a Physical Society meeting, after one of Becquerel's reports in 1896.290 The researches of Becquerel were in any case well known in scientific circles in 1896-7. Marie Curie's experimental work on uranium rays291 began in December 1897 in accomodation at the Ecole Municipale de Physique et de Chimie industrielles where her husband worked. She achieved more quantitative estimates of the electrical intensity of the radiation than those of Becquerel by means of an apparatus which made use of the piezo-electric effect studied by P. and J.Curie in the 1880's. A layer of powdered uranium-bearing material placed on a charged metal plate caused electrical leakage across an air gap and a rising accumulation of charge on a parallel plate which was connected to an electrometer. This charge was continually balanced by adding successive weights, by hand, to a piezo-electric quartz thus maintaining a more or less null deflection of the electrometer. The apparatus was calibrated by means of a known. charge and the current flowing could be calculated from the weight/time ratio; this method of measuring small currents 292 had been described in the thesis of J.Curie. It gave results which Marie Curie claimed to be accurate to 2% -11 of the values of the minute currents of order 10 amps involved during the first few months of the work.293 Problems of quantitative accuracy arose later when large weights had to be added in a short time to compensate for currents produced by the intensely emitting substances which the Curies were to discover. 100 If uranium radiation were a kind of short-wavelength or X-ray phosphorescence, an Becquerel supposed, it should diminish in time, even if slowly, and should be excited by irradiation. If there were similarities with the storage of light shown by thermo-luminescence then heating should have some effect. These points which had occurred to Becquerel and others were probably in Marie Curie's mind when she began by seeking the effects of heating, and of irradiation by light and X-rays, upon uranium. The intensity of the uranium rays, on re-examination with the sensitive apparatus after treatment as above, remained always unchanged.294 Her lack of success in finding a straightforward answer to the question of the origin of the radiation was compensated by an important discovery as her research turned towards other materials. On surveying as complete a list as possible of other metals or their compounds she found that thorium too emitted rays of the same order of intensity as uranium. Her first publication on uranium rays 'Rayons 6mis par les composes de l'uranium et du thorium'295 shows that she had noted that the only two elements exhibiting this property possessed the greatest atomic weights; she had in addition linked this point with current research on X-rays and their secondary rays to give something of a theory of the origin of uranium and thorium rays. Working independently of Mme.Curie and at about the same time, G.C.Schmidt also surveyed many materials, discovered that thorium emitted similar electrically detectable radiation to that from uranium, and claimed priority.296 With E.Wiedemann at Erlangen he had earlier studied experimentally various kinds of fluorescence, phosphorescence and thermoluminescence; he had discussed the theoretical basis of these phenomena in terms of vibrating ether envelopes around molecules, vibrations of atoms and their valency charges, and ionisation or definite 297 chemical separation of atoms in the molecule. Schmidt sought a relationship between three phenomena: the photographic effects of various substances, including uranium, as described by Colson, Russell, Muraoka and others; the 101 electrical conductivity produced in gases by uranium, and lately thorium, and their compounds; and the photoelectric effect involving a loss of negative charge or an acquisition of positive charge by certain metals and minerals upon irradiation with light of certain kinds.298 He stated that he had followed the work of Elster and Geitell who had shown experimentally both that metals exhibiting photoelectricity did not emit radiations electrifying the air in the manner of uranium and that uranium salts were themselves not photoelectrically sensitive.299 Schmidt used a modification of the apparatus used by Elster and Geitel300 for some earlier studies on the photoelectricity of minerals to show that thorium compounds too gave no increase in air conductivity when irradiated with light. We can see that this would indicate that thorium radiation was neither of photoelectric origin nor of a kind of phosphoresence which could be excited by the light employed. Furthermore, none of the many photographically active substances studied in 1896-7 proved to be electrically active. Only uranium and thorium rays possessed both electric and photographic properties, and Schmidt noted the similarities of these radiations in a fuller account 301 of the research, of 1898: thorium rays were reflected and refracted but not polarised; Becquerel's conclusion that uranium rays exhibited all three of these properties, still stood; and it was accepted that X-rays exhibited none of these properties. Schmidt's only hint of an explanation of the photographic and electrical properties of the radiation was that 'Es scheint als ob dieselben an das hohe Atomgewicht Uran = 240, Thorium = 232 gebunden sind' .302 Although it has a place of importance in the progress towards the modern theory, there was no reason for Schmidt to attribute great significance to this interesting point. If he had written that the radiation came from within heavy atoms this would not have been a novel conclusion at the time. For E.Wiedemann believed that Schmidt's spectroscopic work on fluorescent vapours indicated that a single atom of a metal could behave as an electrically complex emitter of electromagnetic 102 radiation.303 Having provided facts and conclusions vital to other scientists, Schmidt left the subject with Becquerel's question of the origin of the energy unanswered and continued his research on the complex problems of electro- and photo-luminescence. Marie Curie was working along the same lines as Schmidt but had made much more of the study than he by the time their first papers on the subject were published 304 in the spring of 1898. In her first paper she clearly assumed, in agreement with Becquerel's conclusion for uranium, that the property of emission of the new radiations attaches to the elements themselves. This enabled her to dismiss phosphorus from the active class, although the white allotrope produced a strong air conduction effect, since compounds of this element were not active. Phosphorus never really became involved with radioactivity though the cause of the conductivity produced by it, whether due to ions, fumes, or radiation, continued to be 305 a point of discussion into the twentieth century. It is not clear whether she was aware, as Schmidt was, of Russell's work on the photographic activity of metals and organic substances, but Mme.Curie seems to have • examined only inorganic specimens. She noted that the activity of all minerals could be attributed to the presence of an active element, namely uranium, thorium, or one of 306 three weakly active rare earth elements, and that uranium salts were active 'd'autant plus qu'ils contiennent plus d'uranium'; her published list of activities, however, shows no quantitative proportionality between uranium content and activity. What was 'tres remarquable' were the activities of the minerals chalcolite and pitchblende, 52 and 83 respectively, compared with impure uranium's reading of 24. The reader, having been led thus far, was presented with the surprising deduction that 'ces mineraux peuvent contenir un element beaucoup plus actif que l'uranium', and it seems that 'element' here does have the meaning of a definite chemical element. Certainly Rutherford could not accept this conclusion; as has been noted307 he attributed the effect to the large surface 103 area produced by finely powdering the substance. But Marie Curie was aware of this kind of effect, had realised that only the surface layer of uranium emitted the rays and that thorium differed in so far as its deeper layers contributed to the radiation; the important factor of consistency of readings no doubt satisfied Pierre Curie, who was always wary of premature publication. Further evidence came from the preparation by Debray's method of an artificial chalcolite308 which was found to possess an activity no greater than that of an ordinary uranium salt. It seems that at the time of this initial publication Mme.Curie, and perhaps P.Curie, had decided that the emission of the penetrating radiation was a fundamental property of an element independent of its chemical and physical state. Becquerel had already said this, but Marie Curie's suggestion of a new element seems to involve the further step of assuming that the magnitude of the conducting effect of the radiation was specific to the emitting element. This, without further explanation, she called the 'activite' of the element or compound. Certainly the high readings of some minerals comprised the only evidence given for the existence of a new element in the 309 Curies' paper cited by Rutherford, which was Marie Curie's 310 second publication on the subject. The information that chalcolite and pitchblende emitted a radiation qualitatively different from that of either uranium or thorium compounds, as determined by the fraction transmitted by an aluminium sheet, had indeed been given in the first note but without interpretation. Such a qualitative difference may appear to be evidence for the existence of a new element, as good as, or better than a high activity; it seems more akin to the emission of the visible radiation of a distinct spark spectrum which constituted the most acceptable evidence and was the chemist's 'court of final appeal'311 for the proponent of a new element. Perhaps she felt that the considerably more penetrating nature of the rays from a thick layer of thorium oxide, compared with a thin layer,312 impaired the value of this as a criterion for identification 104 of the emitting element. But it is possible that qualitative differences in the radiations may have played some part in Marie Curie's bold deduction of the presence of a new element.313 For the consideration of such differences seems to be involved in her theoretical explanation of the origin of the radiation. G.Sagnac, in the months before Mme.Curie's first publication on the rays of uranium and thorium, had been continuing earlier work of others and himself on X-rays by examining their effect in causing gases to become electrically conducting in the presence of metal plates.314 From several series of experiments he concluded that X-rays entering or leaving solid materials always produced easily absorbed secondary rays or S-rays. Thus the observed conductivity or ionisation produced in a gas by X-rays is the sum of the effects of the X-rays and S-rays; this differed from Perrin's view of 1897 that the additional conductivity resulting from the introduction of a metal plate arose from an ionisation occurring at the gas-metal interface struck by X-rays.315 Sagnac examined the S-rays from a variety of substances, using fluorescent, photographic, and electroscopic means of detection. By the beginning of 1899316 he saw the absorption and emission of X-rays and S-rays by chemical elements in terms of their similarity to normal spectroscopic absorption bands though the comparison was not a clear one; and it seems that this idea of an incipient X-ray spectroscopy was being formed during the previous year. In her initial paper of 1898 Mme.Curie mentioned the researches of Sagnac and pointed out that the properties of uranium and thorium rays are 'tres analogues' to those of the secondary X-rays; the researchers at Cambridge had 317 also taken up this analogy, as we have seen. MarieCurie tells us that she had herself examined the secondary rays from uranium, pitchblende and thorium oxide and had found these to have a greater discharging effect than those from lead.318 Her proposed explanation of uranium and thorium rays was that: 105 Pour interpreter le rayonnement spontane de l'uranium et du thorium on pourrait imaginer que tout l'espace est constamment traverse par des rayons analogues aux rayons de Rantgen maisbeaucoup plus penetrants et ne pouvant gtre absorbes que par certains elements a gros poids atomique, tels que l'uranium et le thorium.319 It was well known that bodies of greatest density, or atomic weight, absorbed X-rays most effectively. Marie Curie was now postulating the existence of a highly penetrating radiation which was absorbed and then re- emitted in less penetrating form by the elements of highest atomic weight only. Why lead, with an atomic weight of about 210, should show no sign whatever of activity whereas thorium (230) was more active than uranium (240) was not explained; however, the relatively high activities of some minerals may have suggested that the atomic weight of the new element might be greater than 210. Mme.Curie's theory of the origin of uranium and thorium rays was disposed of by her own hand, and those of others, by the end of the year 1898; nevertheless her suggestion of the existence of a new element proved to be doubly justified. Following her original suggestion Marie Curie was joined by Pierre Curie in the ensuing chemical search for the new active element and the couple were advised and aided by G.Bemont, Pierre's counterpart in chemistry at the Municipal School. After three months they were able to report to the Academy through Becquerel, 'Sur une substance nouvelle radioactive contenue dans la pechblende 1320 and went so far as to give a name to the new element, though, as they noted, E.Demar9ay was unable to confirm it spectroscopically. They had started with the mineral pitchblende, two and a half times as active as uranium, and applied the usual successive dissolution and precipitation techniques of inorganic chemical analysis. After each operation the more active portion, measured with the piezo-electric electrometer, was selected for further analysis. Finally, after separations which were effective though incomplete the highest activity resided with the element bismuth. Continuing fractional dissolution and reprecipitation 106 gave slowly increasing activities, but sublimation of the active bismuth sulphide gave a substance of the highest activity, 400 times that of uranium. As is well known, Marie Curie in this publication named the possible new element 'polonium' after her country of origin. Polonium obstinately refused to exhibit a spectrum, and had a chequered history;321 however an element of this name survives today. Most of the products set aside in the pitchblende analysis must themselves have been active - a number of quite novel chemical avenues could now be followed. By the end of the next academic term, in December 1898, the Curies and B6mont showed how successful a path they had trodden by announcing the discovery of a . second new radioactive element, on this occasion with far stronger evidence to support their claim.322 The direct inorganic analysis guided by continual electrical measurements led to barium chloride of activity 60 times that of uranium; the assumption was that a new active element chemically similar to barium was present. A fractionation procedure of dissolution in water and partial precipitation by alcohol produced progressively more active precipitates. The authors noted that this was evidence of the existence of a new active element whose chloride possessed different solubility characteristics from those of barium chloride. They were able in this way. to attain a substance of an unprecedented activity of 900, before the materials ran out.323 100 kg. of pitchblende residues from Joachimsthal (lacking uranium which had been extracted for use as a colouring material) had already been acquired.324 The strongest evidence for a new element which they produced at this stage was provided by the rare-earth spectroscopist E.Demarpy325 who stated that he had actually found a new line in the spark spectrum of the active substance. And the Curies and Bemont noted that 'L'intensite de cette raie augmente done en meme 326 temps que la radioactivite' which constituted 'une raison tres serieusel for attributing the new line to the radioactive portion of the specimen. They named the new element 'radium'. A possibly marginally greater 107 atomic weight of the active barium, compared with ordinary barium, was obtained by determining the chlorine in the anhydrous chloride. The expectation of an atomic weight greater than that of lead was indeed to be realised. All of the evidence, radioactive, spectroscopic and 327 gravimetric, was fully confirmed within four years. In the original paper on radium of December 1898 the Curies described radioactivity for the first time as 'une proprietb atomique, persistant dans tous les tats chimiques et physiques',. 328 they henceforth persisted in the use of this expression, throughout the controversies in the next few years concerning the origin of the phenomena. Just as 'atomic weight' need mean no more than 'relative combining weight' the expression 'atomique' may here mean no more than 'elemental'. The authors elaborated no further, perhaps with good reason. Marie Curie's review article on 'Les Rayons de 329 Becquerel et le Polonium' appeared shortly after the announcement of the discovery of radium, contains no mention of that substance, and was therefore probably written between July and November 1898. Its contents give us an indication of the meaning of the description of the emission of rays as an atomic property. One of the reasons which Marie Curie gave for excluding white phosphorus from the class of radioactive substances, despite large readings on the measuring apparatus, was that it was active neither when in chemical combination nor as the red allotrope so that 'on ne retrouve done pas le caract6re d'activit4 atomique independante des 330 6tats physiques et chimiques'; this is similar to the statement made at the December meeting of the Academy. But in the earlier review we are also told that since uranium exhibits constant readings, independent of its chemical or physical state, its radiation appears 'comme une propriete mol6culaire, inherente a la mati6re miime de l'uranium'.331 A possible origin of the rays which could be described without contradiction as both atomic and molecular might be the Ur-Ur bond; chemical valency in general might also be described thus; but 108 Mme. Curie did not discuss such points. She was never to describe the phenomenon as molecular after 1898. But others were to do so with attendant difficulty or confusion surpassing that found here. Marie Curie's speculations of 1898 were aimed more at the problem of the 'Degagement d'energie par lee corps radioactifs',. 332 she considered several possibilities for the source. The hypothesis that the radiation was a phosphorescence of long duration previously excited by light she dismissed firmly; we have seen that Becquerel had moved in this direction in 1896. The possibility, briefly mentioned, that the radiation 'est une emission de matiere' accompanied by a loss of weight of the active substances333 seems suggestive of future work and reminiscent of W.Crookes"radiant matter' in discharge tubes; but she may have meant no more than a release of vapour. This is shown by the heading 'Emission de rayonnement lie a un etat chimique de la mati6re radiante' under which she now discussed334 the photographic effects studied by Colson and Russell in terms of the release of vapours. Thirdly she wrote that the source of the energy could come from the evolution of the elements in the manner suggested by Crookes - the elements of greatest atomic weight could still be in the process of formation. Much was left unexplained concerning the original protyle and the production of the radiation; Crookes was himself thinking of the problems of radioactivity at this time but in other ways. Fourthly, Mme.Curie repeated more clearly her own earlier theory, claiming that there was nothing improbable in supposing that 'l'espace est le siege de transmissions d'energie, dont nous avons aucune idee'335 and that these ultra-X-rays might be transformed into detectable, less penetrating secondary radiation by heavy atoms. However, the sun could not be the source of such rays, since the interposition of the whole body of the earth would surely absorb a proportion of these and cause a reduction in the intensity of uranium rays, yet the midnight and midday readings were equal. She was more concerned to argue that although such a theory was 109 in accord with the principle of Carnot there were authorities who held that such a principle need not apply 'avec un mecanisme tree petit'. Mentioning the opinion of Helmholtz on this point, Maxwell's 'demon' of the kinetic theory of gases, and the work on Brownian motion of L.G.Gouy her final comment was that: Dans cette maniere de voir, le rayonnement de Becquerel pourrait 8tre consid6re comme un reflet des mouvements non coordonn6s de molecules matgrielles.336 Thus descriptions of the phenomena as atomic or molecular were each consistent with such an external energy supply. Her connection of the radiation to random molecular motion has implications which were not followed up: G.G.Stokes, J.J.Thomson and others at this time tentatively accepted a theory that X-rays consisted of irregular ether- pulses; but she made no mention of these scientists nor did she state which, if any, of the five possible theories she preferred. Between the times of writing and publication of Marie Curie's review others indicated their interest in the growing problem of the energy source, heightened indeed by the Curies' discovery of highly active substances. William Crookes independently put forward almost the identical molecular explanation of the origin both of the energy and of the attendant phenomena of radioactivity. And J.Elster and H.Geitel had gone so far as to devise and perform experiments which cast doubt both upon Crookes' theory and on Marie Curie's alternative of unknown radiations from space.337 110 4. Theories and trends (1896-9) In his address as President of the British Association in September 1898 William Crookes338 covered a wide variety of topics as expected, from the beneficial effects of chemical fertiliser on the world food problem, to the continuing controversy over the psychic researches whose validity he accepted. As for uranium rays his early interest in these seems to have been increased by Mme.Curie's announcement of a new active element. In his account of recent development in physical science Crookes devoted some time to a discussion of the 'radiant activity' of uranium, thorium, and the new body which possessed this activity in 400-fold degree, polonium: 'like uranium, it draws its energy from some constantly regenerating and hitherto unsuspected store, exhaustless in amount'.339 With regard to the 'haunting problem' of the nature of this store Crookes adopted the view of proponents of the kinetic theory of gases, such as Johnstone Stoney, who believed that the energy of molecular motions might be made available, as with Maxwell's imaginary demons, contrary to 'accepted canons'. In this Crookes independently made the same suggestion as had Marie Curie. But he went into detail, in a manner which she never adopted, by suggesting a mechanism involving the atoms of the elements by which the emission of rays allied to X-rays could spontaneously and perpetually occur. Crookes pointed out that faster moving molecules were separated from slower ones in the case of the evaporation of a liquid and in the separation by diffusion of a lighter from a heavier gas: Let uranium or polonium, bodies of densest atoms, have a structure that enables them to throw off the slow moving molecules of the atmosphere, while the quick moving molecules, smashing on to the surface have their energy reduced and that of the target correspondingly increased. The energy thus gained seems to be employed partly in dissociating some of the molecules of the gas ... and partly in originating an undulation through the ether, which, as it takes its rise in phenomena so disconnected as the impacts of the molecules of the air, 111 must furnish a large contingent of light waves of short wave-length. The shortness in the case of these Becquerel rays appears to approach without attaining the extreme shortness of ordinary Rtintgen rays.340 Kinetic theory indicated a large supply of energy, translational and vibrational, contained in the air. Crookes' and Marie Curie's speculations were within months subjected to experimental tests not of their own; and each theory was found wanting. J.Elster and H.Geitel, scientific collaborators in Wolfenbuttel, had throughout 1896 attempted to determine whether the electrical effects produced by uranium were related to photoelectricity.341 In experiments, followed by G.C.Schmidt for thorium,342 they showed that no relationship existed: uranium salts gave no photoelectric effect, and photoelectrically sensitive metals did not emit invisible radiations. Nearly two years later Crookes' address of September 1898 which they read in Nature343 induced them to compose and send in their paper 'Versuche an Becquerelstrahlen'344 before the end of that month. Reporting experiments begun, in part, earlier, they noted that they too had considered the surrounding air to be a possible supplier of the radiated energy; not as Crookes imagined but by means of a chemical reaction between one of its constituents and the uranium salt used. If the air were the source then a decrease in its pressure should reduce the intensity of the radiation. As they remarked, Beattie and de Smolan at Glasgow had shown that the conductivity produced by Becquerel rays in air indeed decreased steadily to a very small value as the pressure of the air was reduced. Unlike the Glasgow researchers, Elster and Geitel interpreted this result in ionic terms- fewer of the ions required for transport of electricity would be produced in a rarefied gas.345 Thus the reduction of the air pressure should give a lower electrical reading whether or not the intensity of the radiation diminished. They therefore placed the radiating substance in a vessel connected to a vacuum pump and examined the rays emerging through an aluminium window. No significant variation of 112 intensity was to be found using the photographic method with a long period of exposure - not a reliable quantitative method. This procedure was apparently employed prior to Marie Curie's first publication on the subject in April 1898; after this Elster and Geitel improved it by using the more active natural pitchblende, enabling electrical measurements to be made. These confirmed the conclusion that the radiating activity was unaffected by the surrounding air. We know that Rutherford, with his deeper studies of the conduction of electricity through irradiated gases, had already assumed as much; this is shown in his paper on uranium rays,346 then not yet published, in which experiments using a variety of gases and pressures were described. Elster and Geitel admitted that their result did not entirely demolish Crookes' hypothesis, for even the best vacua contained millions of molecules which might still supply sufficient energy for radiation and ionisation. Crookes published his views in France soon afterwards347 and was to maintain these even when the energy problem became much greater in the following years. Elster and Geitel indicated348 that by July they had also arranged to test Marie Curie's theory of April 1898. They believed that no material could be completely transparent to any radiation *and suggested that a thickness of more than one hundred metres of solid rock would surely absorb to a noticeable extent the ultra-X-rays from space imagined in Mme. Curie's hypothesis; this should affect the 'secondary' or uranium rays. Using the same portable electroscope and the same piece of pitchblende they recorded no difference in readings when the apparatus was taken 300 metres below ground into a mine. A photographic experiment carried out for them at the bottom of the Schact Kaiser Wilhelm II, 852 metres deep, also showed no variation of intensity. Their conclusion was much firmer than with Crookes' hypothesis: Nach diesen Versuchen erscheint uns die Hypothese der Erregung der Becquerelstrahlen durch andere im Raume prLexistirende Strahlen im hiSchsten Grade unwahrscheinlich.349 113 However, they were able to confirm the Curies' chemical extraction from pitchblende of a highly active substance. This, following the discovery of thorium rays, appears to have been seen in 1898 as the most important experimental result in the field since the original researches of Becquerel. The development of the subject of uranium rays by Marie Curie during the early part of 1898 has been regarded as marking the conclusion to a period in which interest in these rays had declined to a low ebb partly because of their submersion in a 'morass' of other radiations.350 This obscurity, as has been shown, was complemented by a confusion of uranium rays with the Russelleffect. It is true that in France there was a period of several months in 1897 when Becquerel had turned to studies of the Zeeman effect, and Marie Curie had not yet taken up her investigations. But it may be pointed out that elsewhere in Europe one can trace something of a continuing concern with the subject during 1896-8. As we have seen, in 1896 several French physicists showed an interest in uranium rays, which is indicated by their published reviews and discussions. In England J.J.Thomson's351 comments on the subject were followed by some experimental studies of E.Rutherford and C.T.R.Wilson in 1897. G.G.Stokes too discussed the phenomenon with S.P.Thompson in 1896. Stokes published a theoretical explanation of the radiation in mid-1897352 and during this year he discussed at the Cavendish Physical Society the work of W.J.Russell on the photographic effects of metals; J.J.Thomson reported this in October 1897.353 Stokes also discussed the subject in communication with Crookes who had the phenomenon in mind in February 1898.354 Crookes had been attempting experimentally to obtain a radiometer effect from uranium radiation in August 1897; but the definite results obtained were attributed to temperature differences.355 Kelvin and others at Glasgow published their researches on the electrical effects of uranium in 1897. And one can trace a line of minor interest, if not of continuing experimental work, in Germany, where the publications of Elster and 114 Geitel of 1897, were followed by G.C.Schmidt's studies on thorium rays. These, together with the distinction he made between the effects of the new radiations, of photographically active substances, and of photoelectricity, were announced in February 1898.356 And J.J.Thomson in the previous month made his important speculation as to the origin of uranium rays.357 One can thus see an underlying interest in uranium rays which was to be considerably excited by Marie Curie's announcement of the discovery of polonium in mid-1898. As studies of uranium rays and radioactivity developed during the period 1896-9 the various theories which were put forward can be seen as more or less related to Becquerel's early description of the phenomenon as a metal phosphorescence after he had isolated uranium as the source. G.G.Stokes, the British authority on phosphorescence and fluorescence, in advising S.P.Thompson concerning the latter's discovery in February 1896358 of the emission of penetrating rays from various chemical compounds, also provided at this time a mechanical molecular explanation. Stokes took the rays to be like X-rays 'transversal vibrations of excessive frequency' and likened their emission to the phenomenon of 'calorescence' in which heat radiation of high intensity could raise a body to incandescence. Stokes regarded fluorescence 'as a disturbance extending from more limited to more extensive molecular groups'359 but calorescence and Thompson's new phenomenon360 appeared to be the reverse of this: I look on calorescence as an agitation passing from wider to more minute molecular groups. In your discovery, I think we have something of the nature of calorescence; only that whereas in Tyndall's work the disturbance was excited in the first instance in wider molecular groups, in yours the 'wider groups' are already something like the chemical molecules of the peculiar substance.361 It follows from Stokes' explanation at this early stage that if the wider vibrating groups are something like chemical molecules then the smaller groups should be something like the atoms in a molecule. That Becquerel was soon able to trace the emission to uranium metal 115 might have led Stokes subsequently to such a conclusion concerning uranium atoms. But in his Wilde Lecture 'On the Nature of the Röntgen Rays' delivered in July 1897362 he was evasive on this point, while incorporating most of the new evidence, and expounding a modified theory of the nature of X-rays. In order to explain the perpetual emission of rays by uranium without the necessity of irradiating the metal Stokes told his audience: My conjecture is that the molecule of uranium has a structure which may be roughly compared to a flexible chain with a small weight at the end of it.363 Natural vibrations travelling from the head to the tail of the molecule would produce ether vibrations 'not of a regular periodic character'. Stokes still saw X-rays as transverse disturbances of the ether; however, these were now characterised not by an extremely short wavelength but by consisting of completely irregular pulses. Uranium rays would thus lie between visible light and X-rays in their regularity. The chemical implications of imagining such a 'uranium molecule' are not discussed. Stokes entertained this kind of explanation of radioactivity at least until 1900 without publishing further on the matter. Towards the end of 1899 he sent Becquerel a copy of his Wilde Lecture and discussed theoretical ideas of radioactivity; Stokes maintained an explanation in terms of a comparison with the normal visible phosphorescence of uranium compounds. He considered that both phenomena pointed to 'the existence of a molecular group which is roughly speaking isolated, in the sense that vibrations going on in it are not very quickly communicated to the neighbouring structure'.364 Although, as Becquerel had replied,365 the expected effect of temperature change on the emission failed to appear Stokes continued to entertain what he called the 'wagtail' theory. When the self-luminosity of radium came to his notice in 1900 he corresponded with W.Crookes concerning experiments which might distinguish between his wagtail theory and Crookes' 'bombardment hypothesis' 366 and continued the discussion as the phenomena and ideas became more complex in 1901. 116 We have seen that Stokes attempted to explain uranium rays in terms of molecular vibrations akin to those assumed for phosphorescence, that his ideas changed little during the period 1896-1900, and that they began independently of Becquerel. By the end of 1898 Marie Curie had explicitly rejected the earlier statement of Becquerel that the phenomenon might be a long lasting invisible phosphorescence, together with S.P.Thompson's label of hyper-phosphorescence. It is notable however that her own first analogy of uranium and thorium rays with secondary X-rays seems similar to the phenomenon of fluorescence. Apart from Kelvin's guess in 1897 that the effects of uranium might be due to carbides367 two other possible sources of the rays were seriously considered during the first three years of their investigation. Firstly there was the atomic-molecular hypothesis independently suggested by W.Crookes and Marie Curie involving the impact of gaseous molecules upon sensitive atoms which then produced radiation. Secondly, following J.J.Thomson's setting out of corpuscular atomic structures towards the end of 1897 there was his suggestion that some kind of rearrangement of the constituents of a 'complicated structure' like the uranium atom might be the source of the rays. Remarkable speculations which fitted with this latter view were soon published in Germany. 368 Rutherford had tentatively adopted the above explanation provided by Thomson. The former's new colleague at McGill University, R.B.Owens, attempted in 1899 to incorporate experimental results on the non-homogeneity of thorium rays and similar radiations into this theory. He wrote: Certainly it would be difficult to formulate a theory for the production of such rays which would account for only a particular number of kinds being produced. If x-rays and the radiations from uranium, thorium, polonium, &c. are disturbances in the aether occasioned by the internal motion of certain constituent parts of the atom, as had been suggested, it might be expected that such disturbances would shade off with some degree of regularity from a more intense to a less intense kind...369 But it was left to Elster and Geitel370 to put forward in 117 1899 speculations on the chemical implications of radioactivity; these may have influenced others, in a manner not previously brought to light. Following their dismissal of Mme.Curie's first hypothesis of the source of the energy of uranium rays and their doubts as to Crookes' molecular bombardment hypothesis, Elster and Geitel added further experimental evidence, all negative, some of which had been previously obtained by others but not published. Without definite influence upon the radiation from pitchblende were sunlight, Lenard rays, temperature changes, and being kept in darkness for months. They noted the remarkable luminous effect of the Curies' radium on a fluorescent screen. Their conclusion was that since the emission of energy from all compounds of an element could not result from a chemical reaction the source must be the atoms themselves of the elements concerned. Now this was similar to what the Curies had said in 1898. And their further speculation that the atom of a radioactive element behaves like a kind of unstable molecule emitting rays on returning to a stable state is no more than J.J.Thomson and E.Rutherford had written shortly before. Elster and Geitel may or may not have derived this idea from Thomson's publication of January 1898 or Rutherford's of January 1899; Elster had replied to Rutherford's request for advice on demonstration apparatus and indicated that he had read Rutherford's paper on 'Uranium Radiation etc.' at the time of writing, 10th February 1899.371 Elster and Geitel's final deduction in the paper of 1899 was that the change of a substance from an active to an inactive state might necessitate a change of elementary properties: Der Gedanke liegt nicht fern, dass das Atom eines radioactiven Elementes nach Art des Molecules einer instabilen Verbindung unter Energieabgabe in einen stabilen Zustand Ubergeht. Allerdings Warde diese Vorstellung zu der Annahme einer allmAhlichen Umwandlung der activen Substanz zu einer inactiven nbthigen and zwar folgerichtiger Weise unter Aenderung ihrer elementaren Eigenschaften. 372 This conclusion says, almost in so many words, that a transmutation of one chemical element to another of diffe'rent properties should be taking place as the radioactive 118 radiations are emitted. Rutherford had considered that the energy of the emission from uranium was so 'extremely small' that radiation could continue for long periods 'without much diminution of the internal energy of the uranium'.373 This avoids Elster and Geitel's conclusions by emphasising the smallness of the effect. Now since the existence of the highly active radium could not safely be denied in 1899 Rutherford could possibly have entertained these speculations of the German scientists, which indeed could have followed from his own; it is probable that he knew of them374 and that he came to do so whilst studying the strange properties of thorium rays at McGill University in 1899. We have seen that at this time Rutherford accepted the idea that the ether vibrations emitted by radioactive substances could be attributed to rearrangement of the corpuscles constituting the chemical atom and that he probably knew of the speculation that atomic transmutation might be occurring. It has also been noted that one of the speculations put forward by Marie Curie, published early in 1899 and upon which there appears to have been no later comment but her own, was that the heaviest elements may be in process of evolution. All ideas on the source of the radiations, on their energy, and nature, were to be complicated by two discoveries announced towards the end of 1899. One of these, that Becquerel rays could be magnetically deflected, showed the rays possessed an unexpected property which linked them more closely to cathode rays than to X-rays. And cathode rays were thought by J.J.Thomson, his disciples, and some others, to consist of the material particles which constituted the chemical atom. But the increasing complexity of the new phenomena allowed no easy conclusions. 119

CHAPTER 3

EMANATIONS AND RADIATIONS

1. The ma netic deflection of the Becquerel rays 1 9 -1900) The chemical isolation of highly active substances by the Curies in 1898 provided new opportunities for physicists and chemists to pursue experimental studies. One cannot say whether G.C.Schmidt, E.Rutherford or others interested in the subject would eventually have hit upon the existence of the new active elements but once the Curies had opened this field others soon followed. In this Chapter we shall trace the complex form of the development of radioactive studies during the period 1899-1901. It will be seen that the discovery of the new highly active substances heralded the first magnetic deviation of radioactive rays. This had far reaching theoretical implications particularly, in combination with further new discoveries discussed in Section 2, for radioactivity. F.Giesel, chemist at the Buehler quinine manufacturing company in Braunschweig, was one of those who had studied 1 in new X-ray fluorescence and photography. His interestnterest in the related area of radioactivity began by January 1899 when he spoke in the discussion of Elster and Geitel's paper at Brunswick.2 During this year he followed the analytical procedure of the Curies and Bemont to prepare polonium, and claimed to have independently discovered active barium compounds which possessed the novel property of spontaneously illuminating a fluorescent screen;3 he was able to provide Elster and Geitel with such a highly active sample.4 Giesel had obtained his uranium residue starting material from De Haan, chemical manufacturers of Hanover5 and the first commercial radium-barium samples were soon advertised by this company6 to whom Elster advised Rutherford to apply.7 The physicists Elster and Geitel used an active sample provided by Giesel to determine whether the electrical conductivity known to be 120 produced in air by Becquerel rays could be altered by a magnetic field.8 A peripheral idea involved in their apparently successful attempt led Giesel himself to a crucial discovery. It had been known for some years that the electrical conductivity produced in a gas, by glowing metals for example, could be suppressed by the application of a magnetic field whose direction did not coincide with that of the current.9 The explanation, if based on the assumption that molecules, ions, or particles of any kind carried the charge, was that the lateral force suffered by these current- carrying particles in the magnetic field would deflect them out of the line of conduction. A marked reduction in conductivity was indeed obtained, but more interest attaches to the control experiment which Elster and Geitel devised to ensure that it was not a deflection of the rays themselves which gave the observed effect. The Becquerel rays were thought to be similar to secondary X-rays but there was nevertheless the possibility that the rays might be magnetically deviable like Lenard or cathode rays. The property possessed by Giesel's very active barium salts of exciting phosphorescence in a screen was valuable for the straightforward experimental arrangement of Elster and Geitel. They placed the active substance in an evacuated glass vessel; the emitted rays passed both through the glass walls and an aluminium plate upon which rested a barium platinocyanide screen 1.5 cm. distant from the source. The visible phosphorescence excited in the screen was unaltered when their iron horseshoe electromagnet was switched on. They concluded that the rays were undeflectable by a magnetic field and hence different from cathode rays; all of the properties of the Becquerel rays were thus comparable with those of the X-rays. Others also considered this point. In France the Curies had earlier, in 1898, similarly obtained negative results, unpublished, in seeking an effect 10 of magnetic and electric fields on radioactive rays. And 11 Becquerel later stated that towards the end of 1899 he too had been seeking independently some such influence, and had in fact found one: on placing a fluorescent screen 121 at one pole and an active sample at the other pole of a magnet, the application of the field produced a concentration of the fluorescent patch into a smaller area. All experiments on the subject of radioactivity bore repetition at this time, autumn 1899. Thus Giesel repeated12 Elster and Geitel's experiments in simpler fashion without a vacuum but with a more powerful magnet.13 Using a phosphorescent screen placed upon the poles of a vertical horseshoe magnet and an active freshly prepared polonium specimen placed beneath the screen he became the first to succeed in obtaining quite definite results. Upon switching on the magnet, the luminous spot was displaced in a blurred fashion but in a definite direction in relation to the field. Giesel also produced variously shaped images by the photographic method; exposures of up to ten minutes were all that was required for a fixed record of the effect. Having achieved these results Giesel at first published no interpretation, but various explanations were not long in coming. Two privatdozenten at Vienna who also followed the work of Elster and Geitel moved towards a curious misinterpretation of their own results and expressed an interesting if short-lived speculation bearing on the source of Becquerel rays. S.Meyer and E.von Schweidler extended their work on the magnetic properties of the chemical elements, which included correlations of atomic magnetism with the periodic table, to a study of the magnetic properties of radium 14 preparations and the rays emitted by these. They used both Curie and Giesel barium-radium and bismuth-polonium preparations placed 12 cm. from the air-gap through which the rate of electrical discharge was measured electroscopically. On applying the magnetic field this rate was considerably reduced; the Curie polonium however was unaffected. The results of varying the intensity of the field indicated that at least two different effects operated to reduce the conductivity of the air-gap. They 15 provisionally accepted the statement of Elster and Geitel that the rays were not deviated by the magnet, without knowing that Giesel had proved this wrong, and speculated 122 that the magnetic field might act directly upon the radium in reducing its radiation: so ware die Ursache diener Erscheinung nur in einer direkton Beeinflunnung der Emission der Substanz selbst zu suchen wenn eine Ablenkung der Strahlen nicht stattfande.16 In a matter of days they sent off a second paper now mentioning Giesel's work on the magnetic deflection of rays not yet available in print which they had confirmed for themselves. With their powerful electromagnet they had been able for example to bend the rays in a tight semicircle back to the screen upon which the active substance stood; they noted that the direction of the effects was entirely similar to that of the negatively charged cathode rays.17 However, their initial conjecture that the magnetic field might influence the emitter itself, ill-founded and temporary though it was, gives us an indication of one way in which radioactivity could be understood towards the end of 1899. The possibility that a magnetic field might affect the property of radioactivity was not lightly to be discounted. It is true that none of the various attempts to influence radioactivity by a variety of physical and chemical means had given any positive result and that magnetism was to remain on this list. Yet it is interesting to note that an effect somewhat analogous to that at first assumed by Meyer and von Schweidler had recently been detected. Zeeman's discovery made at the end of 1896 was accepted as demonstrating that a magnetic field could influence the source of atomic spectra, altering the frequency and direction of vibration of the radiation. J.J.Thomson had used this discovery to support 18 his corpuscular atomic theory of 1897. In 1899, when the Continental workers were studying the effect of magnetism on the new atomic property of radioactivity and upon Becquerel rays themselves, he was defending this developing theory against criticism.19 In his paper 'On the Masses 20 of the Ions in Gases at Low Pressures' he insisted on the corpuscular structure of the chemical atom and, probably arguing against the view that all observations 123 could be explained by means of a small number of free electrons or valency charges, he again pointed to Zeeman's discovery, now extended. Since many spectral lines exhibited an effect of the same order each atom must contain many corpuscles despite the fact, shown by e/m estimates for ions in gases, that very few could be removed.21 At this time Thomson considered that Becquerel rays themselves originated in the motion of these electrified particles. Although the hope of finding evidence to support the theoretically possible link between magnetism and radioactivity apparently remained with him for several years22 the negative interpretation of the experimental results of 1899 remained unchallenged. Thomson believed that both chemical and electrical actions essentially involved 'the splitting up of the atom, a part of the mass of the atom getting free and becoming detached'.2 3 The positive discovery that radioactive substances did not simply radiate soft X-rays but released streams of subatomic corpuscles turned out to be of extraordinary interest to those such as Rutherford who entertained Thomson's ideas. However, R.J.Strutt later wrote24 that he clearly remembered that this striking interpretation of Giesel's results was at first acceptable neither to Thomson nor Rutherford on account of the high penetration of these radioactive rays. The former wrote to the latter: I see Giesel makes out that the radiation from polonium is affected by a strong magnetic field, if this is so it might be worth while trying whether your emanation from thorium were so affected.25 Rutherford replied in January 1900: The results of Giesel & Becquerel are very interesting and remarkable. I expect the 'emanation' in thorium is also true for polonium & radium when prepared in a special manner & that the deflection due to the magnetic field is due to the action on a charged particle cast off from the active body.26 Within a few months Rutherford indeed saw the phenomenon as a direct emission of cathode rays and had made certain deductions from this. He had previously adopted a tentative comparison of the beta and alpha uranium rays with X-rays 124 and the secondary radiation produced therefrom27 but had given no decision between Thomson's soft X-ray or Sagnac's Lenard-ray view of the secondary radiation. The demonstration during this period by P.Curie and G.Sagnac28 that secondary X-rays in fact carried a negative charge may have provided Rutherford with indirect evidence for the new view of radioactive rays. The phrase 'secondary X-rays' which he used29 to describe the non-deflectable radium rays may thus have acquired the new meaning of actual X-rays produced as a result of the impact of cathode rays. Yet this slightly modified causal link between the two known types of radioactive radiation was soon to be completely broken. Extensions made by Becquerel to his magnetic results provided direct evidence of the nature of the deflectable rays during the first part of 1900. By June of that year Rutherford was able to write that the property possessed by some active substances of 'naturally emitting a kind of cathode rays'3° did not contradict Thomson's theory of regrouping and vibration of the constituents of the atom. But we note that such a mechanism had been intended to explain the production only of electromagnetic radiation. Rutherford's assumption that the radiations in question were 'cathode rays of low velocity',31 which agrees neither with Becquerel's contemporary studies nor with Rayleigh's later comment on penetrations, illustrates the difficulties of this novel aspect of radioactivity. The successful magnetic deviation excited an interest corresponding to its initial problematical nature; it gave a further impetus to investigations of the rays themselves. A number of physicists concerned with the cathode rays, such as R.J.Strutt (later Lord Rayleigh), P.Villard, E.Dorn, W.Kaufmann, used samples of radioactive substances in their researches from about 1900, with the attainment of significant results pointing towards an electromagnetic theory of matter by 1901. And the effect of the magnetic discovery upon the French students of radioactivity was considerable not only in exciting the great activity in investigating these rays which arose during 1900 but also in the changes 125 of view which the rapidly developing experiments helped to produce. One can, for example, interpret the discovery of the emission of cathode rays as marking the quiet beginnings of a severe conflict of the ideas of P.Curie both within himself and against his colleagues. In the discussion following a report to the French Physical Society32 in which Becquerel announced his successful deviation of the rays by an electrostatic field, further demonstrating their similarity to cathode rays, P.Curie spoke. He thought it surprising that radium should emit radiations having the properties of cathode rays as well as X-rays since it was generally agreed that while X-rays were the propagation of a disturbance cathode rays were a flux of ponderable matter. Despite his own considerable researches on the electrical nature of the radiation, after two years he found himself forced to reject this interpretation of cathode rays. This was in reaction against views, implied by the emission of subatomic masses, which several scientists began to develop with growing supporting evidence following the magnetic deflection of radioactive rays. By March 1899 Becquerel, re-entering the field, had accepted Rutherford's point published in January that uranium rays did not behave like ordinary light with regard to polarisation, refraction and reflection; they were more like X-rays.33 Becquerel continued investigations of the nature of these rays by examining their penetration, secondary radiation, and the possible effects of magnetic and electric fields. It was the work of this scientist, who was fortunate in having access to the Curies' most active specimens, which contributed perhaps most to a clearer understanding of the nature of the rays during the year following their first deflection. Becquerel's initial attempts to obtain an electrostatic deviation during 1899 failed as he perhaps expected since the field applied was known to be insufficient to deflect ordinary cathode rays.34 But his magnetic studies35 soon gave highly significant results. The Curies were first in making a valuable correlation of three aspects of the rays namely deviability, penetration, and conductivity. Their polonium 126 radiation was easily absorbed and remained undeflectable and making use of radium they were able to show that whilst the deviable portion of magnetically analysed rays was more penetrating than the non-deviable portion, it contributed little to the total radiation as measured electrically.36 Becquerel immediately moved the study to a more quantitative level. He pointed out37 that if the radium rays consisted, like cathode rays, of electrically 38 charged material particles then the well-known equation H.R = v.m/e held true. H was known, R was found by photographic impression; this gave a value for v.m/e of the same order as determined for the cathode rays by J.J.Thomson, W.Wien, and P.Lenard. Becquerel deepened the correlation between deviability and absorption: he found that the interposition of screens before the detecting plate gave a kind of absorption spectrum in which the most easily deviated rays were the most easily absorbed.39 He expanded the point in a paper 'Sur la dispersion du 40 rayonnement du radium dans un champ magnetique'. The sharpness of shadows was unimpaired on covering the radium source with an aluminium screen which indicated that aluminium was truly transparent to the rays and not a re-emitter of secondary radiation. Becquerel noted minimum values of HR for the radium rays (dispersed as well as deviated by the magnetic field) to which screens of various materials and thicknesses behaved as transparent; to rays having HR below a certain value, all screens were opaque. It was still debatable at the time whether or not the aluminium screen used to allow Lenard rays to escape from the discharge tube acted as a window; penetration of opaque bodies by cathode rays seemed unlikely to Crookes, 41 for example, at the end of 1900. J.J.Thomson's answer in 1896 to Lenard's claim of 1895 that ether waves, but not material particles, could penetrate a metal window seems to have been that X-rays together with a re-emission of charged particles were responsible for the observed 43 effects.42 From Thomson's discussion of 1897 one might deduce, considering the cm. penetration of cathode rays into ordinary air, that aluminium might not function as a 127 window if thicker than about 0.001 mm. Becquerel's aluminium screen was 0.2 mm. thick; if the particles were all of the same charge and mass his experiment would seem completely to correlate velocity with penetration. A reviewer in an article on 'Becquerel rays. Confirmation of the materialist theory of the deviable rays of radium' seems to have been first to publish the clear inference that 'the particles which strike the plate furthest from the source will be those possessing the greatest speed, and it is natural that they should also be the most penetrating'.44 Radioactive studies in 1900 thus lent temporarily a sharp clarity to the projectile character of the cathode rays. R.J.Strutt staked his claim in the matter writing to Nature45 from the Cavendish Laboratory that the magnetically deviable rays from radium exhibited, like cathode rays, a coefficient of absorption very approximately proportional to the density of a series of materials ranging from solids to gases, but that this coefficient was only about 1/500 that of the cathode rays. Hence 'One must suppose either that the particles constituting them are much smaller, or that their velocity is much greater'. Becquerel's results together with the Curies' demonstration that radium exhibited a continuous emission of negative electricity, spontaneously acquiring a high positive charge,46 provided clear evidence that radium emitted negatively charged particles of matter. That the question of the size of these was not simple is further demonstrated by the suppositions of the Curies themselves. They thought it reasonable to infer that 'le radium est le siege d'une emission constants de particules de matiere electrisee negativement'47 which could traverse either conducting or dielectric screens without becoming discharged. However, one can see that the Curies at least were not yet thinking in terms of subatomic corpuscles for, knowing the rate of loss of charge, they gave in March 1900 what was probably the first published estimate of the loss of mass involved on the assumption that e/m was the same as in electrolysis.. They did not say how these particles could penetrate screens. 128 Such an emission of negatively charged atomic or molecular masses gave an estimated weight loss amounting to 3 milligram-equivalents in a million years. Attempts to detect such losses from ever more active specimens were to occupy the Curies and some others during the next few years. However, the Curies' view of the rays soon altered in a way perhaps similar to that of Rutherford. This shift in opinion followed the current rapid experimental progress. For later in that month Becquerel effectively consummated the study by his 'Deviation du rayonnement du radium dans un champ electrique'.48 He was thus able to confirm that the projected particles were by no means atomic in mass; the variability of this mass was not yet an experimental question. Becquerel's photographic method provided measurable and reversible deviations of rays passing through a narrow slit in a small lead container; the quantity e/mv2 could thus be calculated. As we have seen the value of v.m/e could be found by magnetic deviation experiments so that values of v and e/m could now be separately determined. The 'point delicat' was to ensure that the values obtained from magnetic and electric experiments both applied to the same part of these deviable rays, which exhibited a considerable range of dispersion. Becquerel achieved this, as far as possible, by selection of the rays using partially absorbing screens. Thus he 7 obtained values for v of 1.6 x 1010 and for e/m of 10 for a portion of the deviable radium rays, similar indeed to cathode rays. For the latter Becquerel cited values of v up to 0.81 x 1010. He made no explicit comment on the possible subatomic nature of these particles, nor on their uniquely high velocities, but estimated the rate of decrease of mass which would be caused by their loss as far lower than that of the Curies; one would expect this from the emission of particles lighter than atoms but carrying the same electrolytic charge. From the value of mv2 for the particles Becquerel estimated that the power emitted by the rays was a few ergs per square cm. per sec. Such a value was very soon shown to be excessively low: this portion of the radium radiation carries but a fraction 129 of the total radiated energy. Nevertheless, the experimental deflection of these rays made a permanent impression upon all of the various views of the source of radioactivity, as elaborated below in Section 3. Even while this radiant success was in progress Rutherford came to attribute his own interesting results on thorium rays to something more than a radiation - to the release of a material gas-like substance which he took as the cause of another new phenomenon, 'excited' or 'induced' radioactivity. The nature of induced radioactivity was to become. an area of particular disagreement between the Curies, Becquerel, Rutherford, and others, but also a point of progress.

2. The discovery of induced radioactivity (1899) Yet another branch of radioactive research unfolded as a series of novel experimental results began to appear. In parallel with magnetic studies, several scientists worked in this new field on similar lines, at first independently. In some cases the extraction of the new intensely active substances was an essential factor, in others ordinary compounds sufficed. The discovery of the radioactivity induced by radium and polonium was made by the Curies towards the end of 1899, and the title of their note 'Sur la radioactivite provoquee par les rayons de Becquerel'49 indicates their initial view that this effect was due to the direct incidence of radiation. They found that samples of polonium and radium with activities 5,000 to 50,000 relative to uranium could produce, in all substances tried, 130 temporary activities ranging from 1 to 50, the higher values being obtained with longer exposure to the rays; the effects diminished to about one tenth in two to three hours. The new phenomenon which they named 'radioactivite induite' created many scientific problems for the Curies. As they stated, the value of the activity produced seemed not at all to depend on the nature of the surface made active. However, they were unable to explain this fact. Could the effect be due to vapour or to the deposition of dust particles from the active substance? They thought that the steadily declining induced activity could not be attributed to non-volatile particles of the activating radium-barium chloride for the activity of the latter did not so decline; furthermore, washing the activated surface with water should remove this soluble salt, but the induced activity remained unaffected by the process. Neither could it be attributed to a vapour, for an activity was apparently produced by radiation which had passed through the aluminium window of a metal box within which the activating specimen was sealed. Rutherford at this time did in fact attribute induced radioactivity to an active gas-like substance which could penetrate metal screens. E.Dorn shortly afterwards in 1900 agreed50 that some kind of active gas was responsible, but pointed to the possible presence of pores in thin metal screens. One must note that the problem may also have been complicated by external traces of radium as contemporary scientists may soon have realised. However, the Curies in 1899 concluded that 'Le phenomene de la radioactivite induite est une sorte de rayonnement secondaire du aux rayons de Becquerel',51 though distinguished by its longer duration from the direct emission of secondary rays. Becquerel evidently considered that this fitted with views he had held since 1896 and he commented52 that the new phenomenon should be placed alongside the production of secondary rays of low penetration by thorium and uranium rays; this he considered53 had led to the earlier false deductions by himself and Schmidt that diffuse reflection of these rays took place. Thus induced radioactivity and the emission of secondary 131 rays were respectively similar to phosphorescence and fluorescence. Becquerel did not say how such an analogy could explain why different substances exhibited the same magnitude and duration of induced activity. The Curies themselves merely expressed surprise at this and relegated to a footnote their suggestion that the condition of the surrounding air might have some influence in causing irregularities in the induced effect. Similar points had already been noticed by Rutherford in his work on thorium, and had played a vital part in his own sharply differing interpretation of 'excited radioactivity'.

Whilst Rutherford was studying the electrical effects of uranium radiation at the Cavendish Laboratory in 1898, G.C.Schmidt and Marie Curie had announced the discovery of a similar radiation from thorium. Rutherford in his paper on uranium rays54 wrote of his own attempts to study the new thorium radiation. It appeared to be complex and of a different kind from that of uranium as shown by its penetration of aluminium screens. Unlike uranium salts thick layers of thorium nitrate gave a greater proportion of penetrating rays than thin layers but 'On account of the inconstancy of thorium nitrate as a source of radiation, no accurate experiments have been made on this point'.55 The rate of leak varied 'very capriciously'. This was the problem which Rutherford took to Canada towards the end of 1898. In his first preliminary paper from McGill University some eight months later he again noted that thorium was far from exhibiting the constant radiation which was such a notable property of uranium: The inconstancy of the radiation from thorium oxide was examined in detail, as it was thought it might possibly give some clue as to the cause and origin of the radiation emitted by these substances.56 It was R.B.Owens, Macdonald Professor of Electrical Engineering, working with Rutherford on the thorium rays in 1899, who found the first clue as to the variability 132 of thorium radiation readings - the marked effect of slight currents of air.57 With thorium placed in a closed box the measurements remained constant, but on opening a door in this box the readings were consistently lower, and were further diminished if somebody opened or closed the laboratory door; the readings recovered on standing for some hours in the closed box. The authors mentioned that they had performed a large number of experiments, for example examining the effects of blowing air over the surface of the material, but that they had found 'no clue' as to why the oxide of thorium should exhibit the phenomenon. The explanation which they provided is fascinatingly unclear. The effect of an air current was not due to removal of the ions but due to its action at or near the surface of the substance: It appears as if in the pores of the thick layer of thorium oxide some change takes place with time, which increases the intensity of the radiation, and if the result of the action is continually removed, the intensity of the radiation is diminished.58 In his next paper on 'Thorium Radiation'59 Owens detailed how they had overcome the variability problem to study what appeared to be the true penetration characteristics of thorium rays. He concluded that these were composed of many distinct types which was consistent with the suggestion that 'the internal motion of certain constituent parts of the atom' produced such 'disturbances in the aether'. We shall examine his expansions and enigmatic explanations of the original air-current phenomenon, as well as Rutherford's more successful alternative. Necessarily to anticipate this discussion, here are Rutherford's words of introduction to his description of 'A Radioactive Substance emitted from Thorium Compounds':60 In addition to ... ordinary radiation, I have found that thorium compounds continuously emit radioactive particles of some kind which retain their radioactive powers for several minutes. This 'emanation', as it will be termed for shortness, has the power of ionizing the gas in its neighbourhood...61 Two points turned. out to be most important for the continuation of this research towards the successful 133 theory of 1902-3, both of which became the subject of dispute. These were firstly that the 'emanation' from thorium compounds consisted of a material vapour which was not thorium vapour, and secondly that this emanation produced, upon any solid body, a radioactive deposit of a distinct chemical nature. This deposit could be concentrated upon any object negatively charged. Since the discovery of the thorium emanation, I have always taken the view that the emanation consists of matter in the radioactive state present in minute quantity in the surrounding gas.62 Thus wrote Rutherford a few years later in 1903 when his theory was widening rapidly in scope. We note that this statement is borne out by the earlier publications, and is not tautological in the later context. The vital conception of a material emanation seems to have appeared during the period May to July 1899 and that of an active deposit probably during that period also.63 Although Rutherford chose to discuss each aspect separately in his publications, he evidently studied these in close parallel. This is indicated for example by Thomson's reply to Rutherford's communication with Owens, who was spending the summer of 1899 at the Cavendish Laboratory. And here began an infrequent correspondence between Thomson and Rutherford which seems to have developed into something of a disagreement, lasting until 1903. The views of the former fluctuated somewhat, as will appear; but some of his information, theoretical ideas and suggestions for experiments, though historically neglected, clearly influenced Rutherford. Thomson first commented on the points which he thought were of particular importance or difficulty: I have today been reading the paper on thorium oxide you sent to Owens - the results are certainly very surprising. The points that struck me most were the effect of the air currents, & the necessity of the plate which is to be made active being negatively electrified. It seems to me that it would be worth while trying blasts confined to various strata between the plates ... It seems to me that it might be urged that the effect was due to the gas close to the surface of AB being very intensely ionised & giving out a kind of radiation which produced a phosphorescent effect on the plate...64 134 Another point which seemed to 'want settling' was what part the negative charge on a plate played in its activation. Thomson suggested that clarification might be achieved by the interposition of a metal gauze, positively charged. This would prevent a positively charged 'emanation' drifting across to be deposited upon the negative plate, but would not prevent irradiation of this plate. Perhaps in response to these comments Rutherford performed further experiments on the connection between emanation, induced activity and electric charge during the next few months and was able to publish some answers. With regard to the emanations Thomson continued: It is remarkable that the emanation should stand bubbling through strong 1-4SO4 & yet this substance should destroy the activity of a plate. As the emanation moves so slowly it presumably is large. Hence it must get through the Al. foil by some chemical or quasi chemical process like the 11,01 in Russell's experiments. If this is the ease perhaps gilding or silvering the aluminium might make it opaque to the emanation. I see you tried some experiments & did not get a cloud by expansion - did you use Wilson's apparatus for getting very sudden expansions... The use of this apparatus could indicate the size of the particles of emanation which might be larger than ordinary ions; Wilson had obtained a fog produced by 'something given off from metals'. Thomson's closing question on the subject was 'If the active plate is very highly polished can you see any trace of an alteration of surface 65 under the microscope'. Rutherford withheld discussion of this last point and of the nature of induced activity for his second major paper on thorium; his first, dated September 1899, dealt almost exclusively with the emanation. Now Owens working with Rutherford had by mid 1899 obtained various interesting experimental results with thorium rays. In his paper on 'Thorium Radiation' he described his attack on the early problem of fluctuating readings. By maintaining a very still condition of the air he was able to conduct an examination of the thorium rays by their penetration of aluminium screens. His readings told him that the radiations consisted of at least two different 135 penetration types, only the most absorbable kind being homogeneous. He thought that such complexity fitted with the theory of internal atomic vibrations, as has been noted. Investigations of the air effects themselves he treated separately in ingenious experimental style. For example, removal of the air gave marked reductions in conduction; on the other hand agitation of the air within the chamber by means of fitted vanes produced small increases in readings. His explanation of this was that the radiation from within the compounds 'changes the nature of their surfaces, forming in the neighbourhood a more active material' 66which could be removed; it is easy to read Rutherford's 'emanation' into this part of the study. In a set of related experiments Owens found that using sheets of ordinary writing paper as screens over the substance gave 'very curious results'. A thin layer of thorium oxide gave a regular absorption curve, as with aluminium foils, indicating the homogeneity of a portion of the rays. But with a thick layer of oxide, not only did fifteen successive sheets of paper fail to diminish the current below the 50% reduction produced by the single first sheet, but 'a very considerable time was required for the current to come to a steady value as successive layers were added'. In his opinion: The explanation may possibly be that the penetrating radiations from a thick layer of the oxide in passing through the paper causes it to give off a secondary radiation comparable in its ionising effects to the more absorbable kind that fails to get through.67 The account thus expressed in terms of direct radiations only was evidently neither complete nor satisfactory. Rutherford took over the research entirely as Owens left for Cambridge in the summer of 1899 with many questions unanswered. Some five months after this, having reached a second point of publication, he wrote to his fiancée: I sent off on Thursday another long paper for the press which is a very good one, even though I say so, and comprises 1000 new facts which have been undreamt of.68 136 It is of interest to outline these facts, and the related experiments, and hypotheses of different levels. The letter in July from Thomson, who communicated the publications to the Philosophical Magazine, shows that Rutherford was already using his new emanation theory to good effect. By this means he was able coherently to explain several lines of experiment, namely, fluctuations in readings due to air currents, the differences between thin and thick thorium oxide layers with regard to this effect, and the time-dependent paper screen results. Thus Rutherford dropped Owens' suggestion of secondary rays in connection with paper screens together with his conclusion that the radiation was complex. 'At first sight' he wrote: it appears as if the thorium oxide gave out two types of radiation, one of which is readily absorbed by paper and the other to only a slight extent.69 However, the curious results: receive a complete explanation if we suppose that, in addition to the ordinary radiation, a large number of radioactive particles are given out from the mass of the active substance. This 'emanation' can pass through considerable thicknesses of paper.7O In a manner very similar to that of his experimental researches at Cambridge some three years earlier on the ionisation of gases, which extended continuously to the period now under consideration, Rutherford effected con siderable clarification and progress. By using a slow current of air he removed the *conducting gas from its source for examination in a separate vessel. His recent studies told him that: If the ionised gas had been produced from a uranium compound, the duration of the conductivity, for voltages such as were used, would only have been a fraction of a second.71 Yet his electrometer showed that the gas withdrawn from thorium oxide remained conducting for up to ten minutes. The radiating particles, or emanation, whose presence in the gas was taken as the cause of the conductivity, passed unchanged through cotton wool, water and acids, unlike 72 ordinary ionised gases. As with Russell's photographic actions the emanation passed through foils of metal but not of mica. However, Rutherford stressed that hydrogen 137 peroxide vapour had 'purely a chemical' action on the photographic plate and failed to ionise, and thus make conducting, the gas carrying it. He stated that the radiation from the emanation, not the emanation itself, caused both electrical and photographic actions; though it appears that source and radiation had not been experimentally separated. One of the most interesting points concerning this radiation from the emanation was its exhibition of a rapid, regular 'decay' with time. The conductivity of the carrying gas, taken as the measure of the 'intensity of the radiation emitted by the radioactive particles',73 declined exponentially falling to half the initial value in about one minute. From the phrasing of the discussion, and on consideration of his later researches74 it seems that Rutherford assumed it to be the radiation from each individual particle of emanation which 'diminishes in a geometrical progression with the time'. Ionic and emanation theory fitted certain experimental observations well: the air surrounding an envelope of paper enclosing a thick layer of thorium oxide was drawn continuously into an attached vessel effectively fitted with electrodes and electrometer. The readings, starting at zero, increased gradually and after a few minutes reached a steady value of electrical conduction. The electric current decay curve obtained after stopping the airflow matched the asymptotic growth curve perfectly. Rutherford was able to account for the latter as a balance between the increasing current caused by a constant supply of new radiating centres which had diffused through the paper from the thorium, and the decreasing current due to the geometric decay of intensity. Rutherford boldly expressed the rate of decay in the form dn/dt = -A.n , where n was the number of ions. The observed growth curve was indeed of the theoretically -A.t deduced form i/I = 1 - e , where I is the maximum current, attained at the steady state.75 This is the first example of the growth and decay curves later to be associated by Rutherford and Soddy with all radioactive substances. Here also was the first recognition of a decrease 138 in the radiation from what, to Rutherford, was a definite radioactive substance. This perhaps added strength to his year-old suggestion regarding the energy of uranium rays76 that these should eventually die away; a view already in conflict with the French scientists' acceptance of the fundamental constancy of this atomic property. One may ask what significance can be attached to his repeated statement77 'that the curve of rise of the current is similar in form to the rise of an electric current in a circuit of constant inductance', which seems also to imply a similarity in the current decay curves. Was he attempting to say something new about the emanation, about electricity, or making a merely algebraic comparison? The first of these possibilities is evidently in some way true. As for the second, it is remarkable that the newest view of electrical conduction in metals visualised its mechanism as a kind of diffusion process somewhat analogous to that of ionic conduction in gases. About six months after Rutherford wrote his paper on thorium emanation J.J.Thomson divulged 'Some speculations as to the part played by corpuscles in physical phenomena' to the 'wider circle' of the readers of Nature.78 He pointed out that the recent demonstrations by Giesel, Curie and Becquerel of the magnetic deflection and electric charge of the rays from radium demonstrated the presence of corpuscles in this substance. Thomson'.s major point was an explanation of the electrical conductivity of metals in terms of a gas-like diffusion along the wire of subatomic negatively charged masses or corpuscles, temporarily dissociated from the fixed 'molecules'. He considered that this had consequences for the relationship between electrical and thermal phenomena. The older theory of jumping tubes of force was not explicitly mentioned here. Now Thomson in his earlier letter to Rutherford of July 1899 which contained the comments on the emanation quoted above, did not mention the theory of conduction in metals but only the work on conductivity in gases: I am inclined to think that at low pressures negative electricity is always carried by the small corpuscles however the electrification 139 may be produced - while the positive charge remains on the big atom - this idea leads to very interesting views as to the structure of the atom.79 In the published note Thomson recalled his first demonstration of the existence of corpuscles in cathode rays in 1897 and added that 'Ever since then I have indulged in speculations' concerning their presence in ordinary matter.80 But it is not clear whether Rutherford knew of the corpuscular or electron-gas theory of metallic conduction nor whether his own researches of 1899 had any bearing upon it. If one seeks an influence in the opposite direction the question arises whether Rutherford's identification of formulae involved the notion that each radiating particle of emanation behaved like a miniature or atomic version of the radiating electric circuits which he had studied during 1894-6. He was still concerned with the subject and it was very much in his mind during the month in which he wrote up the paper on thorium emanation, as shown by his letter to Mary Newton: I am giving a course of postgraduate lectures this year on Electrical Waves and Oscillations which will give me a good deal of trouble to arrange. This is the first thing of this kind ever done here and rather surprised them when I suggested it.81 Although his explicit identification of the formulae may be no more than a mathematical guide, our discussion indicates connections between radioactivity, and former and contemporary electrical studies. The same points apply to Rutherford's publication on the excited activity with its much slower decay; this is shortly to be discussed along with the accounts of possible mechanisms which he fortunately detailed. It may be noted that Pierre Curie was soon to develop explanations of the phenomena on the basis of a different algebraic comparison, firm to the point of analogy, between the observed decline of radioactivity and the fall of temperature due to loss of heat energy from a cooling body. Two questions posed by these studies of the emanation concerned its origin and its nature; the need to answer these served as a stimulus to experimental research. 140 The emanation seemed to be produced spontaneously and at constant rates which were different for differing thorium compounds. Rutherford's conversion of the nitrate to the oxide by gentle heating produced a considerable increase in the current due to the emanation, and the direct radiation increased proportionately. On the other hand, maintaining the temperature at a white heat caused a steady decline in the discharge due to the emanation down to 1/20 of the initial rate for the oxide, apparently without so diminishing its direct rays. Rutherford provided no explanation at this time but kept the problem in mind. What then was the nature of the emanation, of the radiating particles comprising it? As for the size of these particles, the fact that they passed unaffected through cotton wool and metals was against the possibility that they were a fine dust, as were also the results of water vapour condensation experiments; Thomson's suggestion regarding Wilson's more powerful method was mentioned by Rutherford82 as a future possibility, but he may not have tried it.83 The results indicated that the particles constituted 'a vapour given off from thorium compounds'. Could it be the vapour of thorium metal? There was 'reason to believe that all metals and substances give off vapour to some degree'.84 As Rutherford knew, this might be the vapour of the metal or substance itself, or of hydrogen peroxide. We have noted the dismissal of hydrogen peroxide vapour85 on the grounds of its direct chemical photographic action and its lack of conductivity; other differences can also be seen, such as the need for a clean unoxidised surface to obtain the Russell effect with metals which contrasted with the production of an emanation by the oxide and compounds of thorium. That the emanation might be the vapour of thorium itself was a possibility which he thought did agree with its declining radioactivity. And he again pointed hopefully to future experiments which were in this case actually completed and published more than a year later, though not with thorium; these were measurements of the rate of interdiffusion of the emanation into other gases to determine its density and 141 molecular weight.86 But even without such evidence he felt able to make the cryptic but significant comment that 'The emanation from thorium compounds ... has properties which the thorium itself does not possess'.87 Rutherford's knowledge of such properties was a part of his discovery that by association with the emanation any surface whatever could be made to emit radioactive rays, different from those of uranium or thorium in being more penetrating. The surface activity lasted for several days; it could be concentrated upon a negatively charged body. Now the particles of emanation were themselves electrically neutral. Their discharging effect was entirely uninfluenced by application of the electrostatic field which, as Rutherford had successfully demonstrated in 1896,88 would create a current and thus destroy the ionic conductivity of a gas. We can see that this should dispose of the hypothesis that the particles of emanation were themselves simply clusters of molecules of the surrounding gas about ions, though Thomson later adopted the idea for a time. Hence Rutherford, whose experiments were still in progress, assumed in September that it was 'the positive ion produced in a gas by the emanation' which possessed 'the power of producing radioactivity in all substances on which it falls' .89 But he had progressed beyond this view by the time of completion of his next publication on the 'Radioactivity ,90 Produced in Substances by the Action of Thorium Compounds in November 1899. Rutherford appears to have retained the opinion that the emanation was not thorium vapour without providing further evidence. It is tempting to interpret his experimental results in terms of the later supposition that this new 'excited' radioactivity was itself due to a third material substance. Even if he entertained the idea at this early date Rutherford did not mention it in his publications, preferring other hypotheses; yet the evidence seems suggestive. Naturally adopting the measurable characteristics of its emitted radiation as a means of identification, Rutherford noted that the rays from the excited activity were of a longer duration than those of 142 the emanation and more penetrating than thorium or uranium rays. Another distinction was the chemical one: to repeat Thomson's earlier comment of July, 'it is remarkable that the emanation should stand bubbling through strong H2SO4 & yet this substance should destroy the activity of a plate'. But Rutherford in his sections on chemical and mechanical actions on the radioactive surface91 now stated that the induced or excited activity was not in fact so destroyed but had been simply dissolved from the plate by this acid, or by hydrochloric acid, in which it was afterwards to be found; other reagents such as nitric acid or caustic soda had far less effect. He reported without comment the interesting observations that a microscopic examination (suggested by Thomson) revealed no surface changes although the intense activity could be removed by scraping; and that raising the temperature to white heat had little effect.92 These chemical and mechanical approaches were to enjoy an early and successful future development with the aid of Frederick Soddy, a trained chemist; they also bore a resemblance to Rutherford's past researches on surface magnetism93 performed five years earlier. And his demonstrations that the intensity of the induced activity was roughly proportional to the amount of emanation, by passing the emanation along a tube containing a series of negative electrodes, were very similar in techniaue to earlier studies of the duration of the conductivity of ordinary ionised gases.94 The experimental and theoretical continuity is also illustrated by other experiments, by the strong element of explanations in terms of the mechanical interplay of ions and other particles, and by the lasting links with the researchers of the Cavendish Laboratory. Rutherford summarised his weighty paper by considering three possible explanatory mechanisms of the phenomena of induced radioactivity, one of which he adopted. The hypothesis that the excited radiation was a kind of phosphorescence produced in direct response to thorium radiation could explain neither the production of activity outside the incidence of the radiation nor the concentration upon 143 a negative electrode; he therefore dismissed it. Too abruptly perhaps, for Rutherford made no mention of the different phosphorescence hypothesis, with possible experimental tests, which Thomson had privately suggested in July. The latter supposed that gas molecules close to the thorium surface might become so intensely ionised that they emit a type of radiation capable of producing a phosphorescent effect on the plate; Thomson had not forgotten this by 1903, when he published a similar explanation of induced radioactivity as one possibility.95 It is clear that the identity of the induced radiation whatever the material of the excited surface militates against this. A second idea was that the positive ions produced in the surrounding gas by the rays from the emanation particles could deposit upon any surface; certainly there was no problem here in accounting for concentration by an electric charge, nor an identity of deposit. This had probably been Rutherford's favoured view in September; now he wrote that the hypothesis 'at first sight seems to explain many of the results'.96 In consequence of the intense rays (presumably of X-ray type) close to a radiating particle of emanation, ions may not only be produced but 'the charges on the ions set in violent vibration'. The observed surface effects would occur as the ions 'gradually dissipate the energy of their vibration by radiation into space'. What became of the negative ions was not stated. However, observations at low pressures, where even a large negative charge in fact failed to concentrate the activity, were not explained by this theory. And testifying against it was the fact that an increase in the distance between the emanating thorium layer and the plate to be activated from 3 mm. to 3 cm., which gave the expected higher current attributable to the creation of more positive ions, yielded no increase in the plate's activity. Rutherford made his decision: 'The theory that the radioactivity is due to a deposition of radioactive particles from the thorium compounds affords a general explanation of all the results'.97 The emanation itself was thus deposited. He overcame the difficulty of explaining how 144 this neutral substance acquired the charge necessary for movement to a negative electrode by supposing that positive ions, generally present in excess in gases due to their lower velocity, diffuse to the surface of the emanation particles. At low pressures a lack of ions, indicated by the observed small currents, would allow active neutral particles to diffuse to the sides of the vessel unaffected by the negative electrode. The implication of these statements seems to be that excited activity consisted of deposited particles of the emanation possibly sometimes attached to the positive ions of various gases. But did larger particles such as dust or smoke which Rutherford knew caused rapid ionic recombination,98 acquire the positive charge that they apparently should according to this hypothesis? And why did the rate of decay of radiation become some 660 times slower when the gaseous particles were deposited, though obeying the same law; why were there marked differences in chemical behaviour, for example with sulphuric acid; did these point to the existence of a third radioactive substance? Rutherford did not say so. And he avoided any description of the nature of the particles of emanation themselves, and of the mechanism of their production. These were some questions which Rutherford's discoveries of '1000 new facts' appear to have posed. Research students and collaborators were to aid him in answering them, as new information arriving from Europe created still more problems. In a late footnote to his paper on induced radioactivity, Rutherford remarked that he had .received the note of 6th November in which the Curies had announced their discovery of 'radioactivite provoquee par les rayons de Becquerel',99 an effect produced by the new substances whose activities had now reached a level 50,000 times greater than uranium. Rutherford stated that Curie had, without electrical studies, attributed the results to a kind of phosphorescence excited by the I00 radiation. He pointed out that 'in the case of thorium the author has shown that such a theory is inadmissible' so that further comparisons were required. The magnitude 145 of the activity induced by radium of several hundred uranium units, as reported by the Curies, appears to have been similar to Rutherford's estimate for the maximum electrically concentrated activity produced by thorium.101 The latter's suggested general comparison between radium and thorium was taken up directly by E.Dorn and by his student F.Henning, in Halle. In December 1899 Thomson wrote to Rutherford that he had sent on this second paper on thorium to the Philosophical Magazine. He also told him of Giesel's magnetic deflection of polonium rays, whose implications we have considered. Thomson found Rutherford's results 'exceedingly interesting' but saw fit to suggest a different mechanism for the acquisition of the emanation's charge, which in effect reversed the positions of emanation and ions: the idea that I got on reading the experiments was that the radio-activity was due to thorium vapour or emanation which was carried by the + ions - I mean that the emanation in the field tended rather to condense round the positive ions than the negative ones, as we might expect an electro-positive substance to do.102 Evidently Thomson was not satisfied that the emanation was not thorium vapour - a metal should tend to attach itself to positive ions. And others' opinions of Rutherford's results were not quite so straightforward as has been made out. John Zeleny had been at the Cavendish with Rutherford under Thomson, who had written to Rutherford in 1898 when both students had left the Cavendish, 'I hardly think at present that we have any in the new lot as good as Zeleny & Wilson the amount of glass they break at present is appalling'.103 Zeleny had been first to announce, in that year, that the negative ion in a gas generally travelled with a slightly higher velocity than the positive.104 Indeed, he had made an observation, which seems unfavourable to Rutherford's emanation-charge theory of 1899, that a metallic body suspended in a current of ionised air generally acquired a negative charge, the air being left positive.105 Zeleny attributed this to the differing ionic velocities. He 146 explained that such a body could acquire the positive charge sometimes observed only if another metal were immersed in the same quasi-electrolytic medium. This fits with Thomson's view. An implication of Rutherford's hypothesis seems to be that the emanation behaved not as suspended particles but as a part of the gas. However, it was Thomson who was soon to argue in this way, on the basis of fresh evidence. In March 1900, soon after reading Rutherford's papers 'with very great interest', Zeleny wrote 'I am about ready to believe that most anything is possible. The facts you present are certainly strange' but 'more light on the nature of those things would be of still greater value'. He said he would be 'interested in your work on the energy for producing an ion. I see that you are in for getting ahead of J.J.'.106 Rutherford had begun experiments to this end towards the close of 1899 partly if not wholly in order to clarify the question of the energy of radioactivity. His associated speculations are of very great interest to us, as are contemporary discussions by others. However, Zeleny's latter suggestion turned out to be wide of the mark with regard to Rutherford's attempt to quantify the energy required for producing an ion, as the following account will show. 147 3. The source of radioactivity (1900) Attempts to quantify the energy emission from radioactive substances, together with a more advanced understanding of the radiations, led to interesting speculations and anticipations. These are worthy of examination for the improved perspective of radioactivity which they provide. To that end we will consider formerly neglected discussions set out by researchers in the field as their experimental studies developed from the end of 1899 through 1900. Generally during this period theories were as easy to knock down as to set up; Rutherford indeed found this to be the case with the hypotheses which he preferred. Nevertheless, even at this early stage the view that atomic change was the source of radioactivity began to appear in sharper focus. The earliest explanations of radioactivity were perforce qualitative; they likened the phenomenon either to fluorescence or to a long-term phosphorescence. The respective problems of the origin of the initiating radiation or of the nature of the required ancient store of energy, among other difficulties, had not been solved. The subsequent suggestion, made most clearly by Thomson in 1898, that a subatomic rearrangement might be involved, had implications not to be fully realised until further chemical researches, to be discussed, had come to fruition. However, we may recall that the physicists Elster and Geitel had early in 1899 already expressed their willingness to sacrifice, in theory, the chemical atom to provide the energy of radioactivity. Their experiments demonstrated the inadequacy of Crookes' and Marie Curie's hypotheses, each of which required external, though differing, energy provision. These results confirmed the atomic nature of the phenomenon and induced their proposal that the radiated energy might come from a transmutation of the chemical elements.107 Such a suggestion did not escape strong criticism put forward with the support of apparently positive experimental evidence. Both phosphorescence and all chemical reactions were known to be highly dependent 148 upon temperature; E.Wiedemann and G.C.Schmidt108 had indeed supposed that these two types of phenomenon were closely related. In turn re-linking these processes with radioactivity, 0.Behrendsen published his 'Beitrgge zur Kenntniss der Becquerelstrahlen',109 which exhibit both an understanding of and a direct challenge to Elster and Geitel's solution to the 'Energiefrage'.110 The mode of attack on his contemporaries' views was a study of the effect of temperature upon radioactivity. Behrendsen used a gold-leaf electroscope to determine the intensity of the radiations emitted from specimens placed within the electroscope vessel and an apparatus by which the entire electroscope could be cooled. He satisfied himself that pitchblende and its active sublimate each gave radiation which increased steadily with temperature from -50 to +130 degrees C. and that Moissan's uranium metal exhibited a decreased intensity only on cooling. However he considered that convection of the ionised air on warming might have influenced the result with uranium.111 Raising the temperature of pitchblende to red heat markedly decreased the intensity. This he attributed to a chemical decomposition, which illustrates his general assumption, or conclusion, that radioactivity was the manifestation of ordinary interactions between atoms and molecules. The slow conversion of a complex compound such as pitchblende, which contains uranium, the Curie elements and acids, into a more stable one, should indeed be influenced by temperature variations; radioactivity was to be compared with thermoluminescence. The Curies and Elster and Geitel, he wrote, are of the opinion that Becquerel rays were in fact not the result of any chemical change so that the emission was an 'Atomeigenschaft' of the radiating element. They had said that the atom of a radioactive element might itself be constructed like a kind of molecule, able to emit rays while being transformed into a more stable entity.112 Gegen diese Ansicht scheint mir vor allem zu sprechen, dass die Annahme eines instabil gebauten 'Atomes' nicht mit dem Atombegriff als solchem sein diarfte.113 149 Retaining his conception of the atom, and assuming the energy to be stored within the material, Behrendsen supposed it possible 'dass die Atome der radioactiven elemente die Fghigkeit besgssen, miteindander und auch mit fremden Atomen zu instabil gebauten Molecalen zusammen zu treten' like the allotropes of sulphur or selenium. This interpretation fitted with his thermal results. However, in his argument114 that the salts of uranium gave a greater total radiation than the metal itself, which should dismiss the Ur atom as the source, he included the visible. Now Becquerel's original discovery had involved the creation of a clear distinction between uranium rays and ordinary phosphorescence. Behrendsen's paper thus may only have served to spread wider the suggestions of those whom he criticised. His experiments and conclusions were put in further doubt on several counts as scattered studies of the effect of temperature continued. F.Himstedt's report one year later 'Uberl 5 Versuche mit Becquerel- und mit ROntgenstrahlen' noted the view which attributed the energy of radium to a slowly proceeding chemical reaction. An investigation of the influence of cooling upon the intensity of the rays was therefore of considerable interest. His results however gave no support to such a theory. For whilst luminous paint was completely extinguished at the temperature of liquid air radium radiation, as measured with an electrometer, showed but a small diminution which itself might be attributed to additional absorption of the rays by the liquid air. Himstedt's conclusions were more in accord with the conclusions of the earliest students of radioactivity than with Behrendsen's. Others had considered this approach to radioactivity as worthy of investigation. Before radium became important, Rutherford had measured the activity of uranium at 200 degrees C. but found little difference in the rate of discharge. His opinion was that 'The results of such experiments are very difficult to interpret, as the variation of ionization with temperature is not known'.116 Marie Curie had found that the activity of 150 uranium was always unchanged on returning to room temperature after heating or cooling.117 G.G.Stokes, who entertained what can be called a 'sub-phosphorescence' explanation of radioactivity, had corresponded with Becquerel in August 1899 concerning uranium rays. Stokes thought it 'very probable that the efficiency of a substance (suppose metallic uranium) which emits them would depend very materially upon its temperature'.118 Becquerel replied119 with a description of experiments using uranium which he had performed two years earlier but had not published. These involved arrangements similar to those described by Behrendsen. Becquerel on the contrary cautiously remarked that the photographic method of detection was itself sensitive to changes in temperature, so that further study was required. He similarly noted that an electrometric method gave only a small difference between +100 and -20 degrees, which might be attributable to air currents of convection, and that smaller changes in temperature appeared to have no effect. These attempts to find a thermal influence upon the radiations from active substances were continued by various scientists.120 The generally negative results were more than once cited positively, as theoretical debates intensified during the period 1899 to 1903. But these five years saw at their beginning the discovery of the new phenomena of the radioactive vapour-like 'emanations' released from thorium, then radium, then actinium, and the development of an understanding of these. Rutherford in his earliest publications on thorium emanation, issued in 1900, noted that temperature here had a definite effect: he considered that heating the thorium oxide source greatly reduced the rate of production of emanation.121 Behrendsen however insisted on his earlier conclusions, now with regard to 'Das Verhalten des "Radiums" bei tiefer Temperatur' .122 His experimental diminution of the electrometric intensity of radium radiation to half value at the temperature of liquid air led him to state that this behaviour was precisely similar 151 to that of visibly phosphorescent bodies. He thought that the spontaneous decrease in the radiation from Giesel's polonium confirmed the comparison. Yet contemporaries probably saw his case with radium to be weakened by new discoveries. The complex nature of the composition of the rays was becoming clearer during 1900; temperature experiments ought therefore to be more dis- criminating in this respect. And a preliminary point in another direction is Elster and Geitel's note of January 1900 that radium released a volatile constituent on heating;123 this may have been seen merely as comparable with the sublimation of polonium from bismuth. Their paper partly anticipates Dorn's description later that year of a gaseous radioactive emanation associated with. radium.'24 Future attempts to clarify the source of radioactivity by seeking an effect of temperature were compelled to take the emanations into account. It is a sign of the complexity of changes in the interpretation of radioactitrity occurring during the year 1900, that developments of Becquerel's views, which we now consider, were produced largely by factors other than those just described. Neither his experiments giving negative or inconclusive effects of temperature change, nor the discovery of the emanations, appear to have been immediate influences in his definite shift from the original phosphorescence analogy of 1896-9. Instead, experiments on the radiations themselves which indicated that they consisted not only of ether vibrations but also of an actual emission of material particles, seem to have been most significant. Thus in March 1899, after he had accepted that uranium rays were not after all like light, Becquerel in his 'Note sur quelques proprietes du rayonnement de l'uranium et des corps radio-actifs'125 considered again the energy of radioactivity. If uranium did not lose energy in producing its rays then this metal might be in a special state like that of the iron in a magnet maintaining a field around it through which it could create the observed effects by the transformation of some external energy source. However, he accepted the 152 Curies' view that photographic and phosphorescent actions of the new highly active substances did in fact constitute a spontaneous release of energy 'dont on ne voit pas la 126 source ailleurs que dans la substance radio-active'. Since this was of small magnitude: it ne serait pas contraire a ce que nous savons sur la phosphorescence, de supposer que ces substances ont une reserve d'energie relativement consid6rable qu'elles peuvent 6mettre, par rayonnement, pendant des annees, sans affaiblissement sensible; toutefois i1 n'a pas etc possible de provoquer par des influences physiques aucune variation appreciable dans l'intensite de cette emission.127 The communication by Becquerel of his unpublished temperature experiments and earlier papers to Stokes was sufficient to make the latter drop his phosphorescence-radioactivity link by September 1899.128 Yet Becquerel himself apparently retained the idea till November. This is indicated by his application of the terms phosphorescence and fluorescence respectively to the new temporary induced radioactivity and to secondary rays.129 But the analogy was not to be stretched beyond this point. Becquerel's new corpuscular view of the rays from radium led him to a calculation of the energy and the matter carried off in this way. His respective estimates, announced in March 1900, of 1 mg. in 109 years and a few calories per cm.2 per year from a radium sample, were sufficiently small to allow the continued belief that the radiated energy 'peut titre empruntee a la matiere elle meme' 130 without measurable loss of weight. In the concluding comments of his paper read at the International Congress in Paris in August Becquerel cited the same estimates and gave something of an expansion of his views. Since the loss of mass involved was immeasurably small n'y avait aucune contradiction entre la spontangite du rayonnement sans cause apparente, et le principe de la 131 conservation de 11 6nergie'. The apparent assumption of an equivalence of mass and energy seems not to alleviate the contradiction with Carnot's principle, but he made no mention of this. His final word here was that 'Le 153 phenomene d'emission materielle pourrait etre du meme ordre de grandeur que l'evaporation de eertains matieres odorantes'.132 Becquerel's firmest expression, vague though it was, of this approach appeared in a review paper on 'The radio-activity of matter' published in Nature some six months later, in February 1901.133 A 'material emission' of the order of 'certain scented substances' had now become 'the first cause of the observed phenomena'. Becquerel mentioned aspects of Rutherford's 'very penetrating "emanation"' from thorium without making it clear that the emanation behaved like a vapour: he had not yet taken the opportunity of fitting this with his 'material emission' hypothesis. Yet towards the end of that year, by considering both of these in combination with the 'new horizons'134 opened up by developing studies on induced radioactivity, Becquerel arrived at his own important unifying theory of atomic disintegration.135 Such developments were followed by some scientists, but not by all. F.T.Trouton writing to Nature from the Physical Laboratory, Trinity College Dublin, in March 1900 put forward a 'Suggested Source of the Energy of the "Becquerel Rays"'136 which seems very similar to the magnetic analogy mentioned and rejected by Becquerel himself one year earlier. Trouton suggested that because of the difficulties arising from the supposition of a continuous release of energy from the active material 'the possibility should be kept in view of the real source of the energy being found in the things themselves in which these effects are manifested'. Including in the expression what others called 'rays' he wrote that 'the emanating influence would be looked upon rather in the light of lines of force than as a wave propagation'. Ionisation and other effects 'would then be viewed as due to what might be called a Becquerel field of force'. The Curies' observation that a phosphorescent screen became 'exhausted' after a time, but could be rejuvenated by exposure to light, he interpreted as an indication that some or all of the energy originated in the screen itself. According to his magnetic analogy 'forces should exist between the acted-on substance 154 and the source of the "Becquerel Rays"'. One cannnot say that these ideas were influential, but they serve as an epitome of one undercurrent of conjecture on the subject during and beyond137 the period under consideration. It is interesting to note that W.Crookes earlier138 and P.Curie with A.Laborde later, each sought experimentally, and unsuccessfully, just such a force; they, however, were thinking in terms of the pressure of radiation truly emitted from the radioactive source. The French scientists came to a remarkable and quite different discovery139 in the attempt. Like all active researchers in the field, the Curies seem always to have regarded 'radioactivity' according to its name, which they had coined. But it is perhaps an indication of the appeal of the interpretation described above that the Curies, shortly after their discovery of induced radioactivity, still found it necessary to stress the point that energy was being released. Such was the conclusion of their note of 20th November 1899 on the 'Effete chimiquesproduits par lee rayons de Becquerel'.140 This may be the research which prompted Trouton's paper for the Curies here described their observations with the fluorescent barium platinocyanide screen. The colouration of glass and particularly the newly observed (by the spectroscopist E.Demargay) transformation of oxygen into ozone by radium was 'une preuve que ce rayonnement represente un degagement continu d'6nergie'. Trouton's comment concerning radioactivity that 'we have no conclusive evidence that the effects are those of waves',141 which may be associated with the recent demise of Becquerel's earlier description of the rays, remained true for at least a decade. However, the Curies as well as others accepted the view that both particles and ether vibrations were involved. Following the successful deflection of a portion of the radium rays, their detection of the spontaneous acquisition of a positive electrostatic charge by an insulated specimen of radium told the Curies that this element continually lost negative electricity.142 'Or, 155 jusqu'ici' they wrote 'on n'a jamais reconnu l'existence de charges electriques non liees A la matiere ponderable', hence radium must be the seat of a constant emission of negatively charged material particles. They cited the work of J.Perrin who had detected the transport of a similar charge by the cathode rays but the Curies made no reference to J.J.Thomson's related conclusions that these particles were smaller than atoms. Indeed, their estimate of the continuous loss of mass which should result from the emission was based upon an assumed e/m ratio 'le meme que dans l'electrolyse'; thus in March 1900 they took the particles to be atomic in size. A few weeks after their note was read Becquerel demonstrated an almost total similarity between the deviable radium rays and Thomson's corpuscular cathode rays.143 Becquerel's studies may have been a cause of their acceptance, as far as it went, of the subatomic corpuscular view of radium rays. For in Marie Curie's review paper on 'Les nouvelles substances radioactives' read in June 1900,144 and in the Curies' joint paper presented to the International Congress in Paris two months later,145 discussions on the origin of the energy of radioactivity centred on two possibilities neither of which involved the emission of atomic particles. At the Congress they squarely put the question 'Quelle est la source de l'energie des rayons de Becquerel? Faut-il la chercher dans les corps radioactifs eux-mgmes ou bien a l'extorieur?'146 and answered it by suggesting that one group of radium rays might be secondary to the other. Support for this view may have been derived from current researches by P.Curie and G.Sagnac, and E.Dorn which showed that secondary X-rays, easily absorbed, were themselves cathode rays;147 this implied an apparent symmetry between the production of X-rays and cathode rays. Despite the refutation by Elster and Geitel148 the Curies extended Marie's earlier speculation, and suggested that the primary Becquerel rays, whichever these were, might themselves be secondary to some unknown exterior radiation, again with the rider that this would be contrary to Carnot's principle. 156 Yet their ideas had developed, and discussions were couched in slightly different terms from those of the previous years. For the Curies now debated for the first time an alternative hypothesis which was based upon J.J.Thomson's view of cathode rays as subatomic particles; it involved the discovery, to which they had themselves contributed in respect of the electric charge, that radium emitted such particles. Although Marie Curie, in her June reviewl49 made the disclaimer that the origin of the radiation from radioactive bodies was no better understood than when uranium rays alone were known, yet she evidently felt able to express one qualitative hypothesis fully and clearly; from this one may surmise, despite later events, that she perhaps even considered it favourably. Thomson, she explained, had shown that if one took the cathode rays or 'matiere radiante' of Crookes to be electrically charged material particles then 'ces particules transportent a poids egal 1000 fois plus d'electricit6' than hydrogen in electrolysis. Thomson had concluded that 'Ce ne seraient done meme plus les atomes libres de la chimie, mais des sous-atomes bien plus petits encore, et anim6s de vitesses prodigieuses'.150 Radium itself therefore seemed to behave like a spontaneously excited cathode. Thus Marie Curie arrived at her first speculation on the disintegration of matter. Although the loss of mass would be too small to detect, 'La matiere radioactive serait done de la matiere oil regne un etat de mouvement interieur violent, de la matiere en train de se disloquer'.151 Setting out the chemical implications more clearly than had J.J.Thomson, for example in his 'Speculations as to the part played by corpuscles in physical phenomena' published the previous month,152 and with the advantage of a year and a half's new evidence over the conjectures of Elster and Geitel on radioactivity and atomic change,153 she wrote: La theorie materialiste de la radioactivite est tree seduisante. Elle explique bien les ph6nomenes de la radioactivity. Cependant, en adoptant cette theorie, it faut nous rosoudre a admettre que la matiere radioactive n'est pas a un 6tat chimique ordinaire; les atomes n'y sont pas constitue's a 157 11 6tat stable, puisque des particules plus petites que l'atome sont rayonn6es. L'atome, indivisible au point de vue chimique, est divisible ici, et les sous-atomes sont en mouvement. La matiere radioactive eprouve done une transformation chimique qui est la source de l'energie rayonnee; mail ce n'est point une transformation chimique ordinaire, car lee transformations chimiques ordinaires laissent l'atome invariable. Dans la matiere radioactive, s'il y a quelque chose qui se modifie, c'est forcoment l'atome, puisque c'est a l'atome qu'est attachee la radioactivite.154 Here indeed can one see the elements of a theory involving both atomic disintegration and transformation. It has not generally been appreciated that Marie Curie expressed such a view at this time. The fact that she did so illustrates the early trend; but from this the Curies turned away. The ideas proposed by Marie Curie were readily taken up, or independently conceived, by others in the following year or two;155 Rutherford's experimental researches developed along a path different from the Curies', but he may have entertained such thoughts. However, by this time, June 1900, Rutherford had been concerned to point out that the emission of corpuscles or cathode rays was not a property of all radioactive substances. Mme.Curie knew that her polonium, though highly active, gave none of the deviable rays;156 she did not mention this in her review paper; it was not to be her only theoretical problem with polonium.157 In his joint paper with the Demonstrator R.K.McClung on the 'Energy of Rdntgen and Becquerel Rays, and the Energy required to produce an Ion in Gases'158 Rutherford remarked that Giesel's polonium emitted deflectable rays but Curie's did not,159 that Becquerel had 'found no trace of magnetic action in uranium radiation',160 and that magnetic experiments at McGill had deflected neither uranium nor thorium rays. The point was clearly made: This emission of rays similar in character to cathode rays of low velocity is very remarkable, but does not seem to be a necessary accompaniment of a radio-active substance ... The rays which are deflected by a magnet seem to be present or absent according to the mode of preparation of the substance, and depend possibly on the age of the specimens.161 158

This illustrates a difference in emphasis from that of Mme.Curie. Rutherford tells us that the aims of the study were to determine experimentally the energy required to produce an ion in a gas, 'and to deduce from it the energy of the radiations emitted per second by uranium, 162 thorium, and other radio-active substances'. An intermediate, quantitative study of X-rays was the basis of his method. By January 1900 he had obtained a measure of the small heating effect of X-rays playing upon a specially absorptive platinum bolometer to an estimated accuracy of 163 2%, as he helped McClung to master the complex techniques. The assumptions were that X-rays manifested their energy as a heating effect when absorbed by metals,164 but as an 165 ionisation-conductivity effect when absorbed in a gas. Comparing these, and knowing J.J.Thomson's recent value of the charge on an ion, Rutherford calculated the energy required to produce an ion in the gas; conductivity measurements provided the required estimates of the energy released by radioactive substances. Referring to his own and others' work on ionisation Rutherford argued that 'the same energy is required to produce an ion whatever the gas:166 This conclusion played a part in his speculations, the more fascinating for their confusions, upon the structure of the matter from which ions were produced; these speculations in turn had implications for the source of radioactivity. He implicitly adopted the purely electrical view of atomic and molecular bonding:167 from the experimental value of 3.8 x 10-10 erg for the minimum energy required to 'produce a positive and a negative ion from a neutral molecule ... against the forces of electrical attraction', he calculated the distance between ions before separation as r = 1.1 x 10-9 cm. Rutherford took this to be a significant value supporting Thomson's ionisation theory. For 'The average diameter of an atom, calculated from various methods, is about 3 x 10-8 centim.' which was 'very much greater' than the above ionic distance 'in a molecule'.168 Now Kelvin in one of his later lectures on 'The Size of Atoms, had in 1883 apologised: 159 I speak somewhat vaguely, and I do so, not inadvertently, when I speak of atoms and molecules. I must ask the chemists to forgive me if I even abuse the words and apply a misnomer occasionally.169 One reason for this seems to be that none of the phenomena of optical dispersion, contact electricity, capillarity, gas viscosity and diffusion used for the estimates specifically involved the atom of chemical combination. Rutherford made no such apologies for his variable usage of the terms atom and molecule; sometimes it seems quite clear. Part of his aim may indeed be construed as the making of a case for Thomson's corpuscular theory of matter against 'the atomic theory, as ordinarily understood',170 radioactivity being the test. We know of Rutherford's open attack to this end upon 'the chemists' in 1901.171 He considered in 1900 that his own results on radioactive energy, and those on the emission of 'a kind of cathode rays' from active substances, indicated 'that the present views of molecular actions require alteration or extension'.172 However, apart from the employment of such imprecise expressions his explanations were acknowledged as being unable to cover the experimental results. According to Rutherford, Thomson considered 'that an atom is not simple, but composed of a large number of positively and negatively charged electrons';173 we note that the latter had not publicly stated this exact view. The problem of the positive charge was a difficult one for it appeared that only negative electrons or corpuscles were detectable experimentally; Thomson's usual supposition from the first was that the space which the corpuscles occupied behaved somehow as if it possessed a positive charge. Nevertheless, for the purpose of calculating theoretically the energy within an atom which might be available for the manifestations of radioactivity both Thomson and Rutherford appear to have assumed a structure consisting of pairs of oppositely charged electrons.174 Rutherford and McClung measured the energy emitted from uranium oxide as 0.032 calories per year per gm. and found that from thorium oxide somewhat greater. 160 Becquerel's earliest work told them that radioactivity persisted unaltered for years and 'appears to depend on the uranium molecule alone, and not what it is combined with';175 this is a phrase in which 'atom' would seem to be more suitable than 'molecule' particularly since chemical combination was in question. The energy measurements induced Rutherford's clear dismissal of normal chemical change as the source: 'It is difficult to suppose that such a quantity of energy can be derived from regrouping of the atoms or molecular recombinations on the ordinary chemical theory'.176 Now his estimate of at least 300,000 calories emitted per gramme of uranium over ten million years involves assumptions and calculations not explicitly revealed. One can however infer some of these, knowing his view that 'a greater concentration or closeness of aggregation of the components of such a complex molecule'177 as that of uranium 'would possibly be sufficient' to supply the required energy, and that these components were positive-negative electron pairs. There should be 1000 (or 500?) such pairs, taken to be existing independently within, and in fact constituting the hydrogen atom; hence 200,000 per Ur atom. Rutherford's measured energy of ionisation being about 2 x 10-10 erg, the energy per Ur atom is thus 4 x 10-5 erg. Using Avogadro's number178 the energy per gramme of Ur is about 2 x 1019 ergs or 5 x 1011 calories. If we assume that, say, 0.1% of this becomes available for radiation during the attainment of a greater 'closeness of aggregation', the resulting 5 x 108 calories, at the measured emission rate of 0.032 calories per year, would last about 1010 years. Rutherford in fact stated his supposition that uranium had already been radiating for 107 years. It is therefore reasonable to suppose that he went through some such procedure as the rough calculation above. One interesting point implicit in Rutherford's assumptions is that the atomic-ionic energy contained in unit weight is the same (5 x 1011 cals per gramme) for all elements. But what did this imply for the element radium? 161 The Curies' statement that they had used radium specimens 100,000 times more active than uranium told Rutherford that about 3,200 calories per gramme would be released per year. We have seen how Becquerel explained his far smaller estimate. But Rutherford, try as he might, could find no plausible answer to his own larger problem: It is evident that, unless energy is supplied from external sources, the substance cannot continue emitting energy at such a rate for many years, even supposing a considerable amount of energy may possibly be derived from rearrangements of the components of the molecule.l79 This statement may be no more than the rejection of an ordinary chemical or phosphorescent supply, depending on the meaning of 'molecule'. However, he made his view entirely clear that an electronic source though 'many thousand times greater' was still insufficient: The energy that might possibly be derived from regrouping of the constituents of the atom would not, however, suffice to keep up a constant emission of energy from a strong radio-active substance, like radium, for many years.180 A direct comparison with our calculation for uranium suggests why not, though Thomson in 1903 theoretically extracted more than enough energy from this source. As for the third possibility that 'the radio-active substance in some way acts as a transformer of energy' from its surroundings, he thought that 'this does not seem probable 181 and leads us into many difficulties'. Rutherford's last word for some time on the problem is enigmatic: It is of importance that experiments to test the constancy of a powerful radio-active substance, like radium, should be carried out at definite intervals. If the radiation should keep constant from year to year, it would be strong evidence that the energy of the radiation was not derived at the expense of the chemical energy of the radio-active substance.l82 It seems that if the activity remained constant the source must be external. If the activity in fact declined, this should indicate a chemical source for the energy - but of what kind? Direct observation was not to be the way in 162 which the apparent constancy of the radiations from radium, uranium, and thorium was eventually broken down. In the prelude to this much-disputed advance, the mysterious thorium emanation with its rapidly declining activity played a part, as did quite novel studies on the chemical side of radioactivity which were now growing apace.

4. Emanations and the X-substances (1900-1) In May 1900, shortly after F.T.Trouton sent his letter concerning radioactivity from Trinity College Dublin to Nature, the better known physicist G.F.Fitzgerald wrote from the same address to Rutherford. Expressing his interest in the experiments on thorium emanation Fitzgerald remarked: that Debierne says he has isolated a substance he calls actinium which he thinks is the active material in the thorium experiments. This actinium gives out something that is magnetically deflected, but I am not sure that this is not almost always present to a small extent in all these cases and that it is merely the very powerful ones in which it has been observed.183 Was the well-known activity of thorium then not its own? And was the emission of corpuscles or 'disembodied electrons'184 a general property of radioactive substances after all? I shall in this Section discuss questions like 163 the first of these as they began to be asked with increasing persistence during the year 1900. In exploring the expanding web of experiments, ideas, and communications concerning the precise chemical nature of radioactive materials we shall see how the notion of radioactive induction was invoked and extended by some with a view to explaining the observations. The new chemical-radioactive studies can be seen as a vital step towards the more successful transmutation-disintegration theory of 1902-3, or alternatively as providing a large amount of empirical information which helped to broaden the scope of theoretical explanations of radioactivity. Outside the confines of radioactive studies, though loosely related to them, there was no lack of discussion, continuing from the earlier period through 1900, regarding the possible transmutation of the chemical elements. Lockyer repeated some of his old astronomical-chemical arguments in a new book on Inorganic Evolution.185 On the physical side, Fitzgerald's note on 'The Theory of Ions' appeared in Nature in September.186 He viewed favourably the varied evidence for an electronic structure of all matter, and hence thought that there seemed to be 'no impossibility in the dreams of the alchemist, and an element of one kind may some day be transmitted into that of another'. No mention of radioactivity was here made by Rutherford's recent correspondent. Discussions of purely chemical transmutations at this time, and earlier, are exemplified by F.Fittica's claim and C.Winkler's refutation 'On the alleged transformation of phosphorus into arsenic',187 followed by Fittica's rejoinder 'On the transmutation of phosphorus into arsenic and antimony'.188 W.Crookes corresponded with the American chemist Dr.Etmens concerning the latter's claim of 1897, to have converted silver into gold by hammering; Crookes thought it doubtful.189 And at the end of 1898, before the American Chemical Society, F.P.Venable on general grounds vigorously attacked 'so dangerous a doctrine' as that of a changing chemical atom, which had been suggested to explain variable valency. His view was that: 164 It will be an unfortunate day for chemists when the unchanging atom is given up. Chaos will indeed enter into all of our theories when this, the foundation rock, is left at the mercy of every shifting tide of opinion and can be shaken by all manner of unfounded hypotheses.190 It is possible that a considerable proportion of chemists clung to the atomic theory in this way. By 1900, radioactivity had already begun to impinge upon this view; the phenomena evidently required new concepts of one kind or another to explain them. With the Curies' guidance, the chemist A.Debierne followed another line of inorganic analysis in the pitchblende residues. In October 1899 he reported 'Sur une nouvelle mati6re radio-active'191 which appeared to be chemically identical to the element titanium and comparable in its high radioactivity with radium (100,000 Ur). By April 1900 he had taken the step of naming a new element, 'Sur un nouvel element radio-actif: l'actiniumi.192 At this later date, however, Debierne associated his perhaps questionable193 new element in its chemical properties with thorium. He noted that his actinium, which emitted magnetically deflectable rays, caused very weakly the permanent induced activity discovered by the Curies. Now the latter had published descriptions only of a temporary induced radioactivity. However, without discussing duration, they had indeed been first to pose the vital question of whether 'la radioactivity, en apparence spontanee, n'est pas pour certaines substances un effet induit'.194 Debierne was one of those who followed such a suggestion and who also becelme enmeshed in the connected problem of whether certain ostensible radioactivities should be attributed to active impurities. Debierne's approach is illustrated by his conjecture that thorium, weakly active, might owe its activity to traces of a chemically similar foreign substance like actinium, and by his opinion195 that Rutherford's results with thorium also might lead to this conclusion. Study of Rutherford's publications on thorium emanation has revealed no grounds for the 165 statement; he himself could see none.196 Nevertheless, Debierne announced that he intended to start with thorium compounds themselves, rather than with mineral residues, and to separate from these either an inactive thorium or the strongly active foreign substance, actinium. Perhaps Debierne expressed his intention in the alternative form because he realised that the preparation of an inactive thorium would be an impossibility if it owed its activity to actinium, and if this substance really induced a permanent radioactivity upon adjacent materials. Several scientists struggled among the confusions between induction, element, and impurity. F.Giesel's experimental studies ran parallel to and sometimes ahead of those in France. In August 1899, two or three months before announcing the first magnetic deflection of the rays, he published 'Einiges Uber das Verhalten des radioactiven Baryts and Uber Polonium'197 which effectively began a new line of research; though at first he provided no theoretical interpretation of his results. Firstly, the activity of radium was not constant; after crystallisation it rose from a very low level as indicated by a fluorescent screen to a constant maximum during the course of days or weeks.198 Secondly, the activity of a solution of the chloride gradually faded. And his polonium sulphide precipitates had completely lost their radioactivity after two months. Though making no claims, Giesel at the end of 1899 seems to have been the first explicitly to describe a new method of induction, the artificial activation of salts by admixture in solution. His preparation in this way of an 199 artificially active bismuth had significant consequences. For this seems to be the beginning of his long-held belief that polonium was merely induced bismuth. Debierne too made progress in experiments on this new form of induction and cited Giesel and Rutherford in his note 'Sur du baryum 200 radio-actif artificiel'. Debierne considered that the far stronger effects which he found with the solution method were due to the more intimate contact of the substances concerned. Ordinary barium, precipitated as 166 sulphate from a solution containing actinium, turned out to have an activity several hundred times that of uranium; this could then apparently not be altered by chemical reactions. His somewhat surprising conclusion was that: The radioactivity of barium rendered active by contact is an atomic property in the same way as that of radiferous barium, since it persists in all chemical transformations.201 The artificially active barium could, like radium, be concentrated to comparably high activities by crystallisation, but Debierne held its lack of a spectrum to be one of the 'important' differences between the two. The fact that its activity diminished spontaneously to one third in three weeks provided a further distinction from both radium and actinium, each of whose activity steadily increased to a maximum after preparation. Its vanishing radioactivity did not deter him from claiming to have produced: par induction un baryum radio-actif, qui se distingue nettement du baryum et du radium et qui se presente comme un terme interm6diare entre ces deux elements.202 He was sure that his results were not due to traces of actinium or radium. However, perhaps partly in his own defence, Debierne accused B.von Lengyel of committing this very error. For the latter in his report from the Chemical Laboratory of the University of Budapest 'On radio-active barium',203 had stated that he found uncon- vincing all of the evidence for new chemical elements based on radiatiom measurements. Even in the case of radium atomic weight and spectroscopic determinations were not sufficient. The experimental preparation of an artificial radium or of a radioactive barium would tend to show that radium was not an element; in his attempt to do this von Lengyel considered himself successful. He claimed that the active barium compounds extracted from a melt of uranyl nitrate and barium nitrate crystals, exhibited in qualitative fashion the increasing radioactivity possessed by the radium of others: It appeared that one can transform ordinary barium into a radio-active form which apparently 167 possesses all the properties of radio-active barium observed by different experimentalists.204 This conclusion was immediately criticised by F.Giesel in his note 'Veber radioactives Baryum and Polonium'.205 He stated that uranium nitrate owed much of its radioactivity to highly active impurities such as radium and that these would simply have contaminated Lengyel's barium. It is notable that the constancy of uranium rays, so important for radioactivity up to this time, was by implication discarded by Giesel as casually as that of thorium had been by Debierne. With regard to active 'impurities' the subject was interconnected in such a manner that by saving radium Giesel seems in a way to have forfeited uranium. Whether he was aware of this it is not clear. On the other hand, he considered that the Curies' claims for polonium were weakened by a different factor, by their own discovery of radioactive induction.206 As yet another new element came into contention Giesel exhibited the same cautious tone. Both the possibility of radioactive induction and of contamination by active traces played a part in his continuing criticisms of the assertions of K.A.Hofmann. From the chemical laboratory of the Academy of Science in Munich, Hofmann and E.Strauss in November 1900 announced their discovery of several new radioactive elements. One was close to lead in its chemical properties, 'das radioactive Biel', 'radiolead'; the others were rare earths.207 Within weeks Giesel pointed out that traces of radium, polonium or actinium, chemically undetectable, were the likely causes; similarly Debierne's artificially active barium precipitated from actinium solutions had merely carried down a trace of the latter element, undetectable but for its radiation208 Hofmann and Strauss concentrated their researches upon the radiolead; their determination of a chemical equivalent of 65, hence anatomic weight of 260, seemed highly significant (lead 207)209 but the value was never confirmed210 and the statement was not repeated. Then in February 1901 they claimed, apparently in reply to Giesel, that qualitative chemical tests had ensured the 168 absence of all three new active elements from their radiolead; this missed the latter's point. In the face of criticism these incipient radiochemists appear to have sought all possible means of qualitatively identi- fying their materials. The title of their paper, 'Veber die Einwirkung von Kathodenstrahlen auf radioactive Substanzen',211 indicates one such method. The idea was not new, but their attainment of seemingly definite results had been anticipated, in part, only by one other scientist. During the preceding summer, P.Villard had spoken of the induced radioactivity of bismuth;212 he had extended the Curies' discovery of temporary induced radioactivity by producing a similar though weak effect in bismuth by the action of cathode rays alone. Villard expressed the hope that researches using the vacuum tube could lead more readily than solution studies to a simple explanation of radioactivity. We note Villard's view, which appears to relate current researches directly to Prout's hypothesis, that the cathode rays or radiant matter of the Crookes tube consisted not of sub-atomic particles but of the lightest element hydrogen.213 His work in 1900 on the penetration of gaseous hydrogen immediately influenced the ideas of Crookes himself on radioactive rays;214 while that on induced activity was taken up in 1901 at the Cavendish Laboratory in the form of researches 'On a kind of Radioactivity imparted to certain Salts by Cathode Rays'.215 These studies appeared to be of theoretical importance for radioactivity at least until 1903. Hofmann however made no reference to Villard's work as he explained early in 1901 that by the influence of cathode rays he could revive the activity of radiolead. This, unlike radium radiation, had vanished entirely in a few months; the renewed activity lasted for several weeks. He considered that these observations constituted conclusive evidence, in addition to the chemical tests, that polonium too was absent; for the similarly declining radiation of this substance could not be restored. Iiofmann gave a vague theoretical explanation of such 169 effects in terms of vibrations of small wavelength excited within the metal atome.216 This could not have impressed Giesel, for his extensive experiments with the radiolead kindly sent to him by Hofmann confirmed none of its distinguishing features217 not even the effect of cathode rays. Furthermore, he turned the idea of induction against Hofmann by precipitating an articifially radioactive lead sulphide of declining activity from a solution containing radium.218 The dispute continued into 1904, with claims,219 rejections,220 and further claims,221 to have found a distinguishing feature. By 1903, in order to back his assertion of 'Radio-active lead as a primarily active substance'222 Hofmann claimed to have characterised this material by analysing its radiation, despite his employment of techniques unrefined compared to those of some physicists. He still assumed that the rays could directly induce radioactivity into other substances. According to the theory developed at that time, and now accepted, each radioactive element is distinguished both by the character and by the rate of decline of its rays, but not by any inductive property. Experimental problems of isolation and identification certainly continued to exist within the area covered by this theory. Nevertheless, the disagreement concerning Hofmann's preparations was settled more or less in his favour during the following few years. Shortly before the radiolead dispute began William 223 Crookes read a paper on 'Radio-activity of Uranium' which was to have a more direct influence than those discussions upon the development of the understanding of radioactivity. It was he who in May 1900 first pointed to the problem of entrainment of radioactive traces on precipitates, a legacy of his own spectroscopic studies of the rare earths: a substance present only in traces tends to follow the analytical reactions of the bulk material even if chemically dissimilar. Crookes did not mention radioactive induction though he probably read of it in Debierne's papers on active titanium and actinium, 170 which he cited.224 Confused though he admitted himself to be during the following year,225 on account of the results of others together with his own difficulties with radioactive fractionations,226 Crookes seems never to have entertained the possibility of direct induction; perhaps entrainment and diffusion made it seem unnecessary. His laboratory notebooks record some attempts to prepare radium and polonium from pitchblende in October 1898,227 to activate barium using uranium solutions in January 1900,228 and the vital fractionation of uranium in the following month.229 Crookes tells us that his original intention was simply to purify uranium for use as a photographic standard against radium or polonium preparations.230 But on application of a known purification method the portion of uranyl nitrate dissolving in ether turned out, unexpectedly, to be entirely inactive photographically while the aqueous layer, normally discarded, gave a strong effect. He directed his remarks against the Curies' statement that the activity of uranium or thorium was a constant property of the metal independent of its state of combination.231 Crookes had shown on the contrary that the radioactivity exhibited by uranium belonged not to itself but to quite another substance which he designated UrX. This conclusion was supported by marked differences in photographic intensity between various commercial samples.232 Thorium too had begun to separate into portions of different activity upon fractional crystallisation; he remarked upon Debierne's supposition that actinium was the true source of thorium rays. It appears that this notion became persuasive for a time. Crookes informed P.Curie of his results and suggested on the basis of extractions from a barium-free pitchblende that radium might not really resemble barium.233 Curie replied that he had not seen Crookes' paper but that 'it is absolutely certain that the substance you have extracted from your mineral is not radium'. Curie's answer was actinium: he noted that Debierne had detected it in commercial uranium specimens and had already succeeded in decreasing the activity of these; and he believed that 171 the results of von Lengyel and Giesel234 of May and June could also be explained by the presence of this material.235 In his reply, Crookes did not dispute the point;236 indeed a statement made in 1902 shows his acceptance of the identity of his UrX and Debierne's actinium.237 By that time the influence of Crookes' publication had run through Becquerel and Baskerville to Soddy and Rutherford. The American chemist C.Baskerville, in his paper 'On the existence of a new element associated with thorium' delivered in August 1901238 cited Crookes on UrX and thorium. Baskerville too, like Debierne before Crookes, thought that thorium's activity might possibly lie with actinium; he noted that Rutherford's results on the radioactivity induced by thorium made photographic comparisons difficult, but discussed this no further. Five years earlier he had used the designations Th and ThX for the two separated elements.239 These results and symbols featured in the future progress of the science though this belief in the ubiquity of actinium did not. Becquerel too had wished to prepare a pure uranium, in his case for (successful) attempts to deviate magnetically its rays. In a 'Note sur le rayonnement de l'uraniume of June 1900240 he drew from the studies of Debierne on actinium in thorium and von Lengyel on the activation of barium by uranium. Becquerel described an actual lowering of uranium's activity by two successive additions of barium chloride to its solution, each followed by precipitation of the barium as a now active sulphate. He understood apparently independently of Crookes that entrainment of an active material, perhaps actinium occurred here but concluded that uranium did emit a radiation of its own. Becquerel was not so sure of this after a series of 28 entrainments performed241 during the following weeks yielded continual diminutions in uranium's activity, successively smaller, with some irregularities. He acknowledged Crookes' attainment of a completely inactive uranium nitrate, to the Paris Congress in August.242 It was Becquerel's recognition during the following year of the self-recovery of this activity which produced a leap 172 forward in radioactive research. As the Curies saw the position in August 1900243 the experiments of Millard on activation with cathode rays showed that one could 'creer la radioactivite sans faire intervenir une substance radioactive';244 Debierne's 'baryum active' by actinium was unaffected by chemical change, 'son activite est donc une propriete atomique'.245 They thus considered radioactivity to be an atomic property which could be induced upon the atoms by an external agent; the source of the energy released, once this property of radiating had been acquired, was another question.246 As for Crookes' work on UrX and inactive Ur the defence against the implied criticism was clear. It was an associated element, probably actinium, to which the radioactivity of uranium must be ascribed. Owing to the difficulty of obtaining this element free from actinium uranium would simply have 'l'apparence d'un element atomiquement radio- actif'; there was no 'contradiction avec l'idele que la radioactivite est une propriete atomique' .247 It may appear however that the underlying evidence for this idea was now in considerable disarray. Moreover, the Curies' notion of radioactive induction appears to have been at a point of transition. Rutherford's results on thorium emanation and its electrically sensitive inducing effect, which they were unable to repeat in regular fashion with radium,248 together with solution studies, had probably caused the Curies to discard direct radiation as a simple cause of induced radioactivity. Their readers waited until the spring of 1901 for the appearance of a coherent alternative. The physicist E.Dorn at Halle, having recently published on X-rays and radium rays, had already attained some experimental success with radium induction before the time of the Paris Congress. To radium and polonium he applied the techniques used by Rutherford upon thorium. With French and de Ha?n German samples he succeeded, where Rutherford could not, in demonstrating that radium and to a small extent polonium released a gas-like emanation with the inducing property. His discussion 'Veber die von den 173 radioactiven Substanzen ausgesandte Emanation'249 indicates his use of Rutherford's term. Dorn also accepted the view that the cause of induced activity was the deposition of emanation.250 Yet he may not have grasped or adopted this completely for in his reported experiments, some of the earliest, on the electrolysis of radioactive solutions, he applied the Curies' expression 'secondary activity' to activated electrodes without visible deposit.251 And he could not understand why a wire sealed in a glass tube after activation by radium should lose its activity in a day, unless the glass were in fact permeable to the emanations in an undetectable degree.252 The different rates of decay showed him that there were qualitative differences between the emanations from thorium and active barium, and between the secondary activities. His discovery of large, if irregular, increases in electrometer readings upon moistening thorium oxide or radium specimens seemed to him of particular interest; radioactivity was thereby placed in close connection with a 'physikalisch-chemischen Prozess'.253 Dorn's student F.Henning continued electrical researches upon the emanations and upon aqueous solutions during the following year, 1901,254 but drew no significant conclusions; he seems to have thought that the particles of emanation could disappear spontaneously in the air.255 We recall Rutherford's assumption that the intensity of radiation from each particle slowly declined. On the basis of research upon the emanations, only ' Rutherford and his collaborators were to make progress. His study of the energy of radioactivity, completed by mid-1900, was problematical in various ways. Dorn had accorded due acknowledgement to Rutherford regarding emanation studies and now demanded256 the same from the latter regarding the energy of X-rays; priorities were in fact given, in the full published paper.257 Happily, Dorn in his 'Bemerkungen au der Mitteilung von Rutherford und Mc. Clung Tiber die Energie der Becquerel- und ROntgenstrahlen etc.'258 agreed with the values therein. But J.S.Townsend, as we have seen, disagreed with estimates used at a succeeding stage of Rutherford's reasoning towards 174 the energy of radioactivity. The question of its source seems not to have had an answer, nor was there any clear direction of research which might lead to one. The same was not quite true of the emanations: to the standing questions of their nature and means of production Rutherford redirected his attention. His examination of the 'Einfluss der Temperatur auf die "Emanationen" radioaktiver Substanzen',259 dated March 1901, constitutes an extension of his researches of 1899 upon thorium.260 He reported that the rate of production of emanation from both thorium and radium increased steadily with temperature up to a red heat, above which it was almost destroyed, not to be restored. This latter statement was to be very significantly revised within the following months. Somewhat like the Curies, Rutherford noted that there were irregularities in the production of induced radioactivity by radium emanation; it could be confined less readily to a cathode than thorium emanation especially when provided, in large amounts, by heating the radium. Rutherford concluded, adopting an expression similar to that of Dorn whom he cited, that the two emanations were probably produced by a chemical process 'einem chemischen Vorgang im Material'261 J.J.Thomson was prepared to say much more than this concerning the emanations, though not as yet in print. His reply of 12th April to Rutherford's enquiry262 concerning Tait's Chair at Edinburgh contains a comment which may seem familiar in the context of the contemporary chemical- radioactive researches: I suppose you have seen Debierne's work on actinium, a substance which is closely associated with thorium, and which has extraordinary powers of producing induced activity; do you think there could have been any of this in your experiment on the thorium radiation?263 This was Rutherford's second private warning concerning actinium; one of the main tasks of the chemist whose help he enlisted in the following months was to see whether the emanation really came from thorium, or not. As for the emanation itself Thomson now thought he knew 'pretty clearly' its nature and the mechanism which produced its observed 175 electrical properties. We have seen264 that he had reversed Rutherford's explanation both of the concentration of induced activity or emanation upon a cathode, and of the failure of this at a low pressure. Rutherford had postulated an excess of positive ions clustered around the emanation particle, Thomson the attachment of electro- positive emanation particles around positive ions. The failure of electrical concentration in rarefied gases was attributed by Rutherford to a scarcity of air molecules and hence ions; this outweighed the increased mobility of a charged particle. Thomson needed far fewer ions than Rutherford both in his old account of 1899 and in his new one of 1901: I was much interested in your paper on the effect of temperature on the emanation which I was reading last night. I think your experiments show pretty clearly what the emanation is - does not the following view explain most of the effects - suppose that thorium or radium gives out a gas (the emanation) & that this gas is radio-active in the same way as radium i.e. by giving out negatively electrified corpuscles - the effect of this emission of corpuscles will be that the particles of the emanation will behave as if they had a slight + charge. The equivalent charge will be only slight because though the emission of the corpuscle will momentarily leave the emanation with a + charge this charge will soon be neutralised by a negative ion from the surrounding ionised gas The equivalent charge ought to be less at a high pressure than at a low (or rather at a very low pressure) for the smallness of the charge is measured by the quickness with which it is neutralised & at a very low pressure there would be few ions to do this. I should not expect the effect of pressure to be great until the pressure got very low as until then the diminution of the number of ions would be compensated for by their increased mobility.265 Thomson was perhaps first thus to combine the eighteen- month old knowledge of the corpuscular nature of the rays with emanation studies. His account entered strongly into Rutherford's publications266 after initially suffering setbacks. One of these appeared in conjunction with the justification of Thomson's description of each emanation as a 'gas', which Rutherford announced within weeks, in ful- 267 fillment of a promise of 1899. Rutherford's experimental success as he began to answer the second major question regarding the emanations - their precise nature - owed much 176 to radium emanation, effectively unknown in 1899; its radiation lasted for days, rather than the minutes of thorium emanation. His paper with Miss H.T.Brooks268 on 'The New Gas from Radium'269 and his note to Nature on 'Emanations from Radio-active Substances', of May 1901,270 describe the diffusion experiments on radium emanation which now gave a value for its molecular weight. This, being between 40 and 100, excluded the vapour of radium. 'We must therefore conclude that the emanation is in reality a heavy radioactive vapour or gas'.271 The correct modern value of 222272 certainly would not have excluded the vapour of radium; nevertheless, the conclusion held firm. Of great interest are two points which Rutherford briefly made in the closing statement of both papers, each of which weakened previous hypotheses. Firstly, the emanation emitted a radiation 'apparently similar in character to easily absorbed ROntgen rays' and presumably not the charged deviable rays required by Thomson; Rutherford was later273 to make sure of this, with important consequences. Secondly, this emanation 'in some way manufactures from itself a positively charged aabstance, which travels to the negative electrode and becomes a source of secondary activity'.274 This statement is apparently not entirely consistent with Rutherford's and Thomson's previous notion that it was a deposited layer of the emanation itself, positively charged by association with ions of any surrounding gas, which produced the excited radiation. Had the differences between emanation and deposit in their chemical behaviour, and the reality of the new gas, convinced Rutherford of the occurrence of a second strange chemical process? It was months before radiochemical studies on thorium began in earnest at McGill, and nearly a year before his first and barest hint of an observed transmutation appeared in print. But, considering the background of speculation in the field, we are perhaps entitled to ponder privately upon Rutherford's ideas of May 1901; though at this time he would only say that 'Space is too short to enter into the interesting 177 question of the possible explanation of these complicated phenomena'.275

The researches made by Elster and Geitel during 1900-1 came to stand between Rutherford and Thomson in 1902; praised and used as support by the former, they led the latter into criticisms and doubts.276 J.Elster and H.Geitel maintained their early interests in radioactivity; they were pleased to incorporate Rutherford's ideas on emanations and active deposits into another of their areas of study, the electrical phenomena and conductivity of the atmosphere. A slow spontaneous increase in the natural conductivity of an enclosed portion of air over several days led them, towards the end of 1900, to suggest that natural atmospheric conductivity might not be produced by solar radiation as formerly supposed.277 Instead, the rays from traces of emanation in the air, and from the resulting induced activity on the walls of a containing vessel might be the cause. They found observations to support this view, again going underground. Here they found abnormally high conductivities in the air from caves and cellars278 where there was no possibility of contamination from their laboratory. And the final comparisons, made by October 1901, with Rutherford's results on the emanations were the concentration of induced activity upon a negatively charged wire simply placed in the open air,279 and the removal of this activity from the wire by mechanical or chemical means,280 where it then continued to decay. They did not explicitly discuss the important question of whether the active layer was a deposited material, or the surface itself put in a radioactive state; but the answer implied by their exper- ments and discussions would seem to be that it was both. For they used specific methods directed at metals to 178 remove the activity from their surfaces,281 and yet stated that the production of surface activity was 282 the same for a variety of substances. The problematical theory which they devised to encompass these results was one of creation and disintegration of radioactive matter: though unusual in certain respects, it was not the only one of its kind. 179

CHAPTER 4

DISINTEGRATION, INDUCTION, TRANSFORMATION

1. The emergence of induction and disintegration THiiories (1901=7) Dispersed among the various works on radioactivity published during 1901-2 all of the pieces which were shortly to coalesce into a coherent theory may be dis- cerned; later priority claims testify to this. Also present were other persuasive concepts which were to handicap their employers greatly; but this only became clear after the event. At the end of 1901 the physicists Elster and Geitel provided discussions which seem typical of the period in being both suggestive and incomplete. Their comments were contained in a paper whose main purpose was to describe 'Recherches our la radioactivite induite par l'air atmospherique1 .1 During the past year they had linked their discovery of atmospheric-induced radioactivity with Rutherford's conclusion that the emanations and induced activities of thorium and radium were distinct materials. But now the German workers were much attracted by an alternative view which did not require the existence of special substances. Ironically, this occurred just at the time when Rutherford and Soddy were making great progress on that basis. Though it had obvious weak areas Elster and Geitel's thesis was persuasive. For it pointed towards a fundamental explanation not only of the universal induced atmospheric activity but of natural radioactivity also. All gases, they thought, possessed an ionic constitution and could thus provide positive ions which 's'unissent aux electrons nogatifs du conducteur blectrise'. The result was 'une sorte de combinaison instable qui se de-bruit par l'6mission des electrons, c'est-6.-dire par la production de rayons de Becquerel' as required.2 Accordingly, induced activity would be due to a temporary compound derived both from the surrounding gas and the metallic anode. Their approach towards a theoretical advance was 180 to ask 'd'une maniere generale si l'on peut distinguer l'une de l'autre les radioactivites primaires et induites'.3 In order to assimilate these two phenomena they argued from the 'lois d'energie', as they had in 1899, that the apparently permanent activity of the elements uranium, radium and thorium in fact suffered an imperceptibly slow decline. The particularly rapid decay of induced radioactivity was simply attributed to the 'tres petite quantite de matiere recueillie'. Of great interest is their idea that this material was actually in the course of creation - that one might be witnessing 'la veritable elaboration d'une substance active'. The implications of this for uranium and the other active elements are fascinating indeed, but were not discussed. On the other hand they ascribed the observed decay of induced activity to an emission of electrons by which the material 'se detruit' or 'est tres vite ramenee a l'etat indifferent'. Whilst it may be described as a theory of disintegration this account skips over the intermediate chemical stages so crucial for other workers. Even with the limited discussions which Elster and Geitel provided there were certain problems some of which were mentioned and some not. The theoretically implied but unobserved effect of dilution or quantity upon the rates of decay falls into the latter category. In addition their statement that the Curies' radium-induced activity, excited 'par le contact immediat' or via fluid media, 'n'est point une propriete durable'4 seems to admit two distinct types of induction. And in the open admission that their theory could not account for the extraordinary high atmospheric activities observed in certain caves may be seen their reason for not completely rejecting Rutherford's special emanation hypothesis. Having failed to produce the predicted induced activity by attracting the negative ions of the air on to a positively charged wire5 their studies of the different natural conductivities of air from caves and from the laboratory finally convinced Elster and Geitel of the existence of a primarily radioactive gas in the air.6 181 But by then in mid-1903 such phenomena had already been absorbed into a disintegration theory more successful than theirs. That others were moving in that direction towards the end of 1901 is indicated by Elster and Geitel's remark that their 'idee se rapproche beaucoup' with the analogous considerations presented recently and independently by W.Nernst and H.Becquerel. The comparisons are interesting but not simple. Nernst gave only the briefest comments on radioactivity to conclude his discussion 'Veber die Bedeutung elektrischer Methoden and Theorien fUr die Chemie'.7 He supposed that electrons escaping from their dynamic equilibrium with metallic elements constituted the Becquerel rays. And whilst Becquerel himself would certainly have agreed with Nernst that uranium rays consisted in part of electrons yet on the basis of his own experiments he postulated a much more severe dissociation of the active material. In advance of my examination of his and other theories of radioactivity it may be helpful roughly to classify these according both to the kind of disintegration envisaged and to the use made of the notion of induction which can now be seen as a completely false trail. Elster and Geitel's indifferent attitude towards induction has already been noted. They shared with G.Martin and H.Becquerel the idea of a complete or destructive atomic disintegration. The latter was the only one of these to employ the conception of induction by contact in his explanations. W.Nernst, J.Perrin, W.Crookes and J.J.Thomson all seem to have believed or implied that an atom or molecule lost sub-atomic electrons or corpuscles only to pick up others from the surrounding material, so restoring the original situation. On the other hand Becquerel, J.Stark, and Rutherford and Soddy viewed the dissociation of radioactive atoms or molecules as passing through a series of irreversible steps. The latter two scientists in collaboration argued most forcefully concerning the chemical consequences of such a process. The case of the Curies is an interesting one. At the time of her lecture of mid-1900, which contained 182 lively discussions on the relationship between atomic change and corpuscular emission, Marie Curie would appear to fit moderately well into the last group of the above scheme. Within a year however all such considerations had been effectively shelved. Reasons for this are uncertain, but one can point again to the unresolved contradictions regarding the non-corpuscular rays of polonium. It is also just possible that one or both of the Curies came to realise the chemical implications of Marie Curie's speculations only gradually. In any case, P.Curie and A.Debierne were able to work experimentally towards an apparently superior theory uniting both inductive and radiative phenomena. Their first step, however, involved the partial disconnection of a prematurely formed link between these two. In a paper of March 1901 'Sur la radioactivite induite provoquee par les eels de radium'8 they described experiments performed with thorium, radium and actinium in sealed vessels, which proved con- clusively that radioactivity could be induced without direct irradiation. Significantly, the hint of a remaining bond can be seen in their comment that polonium was an exception in producing neither induced activity nor the deviable radiation, two facts which might be related. They stated that it was too soon to accept Rutherford's theory of a diffusing particulate radioactive emanation since other equally satisfactory explanations could be formulated. But they were not prepared to say what these were admitting only that 'La radioactivit6 induite se transmet dans l'air de proche en proche' from source to object and insisting that this process might be connected with the deviable radiation. The adoption and interpretation by Rutherford of the results described in P.Curie's and Debierne's succeeding note 'Sur la radioactivite induite et les gaz actives par le radium',9 read a few weeks after their first, exemplifies the closeness of and the differences between the paths followed by the students of radioactivity. Two months later Rutherford described the electrical examination of 183 10 what he called 'The New Gas from Radium' whose moderately high molecular weight he had estimated with the aid of a standard diffusion method; induced activity was caused by 'a positively charged substance' somehow manufactured from this gas. His approach may be contrasted with the French scientists' suggestion of a gas activated lox radium and with their experimental attempts to grasp the role of the medium through which induced activity was transmitted. Curie and Debierne found that induced activations were unaffected by the use of different gases or by evacuation down to 1 cm. mercury. But in a high vacuum maintained throughout the induction by continuous pumping, as they stressed, activation did not occur and previously induced bodies lost their activity. Suppression of activation failed, however, when the evacuated vessel was simply sealed and left. Curie and Debierne attributed this result to the release of highly radioactive gases occluded within the radium sample; when collected by gently heating the specimen these produced spectacular effects such as the luminosity of the entire containing vessel. Evidently all this fits well with Rutherford's earlier ideas on thorium emanation and with his as yet unpublished study of the effects of heat upon the production of emanations. Yet without again citing that theory the French team spoke with justifiable reserve concerning the only explanation of their results which they mentioned. It might be supposed that 'des gaz ordinaires contenus dans lair slactivent au contact de la matiere radioactive';11 the activated gas could then excite solid bodies by contact. But, as they noted, this accounted neither for the maximum activity's independence of the pressure and nature of the surrounding gas nor for the rapid transmission of the activity along a capillary tube, which appeared to proceed faster than ordinary diffusion would allow. Evidently the idea of a heavy gas, on which Rutherford was shortly to publish, was in even greater disagreement with the apparently rapid transfer through gases than Curies's and Debiern's own discarded conjecture. The French scientists were to find their answer to the problems of the inductive 184 transmission of radioactivity by employing a totally different medium. Researches on this subject differed between Paris and Montreal in technique as well as interpretation in an unfortunate and perhaps unavoidable manner. P.Curie and A.Debierne were moved to comment openly on the 'stat deplorable' to which things had come in the laboratory: the air had become so conducting that only 'des mesures grossieres' with the electrometer could noftrbe made.12 They attributed this situation to the continuous formation of activated gases rather than to dust as previously assumed;13 in another context they reported induced activities of up to 8,000 times the intensity of uranium rays.14 The difficulties of repeating any of Rutherford's work on thorium must have been great. This seems to be confirmed by information recorded in a laboratory notebook15 of the Curies: the 'mouvement propre' of their weight-balance piezo-electric electrometer varied from day to day during some activation studies16 and sometimes made the radiation from thorium impossible to measure.17 In compensation however the extreme activity of the Curies' radium samples soon led them to experimental discoveries which could otherwise not have been made. Within two months of their complaint, Curie and Debierne had been able to construct the most comprehensive theory of radioactive phenomena yet achieved. Its experimental basis developed as they turned from gases to water as the medium of radioactive transmission. Debierne had been one of the first to investigate the solution method of induction in 1900; now with P.Curie in a note 'Sur la radioactivity des eels de radium' of July 190118 he explained that the heating of radium salts produced not only active gases but radioactive water too. Here was their clue. They followed it by showing that water could also be activated simply by placing it in a dish within the same sealed enclosure as a similar dish containing the solution of a radium compound. Perhaps more revealingly water could also be activated by immersing in it a sealed 185 celluloid capsule of the radium salt. Their interpretation was that the celluloid 'joue is role d'une membrane semi- permeable parfaite' allowing the activity but not the radium to pass. Transmission did not occur through a dry celluloid wall. Radioactive water lost its activity within a few days when in a sealed vessel, much more rapidly in an open one, and faster if the surface area was greater. The fact that solutions containing radium itself appeared to behave similarly, with the difference that here the activities declined to a minimum but not to zero, was the final point. The resulting theory was the first which 'permet de coordonner' the rise, decay, equilibrium and transmission of radioactivity. Curie's and Debierne's fundamental assumptions were, firstly, that 'chaque atome de radium fonctionne come une source continue et constants d'energie radioactive', and secondly that this energy thence dissipated itself in two different ways: 1. par rayonnement (rayons charges et non charges d'electricit6); 2. par conduction, c'est-a-dire par transmission de proche en proche aux corps environnants par 1'interm6diare des gaz et des liquides (radioactivite induite).19 The authors made clear the analogy which they saw between this formulation and expressions in use 'dans l'etude des phonomenes calorifiques'; but other points were not so plain. Although they wished to elucidate 'le mecanisme de la propagation de la radioactivite induite'2° Curie and Debierne could or would give no details beyond the phrase 'de proche en proche', or as a translator put it 'from particle to particle' .21 This remained so even after the end of 1901 when they were exploring the spatial aspect of radioactive transmission.22 Having excluded convection and diffusion as major modes of transfer in favour of a step by step process the impermeability of dry solid materials would seem hard for them to explain. And for us the explanation of the apparently rapid transmission along capillaries is equally obscure. By means of their theory Curie and Debierne pushed a multitude of observations into the background. One may note their failure to comment 186 upon the different chemical properties of emanations and induced activities, a problem which worried F.Giesel who mentioned it to the Curies.23 Among other details of which Curie and Debierne knew but gave no account were the electrical concentration of induced radioactivity and the characteristics of the complex radioactive rays. In this respect it seems they placed the phenomena of radioactivity in a hierarchy of importance which was almost the reverse of that adopted by the prononents of disintegration theories. Evidently the authors had good reason to keep their options open. This they did with the claim of July 1901 that the radium atom constituted a constant source of radioactive energy 'sans qu'il soft nbcessaire, d'ailleurs, de preciser vient cette energie',24 and in a footnote they placed Marie Curie's speculations of January 1899: the energy might have been previously stored, or derived from an external radiation, or taken from the surrounding medium, or produced 'par une modification du radium lui- mame'. The last of these conjectures and then the first were soon to become elevated in Paris and elsewhere to positions of the highest significance. But P.Curie heaped harsh criticisms upon those who did this. With his work on the deviable rays from radium, his studies of the chemical deactivation of uranium in 1900, and some points from the above discussions of Curie and Debierne, Becquerel combined theoretically a new discovery of his own. The attempt thus to produce a complete explanation of radioactive phenomena was nowhere ignored. The researches on secondary radiation which he undertook during 190125 had not turned Becquerel from his beliefs that the primary phenomenon of radioactivity was the emission of the deviable rays, that the consequent undetectable loss of mass was the origin of the energy, and that the non-deviable rays (probably meaning the absorbable alpha rays but possibly gamma rays also) were a type of X-ray produced by the primary rays. At the end of that year he clarified his views 'Sur la radio-activite de l'uranium'26 with a detailed description of the processes 187 involved. He considered that 'en se s6parant' the small negatively charged corpuscles of Thomson's theory were matched by large particles, oppositely charged, recoiling at low velocity. These particles, which would not be penetrating, formed the gas-like positively eleotrified material emanation which would deposit upon all surfaces except those similarly charged. Once deposited 'Ce depot de matiere serait capable de se diviser A son tour en particules plus petites qui traverseraient le verre'. Thus in Becquerel's explanation emanations fade into radiations in an interesting manner; however, among other discrepancies, he missed the point that the emanations themselves are electrically neutral. He appears not to have anticipated the view which was later to become important that the alpha rays consisted of rapidly moving large particles; this had already been suggested in 1900 by R.J.Strutt27 on penetration evidence. But Becquerel's account contains the first published expression of the conception that molecules suffer a mechanical recoil upon the release of a corpuscle; such a notion was also adopted by Rutherford during that same month as a new means of explaining the electrical properties of thorium emanation. As revealed in his paper Becquerel's most striking advance lay on the chemical side of radioactivity. Persuaded by the evidence of his own and of Crookes' observations he had come to believe in 1900 that uranium owed its entire radioactivity to a removable impurity. Since then, as Becquerel stated, he had realised that such a conclusion stood in contradiction to the fact that all commercial samples of uranium salts of whatever purity were equally active; it is noteworthy that this 'fact' was by no means so clear to all.28 His observation of June 1900 that old fractionated uranium samples possessed identical activities29 may possibly have been a clue to it. However, Becquerel did not explain the long delay indicated in his announcement that now, eighteen months later, he had reexamined both the deactivated uranium and the barium sulphate specimens activated by precipitation from the solution of uranium. 188 Though the path to its attainment remains obscure Becquerel's discovery that his deactivated uranium had completely regained its activity whilst the activated barium sulphate had entirely lost this power had a direct impact. His interpretation of these results extended the Curie-Debierne induction theory just so far as to make contact with his own ionic speculations; he was doubtless surprised when sparks flew. Becquerel's words reflect his adoption of their distinction between permanent primary radioactivity and temporary induced effects, but his influential statements are in need of analysis: Laperted'activite, qui est le propre des corps activ6s ou induits, montre que le baryum n'a pas entraine la partie essentiellement active et permanente de l'uranium.30 The active barium's decline was thus equivocally explained. Did he mean that the barium had merely become temporarily induced? Or was he suggesting that the barium had in fact extracted one impurity, temporarily active, from uranium leaving another, permanently radioactive, behind? Evidently Crookes' preparation of an inactive Ur would fit neither of these; but it was not to be Becquerel who proved him wrong. One might expect that Becquerel's conjectures as to the means by which uranium spontaneously regained its depleted activity would indicate his preference. But this is not the case; instead he employed what appears to be an uneasy combination of the two. He compared uranium's revival with the well-known rise in activity of freshly precipitated radium-barium salts and to explain both of these he proposed 'L'hypothese d'une auto-induction'. Becquerel suggested that this could occur in mixtures of active and inactive substances 'et mgme a une combinaison chimique de molecules'; this comment may be better understood by replacing 'molecules' with 'atomes'. His final remark on the point was perhaps intended to cover any loopholes concerning the attribution of an element's own activity: 'pour un corps purl elle equivaut a celle d'une transformation molgiculairel. If one similarly substitutes atomic for molecular then there appears one of the vital points against which the Curies reacted. 189 Certainly in their note 'Sur les corps radioactifs',31 read to the Academy by Becquerel himself a few weeks later in January 1902, the Curies attributed to him a theory of 'transformation atomique'. This they attacked in both specific and general terms but in doing so left some of their own difficulties exposed. Their first point was directed against the hypothesis that a process of auto- induction operated to revive diminished activities. 'Certaines exp6riences, mal interpret6es',- they wrote 'conduirent a admettre une destruction partielle de la puissance du radium'. They insisted that on the contrary each of the known radioelements had always exhibited the same unvarying activity when placed in the same physical and chemical state. It must be pointed out that this straightforward statement immediately compounded the problems of polonium which element they therefore committed to the footnotes as an 'exception', branded 'une espece de bismuth active'. The Caries concluded the argument regarding induction with the claim that neither the laws of dissipation of radioactive energy nor the effects of physical and chemical state were known; this appears to be a retreat from the assertions of the previous summer. The second part of their reasoning was directed both against the notion of the emission of material rays and against the related concept of atomic transformation; it was conducted in mainly energetic terms. If the source lay within the radioatom in the form of potential energy then the activity should decline. The Curies considered that this was contrary to their observations. Alternatively, the atom could be a transformer of external energy. By stressing the scientist's ignorance of the medium surrounding him the Curies in effect defended such a viewpoint notwithstanding the evident violation of Carnot's principle. They placed Becquerel's explanation of induced activity and J.Perrin's theory of the origin of radioactive radiations into the internal category and labelled both of these, perhaps questionably, as theories of 'transformation atomique'. Gone was the Curies' acceptance of the material 190 nature of cathode and radioactive rays on the grounds that electrical charge had always been associated with matter.32 Instead they pointed, without calculations, to the negative results of their experiments designed to detect a loss in weight from radium. It is remarkable how completely the Curies' joint statements of 1902 contradict Marie Curie's own apparently favourable discussions of a disintegration hypothesis of atomic transformation published but eighteen months before.33 The verdict seems now to have been that Becquerel's theory was at best premature: their final word concerned the procedures suitable for the attainment of scientific knowledge and implied that there might be no truth whatsoever in such hypotheses. The arguments of the Curies may appear unhelpful but at least these ensured a thorough airing of the issues. It is doubtful whether they stemmed the advance of disintegration theories. Becquerel, for example, repeated his ideas 'On the radio-activity of matter' in a slightly abbreviated form a few months later in March 190234 and maintained these for several years. Before continuing the discussion of comparable speculations which also appeared at this time let us examine the prior statements of J.Perrin which were critically cited by the Curies. In his popular lecture of 1901 entitled 'Les hypotheses moleculaires'35 Perrin attempted briefly and largely qualitatively to explain spectroscopic, chemical and radioactive phenomena in terms of a singular atomic structure. Concentrating the positive charge into one or more central 'soleils' he made the arrangement and orbital motion of corpuscles surrounding each of these account for valencies and spectral frequencies; radioactivity received a brief explanation: Si l'atome est tree lourd, c'est-a-dire probablement tree grand, be corpuscule le plus oloigne du centre - le Neptune du systeme - sera mal retenu dans sa course par l'attraction blectrique du reste de l'atome; la moindre cause l'en dftachera; la formation des rayons cathodiques pourra devenir tellement facile que la matiere paraisse spontanement radio-active... 36 191 Should this necessarily be called a theory of atomic transformation as the Curies said? G.Martin, whose similar view is described below, might not have done so. One can ask the same question of Thomson's explanation of ionisation, involving the temporary separation of a corpuscle, which he held continuously from 1899. And our answer may be provided by his own rejection for a time of a transformation theory of radioactivity in favour of a mechanism of minimal corpuscular ionisation. Crookes too published an explanation of radioactive phenomena which involved corpuscular dissociation without atomic transformation. Its expression, early in 1902, requires some clarification which can be obtained by an examination of the development of Crookes' ideas. We have seen37 that during 1898-9 he had proposed that the energy of radioactivity came from the kinetic energy of the surrounding air molecules. He then discussed with Stokes experiments, such as the influence of air pressure on radiation, which might serve to distinguish between his own idea and the latter's uranium molecule 'wagtail' hypothesis.38 By the end of 1900 they were communicating on the deflecting effect of the magnet upon radium rays. In his recent work on this subject R.J.Strutt39 had rather confusingly used the term 'emanation' for what was more often described as 'radiations'. Stokes adopted the former expression and interpreted Strutt's results in an individual manner. He considered that the 'emanation' consisted of two different portions, namely 'rays', or ether waves, and 'jets', or molecular projectiles. Crookes replied40 that he did not now accept such a dichotomy but was 'inclined to think that all the radioactive actions are to be accounted for by the theory of "bodies smaller than atoms"'. His conception of radioactivity in these material terms involved an unusual interpretation of Thomson's theory in which the corpuscles were supposed to act in the manner of a gas, diffusing slowly through certain materials: It may be urged that Thomson's ultra-atomic particles are only existent theoretically, and 192 no instance is known of such a phenomenon as a gas or projection or emanation passing through matter ... Now however it has been shown by Villard (C.R. June 25, 1900) that hydrogen will pass through fused quartz at a red heat ... Now if a dense body like hydrogen gas will get through quartz, how much more easily will particles much smaller than the ordinary chemical atom get through glass, aluminium and black paper?41 Whilst some physicists had accepted earlier in 1900 that materials were penetrated by high velocity radium projections Villard's belief on the other hand was that hydrogen itself rather than subatomic particles comprised the radiant matter of the vacuum tube.42 Crookes however, after seeing Rutherford's paper on the new gas from radium, in the June 1901 issue of Nature emphasised his point to Stokes: 'I cannot agree that the chief radio-active body in pitchblende is a gas, in the ordinary sense of the word'.43 Instead that position was held by the 'Thomsonian ultra-atomic' particles which, after emerging, temporarily behaved like a gas. Crookes' additional suggestion that some bodies might be capable of 'temporarily fixing additional atoms of electricity - unstable perelectrides' then expelling these surplus atoms of electricity came with an apology: 'Forgive my crude speculations. I feel as it were groping in an unknown laboratory in the dark'. His laboratory work may have furthered this feeling. Among other experiments some attempts made during 1901-2 to prepare an inactive thorium by the successive entrainment method are recorded in his notebooks.44 These show 'EA' and 'PA' (electrical and photo activity) moving irregularly in opposite directions as well as problems with leaking electrometers.45 He felt sufficient confidence in some of his results to publish early in 1902 a paper on 'Radio- activity and the Electron Theory'.46 The view that 'Electrons emanating from radio-active bodies behave like material particles and are impeded by the molecules of the surrounding medium' Crookes now illustrated by the diffuse photographic effects which he had obtained. In this manner he explained emanations and radiations by the same means; other researchers might have considered that he had not. 193 clearly distinguished these entities. Crookes accepted Strutt's suggestion that the emission of subatomic corpuscles was matched by the release of large positive ions and likened this to his own earlier conclusions concerning the electrical evaporation of metals. It could be said that his theory included a minimal atomic dissociation but that he was far from any consideration of radioactivity in terms of atomic transformation or irreversible disintegration. He believed until mid-1903 that the molecules of the surrounding air provided the source of radioactive energy. The young chemist Geoffrey Martin began his communications on radioactivity by expressing views akin to those of Perrin and Crookes but he soon proceeded well beyond them. The first of his series of letters to the Chemical News47 concerned 'Radio-activity and Atomic Weight'. In it he broached the subject of a possible connection between radioactivity and variable valency via a process of ionic interchange, which he saw as the basis of both phenomena. He thus arrived 'at the conception of very heavy metals continually casting out into space light ions, until finally their supply runs short or diminishes'. Evidently no permanent atomic change was envisaged here: if the exhausted metal were chemically 'treated with another body full of such particles ... the heavy element will abstract from it a sufficient quantity to replenish its store, and thus the radio-activity increases again'. One may possibly discern a movement towards his forthcoming depiction of a more destructive atomic process in Martin's letter on 'Prout's Hypothesis and Radio-active Elements'.48 Though this simply raised the question of whether heavy metallic impurities might be the cause of some of the observed deviations of atomic weights from whole numbers, it tended marginally to strengthen the tenuous link between the subjects of the title. It may also be recalled that R.J.Strutt published separate articles on radioactivity and upon Prout's hypothesis in 1901. Martin's publication of most significance for the emergence of atomic disintegration theories appeared 194 early in 1902 shortly after Crookes' paper on ultra-atomic diffusion. Endowed with the title 'The radio-active elements considered as examples of elements undergoing decomposition at ordinary temperatures. Together with a discussion of their relationship TO the other elements',49 its contents may be described as a collection of speculations, some well-worn others premature, all loosely linked to the experimental basis of the hypothesis of subatomic electrons. Martin considered that the long-sought laboratory evidence which could combine with the work of Lockyer to support the notion of a common 'protyle' was now at hand. In his opinion decomposition of atoms occurred not only at the high laboratory temperatures which caused any metal to ionise gases but also spontaneously at room temperature. He cited as experimental support the view of the Curies, which they had themselves revoked shortly before, that 'radio-active matter is at ordinary temperatures giving off electrons (and other particles?)', and put Russell's studies of photo-active zinc and hydrogen peroxide into the same supporting bracket. These points together with the correlation of high atomic weight with both variable valency and radioactivity took him beyond his former position regarding radioactive atoms. He now supposed that these atoms suffered 'incipient decomposition' and were completely 'shattered' into positive, negative and 'inactive' particles; these latter 'which may be very small indeed' comprised 'the unelectrified matter which composes the bulk of the atom'. This statement shows that Martin probably agreed with W.Crookes, L.Boltzmann, W.Sutherland and others in his conception of the atom as a spherical solid mass furnished with electrical or other appendages.50 His idea of atomic decomposition would therefore seem to be more revolutionary than that deriving from the corpuscular theory of matter. Nevertheless Martin's comments evidently covered but a small proportion of the known phenomena of radioactivity. He later claimed priority with the leading question 'Who first suggested that the radio-active elements are elements undergoing decomposition at ordinary temperatures?'51 But the appearance of his 195 paper in 1902 is perhaps best looked upon as part of a general upsurge of discussion concerning the decomposition of atoms. During that year a subtly different set of ideas appeared in a textbook of J.Stark, privatdocent at G6ttingen University, entitled Die Elektrizitdt in Gasen.52 Stark advocated an electronic view of matter of the kind then gaining ground among physicists; an approach which, in part independently of radioactivity, gave hopes for the transformation of the elements.53 Supposing that the chemical atom consisted of equal numbers of negative and as yet undetected positive electrons, each of which comprised a vortex in the ether, he provided explanations of the various phenomena of electrical conduction and chemical valency in terms of electronic dissociation. As for the emission of electrons by radioactive substances he conject- ured in the first place that the electrification thus lost could be regained by the capture of negative electrons from the surroundings. Stark's second suggestion, that electrical neutrality might also be maintained by the loss of positive electricity, indicates his view that the emanations and induced activity consisted of positively charged material particles, presumably atoms or molecules.54 However, he failed to make any theoretical connection between such phenomena and his suggestive ideas on the pressing question of the source of radioactive energy. Stark introduced his answer to the energy problem by calling upon what were becoming standard arguments, based on spectroscopic and ionisation studies, against the indivisibility of the chemists' atom. His conjecture, which was accompanied by rough numerical estimates, that the high temperature of many celestial bodies was due partly to the electronic 'Genesis der Atome'55 effectively inverted the view of Lockyer and Martin that heat was the cause of elemental dissociation. The naturally radioactive substances with their continuous release of electrons and energy Stark saw as remnants of this cosmical process. They were elements which having been stable at the astronomical temperature of their formation now possessed the property of slowly 196 dissociating and recombining into stabler forms.56 But whether these were chemically distinct entities he did not say. As Stark completed his speculative exposition at Easter 1902 the race towards a successful theory of radioactivity was almost won. Rutherford and Soddy were already watching the exothermal creation of new radioactive elements in their laboratory.

2. A quantitative theory of atomic transmutation (1902) I am now busy writing up papers for publication and doing fresh work. I have to keep going, as there are always people on my track. I have to publish my present work as rapidly as possible in order to keep in the race. The best sprinters in this road of investigation are Becquerel and the Curies in Paris, who have done a great deal of very important work in the subject of radioactive bodies during the last few years. Thus wrote Rutherford to his mother in the first week of 190257 soon after seeing the latest paper of Becquerel; he knew also that Crookes was about to publish. Just six years earlier, shortly before the original discovery of uranium rays, he had written of the new X-ray photographs: One of a frog is very good ... The Professor of course is trying to find out the real cause and nature of the waves, and the great object is to find out the theory of the matter before anyone else, for nearly every Professor in Europe is now on the warpath...58 197 Though no single scientist emerged to dominate the study of X-rays E.Rutherford and F.Soddy, physicist and chemist, not only stayed in the radioactive race but ran out clear if disputed winners. The strong field comprised similar mixed teams of P.Curie and A.Debierne, G.G.Stokes and W.Crookes, also researchers of a single discipline such as Elster and Geitel, or Becquerel. By late 1903 the leading pair had forged sufficiently far ahead for one of its partners privately to pour scorn upon such reputable competitors.59 As Rutherford's letter shows, the situation early in 1902 was quite different. The manuscript he had just mailed to London was but the first of a score of papers published during those two years, about half of them jointly with Soddy, which established a position of superiority only gradually. During that period at McGill these researchers developed a theory of atomic transformation which passed rapidly through several phases. Fortunately Rutherford and Soddy provided in their joint publications fairly explicit discussions of experimental studies and theoretical problems concerning the relationship between thorium and ThX. This aspect of radioactivity provided them with the most successful quantitative test of their theory. This furrow has been ploughed deeper by successive writers who have thereby followed the steps leading to the attainment of the 'full' theory of 1903.60 Much is left to be said even along these lines. However, my intention is to place those thorium studies in a wider perspective by viewing them both as a continuation of earlier developments and as a part of the contemporary network of radioactive investigations. Emphasis will be placed on the origins of the theory of disintegration rather than upon its experimental confirmation. We shall see for example that even before the discovery of ThX a notion of chemical transmutation in radioactivity had entered the laboratory; this was itself related to prior discussions. It is notable that the two known members of the uranium series and four or five of the thorium sequence featured together in Rutherford's and Soddy's earliest exposition of the 198 disintegration theory; that all of these contributed to its origins as well as to its extensions is a possibility which should not be ignored. The influence and implications throughout the period of the theory of induction and of the work of scientists such as Becquerel, Crookes, Curie, Dorn, Giesel and others are also considered. Rutherford perhaps felt that to stay the course of radioactivity he would need more than minor chemical assistance. By the end of May 1901 some of the most interesting questions were chemical ones. That these had been created by Rutherford is an indication of that physicist's chemical leanings, the origins of which we have sought in earlier Chapters to unearth. He had subjected different thorium salts to the action of heat and had concluded that the production of emanation was a kind of chemical process highly dependent on temperature. He was sure that radium emanation was in reality a non- radium 'heavy radioactive vapour or gas'. And a notable change in interpretation was that instead of viewing excited activity as a deposit of thorium emanation positively charged, Rutherford now considered that the similar radium emanation 'manufactures from itself a positively charged substance'.61 Whether or not this material difference between emanation and active deposit had come to the fore by virtue of evidence additional to the chemical pointers of 1899 it is not clear. Certainly by May 1901 there had been discussions with a chemist at McGill but these may not yet have been constructive. In the autumn of 1901 F.Soddy, Demonstrator in Chemistry at McGill University since the summer of 1900, joined Rutherford's investigations.62 However, prior to their experimental union the partners had clashed mightily at a McGill Physical Society debate in March 1901. Rutherford commented in a letter to J.J.Thomson, mainly concerning appointments, that in a forthcoming 'great discussion' on the latter's physical corpuscular dissociation theory 'we hope to demolish the Chemists'.63 According to Soddy's report to his biographer long afterwards the physicists were quite unable to do this64 in 199 the face of his own vigorous debating-style attack.65 He spoke against the evidence for atomic dissociation provided by Lockyer and pointed to the weakness in Thomson's early statement on the mie ratio for cathode ray corpuscles; the latter had stated that e might be large as well as m small. Perhaps Soddy became familiar with and less antagonistic towards the more recent studies which provided separate estimates of e. His later statement that he had always been sceptical of 'the electrical theory of matter'66 may not exclude this possibility; a purely electrical view of matter did not become widely acceptable to physicists, including Thomson,67 until after 1901. Despite these theoretical disagreements Soddy though not Rutherford's first choice as an assistant68 no doubt seemed a good one since he was involved in lecturing on gas analysis at McGill during 190169 and claims to have been familiar with this subject and with the inert gases before leaving Oxford.70 A series of exciting discoveries and allied interpretations was soon to make Soddy the champion of transmutation rather than its challenger.

In a lengthy publication entitled 'The Radioactivity of Thorium Compounds.I. An Investigation of the Radioactive Emanation',71 whose abstract72 was read at the Ordinary Meeting of the Chemical Society of London on 16th January 1902, Rutherford and Soddy described the many fruits of their first few months of united labour. Prime among these were two remarkable conclusions. Firstly, thorium emanation belonged to the family of the recently discovered inert gases. And secondly, discovered late in the day we are told, it was not in fact thorium which produced thorium emanation. These deductions constituted part of the answers to the five largely chemical questions explicitly posed; though one cannot apportion the authorship the 200 interested physicist was evidently capable of formulating, if not of answering, each of these. Did the emanation come from thorium itself or from a 'foreign substance'; could the almost non-emanating thorium oxide, rendered thus by excessive heating, recover this property; was the radioactive gas chemically similar to any known matter; did its emission cause a loss in weight; was there anything in the chemistry of thorium to account for its 'almost unique power' of giving an emanation?73 Many of the experimental techniques involved in these new investigations were modifications of or improvements on those employed earlier by Rutherford. For example, in measuring the conductivity produced by thin and thick layers of a powdered thorium compound in combination respectively with an air draught or screening, the researchers were satisfied that their separate estimates of the direct radiation and emanation emerging from the same specimen were accurate to within one or two per cent. Their preferred method for determining the emanation, which avoided waiting for equilibrium, was to pass it at a known speed down a tube along which a number of electrodes were spaced. The successively lower electric currents detected at these points gave a value for the emanation which agreed well with the simpler arrangement; each method was comparative in conception with a 10 gm. sample of thorium oxide taken as the standard.74 These techniques, which can be traced back to the time of Rutherford's first studies of thorium emanation in 1899 or earlier, were vital for Soddy's answer to the question of the chemical nature of this entity. Able thus to estimate the emanation's quantity by means of its radiations, while they lasted, Soddy found that this gaseous material refused to combine with any reagent. The conclusion which the investigators boldly announced was that the emanation belonged to the recently 76 established75 family of inert gas elements. Whilst acquiring this understanding of its nature Rutherford and Soddy enquired experimentally after the emanation's source. In response to their own question 201

'Is the Emanating Power a Specific Property of Thorium?'77 they reported that samples of thorium sulphate from opposite ends of a fractionation each exhibited the same intensity of direct radiation and identical emanating powers. The entailed affirmative answer, though soon to be stifled, was backed by further evidence. For Soddy was able in effect to smooth out the sharp differences in emanating power between oxide specimens subjected to different heat treatments. Such variations had contributed to Rutherford's description of the phenomenon as a heat- dependent chemical process. 'The Regeneration of the Emanating Power by Chemical Processes'78 which they claimed to have achieved was impressive though irregular and incomplete. The ordinary oxide (adopted as the standard at 100%), after de-emanation by ignition (to 10% power), dissolution, then reconversion to the oxide, exhibited a partial recovery of its emanating property. Following such a cycle via the sulphate thorium oxide turned out as high as 40% effective, via the chloride 55%. But a different chemical conversion tried by Soddy gave quite different results which, however, tended to make the situation clearer. Both ordinary and ignited thorium oxides when dissolved then reprecipitated as the hydroxide were endowed with enhanced powers of emanation. And in addition these hydroxides' values rose spontaneously during the course of a week or so from an initial in- equality (hydroxide from normal thoria, 108%; from ignited thoria, 128%) to exact equality at a high level (about 250% of the standard). Furthermore they were able to clarify the effect of moisture79 thus freeing themselves from the influence of Dorn's suggestion of a 'physikalisch- chemischen Prozess'. Rutherford and Soddy were beginning to realise that these chemical and physical influences might only be superficial. They noted, for example, 'that the cause of the emanating power is not removed by ignition, but only rendered, for the time being, inoperative'.80 The authors' statement that the evidence 'certainly seemed to point to the conclusion that the power of giving an 202 emanation is really a specific property of thorium'81 is doubly significant. Firstly its expression gives an unusual illustration of conclusions which were already withdrawn. Now the word 'specific' was practically synonymous with the term 'atomic' which others such as the Curies tended to use in this context. Thus, secondly, the proof of a chemical transmutation may briefly have become apparent. It may well have appeared so to Soddy who remarked, on looking back more than fifty years,82 that he had first realised in 1900(sic) that a genuine chemical transmutation of thorium to an inert gas was at hand. In the published paper one reads the cautious statement that though it was 'perhaps early' for theoretical discussion one of two possible alternatives was: to look upon the emanation as consisting of a gas emitted by the thorium compound. It is not necessary that such should contain thorium, it might conceivably be an inert gas continuously emitted in the radioactive state.83 But this is very similar to their better known assertions made in the spring of 1902 which are generally taken to mark the great innovation of atomic transformation. One might argue instead that radioactive transmutation had been conceived by Rutherford and Soddy as an experimental reality before the end of 1901. Rutherford's recollection long afterwards was that: The great contrast in the physical and chemical properties of thorium X and the emanation gave us the first definite clue that radio-activity was a consequence of the successive transformation of elements and led ultimately to the disintegration theory...84 'Definite clue' is an ambiguous phrase; but Rutherford unlike Soddy evidently refers to the period following the time in about December 1901 when the idea of the direct production of the emanation by thorium became untenable and thorium X appeared. In a section which forms an appendix to their first paper Rutherford and Soddy admitted that 'since the preceding account was written developments have been made 203 in the subject which completely alter the aspect of the whole question of emanating power and radioactivity'.85 The revelation was twofold; its cause was their discovery that both emanating power and direct radioactivity, the latter so far unaltered in all experiments which affected the former, were properties not of thorium but of a different substance. This they labelled ThX. The connection with W.Crookes' earlier conclusions is an interesting one which should perhaps be brought out. Rutherford and Soddy stated, rather surprisingly it seems, that their negative results in fractionating thorium sulphate were obtained before they knew of his similar work.86 Yet it is true that almost everyone else who wrote on radioactivity had referred to Crookes' important preparation of inactive Ur and his discovery of UrX shortly after the announcement in May 1900. Rutherford, however, left for New Zealand to get married at just this time and although he may have read the Curies' and Becquerel's papers to the Paris Congress of August 190087 each of these referred to Crookes' discoveries with uranium alone. Only Baskerville's discussion 'On the existence of a new element associated with thoriuml88 published shortly before Soddy began these studies, and Crookes' own publication, described the latter's failure with thorium sulphate but his partial success in fractionating the nitrate. However, Rutherford and Soddy made no mention of this work on the nitrate; they merely pointed out that the photographic methods used by the above chemists were qualitative and incapable of distinguishing between thorium rays and those from the emanation.89 Nevertheless, the McGill scientists may have owed some debt to these nitrate studies; they were certainly aware of them. In his paper Crookes had mentioned the German commercial source of a highly purified thorium nitrate; Rutherford wrote asking him to forward a request for this material. Crookes replied welcoming Rutherford's promised paper to the Chemical Society, stating that he too was preparing a publication, and reporting the 'curious circumstance' of Becquerel's finding that an old specimen of inactive uranium nitrate 'had reassumed its radioactivity.,90 204 At that time, in December 1901, Rutherford and Soddy were themselves studying the spontaneous recovery or increase of the emanating power of thoria. Now since in their view uranium gave no emanation the question of the revived activity of this element was an open one. Evidently there was much to consider even beyond the late additions and alterations which appeared; here they described to their readers the successful outcome of a new search for ThX, the hypothetical emanating and radiating constituent in thorium. Crookes' attempts to correlate the electrical and photographic activities of thorium with its chemical treatment, which had yielded only irregular results, were thus overtaken at the end of 1901. But the passage of Rutherford and Soddy was not smooth. Their emanation studies began to take on a coherent form as 'it was beginning to be realized' that emanating power depended on the 'previous history', or mode of preparation, as well as upon the chemical nature of a compound.91 But the continuing approach towards the origin of this power was disturbed by further striking irregularities. As they noted in their late addition, whilst the powdered crystals of thorium nitrate were of surprisingly low emanating power (1.8% of standard thoria), quantitatively prepared solutions whose study they had newly taken up possessed a very high power (about 300%) independent of dilution. They interpreted the phenomenon as a 'latent emanating power'92 of thorium nitrate in the solid state. 'Simultaneously with this observation' they remarked, it was noticed that preparations of thorium carbonate varied enormously in emanating power according to their method of preparation .93 They recorded a notable vagary, evidently one of many,94 in which Soddy precipitated thorium carbonate from the nitrate by means of sodium carbonate, partially redissolved the carbonate with nitric acid, presumably removed the remaining solid, then reprecipitated the dissolved portion as hydroxide using a solution of ammonia. 'The result was remarkable: the carbonate had an emanating power of only 6 per cent, the hydroxide one of 1225 per cent of that of 205 the ordinary oxide'.95 It was perhaps equally remarkable that the hydroxide's emanating power then decreased spontaneously to 1/3 value after 14 days whilst the carbonate's remained constant. They ascribed this result to an 'accident' of the conditions but did not let the matter rest. Repetitions of the procedure gave totally different results: the carbonate and hydroxide precipitates were of approximately equal low powers (about 15%) which in the manner usually anticipated rose spontaneously during a week or two to values of 100 to 300%. The first carbonate of very low emanating power displayed no abnormality in its direct thorium rays and was turned into a normal emanating sample of the carbonate on redissolution in acid, followed by reprecipitation. But we are told that 'The production of preparations of such low emanating power led naturally to an examination being made of the filtrates and washingsq6 which should contain no metallic substance whatever. This examination was indeed a fortunate step. Its well-known results have been seen97 as a turning point. For although 'they should be chemically free from thorium' the ammonium nitrate/hydroxide filtrates possessed a definite emanating power and, after evaporation, direct radioactivity too. Subsequent closer tests of the filtrates yielded a mysterious white phosphate precipitate, highly active and 'in very appreciable quantities', but they were able to dismiss this as an irrelevant impurity.98 'The evidence of long series of experiments in two directions' afforded them 'little doubt of the actual existence' though in 'altogether minute amount' of 'a constituent ThX to which the properties of radioactivity and emanating power must be ascribed'.99 There seem to have been several implications of this conclusion. Evidently one of these was that the direct radiations of thorium as well as its emanating power were now in question. Another relates to parallel studies under way in Paris. Curie and Debierne had announced in July that ordinary water could readily acquire induced radioactivity, 100or 'emanating power' in Rutherford's and Soddy's terms. In the light of Curie's experiments and theories it was surely to be expected that all 206 filtrates from thorium or radium would posses an emanating power which might in turn be transformed into direct radiation: Rutherford indeed found that the solid traces in thorium filtrates emitted such rays. But the fact that this activity could be made far higher than that of thorium itself, by factors of up to 1800, appears to confirm ThX after the fashion of its predecessors Po, Ra, UrX and others. Rutherford and Soddy noted that its inconsistent chemical properties, for example solubility in and precipitation by hydroxide or phosphate, could be explained by entrainment of the minute amounts of ThX present. And they made the identification of ThX safer by the technique of penetration analysis of the radiations, a method which was becoming increasingly important. The rays from the active residue were identical with thorium rays and different from the several other varieties; furthermore this residue produced an emanation whose activity decayed at a rate identical to that from thorium. Having thus attributed the entire radioactive phenomena of thorium to ThX they considered naturally that the preparation of a totally inactive thorium would form a desirable confirmation. To this end they reported their actual attainment, by repeat- edly washing with water, of a 20% reduction in the activity of thorium.101 However, the path leading from this point, like that leading to it, was not a straight one. During the course of their later experiments of 1901 Rutherford and Soddy achieved the beginnings of a unification of the two major aspects of thorium's, now ThX's, radioactivity. That the 'straight line' radiation of thoria remained constant throughout wide fluctuations in its emanating power had at first served 'to bring out the fact' that these two powers were independent.102 The apparent difficulty of reconciling this with their conclusions that each was connected with the thorium atom or molecule was soon to be eased. In directing attention to the 'straight line radioactivity, which is generally unaffected by these changes of conditions and previous history' in order to follow 'the progress of the removal of the active material', Rutherford and Soddy flatly 207 contradicted their initial statement with the comment that 'The two phenomena are undoubtedly connected'.103 The empirical correlation, whioh had arisen from the use of solutions, was soon to become of great theoretical significance. From the modern viewpoint one sees a continuous emission of emanation from ThX which is itself continuously produced by thorium; when formed within the various solid compounds of thorium the actual rate of release of the gas is complicated greatly by a temperature- dependent process of occlusion. By April 1902 these scientists suspected all this.104 But in December 1901 they thought that 'the surprising uniformity' of the emanating powers of variously treated thorium compounds, despite the known loss of a large proportion of the supposed emanating source ThX made the process appear: rather as the result of a dynamical change, possibly in the nature of a chemical reaction where the active mass of emanating material is a constant, than as the property of a peculiar kind of matter in the static state, additive with regard to mass.105 Here 'active mass' is a term drawn from chemical reaction kinetics rather than radioactivity; 'effective mass' may be a clearer substitute.

The vision of a strange chemioal reaction which they saw here was obscurely mirrored in Rutherford's parallel and neglected researches on the excited radioactivity produced by the emanations. During the Christmas vacation of 1901 at about the time of completion of the joint paper on thorium emanation and ThX he presented to the American Physical Society in New York papers on the 'Transmission of Excited Radioactivity'106 of thorium and radium, and on 'Excited Radioactivity and Ionization of the Atmosphere,107 To the physicists Rutherford revealed some of the mechanisms he had in mind. Regarding the question of the positive charge of excited activity Rutherford noted his own earlier 208 explanation that this might be caused by condensation of the emanation around the positive ions produced by its radiation, and Thomson's alternative idea of an average positive charge left by the temporary loss of a corpuscle.108 The version he preferred, however, which could better account for the suppresion of the charge effect at low pressure was an extension of Thomson's view: the emission of a material corpuscle or electron at its high velocity of 1010 cm./sec. would impart an opposite impulse to the remaining positively charged molecule. This could fling it contrary to the field against the positive electrode.109 Presumably this molecule would have to pick up another electron or a negative ion before actually adhering to the electrode to form the active deposit. Thus, like Becquerel,1 10 Rutherford saw the first step in the production of excited activity as the violent emission of an electron. But he was as yet prepared openly to discuss the notion of a minimal disintegration only. Rutherford thought that the radiation from excited radioactivity was caused by the recoil vibrations within each deposited molecule. 'Da es unwahrscheinlich ist, days innere Schwingungen von Molekulen verschiedener chemischer Natur sowohl nach Charakter wie naoh Dauer dieselben sind' it was to be expected that the induced activities from radium, thorium and perhaps atomospheric air would differ both in duration and penetration, as they did.111 However, Rutherford's explanation of the electrical character of its transmission left open many questions regarding excited radioactivity. These can be isolated and grouped into three areas. Firstly there were problems concerning atmospheric excited activity. Whilst a chemical audience read or heard that the existence of an atmospheric emanation was most probable112 to the physicists Rutherford spoke equivocally. Although he accepted that its electrical properties and the duration and penetration of its radiation appeared to identify an atmospheric deposit he queried, on the basis of two observations, Elster and Geitel's assignment of the cause of this deposit to an atmospheric emanation. The conductivity of a sealed mass of air failed to decline, remaining 209 constant for a month; and carbon dioxide exhibited the same properties as air.113 The contradiction was soon to be eased114 by C.T.R.Wilson's conclusions, based on pressure-variation experiments, that the walls of the vessel contributed to the 'Spontaneous Ionisation of Gases'.115 But Rutherford's only visible step towards clarification at the end of 1901 was his statement that experiments were in progress to see whether any of the components of the atmosphere, prepared chemically, displayed sufficient ionisation and excited activity to account for the observed atmospheric effects.116 Whilst it is not entirely clear how these might affect the problem such experiments seem relevant to statements published with Soddy on thorium emanation. Their joint abstract boldly denied that the emanation might be a mere sport of induction: The possible explanation that the emanation is the manifestation of excited radioactivity on the surrounding atmosphere was shown to be untenable by a crucial experiment.117 Vernon Harcourt who read this at the meeting and who had been one of Soddy's referees at Oxford seems nonetheless to have been unimpressed. He pointed in a different direction to Russell's work on hydrogen peroxide emanations.118 However, in the full paper Rutherford and Soddy had dismissed the Russell effect as having no connection with radioactivity and had written with more caution regarding induction. They had performed certain experiments which proved the amount of emanation emerging from a source to be independent of the type of gaseous carrier and others which showed that the emanation differed from the trans- porting medium in its resistance to any chemical attack. Bitt they admitted that these facts still left open the possibility that one of the inert gases known to be present in the atmosphere might be 'rendered radioactive in the 119 presence of thoria'. It is of interest that the authors referred to experiments in progress which appear complementary to those mentioned by Rutherford in the context of a supposed atmospheric emanation; they hoped to measure the 210 emanating power of a specimen placed in a current of gas as free from air as possible. No results were published; perhaps this test came to appear unnecessary during the following months. A second group of problems relating to excited radioactivity and its transmission which existed at the end of 1901 may be loosely described as chemical. Whether Rutherford's complete avoidance of any description of thorium emanation as an elemental inert gas was deliberate one can only speculate. It is notable however that he expressed doubt as to the 'Ausstramungsanschauung' or emanation view of atmospheric effects in favour of the 'Elektronhypotheses.120 Yet he did not say what variety of molecules in the atmosphere might be supposed to emit the electron. In his account of 'Versuche fiber erregte Radioaktivitdt' dated mid-January 1902121 Rutherford confirmed that the rate of decline of the radioactivity excited by thorium suffered no change when this deposit found itself in acid solution. Having deduced therefrom that the decay was a process occurring within the active substance he gave no further indication of what this substance might consist. And, thirdly, the process of radioactive decay was itself made to seem more complex by his discovery122 that the weak excited activity produced upon a wire by brief exposure to thorium in fact spontaneously increased in intensity for an hour or two before declining at the known rate. The activity excited by radium showed something similar.123 The curves were unaffected on heating the wire or plate to redness; nor could any secondary emanation be detected. Rutherford concluded without elaboration that this pattern of changing radiation was caused either by a gradual molecular rearrangement or chemical reaction, or a second excited activity produced upon the surface by the first, presumably without an intermediate emanation. Evidently the phenomenon of a rise in activity could not be explained so readily as the usual decay. It is interesting to speculate whether that discovery had an influence upon the emerging theory of atomic 211 transformation which was to bring these diverse facts rapidly to order. This spontaneous rise in activity was the first of three which appeared almost simultaneously. The second occurred in Becquerel's deactivated uranium. And the third involved the vital extraction of ThX. All three took their place in the famous joint publication which followed within months. Rutherford and Soddy probably read Crookes' letter upon their return to the laboratory after Christmas 1901. On checking their partially deactivated thorium, which they no doubt would have done as a matter of course, they found that like Becquerel's nearly inactive uranium it too had regained its full direct radioactivity. That of the extracted ThX had declined almost to zero. They may have seen at the same time the paper which Becquerel read on 9th December 1901. Rutherford mentioned this scientist when he wrote home on 5th January and in the second joint paper with Soddy124 where credit was assigned for the discoveries of the respective revival and decay of the activities of deactivated uranium and activated barium. Becquerel's paper contained far more than this. He was no more satisfied than were Rutherford and Soddy merely to report an empirical discovery; some of the ideas he expressed may have seemed highly significant at the time. As we have seen125 Becquerel was first to publish the valuable suggestion that the mechanical recoil from a corpuscular emission might play a part in radioactivity. He explained radioactive induction in alternative ways either by direct contact or by the deposition of the large positive particles ejected on the emission of electrons, which remnants were themselves subject to further division. The recovery of uranium's activity he attributed to self-induction which might constitute a 'transformation moleculaire'. By ignoring his assumption of a radioactivity induced by contact or direct rays one can extract the consequence that the rise in uranium's radioactivity is entirely due to its own disintegration into smaller atoms themselves radioactive. Doubtless it could also have been deduced that there might exist a linked series of distinct and 212

radioactive chemical products and that the apparently constant maximum activity of uranium or thorium was a resultant or equilibrium value of these changing activities. By the time Becquerel's paper was published Rutherford and Soddy already had an obscure knowledge of such a chemical series, thus: ThX---) emanation—>first active deposit (---? second active deposit); directly after Christmas they added with their own discovery the vital initial stage Th---ThX. Upon the basis of the quantitative experimental examination of this step they were to argue their theoretical case. Thomson's unhappy news regarding Rutherford's candidature for the Royal Society126 arrived at McGill early in May some days after Rutherford and Soddy had sent off their second joint paper to the Chemical Society; its confident title was 'The Radioactivity of Thorium Compounds.II. The Cause and Nature of Radioactivity'.127 It is fortunate that they there included a brief history of the events; (changes with the passage of time were now of considerable importance. We are told that on reexamin- ation after three weeks the thorium hydroxide which they had deactivated to only 36% of the normal value had completely recovered, that the ThX residues 'had almost completely lost their original activity', and that 'At this time, M.Becquerel'e paper ... came to hand': A long series of observations was at once started to determine: (1)The rate of recovery of the activity of thorium rendered less active by removal of ThX, (2)The rate of decay of the activity of the separated ThX. 128 Using chemical methods similar to those employed previously they obtained 'numerous series of observations made with different preparations at different times'. Apart from 'the difficult questions' of 'initial irregularities' in the first few days of each 3 to 4 week series, and an appreciable unseparated or inseparable 'residual activity' left in the thorium, these rise and decay curves matched perfectly. They fitted respectively the related equations 213 - >t ItiIo = e -Xt and It/Io = 1-e with the same X. The 129 authors noted explicitly that these were identical in form with the pair of equations which Rutherford had developed in 1899 to describe the asymptotic rise of current observed on steadily passing gaseous thorium emanation into a vessel, and the geometric decline of activity which ensued once this flow was stopped. The explanation which they formulated may seem startling perhaps because it could be 'put to experimental test very simply'. It was this which made the Th-ThX relationship the main spearhead of their theoretical claims. Like the arrival of particles of emanation in the vessel 'the active constituent ThX is being produced at a constant rate1130 and similarly its activity then suffered a geometric decay. Thus the 'normal or constant radioactivity possessed by thorium is an equilibrium value', a balance of the two processes. One of these effects, the decay of activity of the separated ThX, was directly observable and the other, a continuous growth of ThX itself in the thorium freed from it, almost so. Having refined the separation techniques they proceeded as follows. A double precipitation of thorium as hydroxide from a solution of the nitrate left the ThX in the filtrates. Evaporation of these gave a measure of the maximum or equilibrium value of ThX's activity. A third precipitation of the thorium after redissolution confirmed that it now contained a negligible amount of ThX. Several thorium hydroxide precipitates thus initially freed from ThX were later dissolved and reprecipitated after different periods of time from a few hours up to one month. The fact that the new filtrates always possessed activities agreeing with the normal recovery of deactivated thorium131 provided a striking confirmation that 'The process of production of ThX is continuous' and that this transformation was unaffected by the separation procedure. One of the major differences between the observed measurements and those expected for a continuous arrival or production of active material followed by a geometric decay concerned the initial portion of each curve. The discrepancy 214 was complementary: the activity of freshly removed ThX began by rising for a few days (by about 10%) before decaying geometrically, and the deactivated Th actually lost activity (about 15%) for the same period before rising asymptotically. The thorium deactivated by removal of all its ThX still possessed 46% of the normal activity - a second discrepancy since according to theory thorium should be inactive. Perhaps these were welcome problems, for Rutherford and Soddy were able to use them to justify their hypotheses at the same time helping to tie together the entire incipient transformation series. ThX was known to create excited radioactivity, via an emanation, 'on surrounding inactive matter' so that the 46% 'residual activity of thorium might consist in whole or part of a secondary or excited radioactivity produced on the whole mass of the thorium compound by its association with the ThX1 .132 In brilliant style they tested this supposition by preventing the ThX from producing any excited activity. A series of 23 successive dissolutions and reprecipitatione in 9 days to remove ThX from one tortured thorium hydroxide specimen allowed the excited activity initially present to decay completely, as the constant minimum of their readings showed. This still left the thorium with 25% of its normal activity - a discrepancy smaller but harder to accomodate as will be seen. When left to recover from this treatment the activity of the specimen rose without the initial fall; to clinch the point they showed that the 'difference curve' between ideal and observed curves for ThX, both rising and falling, had a half-value time of 12 hours which was equal to that for the known decay of the 'ordinary excited activity'. There was thus 'no reason to doubt that the effect is the same as that produced by the thorium emanation, which is itself a secondary effect of ThX'.133 The production of thorium emanation from ThX took its place in the new scheme: just as thorium produced the non-thorium material ThX, a 'further transformation° of the latter resulted in the continuous emission of a radiating inert gas; the sometimes highly 215 irregular results could readily be attributed to occlusion or changes in crystal structure.134 Furthermore an analysis of the radiations showed that specimens from which the emanation could not escape possessed the expected high proportion of excited activity. Rutherford and Soddy were not yet ready to say whether the production of emanation imitated the primary change of thorium to ThX in proceeding at a rate independent of the conditions;135 they were prepared only to state that this applied to the decay of the emanation's radiation. This aspect of thorium had given rise to the first notion of its transformation to an inert gas; but now the emanation took second place both in sequence and in certainty. Rutherford and Soddy extended their discussion beyond the confines of thorium: they went so far as to suggest that the apparently constant activities of uranium and radium too were the resultants of chemical changes 'also independent of the conditions'. And describing the radiated energy as a loss after each such change from a supposed store within the system they came to the impressive conclusion that 'All known types of radioactivity can thus be brought into the same category'.136 It may be noted in return that considerations of several varieties of radioactivity had contributed to the theoretical development. It is true that the remarkable quantitative confirm- ations with thorium and ThX increased the power of this theory far beyond any other. However, in its explanations there were weaknesses. Some of these led to a broadening of its success but others to criticism from without. In a final discussion Rutherford and Soddy tried to justify the applicability of expressions such as 'the chemical atom in certain cases spontaneously breaking up with evolution of energy' and 'sub-atomic chemical change'137 to radioactivity but their chemical evidence was not strong. ThX, the most important member of the thorium decay series from a theoretical point of view, was perhaps also the least certain. In their section on 'Chemical Properties of ThX1138 radioactivity measurements correlated with precipitations formed the only evidence they could muster to justify the statement that 'There can thus be no question 216 that both ThX and UrX are distinct types of matter with definite chemical properties'.139 Though they believed that 'transmutation' was the origin of these materials such an expression did not appear in print; nor did they describe these substances as 'elements' known or new. Yet claims of the latter kind based on similar evidence had played a vital part in radioactive studies since their early days. Radium was the only thoroughly confirmed new radioelement. It had been preceded on the scene by polonium and had been followed by actinium and radiolead all of which were of disputed status. As for UrX, Crookes and the Curies thought it might be identified as actinium and Debierne interpreted ThX similarly. As Rutherford and Soddy noted140 there was also the non-radioactive splitting of thorium by Brauner and Baskerville to consider. Since 1899 each claimant had to refute the possibility that his supposed new radioelement was merely an induced activity. We have seen141 how Hofmann defended his radiolead against F.Giesel's accusation of induction. At the time of Rutherford's and Soddy's first publication, in January 1902, Hofmann reported his own results on the fractionation of thorium. These led him to suppose that thorium itself was really inactive and that it possessed only a temporary radioactivity induced by uranium; he claimed to have successfully prepared inactive thorium from minerals which contained no uranium. This work142 was not of a high electroscopic standard143 and was not mentioned by Rutherford and Soddy in their second publication. However, as with the emanation previously, they were at pains to refute any suggestion that the invisible ThX was no more than the manifestation of a temporary activity induced by thorium upon some portion of the neighbouring materials. They described such a view as 'quite untenable'; if it were true any precipitation of thorium should leave active residues in solution whereas experiments showed that carbonate, oxalic acid, and phosphate precipitated thorium leaving non-active solutions; only ammonia was capable of the separation. It should perhaps be recognised that this point taken in isolation 217 may have seemed less than convincing to others; for they failed to dismiss explicitly the possibility that ThX was a trace element in thorium which suffered temporary activation by induction. Rutherford and Soddy at this stage may themselves have entertained the idea of direct induction in order to explain a second questionable aspect of their theory. What was the cause of 'The Non-separable Radioactivity of Thorium',144 the 25% of the radioactivity remaining, when both ThX and excited activity were absent, which did not decay appreciably with time? One hypothesis of the two they described involved the production of 'a second type of excited activity' with a 'very slow rate of decay' by ThX. It should be possible to observe this decay if thorium were continually freed from ThX over a very long period. Notably, their assumption that 'it will not be possible to free thorium from this activity by chemical means' suggests that by 'excited activity' in this instance they meant a radioactivity directly induced into the thorium. And since they referred to it as a 'second type of excited activity ... similar to that known',145 Rutherford and Soddy may also have wavered towards an induction view of ordinary excited activity. They appear, at first sight, to have avoided the consequences of such a belief by instead accepting a second hypothesis: the initial transformation gave two products rather than one. The constant residual activity would accordingly itself be an equilibrium value and the other substance continually produced by thorium should be chemically separable from it. Magnetic analysis of the radiations tended to support this view for the residual activity consisted only of non- deviable rays whereas both ThX and the excited activity emitted mixed radiations.146 But the main reason for their conclusion regarding thorium's residual activity, as they said,147 was the work of Soddy on uranium where both emanation and excited activity were absent. His active investigations of that element had probably been induced by Becquerel. These were thus pursued throughout the course of the ThX studies and contributed directly to the 218 early transformation theory generally associated exclusively with thorium. Soddy concluded his companion paper on 'The Radio- activity of Uranium'148 by turning a current criticism of his own emanation studies against the researches of another; he asserted that the diffusing emanation from uranium discussed by Crookes 'is not a radio-active substance in the accepted sense of the word' but 'an agent similar to hydrogen peroxide in its photographic action'. And Soddy began this piece by announcing that the 'inactive Ur' prepared according to Crookes was actually not inactive electrically but only photographically; it emitted a non- deviable and readily absorbed 'alpha' radiation and thus possessed a 'residual activity' similar in character to that of thorium. The same questions arose here: 1. Is this residual activity to be regarded as a secondary radiation produced by the presence of UrX? Or, 2. Is it caused by a distinct material substance capable of chemical separation? 149 According to the corresponding joint paper one might expect definite answers; these were provided but on the basis of insecure evidence. Soddy considered the first view to be improbable. He kept uranium free of UrX for three weeks by barium sulphate precipitations and then compared it with Ur not so treated. The alpha rays in one sample had thus been given three weeks to decay but Soddy could detect no difference between the two. He concluded that 'it takes at least a year to decay to half value' which was 'not in accordance with what is known of the nature of excited radio-activity'.150 But what indeed was known of excited radioactivity? F.Giesel's comment151 which appeared two or three months before that of Soddy illustrates the difficulty. Having found that a radioactive preparation of lead was still active after a year he yet refuted Hofmann's idea of a new element; although induced activity was usually 'relatively soon lost' Giesel considered it possible that the extreme conditions of induction - great excess of radium and a year-long exposure - might result in an induced lead of greater activity and longer duration than obtained 219

formerly. The fact that Soddy's argument requires the existence of directly induced radioactivity seems to indicate that he too believed in its reality at this time. However, he had no particular interest in maintaining such a view. For his second hypothesis regarding uranium's residual activity involved the attribution of these alpha rays to 'a second distinct type of matter'. Soddy tells us it was Rutherford's idea that the Curies' polonium 'fulfils in almost all respects the functions of this hypothetical constituent'. He expected that the known slow decay of its activity would be observable 'after, but not before, its separation from the uranium producing it'.152 One might represent the suggested process as Ur

Neither of these crucial terms appeared in the second joint communication which contained the core of that theory. At the end of April 1902, when this paper together with Soddy's on uranium were mailed, Rutherford caused Crookes to become the first scientist outside McGill to see those 157 expressions on paper. Rutherford began his letter by thanking Crookes for copies of the latter's two Royal Society papers, which were probably those on 'Radio-activity and the electron theory'158 and on 'The Stratifications of Hydrogen'.159 In his discussion of vacuum-tube phenomena Crookes expressed the views that what he had once called 'Radiant Matter' now passed as 'electrons' or 'atoms of electricity' which were the same as Kelvin's 'satellites' or J.J.Thomson's 'corpuscles', that only a few of these were attached to each 'material nucleus or atom of matter' to constitute a chemical ion, but that there was nevertheless a'protyle' basis to matter.160 Perhaps these and earlier conjectures played a part in persuading Rutherford that Crookes was a suitable recipient of the request 'to facilitate the publication of the paper if difficulties arise over "atomic views". The fact that publication was achieved does not imply that Crookes accepted Rutherford's conclusions, as will be seen. In his letter Rutherford briefly summarised the equilibrium view of thorium's activity, noted the similarity of uranium and radium to thorium in this respect, and explained in a well-known passage that: All these processes are independent of chemical & physical conditions & we are driven to the conclusion that the whole process is sub-atomic. Although of course it is not advisable to put the case too bluntly to a Chemical Society, I believe that in the radioactive elements we have a process of disintegration or transmutation steadily going on which is the source of the energy dissipated in radioactivity.161 These words thus supplement the published 'General Theoretical Considerations' regarding 'sub-atomic chemical change'162 where the authors were doubtless also constrained by the attack which the Curies had recently launched upon Becquerel. Rutherford's usage of the expression 221 'disintegration or transmutation' may perhaps be interpreted on the physical side by comparing it with the respective alternatives of emission or rearrangement of the electrons comprising the corpuscular atom.163 On the chemical side the phrase seems to fit Rutherford's and Soddy's published statements that thorium might undergo the subatomic version either of a chemical 'decomposition' or a idepolymerisations to produce respectively either two or one non-thorium radiating substances.164 From the advanced position which they held in April 1902 Rutherford and Soddy could see novel problems and fresh ways of solving them. The difficult question of residual activity, which had led to the idea of 'decomposition' into two products, was soon to be given a definitive answer. Attained apparently by the head and not the hand the revised transformation theory which they put forward was both a solution and a synthesis. In November 1902 the second of Rutherford's and Soddy's joint papers, slightly altered in structure, appeared for the first time in a leading journal of physics. To their account of 'The Cause and Nature of Radioactivity. Part II'165 was appended a brief section which contained a far-reaching theoretical modification. Trenn166 has argued cogently against Romer that the new view arose within weeks, rather than months, of the submission of the old. One may also note that the revision contains an elementary error; it may therefore have been hurriedly penned. Possibly one of its several consequences initially led them to it. Instead of assuming 'as the simplest explanation' that radiation was 'preceded by chemical change' their new interpretation was that 'Radioactivity may be an accompaniment of the change'.167 The apparently accidental correlation between the radiating and the emanating powers of ThX, which both suffered a geometric decay to half-value in 4 days, now followed logically from the new theory; two pairs of decay- rise curves thus became one. Instead of viewing the decay in Hertzian or vibrational terms, as the loss of excess energy mainly in the form of soft X-rays from a freshly formed product, they now saw the phenomenon in a quite different light. The decline of activity obeyed 'the simple 222 law of chemical change' according to which a single substance is transformed at a rate directly proportional to the amount present. The observed 'decay' was thus essentially a dissipation of the substance itself; a lasting view. On its basis they tried to explain the seeming permanence of the residual activities: 'In the primary change the amount remaining is infinitely great compared with the amount that alters in a short time, and therefore the velocity of reaction is constant'.168 But this account seems unfortunate since the amount of material present in any reaction following that law makes no difference to its proportional rate of disappearance; reasons for the dis- tinctive slowness of the primary change were hard to find. Nevertheless the success of the newer theory is further marked by its consequences that both uranium and thorium should themselves in fact be truly radioactive. Accordingly, Soddy's as yet unsuccessful path to an inactive uranium should not exist, and that to a second 'decomposition' product need not. It would also seem to follow as a probable implication of the accompaniment theory of radioactivity that each radiating material should be in the course of producing a substance different from itself. The statement of Rutherford and Soddy169 that the changes in uranium detectable by radioactivity 'appear to be at an end' with those causing the radiation of UrX is thus an interesting one. For it appears to suggest either the production of an inactive UrY or, since UrX was exceptional in giving for a period of 'many weeks' only the 'cathode rays', a total corpuscular disintegration reminiscent of that proposed by Becquerel. When applied to thorium the assumption that each active element was creating another forced open the gateway to disintegration series of increasing length and complexity. At the same time the theory of induction began to recede. Rutherford's and Soddy's first complete and public rejection of the 'Radioactive Induction' by irradiation or contact accepted by others appeared early in 1903.170 However, the worried note which Soddy wrote to Rutherford in the autumn of that year gives an indication both of their deep involvement 223

with the question of induction and of how difficult it was to disprove its existence. Soddy's discovery that a sealed glass tube of radium emanation produced upon a brass electroscope temporary radioactivity lasting two or three days provoked his opening remark: The new fact, concerning which, or the possibility of which, you were always harping when I was prone to be too positive in the statement of the disintegration theory has I fear arrived.l71 He could evidently see no fault in his own technique and managed to turn the result against a rival, as was his wont, wondering 'how the dickens P.Curie managed to measure the decay of the Ra emanation by this means ... unless the effect is peculiar to brass'.172 Nevertheless, the theory of induced or artificial radioactivity died completely within the next few years to be resurrected decades later in entirely different circumstances. In mid-1902, only a few weeks after he had framed with his partner the new accompaniment version of the transformation theory, Rutherford made an explicit denial of directly induced radioactivity in so far as it related to 'Excited Radioactivity and the Method of its Transmission'.173 It has not been clearly recognised that in this same publication the near full-grown flower of the final modification of the disintegration theory was already evident. Rutherford's investigations of the comparatively long-standing questions of thorium emanation and its active deposit here provided a pictorial account of the disintegration process. Such studies had led during 1899-1902 to a situation in which three related points required explanation, the almost complete concentration of the excited activity upon a negative electrode, the suppression of this effect at low pressures, and new experiments174 which showed that even with powerful electric fields some few percent of the carriers moved in a direction contrary to that of the majority. To recall and to expand upon the developments of those years, Rutherford had relinquished his own early condensation hypothesis for an extended version of an idea of J.J.Thomson. The latter had privately suggested in April 1901 that the loss of an electron left a positive 224 charge whose mean magnitude depended on the speed of neutralisation by negative ions; the pressure-dependent balance between the number and mobility of these ions in turn determined this speed. Rutherford's additional contribution, made by December 1901,175 was to suppose that a recoil at the moment of separation of the electron gave some carriers a sufficient velocity to reach the repelling electrode. This mechanism satisfied all of the above three points. However, at its very time of initiation one sees the beginnings within the scheme of a serious discrepancy. The problem was the lack of a corpuscular type of emission from certain radioactive substances in particular the emanations which theoretically needed it most. Thus in May 1901 Rutherford had published the remark that radium emanation emitted absorbable X-rays (alpha rays) without mentioning any other radiation. Moreover, discussion of the nature of the rays from the two emanations was con- spicuously absent from his later account, dated March 1902, of research performed with Miss Brooks on the 'Comparison of the Radiations from Radioactive Substances' although both substances were actually listed for examination.176 And in June, as an explanation of atmospheric excited activity,177 Rutherford again invoked the hypothesis of 'the expulsion of a negative electron' from some aerial 'carrier' giving as its only justification the fact 'that all the radioactive substances, thorium, radium, and uranium, as well as the excited activity due to thorium and radium, possess the property of spontaneously expelling electrons'.178 But this claim rings rather hollow in view of earlier, contemporary and later events; and it was soon to disintegrate completely. At the end of July 1902 Rutherford finally confessed 'I was at first inclined to suppose that the particle expelled from the emanation was a negative electron' but that 'a close examination' had detected none.179 The gamma rays might have answered here, see below, but presumably he could not detect these either. Still requiring such an emission of negative particles from the emanation he found it in a new interpretation of the alpha radiation. We are told that he had been recently led 'by a mass of indirect evidence', largely 225 concerning absorption characteristics, to the conclusion that alpha rays were not after all soft X-rays. They consisted instead of streams of rapidly moving and as yet undeflected180 particles of atomic size.181 Rutherford noted the earlier suggestion of Strutt, taken up by Crookes in his recent publication, that the alpha rays might be positively charged atoms and mentioned Wien's work on canal rays which also were positively electrified.182 However, he used the recent results of electrolytic studies on radium solutions as support for the adoption of the negative charge which his theory required. The alpha rays were thus negatively charged atoms ejected from radioactive substances. One can observe the considerable contribution of the chemical transformation theory to interpretations of the phenomena of 'Excited Radioactivity and the Method of its Transmission'. For Rutherford now understood that like thorium and ThX the emanation 'consists of matter in an unstable state' which in producing the material excited activity suffered a 'chemical change'. Conversely, a crucial influence of studies of excited activity upon that theory is illustrated by Rutherford's remark that 'The change consists in the expulsion of a negative particle from the neutral molecule'.183 This statement provided a final, permanent and mechanical link between chemical change and radiation within the accompaniment theory of atomic disintegration. How Rutherford succeeded during the autumn of 1902 in demonstrating the characteristic positive electrification of the atomic particles constituting alpha rays instead of the expected negative charge is a well known story.184 His 'Magnetic and Electric Deviation of the Easily Absorbed Rays from Radium' was announced early in 1903. It was closely followed by the extension of these results and ideas towards the important generalisation that in the various series of changes the alpha rays 'are in all cases the first to be produced'.185 It appears, however, that other striking experimental discoveries which arose at this time had a more profound influence than such developments upon the views of contemporary scientists regarding the mysteries of radioactivity. 226 CHAPTER 5

RECEPTIONS, GENERALISATIONS, SPECULATIONS

1. Reception of the disintegration theory (1902-3) The year 1903 was especially important ... Pierre Curie demonstrated the astonishing discharge of heat by this element [radium], which nevertheless remained unaltered in appearance. In England, Ramsay and Soddy announced a great discovery. They proved that radium continually produces helium gas and under conditions that force one to believe in an atomic transformation ... It furnished us the first example of a transformation of atoms.l This later account of Marie Curie hints at the fact that she and Pierre were not easily persuaded of the validity of the disintegration theory of radioactivity and indicates the evidence which to them appeared crucial. But the story of that year is more complex than this record shows. Important and spectacular experimental discoveries were announced in rapid succession: the condensation of the emanations in November 1902, described more fully in 1903, by Rutherford and Soddy; deflection of the alpha rays in February 1903 by Rutherford; the spontaneous emission of heat from radium in March 1903 by P.Curie and A.Laborde; scintillation effect of the radiations in March 1903 by W.Crookes and others independently; and the production of helium from radium, in July 1903. Although the last of these certainly affected the views of the Curies it seems that the previous discoveries and experiments were not without influence. In tracing the development of this period, partly chronologically and in part by taking up parallel threads, we shall see that scientists with different interests received these discoveries and the associated theoretical advances in various ways. The arguments presented by Rutherford and Soddy during 1902 were intricate, contained late appended revisions, and could easily be misunderstood. The new discoveries immediately aroused a tremendous scientific and general interest in radioactivity. This in turn may have hastened the publication of the powerful and lasting notions at 227 which Rutherford and Soddy had arrived before the spring of 1903. Although by that time they had the theoretical side effectively sewn up it was necessary for them to campaign vigorously against misinterpretation and opposition during the rest of that year; this they did in what was more than a mere defence. The Curies were acknowledged authorities on radioactivity. The acceptance of the basis of the disintegration theory by them seems therefore particularly significant. In tracing its course we shall see that this acquiescence involved the almost complete overthrow of their own published views of 1901-2. This occurred despite a tendency, discern- able after their attack upon Becquerel's disintegration theory in January 1902, to keep their theoretical conclusions as general as possible. In the period following that event P.Curie attempted to remedy his proclaimed ignorance of the experimental laws of dissipation of radioactive energy. Moving away from the study of spatial arrangements, which may not have provided regular results, he investigated more closely the effects of time and temperature. Curie began to eliminate the problematical effect of a cooling jacket in absorbing the relevant rays by the ingenious method of determining conductivities produced not in the air but in the surrounding liquid itself. Thus in February he announced, in a paper on 'ConductibilitO des dielectriques sous l'influence des rayons du radium et des rayons de Rantgen',2 that radium rays remained constant over a small temperature range. By November he had extended the study to highly accurate determinations of the rate of dissipation of activity excited by radium. His researches 'Sur la constante de temps caractoristique de la disparition de la radioactivit6 induite par le radium dans une enceinte fermee'3 showed that the decay of the rays emerging from a sealed tube followed closely the equation I = I0 e-t/T with a half-period of 3 days, 23 hrs., 42 mins. He thought that this law's independence of conditions such as the nature of the walls of the vessel and the gas within was sufficiently firm to provide 'La mesure absolue du temps'.4 228 That this also applied whilst the vessel was maintained at widely different temperatures, from -180° with liquid air to +450° in an oven, was a point of great theoretical importance to Curie. For it served both to confirm his own position and to rebut that which he attributed to Rutherford and Soddy. In a paper delivered in January 1903 Curie revealed his views 'Sur la radioactivite induite et sur l'emanation du radium'5 which were based upon further studies of the abnormally rapid dissipation of induced activity from open vessels; this was apparently the only means by which the decay law could be altered. He considered that the radium atom, the unchanging source of radioactive energy, gave no direct rays at all but merely an 'emanation'. Only if this were unable to escape such as in a solid radium compound would it 'se transforme sur place en rayonnement de Becquerel'. Evidently the emanation occupied a position of great importance within Curie's theory. It is notable however that he explicitly rejected Rutherford's usage of the term. Although he adopted the expression 'emanation' as 'commode' Curie restricted the meaning to 'Ponergie radioactive &Ilse par les corps radioactifs sous la forme speciale sous laquelle elle est emmagasinee dans les gaz et dans le vide'.6 Citing Rutherford's and Soddy's revised paper on thorium emanation of November 1902 he remarked that there was insufficient evidence to establish 'l'existance dune emanation de matiere sous sa forme atomique ordinaire', firstly because spectroscopic evidence was lacking, and secondly since the emanation vanished spontaneously from sealed tubes containing it. This last comment is interesting in that it seems to impinge only upon the second version of Rutherford's theory;7 the first had assumed a loss of radiation8 rather than the disappearance of emanation to be the major cause of the observed decay. Whether Curie distinguished between the two versions, both of which were contained in the paper he cited, it is not certain. Curie concluded on experimental grounds that after energy had been released by the radium atoms to their surroundings: dans les gaz l'energie transmise de proche en proche est emmagasinee sous une forme speciale 229 qui so dissipe suivant une loi exponentielle en provoquant la radioactivite des corps materiels.9 Thus he provided an alternative explanation of the results which Rutherford saw in terms of material emanations and active deposits. The observed invariability of the decay law over a wide temperature range allowed Curie a final criticism: Je considere aussi come peu vraisemblable que les effete qui accompagnent l'existence de l'omanation aient leur origine dans une transformation chimique.10 This too was misdirected but again perhaps understandably since the particular paper to which Curie referred concluded with the late theoretical discussion of whether the radiation accompanies or precedes 'chemical change' without mentioning that this was not a normal chemical change.11 Rutherford's reply in an open letter comprising 'Some Remarks on Radioactivity'12 corrected Curie's criticisms. Rutherford pointed out that the chemical change conceived was not 'ordinary' but 'sub-atomic', that he had described both the condensation and the diffusion of the emanation some months before, and that the rays from the emanation were themselves material consisting of 'heavy charged bodies'. It is notable that a condensation of the emanation explains perfectly the single outstanding exception to his own theory which Curie reported. After keeping the sealed tube at the temperature of liquid air he found that the standard rate of decay occurred only after the recovery of the activity from an abnormally low value. Curie vaguely attributed this to the effect of temperature on the walls of the glass vessel. And his description of the remarkable spontaneous rise (for some 30 minutes) of the activity induced by very brief exposure to radium, given in the succeeding publication of February 1903,13 demonstrates the hopeful flexibility of Curie's view. He thought that the explanation of the initial increasing portion of what should be a decay curve might lie 'dans la presence et dans la transformation d'une certain quantite d'emanation'. Rutherford's explanation of this phenomenon had come to be couched in terms which 230 appear similar to those of Curie but which in fact possessed a much more material meaning. But even before the publication of Rutherford's informative reply Pierre Curie had made a dramatic discovery of sufficient force to produce the beginnings of a shift in his own outlook. The path to Curie's revelation of 'La chaleur degagee spontanement par les eels de radium', in a note published jointly with a younger assistant A.Laborde in March 1903,14 was via the latter's apparently successful attempt to detect a mechanical pressure produced by radium radiation.15 These results were instead seen by Pierre Curie as explicable in terms of a small temperature difference, known to affect delicate weighings. This was directly confirmed by means of a sensitive thermometer. And they announced that a radium-barium chloride sample (about 4% Ra) remained permanently at a temperature 1.5 degrees above its surroundings; background variations were but 1/100 degree. The rate of heat production, easily measured both by electrical comparison and calorimetrically, was incredible yet had to be believed. 1 gm. of radium gave 100 calories per hour; or as they significantly put it 1 gm.-atom of radium con- tinously released in each hour as much heat as the combustion of 1 gm.-atom of hydrogen in oxygen. Thus Curie could reason with conviction that 'Le degagement continu d'une telle quantito de chaleur ne pout s'expliquer par une transformation chimique ordinaire'.16 If this was intended to be an additional argument against Rutherford the effect was perhaps the opposite; the points that Curio made were somewhat similar to those already aimed at himself by Rutherford and which were still on their way. If one sought the origin of the heat in 'une transformation interne' this must be more profound than a chemical change and might be due to 'une modification de l'atome de radium lui-mime'. However, since no change in the spectrum of radium was visible each atom must change 'avec une extreme lenteur'. Thus the energy in such a transformation would be 'extraordinairement grande'. His figure was in fact some 60 times greater than Rutherford and Soddy's forthcoming 231 'under-estimate' based upon alpha-ray ionisation measurements.17 Curie failed to make it clear whether he still preferred the alternative hypothesis, which he again mentioned, that 'le radium utilise une energie extOrieure'.18 Within three months he had seen Rutherford's letter, had himself demonstrated the condensation and diffusion of the emanation, 19knew of the scintillation effect of the alpha rays and had accepted their atomic nature. Some of this experimental evidence served to move him theoretically one step further. The concluding remark of his lecture at the Royal Institution2° on 19th June 1903 was that the two competing hypotheses regarding the source of the energy - 'un "element en voie d'evolution' or an unknown external radiation - 'ne sont pas du reste incom- patibles'. Perhaps this was the effect Soddy desired as he wrote to Rutherford regarding their general attack, to be published in May, against the theory of induction: 'I feel it would be unwise to get Curie into a position he was unwilling to go back on, before he has seen all our evidence'.21 Certainly the Curies were not easily moved. For in that June lecture Pierre repeated his own ideas on the transmission of radioactive energy through gases together with some of his reservations on the material nature of the emanation. And Marie, in her D.Sc. thesis on 'Radio-active Substances' which was probably completed in May,22 aimed a blow at the most vital area of the disintegration theory, namely 'ThX'. She asserted that this was no more than the manifestation of an activity induced by thorium upon some inactive substance whose chemical properties might be temporarily or permanently altered to give the results obtained by Soddy; she did not omit these comments from the 1904 edition of her work despite making other important changes. However, the balance of Pierre Curie's newly attained compromise of mid-1903 was soon to be tilted, by the weight of still more experimental evidence, towards the idea of atomic change. F.Giesel, a figure not without influence in the early field of radioactivity, was a representative of those who, 232 even before the striking discoveries of 1903, accepted the existence of atomic disintegration as well as radioactive induction. Perhaps wisely however he offered no coherent theory. The essential part played by the Curies' induction theory in Giesel's chemical controversies over polonium and radiolead during 1901 has been described above.23 The German chemist's concern with the problems of the theoretical side of radioactivity is illustrated in his letter to the Curies of March 1902.24 Having mentioned his new radium-barium bromide fractionation, which was shortly to supersede the chloride method of Marie Curie, he commented upon Pierre's experiments on the transmission of radioactivity through water. Giesel suggested an idea later to be extended by the Curies, that radium might release energy only in the form of a Rutherford emanation which would in turn produce direct radiations as a secondary effect. Regarding induced activities he expressed unease at the possibility that all such manifestations, of which he had seen many, might be due to traces of known active substance; as he admitted, the Curies' demonstration that the activity induced by soluble radium chloride was itself insoluble constituted an exception to this.25 In a paper 'Veber Becquerelstrahlen und die radioaktiven Su.bstanzen' 26 published some months later Giesel advocated a material interpretation of thorium emanation; he compared it to the odour emitted by Musk. It is interesting that he pointed to the characteristic chemical properties of induced radioactivity whilst still expressing uncertainty as to whether this was a deposit of the primary substance. By October 1902 Giesel appears to have combined the Curies' current view that the emitter of radioactive energy was the atom, 'die Arbeitsmaschine' driven by an unknown power, with the prevalent physical theory of electronic dissociation: das Atom dabei nicht bestehen bleiben kann, sondern sich in noch west kleinere Theilchen aufliisen muss, in Ionen (oder Elektronen), welche als Zwillinge mit + und - Elektricitat geladen zur Welt kommen.27 233 He was also inclined favourably toward Rutherford's opinion that since the emanation came from radium itself its study might clarify 'die inneren Vorggnge im Radium- atom'.28 On the other hand his classification of radioactive substances in no way agreed with Rutherford's disintegration theory. Giesel divided these materials into three groups according to their radiation characteristics: intensely active and constant, feeble and constant, and weakly to intensely active with declining radiations.29 His attribution of the activity of the entire third group to inductions by the permanently active elements, and his placement of polonium in this category invoked again the comment of Rutherford that radioactive induction did not exist.30 After this Giesel quietly dropped the idea to work like others within the disintegration theory. These developments high- light the difficulties in understanding induced radioactivity which also troubled others during 1902-3.

J.J.Thomson too expressed an idea of direct induction or self-induction by radiation which was also to be dismissed by Rutherford in this case by straightforward experimental means.31 However, Thomson's suggestion of April 190332 marked not the beginning but the end of a scientific struggle between the two. For more than a year they had differed over the emanations which had played so vital a part in the development of the disintegration theory. Whilst the Curies steadfastly reiterated that the emanations were a special non-material form of energy and spoke against Rutherford's and Soddy's belief that these were a particular kind of matter, J.J.Thomson came to adopt a quite distinct position.33 At first he had accepted Rutherford's idea that thorium emanation was a radioactive material, and had contributed welcome suggestions regarding its gaseous nature and the origin of its acquired positive charge. After some three years of harmony disagreement arose over Elster and Geitel's 234 invocation of a third radioactive emanation as the cause of the temporary activity produced on negatively charged wires in the atmosphere. The tension was almost at its greatest when in May 1902 Thomson wrote to Rutherford of his own experiments on this phenomenon: These results make me doubt whether Eleter and Geitel's induced radioactivity is really due to some rare substance; it seems to me it is probably made from wind and water! C.T.R.Wilson has discovered that freshly fallen rain is radioactive.34 But worse was to come as the criticism tended to expand. Let us briefly follow the tale up to this point and on to its conclusion. Some earlier experiments of Thomson's research students were directed to show what could be done with ordinary materials. For example Wilson announced in 1899 that large uncharged (non-radiating) nuclei could be created in gases by irradiation.35 By the end of 1901 he had demonstrated an apparently 'Spontaneous Ionisation of Gases'36 and J.C.McLennan had written 'On a kind of Radio- activity imparted to certain Salts by Cathode Rays'.37 Thomson's experiments 'On Induced Radio-activity'38 which he described without interpretation in March 1902 appear to continue this trend. Although a negatively charged rod did not become active in a sealed vessel of air, which one can see might be explained by the limited quantity of emanation therein, when the air was continuously irradiated by X-rays with all that implied, then the rod did become active: it is notable that this was entirely contrary to Rutherford's statement of 1899.39 Furthermore, as Thomson reported, chemical substances especially hydrogen peroxide produced large currents when absorbed on paper in a layer around the rod; this may possibly be interpreted as a renewed link between radioactivity and Russell's earlier photographic work. By May 1902 the 'continuation of the experiments' to a related subject showed that 'The Increase in the Electrical Conductivity of Air produced by its passage through Water,40 could be as great as 10 to 12 times the initial value. Such observations led him to compare explicitly 'the "emanation" from radio-active substances' 235 and ordinary air 'put in this highly conducting state' simply by bubbling. Writing to Rutherford on various matters at this time Thomson gave some details of the ionic mechanism which he saw operating here: I think the effect is due to excessively minute drops of water so small that they fall with extreme slowness, & that around each drop there is a layer of ionised gas which is pulled off by the electric field.41 He openly linked this only with 'atmospheric electricity', but the direction of his reasoning is suggested by questions regarding Rutherford's emanations in the same letter: Have you tried whether the emanation is longer lived when it is in a solid or liquid than when in the air; if you let it bubble in very small bubbles through water for a minute will it lose half its radio-activity as it would in air; it seems as if the ability of the emanation to get through a great many layers of paper rather pointed to the conclusion that the emanation does not fade away so rapidly in solids as it does when free.42 Despite the compliment that Rutherford's explanation of radioactivity 'clears up a great deal of obscurity', which would help Thomson's forthcoming book, the latter evidently remained suspicious of the emanation aspect. Besides the above discrepancies and queries it is also possible that he had noticed Rutherford's and Soddy's easy recognition of an atmospheric emanation in their first joint paper43 without the promised44 tests. It seems that Thomson found many of Rutherford's results not unrepeatable but too easily so. He wrote again some days later45 mentioning Wilson's radioactive rain and ascribing Elster and Geitel's 'rare substance' to 'wind and water'. Then into print went Thomson's 'Experiments on Indueed-Radioactivity in Air, and on the Electrical Conductivity produced in Gases when they pass through Water'46 which extended and united the two main aspects of his previous researches on radioactivity mentioned in the title of his paper. Thomson argued squarely both that the existence of a radioactive component in the atmosphere was 'possible' but 'not necessary', and that 'negatively electrified surfaces may become radio-active without the deposition 236 upon them of substances having specific radio-active properties'.47 Let us consider firstly the emanation side of the study. The air which Thomson passed through water in various ways attained an increased electrical conductivity by factors as great as twenty and retained this property for some days; it followed that 'In the air modified by passing through water there must be a continuous production of ions'.48 And he appears to have cast aspersions not only upon the hypothetical atmospheric emanation but also at Rutherford's thorium and radium emanations. Though no-one seems to have known it, Elster and Geitel, the cited targets, had already moved out of range49 before Thomson launched his attack. It therefore fell entirely upon Rutherford. Thomson asserted that although certain extremes of heat and cold destroyed the artificial conductivity an electric field did not: Thus, in this respect, the modified gas resembles a gas mixed with the 'emanation' from thorium. Rutherford has shown that in this case the conductivity is not destroyed by a strong electric field.50 Regarding the second major aspect of the discussion, that of surface induction, he explained the 'induced radioactivity caused by negative electrification' within the modified gas in a manner ironically reminisoent of Elster and Geitel's newly favoured hypothesis. Thomson supposed that positive ions in the gas adhered to the negatively charged wire causing the corpuscles there present to be accelerated into the surrounding gas as 'cathode rays'. Thus was the wire radioactive. He postulated a similar mechanism involving minute water drops surrounded by positive ions to explain the lasting conductivity of the modified air. Some positive ions, for example those from flames, did not produce such effects; but an electrode polarised in solution did so. Evaporation to dryness satisfied Thomson that no active material was present in the water which he had used for spraying and bubbling.51 The inference that all of the emanations and induced activities were not each 'a special kind of matter', as Rutherford and Soddy claimed, was quite clear. The 237 implications left ThX isolated, exposed and highly vulnerable. But Thomson had not expressed the criticism very forcefully in that direction. Perhaps this was fortunate. For by the end of 1902, only six months after the completion of his paper, he had adopted a separation of the atmospheric emanation hypothesis from the transformation theory of radioactivity which he now fully supported. He had evidently not heeded Rutherford's dig that 'although the amount of excited activity ... varies greatly with the weather and amount of wind' its decay law was always the same.53 But perhaps Thomson had come to appreciate the revised discussions of 'The Cause and Nature of Radioactivity'54 before writing his piece on 'Becquerel Rays' for Harpers Monthly Magazine.55 In that article Thomson publicly recognised in the disintegration theory both a solution of the energy problem and an explanation of the chemical manifestations of radioactivity: what is the nature of this energy, and how is it stored? A satisfactory answer to this question has, I think, been given by some quite recent researches made by Professors Rutherford and Soddy in Montrea1.56 Having described the separation of ThX from Th, the recovery and decay of activities, the continuous production of ThX and the equilibrium nature of thorium's constant radiation he concluded: We see now the source from which the energy required to sustain the radiation is derived; the radio-active substance is undergoing a continuous transformation into a state in which it has less energy ... Ordinary thorium is thus steadily being transformed into the active thorium X, while this is continually passing into some inactive form.57 That Thomson envisaged some form of chemical transmutation seems clear. For he suggested that this final inactive substance might be detected in thorium minerals 'by ordinary chemical means'. Most significantly, despite the confusion in the last words of the above extract, Thomson now accepted that the emanations were inert-gas elements58 produced from ThX; and he pointed to the presence of radioactive elements in all helium-bearing minerals. Conversely he also repeated his recent claim regarding atmospheric excited activity 238 that 'this induced radio-activity'59 could be explained otherwise - in terms of ionic clusters and layers. The skepticism of Thomson, thus narrowed, was soon to disappear completely along with the experimental basis of his own conclusions. In a long letter sent to Thomson just after Christmas 1902 Rutherford persuasively described his recent successful condensation of the emanations using liquefied air.60 'The experiment is an extremely simple one to show and works like a charm'; the gas leaving a spiral tube at that temperature 'had not a trace of emanation in it' and, on warming, the emanation 'comes off in a rush - all at once apparently or at any rate within a degree'. Delicately Rutherford phrased his remark that 'Anyone whodoes'nt (sic) believe it is a gas after such an expt. is difficult to convince'.61 Furthermore he had 'proved' that much of Wilson's '"spontaneous" ionization' of air was due to a penetrating radiation from outside the containing vessel, from the walls of the room itself. Thomson presented and may have been influenced by J.C.McLennan's paper on 'Induced Radioactivity Excited in Air at the Foot of Waterfalls'62 also dated December 1902. The latter had written to Rutherford in October63 concerning high voltage experiments at Niagara and others in the laboratory which involved the dropping of water through thorium oxide. McLennan confided that these 'would explain the radioactivity of rain found by Wilson' and 'seem to point against J.J.'s results'; but on repeating Thomson's own experiments, McLennan 'found exactly what he found'. That the conflict was soon resolved for the latter is indicated by his conclusion, published in April 1903, that. 'the consensus of opinion' appeared to be that atmospheric excited radioactivity 'is due to the presence in the atmosphere of some peculiar constituent similar to the emanation from thorium'. By that time in April 1903 when he replied to Rutherford Thomson too had fitted into this category of opinion. He announced, or admitted: I have found a radio-active gas in Cambridge water, or rather in that part of the water 239 which comes from deep wells. Dewar liquefied for me the gas extracted from the water... 64 He made the first open statement to this effect soon afterwards;65 his ionic condensation theory of radioactivity was not heard of again. However, it is not true that he gave up the belief that radioactivity was a property of ordinary matter. Papers on this subject had already begun to appear and the discussion which ensued during the next few years similarly involved both Thomson and Rutherford.66 This provides one reason for saying that Soddy's claim of June 1903, that 'Professor J.J.Thomson and Sir William Crookes have both recently abandoned their former theories in favour of the new hypothesis',67 is something of an oversimplification at least with regard to the former. Besides, in 1898 he had been the first to ascribe uranium's energy source to an internal rearrangement of its atom. It is hard to decide whether Thomson can be said to have retained or revived this conjecture. But it is clear that he now attempted to strengthen it with a rough calculation of the possible magnitude of the available atomic energy: if the radium atom were totally corpuscular and each negative charge of 3.4 x 1010 e.s.u. were 10-8 cm. distant from an equal positive charge then a 1% reduction in the intrinsic energy would suffice to maintain its heat production for 30,000 years.68 To Crookes, however, Soddy's straightforward interpretation fully applies. Crookes too had proposed a theory of radioactivity in 1898 and indeed abandoned it in mid-1903. But, as the placement of the younger chemist's claim in The Times Literary Supplement hints, not before the skeletal explanations of that public figure had been well aired. 240 In the spring of 1903 Soddy sent to Rutherford from London an illuminating letter oontaining his comments on their forthcoming publication: wish you immediately to get into thinking array & to consider it & this letter. Events are moving rapidly here. The announcement of the heat given out by Curie has created quite a furore in the Press, & in yesterday's Times Johnstone Stoney had a letter which I have enclosed. Ramsay told me & from his attitude seemed to think it quite possible.69 Soddy confessed that he was unable to convince Ramsay that the surroundings were not the energy source and in relation to these matters he continued: I mention this to show we are on a flood-tide of interest & I do not want to delay (1) If there is a controversy all our papers should be out. They all predate recent developments ... (2) The fewer 'grand-savants' who make asses of themselves the better for our (personal) relations with them afterwards.70 He also feared that Becquerel, who had made certain claims concerning the deflection of the alpha rays, might come out with a theory 'of his own'; in a way he was right. The events which prompted Soddy's urgent message had begun with the delivery on 16th March of Curie's paper on the heat of radium and continued with Crookes' announcement on the 19th of his discovery of a remarkable scintillating phosphorescence produced by that substance. Crookes attributed this phenomenon to the individual impacts of a: bombardment of the screen by the electrons ([footnote] Radiant matter, satellites, corpuscles, nuclei• whatever they are they at like material masses) hurled off by radium with a velocity of the order of that of light.71 The 'furore in the Press' began with The Times' editorial of 25th March entitled 'The Mystery of Radium' which summarised both Crookes' very beautiful demonstration' of the scintillation effect and the discovery of P.Curie: M.Curie, a French physicist of the highest reputation and attainments, has made a communication to the Academy of Sciences which would have been received with absolute incredulity had it been offered on less unimpeachable authority.72 241 For radium, as they put it, produced heat spontaneously 'without combustion, without chemical change of any kind, and without any change in its molecular structure'.73 The conclusion that radium could 'gather up and convert into heat some ambient form of energy with which we are not yet acquainted' was clarified by Crookes the next day, after Punch had interpreted his discovery in its own terms: 'On Ions'. - Such was the subject of Sir W. CROOKES' most recent lecture. Were they Spanish? Pickled? Boiled or fried...? They were made 'visible'. This was hardly necessary, as in such a case the evidence to the eyes would be less convincing than that to the nose.74 In a letter to The Times of 26th March Crookes explained that the source of ambient energy need not be a mystery. Explicitly reviving his theory of 1898 he again referred to the large store of kinetic energy in the surrounding air and suggested that radium might use the faster molecules in the manner of Maxwell's 'Demons'. Crookes made no mention of the disintegration theory about which Rutherford had written to him a year earlier. Despite the success of the transformation theory in explaining the chemical, radiant and electrical phenomena of radioactivity the supposition that the considerable energy involved was stored within the chemical atoms was the subordinate hypothesis with the least direct supporting evidence. Rutherford and Soddy had turned the observed indestructibility of the ordinary chemical elements to their own theoretical advantage; but this move implied that the atoms of every element contained such a reservoir. The notion of an internal store was evidently at first unacceptable to Crookes but his own answer to 'The Mystery of Radium' was immediately questioned by others in the flurry of correspondence which followed the editorial and his letter in The Times. 'Ignoramus'75 pointedly mentioned the known constant intensity of radium rays in vacuo. On the other hand G.J.Stoney in his opening letter76 not only stated his agreement with Crookes but claimed to have employed just such an explanation in 1893. Stoney's 'Suggestion as to a possible Source of the Energy required for the Life of Bacilli, and as to the Cause of their small Size'77 had been 242 that such organisms might be penetrated by 'swifter moving molecules' and could thus abstract the energy of formation of organic compounds from the surroundings, which would become slightly cooler as a result. The process 'therefore- belongs to the recognized exceptions to the Second Law of Thermodynamics'. In Stoney's opinion the restoration by molecular collisions of radiated energy 'whenever the motion of the electron has transferred energy from the molecule to the aether' was another exception.78 This is the point which seems relevant to radioactive radiation; it indicates how the new energy problem was seen by some in terms of an old but still living enigma. Crookes, Stoney, and 'Ignoramus' were joined by others, some anonymously, as they continued the correspondence into mid-April; the names of Rutherford and Soddy received a mention79 before Crookes withdrew from the argument describing it as unfruitful.80 But perhaps it was not entirely so, for Crookes soon changed his view on the source of the energy. Despite the latter's protest that the fast-moving molecules in a 'vacuum' could well provide radium's energy81 one might say that the comments of 'Ignoramus' among others made their mark. For in his note with Dewar 'On the effect of extreme cold on the emanations of radium' read to the Royal Society during the next month82 Crookes described the use of his new 'spinthariscope',83 whose scintillations he now took to be caused by the impact of 'positive atoms'. His intention was to test the effect of a vacuum on radium, the source of these particles. The 'very good vacuum' obtained by using Dewar's liquid air or hydrogen as condensing agents did not diminish the scintillations nor did the low temperatures thus provided; though the screen lost its fluorescing ability when allowed to become cold. The knowledge that such extreme experimental conditions ought to affect the availability of gaseous kinetic energy may have helped to turn Crookes away from his attribution of radioactive energy to this source. His stated intention of following up 'the important discovery' by Rutherford and Soddy of a condensable emanation from radium salts indicates another likely influence.84 243 In expressing his 'Modern Views on Matter: The Realisation of a Dream',05 some days later, Crookes revealed an important change in his ideas. He now believed that in addition to the process of ultra-atomic dissociation, which he had postulated in 1902, radioactivity involved an atomic transformation. To the historic discovery of radium he credited the coalescence 'into one harmonious whole' of the 'isolated hypotheses' of 'ultra-gaseous' matter, electrons, subatomic particles, X-rays, and 'the emanations from uranium'.86 The twin threads of electrical theories of matter and notions of the complexity of the chemical elements, both of which ran from Davy and Faraday via himself to contemporary studies, he saw thus united. But, apart from the miscon- strued 'emanations from uranium', Crookes' own seemingly attractive joinery appears in its context less than perfect. In the previous year Crookes had believed that the protyle atoms of matter were attended by comparatively few electrons8 7 But now in mid-1903 he considered that 'the electron would be the "protyle" of 1886, whose different groupings cause the Genesis of the Elements'.88 According to what he called 'a Darwinian development of chemical evolution',89 the elements were formed in order of increasing atomic weight presumably correlated with decreasing thermal stability. He stated that radium exhibited 'a spontaneous dissociation' and that its atom 'might be actually suffering a katabolic transformation'.90 Radium, outshining uranium in this respect and being thus the least stable element, ought therefore to have the highest atomic weight. Probably for this reason Crookes chose to adopt the tentative spectroscopically extrapolated estimate by C.Runge and J.Precht91 of 258 ignoring Marie Curie's correct gravimetric result of 225. Crookes' only hint as to the cause of atomic disintegration was that since radium held the position 'next after uranium' in order of original creation and present instability it would be sensitive to 'our terrestrial sources of heat'.92 Crookes may have derived this idea from 0.Lodge whose positive and negative electronic atom of electromagnetic mass ho described. 244 It is one indication of the speed of developments at this time that Lodge had already changed his mind about each of these subjects. In his address similarly entitled 'Modern Views on Matter',93 delivered one week after that of Crookes, Lodge placed reservations both upon the existence of the positive electron94 and upon external influences on atomic disintegration.95 And it is a corresponding sign of rapid movement that he attacked the position, from which Crookes had already shifted, of supposing that air molecules supplied radium's energy. Lodge followed closely the theory of Rutherford and Soddy, probably as revealed in their May publication on 'Radioactive Change'.96 In terms bolder than theirs he proclaimed that 'The transmutation of elements' through 'temporary transitional forms' was a process 'going on before our eyes';97 the loss from an atom of radium with atomic weight 225 of a projected portion with atomic weight 2, which caused it to become the unstable emanation, was 'the main fact of radio-activity'. Such comments mark the decisive end to a period of six months, following his perusal in November 190298 of Rutherford's and Soddy's paper 'On the Cause and Nature of Radioactivity', during which he entertained both alternatives for the energy source. Lodge may also have been influenced by J.J.Thomson whose estimation of the internal energy of an electronic atom he appears to have reproduced.99 These two physicists together with Crookes comprised an important trio of converts from a variety of opinions to the disintegration theory of radioactivity. But the major authority in the field lay with the Curies who had other ideas. Marie Curie's criticism of the chemical evidence for radioactive transformation had appeared in May 1903 and Pierre Curie, his rival induction theory in difficulties, had moved to a position of compromise regarding the energy source by June. Let us again take up their story and its connections. The announcements made by Soddy and Ramsay in July created an impression both deep and wide; upon the Curies the impact was almost conclusive. Soddy now partnered the well-known discoverer of terrestrial helium100 and the 245 inert gas family of elements.101 A preliminary notice reported simply that the 'Gases Occluded by Radium Bromide' contained helium.102 Their subsequent description of 'Experiments in Radio-activity, and the Production of Helium from Radium'103 evidenced a considerable development. By means of low-temperature purification techniques they had succeeded in following the first ever spectroscopically traceable chemical transmutation. Not only had they watched the characteristic yellow line of helium appear, after some days, in a tube of radium emanation but they had at last seen a glimpse of the emanation's own spectrum. Writing to Dewar on 22nd July regarding the publication of joint experiments on the heat of radium Pierre Curie told him that he was '1'ennemi des publications hatives' which also was why Rutherford and Dorn 'ont public avant moi des °hoses quo j'avais faites avant eux'.104 Curie did not include the production of helium from radium, of which he was already informed, among those things. But perhaps he recognised the 'presence simultanee dans certains mineraux de l'uranium, du radium et de l'helium' as such. For in a review of 'Recherches recentes sur la Radioactivito' written during the following months he claimed to have been impressed with this fact 'des le debut de nos recherches'.105 The proposed transmutation suitably confirmed106 sowed the seeds of change which can be seen still preserved in Curie's written words. In the main his review repeated earlier statements for example that the emanation was not 'un gaz materiel ordinaire' but one of the forms of radioactive energy.107 One can see that this could form a gulf between radium and helium. However, in a final section Curie credited the new experiments with 'une importance fonda- mentale'. He accepted that'L'helium pourrait, d'apres cola, etre l'un des produits de la desagregation du radium'.108 At the same time his recognition of the work of Kaufmann seems to have removed the basis of Curie's objection to Becquerel's ballistic hypothesis of the beta rays; Curie now seemed happy to grant the possibility of an electronic-electromagnetic theory of matter.109 His note 246 'Sur la disparition de la radioactivity induite par le 110 radium stir les corps solidest shows that by the spring of 1904 he was actively using the transformation aspect of the theory 'de M.Rutherford' in his quantitative research. Marie Curie was similarly influenced by the helium experiments; she reported these and mentioned the disintegration theory in the 1904 revised edition of her 111 thesis. But her work gives a curiously patchy impression. She largely retained the induction theory, including her view of ThI, though now admitting the emanation as a material gas.112 Whilst she was persuaded that 'transformations atomiques' indeed occurred in radioactivity her final word took the form of a strange though perhaps not unique defence of the unchanging radium atom: Au lieu d'admettre que l'atome de radium se transforme, on pourrait admettre que cet atome est lui-mAme stable, mais qu'il agit sur le milieu qui l'entoure (atones materiels voisins oil other du vide) de mani4re a dormer lieu a des transformations atomiques.113 Marie Curie also appears to have attempted to ensure that she had the last word by asserting that Rutherford had 'franchement adoptee'114 one of her own hypotheses of 1898-9; this she repeated115 in 1906 when there was no alternative to the disintegration theory of Rutherford and Soddy. The aspects of the radium-emanation-helium transformations which thus impressed the Curies also invoked a wider interest, a second wave of publicity. Ramsay was applauded as he made the first public announcement of the creation of helium in mid-July at the Dinner of the Society 116 of Chemical Industry. In the same week The Times117 noted that Sir W.Huggins had found helium lines in 'The Spectrum of the Spontaneous Luminous Radiation of Radium at Ordinary 1118 Temperatures - an apparently independent discovery, though based on hints from Rutherford's recent papers. Huggins seems now to have speculated in a manner reminiscent of the dissociation hypothesis of his old rival Norman Lockyer and privately noted elsewhere that a xenon line 247 was identical with one of the radium spark spectrum lines.119 Unfortunately, however, just as the first of a series of letters on 'Radium and Helium' appeared in The Times Huggins had to admit that radium's luminous spectrum contained not helium but nitrogen lines,120 a discovery notable in itself. 'Verily this is the summit of fame' wrote Soddy as he sent 'a cutting from the current no. of Punch' to Rutherford.121 The relevant extract may have been the interesting disclaimer that: 'RADIUM' wishes it to be distinctly understood that he can throw no light on the present political situation. He adds that there is no affinity between him and TIM HELIUM, M.P. 122 Or perhaps, following Soddy's own popularising article in The Times Literary Supplement on 'Possible Future Appli- cations of Radium',123 the cutting may have been the illustration of a subtle connection between 'SCIENCE AND MATRIMONY' which appeared in the same issue of Punch: He (the accepted one, enthusiastically discussing their projects for the future). 'I think it would be a splendid idea, when we marry, to have the Kitchen fitted with a Radium Cooking Range!!' The Betrothed (who doesn't believe in long engagements, very sweetly). 'Er-ye-es, Darling, but if Radium does not come into use - say, in one month's time from to-day, we won't wait for it, dear, will we?'124 Articles by scientists and others125 concerning the physical, chemical, technological and medical implications of radio-. activity proliferated in non-technical magazines during 1903. The extent of popular interest in the Curies, perhaps not matched in French academic circles, can be judged by Pierre's pained declaration to Ramsay early in 1904: Nous avons etc terriblement deranges dans ces derniers temps par les journalistes, lee gene du monde, les excentriques de toutes les especes male vous connaissez vous-meme ces visiteurs encombrants.l26 According to Soddy's complaint, two months earlier, Ramsay himself enjoyed this kind of action: Ramsay has again been interviewed by the Daily Mail. I can't quite understand it. Sometimes wonder if he forsees the great commercial advantage in the future of being known as the expert on radium, & has done it from this motive or from pure lack of judgement & consideration.127 248 However, the students of radioactivity were evidently most concerned with the professional side of their reputations. This is shown for example by Rutherford's remark that 'a photo of my noble self' in Harper's Magazine gave him 'as much advertisement as is good for me', but that 'These things ... don't count scientifically for it is work that tells'.128 Nevertheless one can argue that events in the broader arena did in certain respects affect those in the smaller. Such an effect may possibily help to explain why feelings about priority ran particularly high during the second half of 1903.129 And a belief in such an influence would certainly account for Soddy's further comment to Rutherford at the end of the year that Ramsay's Press inter- views 'must prejudice our case with the real scientists'.130 The publicity at least ensured that the case was brought to the attention of such persons, but their verdicts were not uniform. Rutherford enjoyed more success in his campaign among the physicists than did Soddy with his fellow chemists. J.Larmor in his capacity as Secretary of the Royal Society had written to Rutherford back in April when the first furore arose: I am glad to hear that you are coming in May: you may be the lion of the season for the newspapers have suddenly become radioactive. I see you again monopolise most of the Phil.Mag. 131 The editors of The Electrician were in favour of the new theory.132 They had commented after 'The extra meeting of the Physical Society, convened last Friday [5th June] at University College to meet Prof.RUTHERFORD', which 'was a crowded and enthusiastic gathering', that: the suggestions put forward by Prof.RUTHERFORD in explanation of some of the mysteries of his subject have special value, and must carry great weight.133 At that gathering Rutherford had received the praises of Lodge and had also answered the doubts repeated by J.D.Everett134 and expressed by S.P.Thompson concerning the source of radium's energy.135 It was the chemist T.M.Lowry who at this physicists' meeting made perhaps the 249 most outspoken objections to the transformation theory. He attacked directly the weakness of its experimental foundations. In return Rutherford pointed to the inadequacy of Lowry's substitute. But the latter's last words on the matter had not yet been heard. Nor had those of his senior associate at the Central College South Kensington, H.E.Armstrong. As a chemist and a founder of the atomic disintegration theory of radioactivity Soddy seems to have experienced a division of his loyalties. Perhaps he still sympathised with those who believed in the indivisible chemical atom as he himself had done but two years earlier: Having failed utterly as I can see to make you realise the width of the gap between our recent work and anything preceding I do not intend to attempt it in this letter ... I must say I sometimes feel however as if I had been a traitor to my own camp and let you ... in by a back door. But for me the chemists' fraternity would have continued to smile hard and long for many a year.136 And as a confrontation with the figure described by his excellent biographer as 'the foremost British chemist of the time'137 loomed near Soddy organised the tactics. Referring to Armstrong's and Lowry's recent publication on radioactivity he wrote to Rutherford: I think they, being chemists, are my fair game & I hope to get an opportunity of asking some questions if they get up on their feet at the B.A. Otherwise I think they are best ignored altogether. So perhaps you will leave them to me if they try to interfere. I shall only engage them under provocation.138 Armstrong, a chemist interested in physical aspects of his subject, was one of the few scientists to set out fully an alternative to the disintegration theory. In a paper with Lowry on 'The phenomena of luminosity and their possible correlation with radio-activity'139 he attempted to explain radioactivity 'from the chemists point of view' and to bring it within the boundaries of his own field. Having outlined the relationships of triboluminescence, floures- cence and phosphorescence to the dynamic equilibria of organic compounds he compared Th and ThX, as 'isodynamic 250 forms of thorium', with the forms of nitrocamphor whose rate of interconversion followed a 'simple logarithmic law'. He asserted that this explanation was 'at least as rational as one which assumes that nature has endowed radium alone of all the elements with incurable suicidal monomania'.140 This bark, which Soddy found noteworthy,141 may derive its bite from Crookes' well-phrased suggestion of a 'fatal quality of atomic dissociation'.142 Armstrong's opinions on radioactivity may be understood, in part at least, by viewing them as an extension or continuation of earlier convictions. Though once a supporter of Lockyer's dissociation hypothesis in its early days,143 Armstrong later took every opportunity to criticise various hypotheses of dissociation. He attacked the electrochemical molecular or ionic dissociation theories of the 1880'9,144 and poured scorn upon the corpuscular atomic dissociation theory of the late 1890' s.145 Similarly, in discussing 'The Conditions determinative of Chemical Change and of Electrical Conduction in Gases, and on the Phenomena of Luminosity' in 1902146 he argued that the occurrence of ordinary reversible oxidation effects made Crookes' explanation of vacuum tube phenomena in terms of radiant matter or electrons unnecessary. On the traditional them© of 'The Classification of the Elements' Armstrong argued along two familiar lines. He believed in the genetic relationship and complexity of the elements but asserted that 'no direct evidence acceptable to chemists has been adduced which in any way justifies the belief that the elements are decomposable'.147 Though Armstrong's expressions might appear very similar to Crookes' current and earlier views the meaning of the term 'decomposable' constituted a point of distinction which radioactivity soon brought to the fore. In addition Armstrong appears to have been the author of a series of anonymous personal cum scientific attacks in Crookes' journal upon Ramsay and his researches on the inert gases.148 One •would expect this to have a bearing upon Armstrong's picture of the radioactive emanations which had been placed within what was for him an ill-conceived family.149 It seems 251 probable also that his view of Ramsay's helium transformation was a jaundiced one, which may account for his continuing resistance to the disintegration theory even after that most impressive demonstration. It was indeed necessary for Soddy to speak against Armstrong at the meeting of the British Association in September 1903 as the former had anticipated. The unusually lengthy discussions which followed Rutherford's paper150 have been seen as marking a turning point. An account of 'How the "Newer Alchemy" Was Received' describes the way in which 'The opposition, brought into the open, was all but demolished by the strength of the demonstrated support for the theory'.151 It is to be noted that Armstrong's subsequent three year absence from public discussions of the sabject152 fits with this interpretation but that Soddy's reference several months later to 'the I expect numerous class of unconvinced chemists'153 apparently does not. Perhaps the members of such a chemical class were able to ignore the electrical results as foreign subject matter, were left unconvinced by the chemical evidence for transformation, and accounted for the most recent demonstrations by R.Meldola's suggestion that radium was in fact an unusual chemical compound containing helium.154 Though F.Richarz, disciple of H.Helmholtz155 and similarly interested in the borders between physics and chemistry, stressed the 'Analogien zwischen Radioactivitgt and dem Verhalten des Ozons'156 such physical interpretations were largely extinct by the end of 1903.157 As for the chemists, The Electrician seems to have portrayed them as a single group and pointed with relish to the disparity between the hypotheses of Meldola and Armstrong.158 There was an air of editorial disappoint- ment that W.C.D.Whetham (at that time reading the proofs for Rutherford's book on Radio-activity) in his reply -Go Meldola merely summarised: the case for the transmutation hypothesis, from the point of view of the physicists. We should have been glad if a physical chemist so well known as Mr. Whetham had given us, rather, a glimpse of the arguments pro and con which arise in the chemical mind... 159 252 Thus physicist baited chemist across the gulf which divided them. Radioactivity which might in theory have formed a bridge of harmony instead served some as a route of attack. But the response grew faint. A few dissenting chemists such as W.Ackroyd of the Halifax Borough Laboratory, who continued to advocate an external source of radioactive energy,160 and the famous Marcelin Berthelot who turned from his hopeful 'Etudes sur to radium' of 1901161 to reservations concerning 'Emanations et radiations' in 1904162 made their voices heard. But they were outnumbered by those such as F.Giesel, W.Marckwald,163 G.Martin, A.Debierne,164 Ramsay, Crookes and Soddy who favoured or employed the disintegration theory. If Soddy's 'unconvinced chemists' formed a majority it was largely a silent one; it may nevertheless have influenced future recruitment and prospects.

2. The mechanism of radioactivity (1903-4) Although Rutherford and Soddy were credited with the discovery of the first chemical evidence of the subatomic nature of radioactive change they held no monopoly of dis cussions concerning the mechanism of the process. This applied both when disintegration was supposed to involve the emission only of corpuscles or electrons and after Rutherford had in 1902 recognised as its most striking 253 feature the high-velocity alpha 'projection of a heavy charged mass from the atom'.165 One can well understand the closing comment of the latter's earliest exposition of the disintegration theory: 'Nothing can yet be stated of the mechanism of the changes involved'.166 We recall that two years earlier he believed that an all-electronic atom contained insufficient energy to support the observed radiation; the newly advocated reduction of mass to elec- tricity167 may not have eased the difficulty. In his first announcement of the magnetic and electric deviation of the alpha rays Rutherford made clear his modified view: There seems to be no doubt that the emission of 3 rays by active substances is a secondary phenomenon, and that the 0( rays play the most prominent part in the changes occurring in radioactive matter.168 His increasing hope of discovering the mechanism of these changes is indicated by the further comment that: The power possessed by the radioactive bodies of apparently spontaneously projecting large masses with enormous velocities supports the view that the atoms of these substances are made up, in part at least, of rapidly rotating or oscillating systems of heavy charged bodies large compared with the electron. The sudden escape of these masses from their orbit may be due either to the action of internal forces or external forces of which we have at present no knowledge.169 From the time of publication of these statements early in 1903 various physical scientists sought to grasp by moans of hypothetical atomic structures the underlying mechanisms and causes of disintegration; this phenomenon was by some emphasised less strongly than other areas of chemical physics. In the case of radioactivity the design of suitably unstable model atoms which would produce the successive elements, appropriate rays and stable end point remained as a recognised problem for decades. The questions of the origins of these atoms and of the causes of their peculiar law and rates of decay similarly stood unanswered. Before discussing the contributions of physicists, who indeed made most of the running with this approach, I shall briefly examine relevant views held by some members of the chemists' fraternity. 254 In his report on 'Inorganic Chemistry' for the year 1904 to the Chemical Society of London P.P.Bedson17° gave pride of place to the latest publication of D.Mendeleef, Professor of Chemistry at St.Petersburg. The propounder of one of the earliest periodic tables had now, some thirty years later, extended the system in An attempt towards A Chemical Conception of the Ether.171 He had added a zero group which included Ramsay's five new inert gases172 and which was headed by the 'ether' as the lightest and most inert element. The noble gases were thus doubly linked with radioactivity, though Mendeleef made no mention of the emanations. For the concluding passages of his little book173 dealt with explanations of radioactivity in terms of the ether. This substance in his opinion possessed an 'individualised attractive capacity, a mean between gravity and chemical affinity'174 which caused its condensation upon the heaviest atoms, those which exhibited the 'photo- radiant' and ether 'emission' phenomena of radioactivity. Similar attractive forces were assumed to explain the known solubilities of the other inert gases in water. Mendeleef's views were not entirely out of the run of current opinion. The idea of a zero group had commonly attended attempts to incorporate the inert elements into a periodic scheme. And he was not alone in proposing the existence of a chemical, material ether. C.F.Brush, for example, announced in 1898 the detection, by thermal conductivity measurements, of 'Etherion: A New Gas' consisting of one or more entire groups of new elements all much lighter than hydrogen and probably filling 'all celestial space'.175 However, its properties were soon ascribed by Crookes176 and Dorn177 to nothing more than water vapour. The leading spectroscopist W.N.Hartley proclaimed in a review of his 'branch of chemical physics' in 1903178 that 'atoms of definite groups of chemically related elements are composed of the same kind of matter in different states of condensation'; and speculated that the 'molecules' of matter in the state of greatest attenuation 'may be imagined to constitute the ether'.179 And Marie Curie's theoretical attempts to 255 preserve the stability of the radium atom in 1904 involved an action of this element upon the 'other du vide' to provide 'transformations atomiques';180 possibly some form of condensation was envisaged here. A reviewer in the Chemical News181 described Mendeleef's work as speculation 'pure and simple' yet put forward somewhat similar ideas himself; and Bedson, looking back to the reception of Mendeleef's earliest periodic system, warned against too dogmatic a critique of the newest one.182 Mendeleef made clear his hopes of creating explanations uniting physics and chemistry183 but he rejected equally strongly the suggestion of any single material or other basis of chemical matter. He considered that he had now provided a superior alternative to the protyle or electron theory having abandoned that development as atrophied more than a decade earlier.184 Mendeleef did 'not require the recognition of a peculiar fourth state beyond the human understanding (Crookes). All mystical, spiritual ideas about ether disappear'.185 He considered that the 'series of recently discovered physico-chemical phenomena', particularly radioactivity, which had prompted his publication and had 'caused many to return to the emission theory of light, or to accept the, to me, vague hypothesis of electrons' were best explained in terms of 'the entrance and egress of ether atoms' and the 'familiar conception of an etherial medium transmitting luminous vibrations, &c.'186 Such a conception owed much to the condensable ether of low density of Kelvin187 whose case will be taken up shortly. Each of these men died in 1907 whilst in opposition to the disintegration theory. The example of Mendeleef may thus seem to be a negative one, the more so since material ethers faded from science during the early decades of the twentieth century. Yet his low opinion of the hypothesis of electrons was shared not only by chemists who like him rejected the closely related disintegration theory but by some who accepted the latter with enthusiasm. W.Ostwald began his Faraday Lecture in the spring of 1904 with an unusual description of that 'venerated master' 256 as a past leader in the theory of force or energy. He ended the address with a discussion of radioactivity which was similarly inclined. Ostwald had fitted the notion of spontaneous transmutation with his well known energetic conceptions of 'Elements and Compounds1188 in which 'Chemical dynamics' had 'made the atomic hypothesis unneccessary' for deducing the laws of chemical combination.189 He therefore depicted the chemical elements in terms of an energy curve in the form of a series of 'stalactites'. 'The elements with the highest combining weight' Ostwald represented by the shortest or rudimentary stalactites on the 'sloping ceiling'; along these a drop of water would run at varying speeds of its own accord. The heavy elements were thereby endowed with the temporary existences required by a theory of stepwise transmutations. He claimed an independent realisation of the idea that their known stability constituted an argument for, rather than against, the presence of a large store of energy within the elements. In his view the attainment of artificial transmutation was prevented only by the practical impossibility of concentrating sufficient power.19° Ostwald's approach gave no clue as to the subtlety of the eventual attainment of this end. Its limitations are exemplified by his description of the complex radiations merely as 'intermediate forms' of energy; and by his attribution of both the radioactive and chemical stability of helium to the same 'exceptionally long stalactite' when a distinction had already been recognised, for example in the case of radium. The importance of energetics for radioactivity cannot be denied on that account since many problems of radioactivity and atomic structure from that time onwards were expressed in such terms.191 However, considerations of energy were never taken as a complete description of any atomic process; not even by Soddy who pointedly refrained from depicting any detailed atomic structure for almost a decade.192 One of the most interesting features of Soddy's view of radioactivity is his attitude towards the corpuscular or electronic theory which became during the period under 257 oonsideration the very foundation of many physicists' understanding of chemical matter. It appears that his more or less continuous skepticism came to a form of fruition in 1904 when it became almost constructive. The clash between Oxford chemist and Cambridge physicist at McGill in mid-1901193 illustrates Soddy's early hostility to the corpuscular theory. It is possible that he had acquired these views whilst at Oxford; one of his referees there was soon to question the validity of his electrical- emanation studies. During the course of the pioneering work with Rutherford the antagonism was suppressed within or absent from Soddy's mind; in their joint publications of 1902 on thorium the emission of corpuscles was cited as evidence in favour of the occurrence of subatomic chemical change. However, the earlier attitude began to reappear in Soddy's lectures and writings of 1903-4 after his return to England. During the course of his dozen lectures on 'Radio-activity'194 Soddy described quite fully both the current cathode and beta ray researches and the theoretical reasoning which led to an electrical explanation of mass or matter. 'How far these calculations possess a real meaning I am not in a position to say' he remarked with the qualification that the electronic hypothesis despite its many 'doubtful points' was yet 'necessary to assist the mind in forming concrete mental pictures of the various relations between matter and electricity'.195 In the ampli- fication of his lectures in book form196 Soddy went a stage farther as he closed similar discussions with a ohemist's claim for independence: It may at once be pointed out that the theory of atomic disintegration, to which, in the succeeding chapters, the study of radio-activity will lead, is independent of the electrical or electronic view of atomic constitution. It postulates no view of atomic structure beyond the original conception of Dalton... 197 He commented that the theoretical dependence of the electrical mass of an atom upon its internal energy served to show: how useless it is to attempt to find numerical relations between the atomic weights of the 258 elements of the Periodic Table. It is notorious that all such efforts have been fruitless, but it is only recently that the reasons for the failure have been indicated.198 Now the Cavendish laboratory had quite recently continued the discussions 'On a General Numerical Connexion between the Atomic Weights'.199 Such acidic remarks200 can therefore be interpreted either as another assertion of independence from the Proutian theme or as an attempt to confound the physicists with their own labours. Against only one aspect of the physical view of radioactivity were Soddy's criticisms effective, but here they were particularly so. From the accepted theory it followed that prior to its sudden disruption an atom of uranium for example enjoyed a long period of quiescence. Physicists had come to realise this via an interesting route, shortly to be discussed. By 1904 they were generally satisfied with the assumption that the period of temporary atomic stability must be explained by a slow progression in the atom's internal corpuscular pattern. Soddy considered such a picture to be totally untenable. His attack was based upon two main considerations. Firstly, a very slow approach towards the point of disintegration should be accompanied by similarly gradual alterations in the physical and chemical properties of an atom. If this were so then 'it should be easy by chemical analysis to separate the homogeneous elements into groups'. But apart from the confused case of the rare-earths there was no evidence of any Each success.201 This argument seems to impinge upon Rutherford's suggestion202 that an outlying electron might be the continuous cause of a disintegration. Against the possibility that any such cause could exist Soddy put forward a startlingly straightforward argument which could not be disregarded: the asymptotic law of radioactive decay could accomodate no stage of a definite timespan. Although 'the average life' of its atoms was a specific feature of each radioelement the law implied that 'some of the atoms break up in the first second' yet others survived almost for ever.203 He himself went beyond 'the original 259 conception of Dalton' to assert that individual differences exhibited by different atoms of the same element were manifestations of an 'extremely rapid motion' of 'the internal parts of an atom':204 the internal movements of the atom must be highly irregular and cannot follow a definite sequence if the law of radio-active change is to hold good. The unstable position appears to be rather the result of a chance collocation of the parts than to be due to the operation of any simple law. An analogy might be drawn from the kinetic theory of gases, in which certain of the molecules are regarded as possessing momentarily much higher and others much lower temperature than the average, and the acting causes are so complex that, although the proportion of the whole at any temperature may possibly be calculated when the total number of systems is exceedingly great, the individual history of any one molecule is quite indefinite. In a radioactive substance a definite fraction of the total assumes a peculiar orientation and disintegrates in each second, but the life of any single atom is quite indefinite. The causes at work appear to be so complex that the results can only at present be described as 'chance' or 'accidental' happenings, in the sense of being impossible to predict.205 One may also quite readily devise kinetic models to imitate aspects of radioactivity for example by depicting an atom as a vessel containing a number of gas molecules; the diffusion or chance escape of a molecule through a minute hole in the vessel would represent one disintegration. A large number of such atoms constituting one radioactive sample would exhibit the required geometrical decay law yet would allow the life of an individual to range unpredictably between zero and infinity. Soddy gave no such crude pictures. But he was of the opinion that the comparison of radioactivity with gas kinetic theory 'is important' in the further respect that: it suggests the question whether all atomic properties are not really average properties, the individual atoms continually passing with great rapidity through phases varying widely among themselves in chemical and physical nature.206 This suggestion was in accord with some contemporary developments in both of the areas specified in the title of Soddy's new post of 'Lecturer in Physical Chemistry and Radio-activity in the University of Glasgow' .207 His 260 conolusions were unaffected by Bragg's notable discovery, 208 announced in late 1904, of the definite and by no means irregular velocities of the alpha particles emitted by 209 210 each element in a decay series. As Soddy had stated, his hypothesis was at least consistent with the known law of decay whereas the current theory of the physicists was not. Though Soddy's criticism of the electrical explanation of radioactivity was indeed accepted by his former physical partner the electronic theory in general did not succumb to his incursions. J.J.Thomson's treatment of thermal and 211 electrical conduction in metals indicates the possible compatibility between kinetic and electronic theories of atomio structure. However when Thomson later defended his notion of radioactive decay it was on quite different grounds. Since the disintegration theory of Soddy involved irregular and unpredictable movements it may appear to provide an uncertain foundation for experimental advance. The physicist E.von Schweidler firmly grasped this nettle with the first formulation of a clear statistical-probabilistic approach. In his paper at the 1905 Congress of Radiology in Liege he showed that the law of geometric decay was deducible from the probability equations for random processes. His 212 major point giber Sohwankungen der radioaktiven Umwandlung' was that as it declined the rate of disintegration should undergo fluctuations according to probability predictions; these, he estimated should lie within the grasp of actual electroscopic detection. Thus in Vienna where Boltzmann (d.1906) had done so much with the statistical theory of gases213 this old approach opened up and contributed to the development of a new experimental path which led quickly back to Rutherford. Soddy's anticipations may have influenced the latter who, however, credited Schweidler with the innovation.214 On the other hand it could be argued that a probabilistic interpretation of radioactive decay pre-dated even the expressions of Soddy. His conclusions may be seen as the ultimate internalisation of the 'kinetic' explanation of 261 radioactivity which had in 1898 been among the earliest to be suggested. One of the turning points in this minor thread came in mid•-1903 at a time when the discrete nature of atomic disintegration was becoming appreciated. The surrounding molecules of gas became, for some, not the primary source of the energy released in radioactivity but the detonators by impact of internal atomic explosions. The statements of Lodge exemplify this stage. At the end of 1902, after reading the latest publications of Rutherford and Soddy, his interpretation of radioactivity apparently included both internal and external factors: in the case of massive molecules their mutual collision or agitation under the influence of ordinary temperature is sufficient to shake away some of the loose electrons, which then fly off tangentially with whatever orbital velocity they may have had: giving rise to phenomena recently discovered under the name of radio-activity..215 He may have been the first to make the valuable suggestion, linking the recent results of Curie and Rutherford, that the heat produced by radium was a result of self-bombardment by its 'massive' alpha-projectiles. Yet in his letter on 'Radium Emission' 216 which contained this point Lodge still put forward the alternatives of an 'assumed necessary stimulus, or external supply of molecular energy'. Thomson's 217 article published one month later, at the end of April 1903, shows that despite his specification of an internal energy supply he too favoured the notion of a kinetic stimulus: Suppose that the atoms of a gas X become unstable when they possess an amount of kinetic energy 100 times, say, the average kinetic energy of the atoms at the temperature of the room. There would, according to the Maxwell-Boltzmann law of distribution, always be a few atoms in the gas possessing this amount of kinetic energy; these would by hypothesis break up; if in doing so they gave out a large amount of energy in the form of Becquerel radiation, the gas would be radio-active, and would continue to be so until all its atoms had passed through the phase in which they possessed enough energy to make them unstable... 218 A similar 'law of distribution' if applicable to the non- gaseous radium atoms would account for their passage too 'into some other configuration'. A response to these views 262 was provided by Lodge, whose brief 'Note on the probable occasional instability of all matter'219 directly followed his acclamation of Rutherford's exposition of the disintegration theory at the London Physical Society in June.220 Considerations of radioactivity appear to have changed Lodge's view concerning Larmor's well-known solution to the problem of the theoretical loss of energy from any atom containing orbiting electrons. This development in return allowed the electron theory to impinge strongly upon radioactivity at a point where it had been unconsciously held at bay. In previous months Lodge had supposed that incessant radiation exchanges made unneccessary Larmor's proposal of a zero vector sum of the electrons' accelerations;221 the process of radioactivity was quite distinct and involved the release of orbiting electrons by molecular collisions.222 But Lodge was now prepared to insist upon the importance for radioactivity of the 'radiation or loss of energy' which 'must occur from every atom'.223 Calculation showed that an electron suffering this loss would move inwards at increasing speed until, as its velocity approached that of light, the mass 'becomes suddenly infinite or very great'. It was this effect which in his opinion constituted the likely cause of the breaking up of an atom. Like Thomson he understood that it was 'only a question of time how long an atom shall last before it reaches this stage'. The significant dep- arture by Lodge is to be found in his concluding comment directed specifically at Thomson that 'the slight constant radiation-loss seems competent to bring about instability and decay irrespective of collisions, and therefore independently of any Maxwell-Boltzmann law'.224 Lodge's discussion had an influence not so much for his explanation of disintegration by increased electronic mass, for which there were alternatives, but for two more general proposals. Apparently following one of these, Thomson indeed dispensed with the probabilistic analysis of radioactive decay. Perhaps gladly so; even Soddy's counterblast implied that the statistical method was no more than a temporary and sometimes unavoidable substitute for definite knowledge. Nevertheless, 263 considerations of that kind had formed a continuous theme in radioactive studios from almost their beginning. Lodge's most influential proposal was to the effect that a steady radiation-loss, understandable in terms of standard electromagnetic theory, was the prelude to disintegration. For a time it raised hopes of picturing and predicting changes within the chemical atom, which at that time were not high. It appears that the approach initially provided by the electron theory towards a complete account of spectral patterns was offset by the growing complications of magnetic studies. For those who attempted to design electronic models of the atom there was also the fundamental difficulty concerning the nature of the positive charge. J.H.Jeans was one of those who included shells of the undetectable positive electrons in the hypothetical model he constructed to explain the above phenomena and others, though not radioactivity. His appeal of 1901 that this theory of 'The Mechanism of Radiation'225 be not even 'judged as an attempt to attain to ultimate truth' has been described as a typical disclaimer of the period.226 Though radioactivity placed additional demands upon the proponents of atomic models its exciting new facts were attended by experimental certainty. In conjunction with the theoretical points made by Lodge in 1903 these results perhaps contributed to the optimism of some expressions which appeared in 1904. H.Nagaoka indeed hoped that 'The rough calculation and rather unpolished exposition' relating to the 'Kinetics of a System of Particles illustrating the Line and the Band Spectrum and the Phenomena of Radioactivity'227 'may serve as a hint to a more complete solution of atomic structure'.228 The Electrician's editor heaped an unwonted amount of praise upon Thomson's 'Structure of the Atom'.229 The task of 'handling mathematically the swarm of flying electrons' constituting the material atom was 'obviously a formidable one'. Yet Thomson had 'made a huge stride towards the goal'.230 He had developed 'lucidly and with great perfection, a wonderful theory of the chemical elements', an account of the 'main laws of the line spectra of a series of elements' and finally 'the most suggestive conception yet offered of 264 the mechanism of the radio-active elements'.231 Yet in the same breath Thomson's electronic arrangement was described as 'an apparently highly artificial conception'. Rutherford, who was impressed by these models, did not long maintain his current confidence that he knew the 'probable ... primary cause' of atomic disintegration.232 In fact deficiencies in the various systems were not hard to find. One may discern a subsequent withdrawal to more reserved attitudes as some of the contemporary criticisms found their mark. Between the time of his death in 1907 and his earliest published comments on the disintegration theory in late 1903 Kelvin's interpretation of radioactivity fluctuated considerably. One might say that his ideas moved in an ellip- tical path since they appear similar at each of these dates. Kelvin's 'Contribution to Discussion on the Nature of the Emanations from Radium',233 read by Lodge at the British Association meeting of 1903, has sufficient faults possibly to have embarrassed the observer. He was the only distinguished physicist to reject, with Armstrong and Lowry, the theory of Rutherford and Soddy.234 Kelvin's individual views were that the gamma radiation was 'merely vapour of radium', that the alpha rays were atoms of radium or molecules of radium bromide which apparently also comprised the emanation, and that the experiments which purported to show a loss of weight from active materials235 were acceptable. He attributed the extreme activity of radium to its possession of an abnormally large quota of 'electrions' neutralising the positive atom. 'But' he noted 'this leaves THE mystery of radium untouched'. Kelvin's atoms provided no obvious energy source: he considered it 'utterly impossible' that the known emission of heat 'can come from a store of energy' in the radium.236 He wrote in similar vein to J.Dewar237 and W.Ramsay238 describing as 'utterly improbable' the hypothesis of 'evolution in the atom or transformation of its substance'. Instead it was 'absolutely certain' that 'energy must somehow be supplied from without' possibly by means of ether waves.239 A.S.Eve has noted240 that Kelvin courageously 'abandoned 265 his theory publicly at the 1904 British Association' to 'fall in line with Rutherford's ideas'. But though true this was not the whole, nor the end, of that story. Kelvin indeed constructed his 'Plan of a Combination of Atoms having the Properties of Polonium or Radium':241 (1)To store a large finite amount of energy in a combination having very narrow stability. (2)To expend this energy in shooting off with very great velocity, vitreously and resinously electrified particles.242 But when interpreted in the light of the atomic theory described in his paper 'Aepinus Atomized'243 the conversion seems marred by a heresy, or contradiction. Kelvin's model atoms of 1901 had consisted of ponderable but interpenetrable spheres244 of various sizes, positive electricity distributed uniformly within. One or more potentially mobile but normally static negative 'electrions' occupied each sphere. However, out of line with the unifying ideal of the electron theory, he felt that one could not assume that electrical forces alone operated between atoms: we must keep ourselves free to add a repulsion or attraction according to any law of force, that we may find convenient for the explanation of electric, elastic, and chemical properties of matter.245 Thus Kelvin's atoms were distinct from each other in several respects namely, size; 'quantum' of positive charge and whether or not completely neutralisable by electrions; and finally 'it is possible that the differences of quality are to be wholly explained in merely Boscovichian fashion by differences in the laws of force between atoms'.246 In 1901 Kelvin appears to have allotted one distinct atom to each chemical element, certainly at least for 0,N,H,C1,C,S and Na;247 and in 1904 he indicated his retention of these earlier notions.248 But his plan for polonium contained no less than sixteen atoms, and that for radium two of different sizes. Kelvin's statement that these substances differed from 'ordinary matter' only in the high degree of their 'shooting'249 thus calls into question his conception of a chemical element. 266 This inconsistency may possibly have been a cause of the reactionary trend to be seen in his succeeding statements. Thus his 'Plan of an Atom to be capable of Storing an Electrion with Enormous Energy for Radio-activity'250 of 1905 involved considerations of thelwork-curve' within a single atom only, albeit of a different, onion-skin, design. And after opening a public dispute in 1906 concerning among other points the manner in which 'radium' could be said to contain helium251 Kelvin reverted to views similar to those he held in 1903. His final statements that the energy was drawn from external heat and that heating effects were mainly produced not by alpha particles, which were charged radium atoms, but by emitted electrions252 isolate Kelvin from current research. Despite the evident flexibility of his approach Kelvin was never quite able to countenance the transmutation of atoms.253 Whilst Kelvin directed his theoretical considerations specifically upon radioactivity two other physicists incorporated the subject instead as a more or less important secondary feature of atomic structures which were designed primarily to explain other phenomena. I propose to discuss a system whose small oscillations accord qualitatively with the regularity observed in the spectra of different elements and by which the influence of the magnetic field on band- and line-spectra is easily explicable. The system here considered is quasi-stable, and will at the same time serve to illustrate a dynamical analogy of radioactivity, showing that the singular property is markedly inherent in elements with high atomic weights.254 With these words H.Nagaoka introduced what he described as a new version of an old story. He took a single 'positively charged particle' surrounded by a revolving circular ring of equally spaced electrons to be perhaps 'the most easily conceivable' system for mathematical treatment; actual chemical atoms would possess a number of concentric rings one corresponding to each of the different spectral series exhibited.255 Nagaoka saw a connection between this spectral hypothesis and radioactivity in the example of radium. Since its spectrum appeared simpler than those of elements 267 of comparable or oven lesser atomic weights then the radium electrons must be arranged in fewer and therefore larger rings. His dynamical analysis indicated that the more electrons there were in a ring the greater its susceptibility to the disturbances which might lead to disruption. Further- more, as he noted, elements of high atomic weight were most likely to contain 'massive rings' and consequently to exhibit radioactivity, the manifestation of instability. This reasoning which evidently explains the relative activities of uranium and radium may appear promising. However, the spectral correlations, electrical neutrality, and mechanical stablility of the structure were all put in doubt by the close questioning of G.A.Schott who claimed to have rejected such a system as both unstable and 'not worthy of publication' some five years earlier.256 He had deduced that the theoretical vibrational instability attributed by Nagaoka to large rings in fact extended much farther and applied to almost all rings bar a handful of the smallest variety. In any case it seems that Nagaoka came close to refuting himself in mentioning Sir Oliver Lodge and the different problem of the radiation loss from an orbiting electrical charge. For he neither followed Lodge in considering this as a cause of instability nor did he attempt to neutralise the difficulty in the vectorial manner of Larmor and Thomson. He merely stated that the loss from a 'Saturnian system' should be 'properly compensated' but did not say how this might be arranged. Other difficulties of Nagaoka's system relating to radioactivity were less readily avoidable than this. The 'disintegration of the ideal atom', he thought, involved the breakage of a ring when its electrons 'will disperse in various directions with great velocities, and the positively charged particle at the centre will also fly off'.257 His failure to indicate whether or how changes in the central particle could explain the release of two heavy positive particles, for example from radium, may relate to the current problem of understanding the nature of the positive charge. On the other hand there appears to be a quite 268 definite conflict with the known fact that alpha and beta rays were emitted separately; a solution would seem hard to find. In considering the stages leading to disintegration Nagaoka assumed that a ring was subject to 'resonance' which 'in course of time, if the disturbance be persistent, will acquire such an amplitude as to break the ring'.258 He was prepared to name as initiators of the resonance both vibrations of other rings within the atom and incident electromagnetic waves. But his discussion regarding such an external cause seems only to weaken the case. He argued indeed that since the destructive higher harmonics could be excited by light of short wavelength it followed that 'actino-electric action259 may be the result of the destruction of atoms' under the combined influence of an electric field and incident radiation; semiconduction effects, known also to be produced by these forces, might be similarly explained. Nagaoka's optimistic attempts to reunite dis- parate phenomena in this way were short lived, thanks largely to Schott. Yet they bore some resemblance to the better- appreciated efforts of Schott's former professor. J.J.Thomson too placed radioactivity in an important supporting role in a plan of atomic structure. Issued in late 1903, his paper on 'The Magnetic Properties of Systems 260 of Corpuscles describing Circular Orbits' contains a partial defence of such systems by examining: problems ... met with when we attempt to develop the theory that the atoms of the chemical elements are built up of large numbers of negatively electrified corpuscles revolving around the centre of a sphere filled with uniform positive electrification.261 Regarding this theory he confided to Lodge some months later that: I have ... always tried to keep the physical conception of the positive electricity in the background because I have always had hopes (not yet realised) of being able to do without positive electrification as a separate entity, and to replace it by some property of the corpuscles.262 This attitude allowed Thomson to go beyond the ponderable positive atom adopted by Kelvin during 1902-4 which was superficially similar to his own. Thomson was thus able to 269 explain physical phenomena, including radioactive transmutations, in almost exclusively electronic terms; the positive charge was a more follower of the encampment of corpuscles. The first point which Thomson considered in his paper on magnetic properties was the problem of radiation loss. His analysis showed that when corpuscular velocities were email compared with the speed of light the radiation diminished very rapidly as the number of particles increased. For example, 6 corpuscles rotating at 1/100 the velocity of light emitted elliptically polarized radiation at only 10-16 of the intensity for a single corpuscle.263 Rather than claim permanence for the arrangement, as he might,264 Thomson instead used the very small theoretical lose in a remarkable attempt to solve two further problems of the system which at the same time united all three. He confirmed W.Voigt's deduction that the magnetic properties of corpuscles set out in rings also tended to nullify each other; but in this case cancellation was complete. The hypothetical system failed to display the known magnetic properties. The master- stroke which might have succeeded was Thomson's proof that this conclusion did not apply if the system were losing energy:265 such a dissipation he related to radioactivity: suppose the atoms of a substance, like the atoms of radio-active substances, were continually emitting corpuscles; the velocity of projection ... being, however, insufficient to carry them clear of the atom ... then, if the motion of the corpuscles were not accompanied by dissipation of energy, the corpuscles would not endow the body with either magnetic or diamagnetic properties; if, however, the energy of the corpuscles was dissipated during their motion outside the atom, so that they ultimately fell with but little energy into the atom, a system consisting of such atoms would be paramagnetic.266 He suggested that if the 'energy of projection were derived from the internal energy of the atom' then experiment should reveal a higher temperature within iron than brass. Such results were never reported; corpuscular theory, magnetism and radioactivity were not to be connected in this way. Thomson waited for the major sequel 'On the Structure 270 of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the results to the Theory of Atomic Structure'267 to make clear the link which he envisaged between a tiny continuous radiation loss and the projection of particles from the atom. In this paper he gave explanations of the periodic table, chemical valency and affinity including the inert gases, and spectral formulae. He again compared the basic structure with Mayer's magnets268 as in 1883 and 1897; the rotations imparted to the latest models tended to stabilise movements from the plane. And this was the clue to Thomson's view of the 'Constitution of the Atom of a Radioactive Element'.269 Standard dynamical analyses showed that the speed of rotation could be critical for such stability. With 4 corpuscles, for example, the planar arrangement would be more stable than the tetrahedral only if the angular velocity exceeded a definite value depending on corpuscular charge, mass and number, and the atomic radius; below this value the stabilities would be reversed. And at this value 'there will be what is equivalent to an explosion of corpuscles'. The increased kinetic energy 'might be sufficient to carry the system out of the atom, and we should have, as in the case of radium, a part of the atom shot off'. The approach of a long-lived atom to this value was provided by the radiation loss which allowed velocities 'slowly - very slowly' to diminish.270 'I think a spinning top is a good illustration of the radium atom' he told Rutherford.271 Thomson did not mention Lodge's more extreme alternative. On the basis of the former's suggestions the approach of a corpuscle's velocity close to that of light appears unneccessary. However, the emission from radium of beta rays travelling at 90% of this speed was well known.272 Rutherford continued to follow the electronic tradition and cited the discussions of Lodge and Thomson on several occasions. The high velocities and independent emission of the beta rays may have been reasons for his leaning towards the hypothesis of Lodge during 1904. Of late Larmor's 271 'lion of the season' Rutherford was now leader by far of the field in his incorporation of these and other facts, many of his own making, into a coherent theory of radioactivity. His attempts on this basis to depict the constitution of the chemical atom suffered from the same fundamental problems as those of his fellows. Yet to the experimental scientist such diseases were fortunately not fatal.

3. Conclusion Early in 1904 Rutherford saw the radium series extending to at least273 seven members. The succession of changes, with its radiations and half-lives, proceeded from Radium (alpha, 1500 years274) to Radium Emanation (alpha, 4 days), followed by 'Emanation X' (alpha, 3 mine., soon afterwards called radium A), 'Second change' (no rays, 36 mins.), 'Third change' (alpha, beta and gamma, 28 mins.), 'Fourth change' (alpha, beta, 200 years) and 'Final product'.2 75 Upon a theory framed only two years earlier was this new and increasing knowledge founded. As might be expected Rutherford was not content to rest upon this base but attempted to move towards a deeper synthesis. He argued that the quite different six-membered thorium series and 272 uranium trio bore a resemblance to the above radium sequence in one important respect: The and probably also the rays of the three radio-elements thus only appear in the last of the series of radio-active changes. It is remarkable that the last change, which is readily detected by the radio-active property, should in each of the three radio-elements be accompanied by the expulsion of a single electron with great velocity... 276 Rutherford's subsequent Bakerian Lecture of mid-1904 contains a description of 'The Succession of Changes in Radioactive Bodies'277 which was the most detailed physical represen- tation of the process of disintegration yet to appear; its expression can be seen as a peak of confidence. The 'single electron' of the 'last change' had become the mechanical key which released the apparently more important 'groups of electrons' or alpha particles. According to Rutherford events took the following course: It may, perhaps, be supposed that occasionally one of the outlying revolving electrons, comprising the radio-atom, lapses into a position which results in a slow loss of energy ... in the form of radiation.278 In the ensuing situation of instability an alpha particle would fly off 'with its great orbital velocity, but the atom still retains the disturbing cause' so that the required repetition would result. Meanwhile, and here Rutherford acknowledged his debt to Lodge, the electron's velocity would be increasing slowly until 'finally in the last stage a sudden lapse into a new state' ejected another alpha particle together with the rogue electron. Radium C (RaC,half-life 28 mins.) and thorium B (ThB, 55 mina) were the crucial substances concerned here. The residual atom then 'adjusts itself again into a position of more permanent equilibrium'279 corresponding to the longer lived product RaD (40 years). That this latter material and UrX emitted only beta rays seems not to fit in with this scheme. Nevertheless Rutherford was sure that 'The experimental evidence as a whole points strongly to the conclusion that the change in which the /3 rays appear is far more disruptive in character than any of the preceding ones'. For not only 273 wan the accompanying alpha particle from RaC more penetrating than its predecessors, but recent electrolytic results on ThB could be interpreted as a revelation of, to use an anachronism, fission products. These were 'to be expected' from the 'violent character' of this particular 280 change. Though branching disintegrations and high alpha velocities were later confirmed for both RaC and ThB, and the actinium series seems to have fitted quite well into 281 the scheme by 1905, yet during that year the fairly clear theoretical picture became much obscured. As the expanded second edition of his book shows, Rutherford continued to think it: probable that thep particle, which is finally expelled, may be regarded as the active agent in promoting the disintegration of the radioatom through the successive stages. A discussion of this question will be given with more advantage later (section 270) when the general question of the stability of the atom is under consideration. 282 He did not, however, provide the promised discussion; nor did he again describe this electron as 'outlying'. One reason for this may possibly have been the chemical implications; another may be that such a description begged the question of the cause of the electron's initial or occasional 'lapse'. Hence Rutherford settled for his earlier argument that although the law of decay could not itself make a distinction negative experimental results283 made it likely that rather than any external detonator it was 'forces inherent in the atoms themselves' which brought about their instability. And he repeated his view that: It seems probable that the primary cause of the disintegration of the atom must be looked for in the loss of energy of the atomic system due to electro-magnetic radiation.284 But, as we have seen, Soddy's criticisms involving the law of decay struck an area which no physicist, save perhaps Nagaoka and Kelvin, had covered. To summarise the problem, if the vector sum of accelerations of a system of electrons is zero then the arrangement is stable, as are the atoms of ordinary chemical elements. If the sum is not zero then decay should occur: but not according to the observed law. 274 For this implied that some atoms disintegrate immediately after their formation. To this argument Rutherford responded with an unhappy compromise, outlined in his published Silliman Lectures of 1906,285 before withdrawing to a safer position. He still thought it most probable that radiation loss was the 'primary cause' of disintegration286 but was aware that a steady drain from all radioatoms 'is contrary to the observed law of transformation'. In attempting to avoid the difficulty Rutherford employed for the first time the following argument: We thus arrive at the conclusion that the configuration of the atom which gives rise to a radiation of energy only occurs in a minute fraction of the atoms present at one time, and is probably governed purely by the laws of probability.287 To this statement he appended a suitably revised version of the single-electron theory in which 'one of the electrons may take up a position in the atomic system which leads to a radiation of energy'.288 Unfortunately such a hypothesis harboured unmentioned problems and contradictions. For since some atoms exploded in less than a second this energy loss could neither be slow nor regular, nor could it consistently be described as the primary cause of atomic disintegration. The fundamental question of the processes leading to disintegration was to be explicitly raised on numerous occasions during the succeeding decades, which saw the development of the nuclear atom, the displacement rule and the discovery of artificial transmutation. Even in the 1920's when probabilistic interpretations of atomic structure began to come into their own it remained unanswered.289 By the end of 1906 despite his announcement of a new and suggestive correlation between rate of decay and alpha velocity, which could readily be related to the underlying concept of atomic stability, Rutherford had reached the final retreat: In the absence of any definite knowledge of the causes which lead to the successive disintegrations of the atom, it does not seem possible at the present time to give any adequate explanation of the modes of transformation observed in radioactive matter.290 275 The words with which he concluded that discussion of 'The Velocity and Energy of the."( Particles from Radioactive Substances' are of great interest: A study of radioactive phenomena has emphasized the importance of the a particle as one of the units of which the heavier atoms are built up, and it is not improbable that the < particle may play an equally important role in the constitution of other atoms besides those of uranium, thorium, radium and actinium.291 This statement appears particularly significant when compared with the closing comments of his Silliman Lectures which had appeared shortly before: It appears by no means improbable that the so- called radioactive bodies may differ from ordinary matter mainly in their power of expelling o< particles above this critical speed. Ordinary matter .., might be emitting a particles at a rate comparable with uranium ... and ... may be undergoing slow atomic transformation of a character similar to radium, which would be difficult to detect by our present methods.292 For these thoughts exemplify two aspects of what may be seen as one broad theme which at that time seemed much more than the mere speculation it had earlier been. A third facet, which can be termed the 'cosmical' completes a trio each complex member of which was to suffer a different fate. Each area of this general idea that the phenomenon of radioactivity was possessed of a universal nature had caught the scientific imagination towards the end of the period of our main concern at the time when the disintegration theory was making its initial impact. Early in 1903 Rutherford293 and Soddy cryptically revealed their appreciation of the cosmical relations of radioactivity. These authors were interested to point out that Lockyer's views on Inorganic Evolution agreed with their own on subatomic change. However, they effectively reversed his dissociation hypothesis by noting that 'he regards the temperature as the cause rather than the effect of the process'.294 One striking implication of this was made clear by W.E.Wilson's estimate, using Curie's figures for the heat from radium, that the presence of this element in a proportion of only 3.6 gm. per cubic metre (about 2.5 p.p.m., of the order of that in pitchblende) 'would 276 suffice to supply the entire output' of heat from the sun. His brief letter to Nature on 'Radium and Solar EnergY'295 was followed by those of others, who hastened to take up the exciting corollary of vastly increased astronomical time scales, under headings such as 'Radioactivity and the Age of the Sun'296 and 'Radium and the Geological Age of the Earth',297 as well as to point out difficulties.298 Rutherford, surveying these discussions shortly afterwards, expressed the opinion299 that the time scale of the sun might be 'from 50 to 500 times longer' than Kelvin's estimates based on the energy of gravitational contraction from a dispersed state. And he followed Joly's view that the physicists' assessment of the earth's quiescent life- period might now be stretched sufficiently to fit the minimum of 100 million years required by the biologists and geologists against whom Kelvin had long argued. To geology, radioactivity made a positive contribution via the minerals which had from the first constituted the source of radium. Their composition was both empirically studied and theoretically explained on the basis of the disintegration theory in the search for the parents and inactive descendants of radium, which had begun in earnest by 1904. 300And a successful approach towards the relative and even absolute dating of minerals was one of the 'Cosmical Aspects of Radioactivity' most confidently described by Rutherford in his departing lecture to the Canadian Royal Astronomical Society.301 A.Schuster, introducing his own speculations on 'Cosmical Radio-activity', joined one aspect of the universal theme with another in a statement typical of the period: The fact that every physical property hitherto discovered in one element has always been found to be shared by all suggests the possibility that radio-activity may be a common property of all matter.302 By that time, in the autumn of 1903, Crookes, Kelvin, Lodge, Thomson and others less well known in the field303 had already assumed as much. However, the attitude of some experimental students of radioactivity was more cautious. The Curies in 305 1900304 and Marie Curie again in 1903 admitted their predilection for 'the idea that it was scarcely probable 277 that radio-activity, considered as an atomic property, should belong to a certain kind of matter to the exclusion of all other', but then made the point that observation showed any general activity to be less than 1/100 that of uranium. By the time the later statement was made R.J.Strutt and others had independently directed their more sensitive experimental attention to the 'Radio-activity .of Ordinary Materials'.306 This, Strutt claimed, was small (1/3,000 Ur) and variable but definite. His comment that to give such an effect one part of radium in three hundred million would suffice indicates the drift of his interpretation. Moreover, the emanations and their active deposits were known to be present in the atmosphere so that. a minute surface activity of all solid materials was naturally to be expected.307 But did any of the observed activity in fact belong to the materials themselves? This question was seen to.be of some importance with regard to general support for the electron theory and, perhaps even more so, in relation to the theoretical problem of atomic stability which beoame acute during 1903-4. As Rutherford succinctly remarked: According to the modern views of the constitution of the atom, it is not so much a matter of surprise that some atoms disintegrate as that the atoms of the elements are so permanent as they appear to be.308 J.J.Thomson indeed entertained notions which implied that ordinary chemical atoms were not permanent. We have considered the theoretical analyses of 1903 which induced him to suppose that even the force of magnetism might derive from the atoms' internal energy which would be finally dissipated as heat. And in a paper describing experiments 'On the presence of Radio-Active Matter in ordinary sab- stances',309 read early in the following year, appeared his conclusion that the ordinary material of the walls of a closed vessel emitted, in addition to the effects of the ubiquitous atmospheric emanation, a specific radiation of its own. This too he thought 'involves a continual transformation of the internal energy of the atom into heat',. 310 so also did the normal dissociation and recombination of the gaseous ions by which the radiation was detected. In this way Thomson 278 gave the universal radiation drain implied by his corpuscular atomic theory some experimental substance before the appearance of his theoretical discourse 'On the Structure of the Atom'.311 We recall that during 1902 Thomson had insisted that radioactivity was due to ionic interactions of ordinary materials in opposition to the view of Rutherford and Soddy that special kinds of matter were involved. And in some sense the difference of opinion was maintained. However, N.R.Campbell in whose hands Thomson loft the subject obtained seemingly positive evidence of 'The Radiation from Ordinary Materials',312 though not of the hoped-for heat emission. This turned Rutherford's strong reservations of 1904313 gradually314 to complete acceptance. Thus Rutherford proclaimed in 1906 that Campbell's results afforded: very strong proof that ordinary matter does possess the property of emitting ionizing radiations, and that each element emits radiations differing both in character and intensity.315 Now this evidence did not stand alone. Rutherford had combined it with two independent observations, which were becoming clear during 1903-4, under the familiar 'general principle' that 'every physical property discovered for one element has been found to be shared by others'. The existence firstly of quite rapid 'rayless changes' and secondly of a rather high 'critical velocity' below which any emitted alpha rays would be undetectable316 each implied that continual unseen transformations might be universally proceeding. All this evidently seemed convincing to Rutherford. But the direct evidence soon faded into irregularity317 and, as one can see, the latter two points merely allowed of a possibility. A further side to the universal view of radioactivity, which may be termed the 'Proutian', has been introduced by quoting Rutherford's words of 1906. Rutherford had by 1904 moved towards what he saw as a specifically radioactive ,318 development of 'Prout's hypothesis based on the observed successive release of alpha particles or helium from radioatoms. There was in his opinion 'no reason to suppose' that 279 radium was 'not an element in the ordinarily accepted sense of the term' so that 'the radium atom is built up of parts, one of which, at least, is the atom of helium..319 He was able readily to combine this with the current electronic version of Prout's hypothesis by asserting that the alpha particles were themselves 'groups of electrons'. The radioatom thus consisted of electrons and large groups of elect- 320 rons. W.H.Bragg accepted such ideas as the theoretical basis of his experiments 'On the Absorption of the «Rays'321 and took them farther. In a letter to Rutherford he argued that the alpha particle 'or some submultiple of it' might be a 'common constituent' both of radioactive substances and of ordinary ionisable gases: Then the electrons would be as it were the soldiers of the army, but the o< particles would be the regi- ments. Might not this account for the atomic weights having a leaning to whole numbers?322 Rutherford in turn extended his previous discussion with. the remark that 'many of the elements differ in their atomic weight by four - the atomic weight of helium'.323 Yet his continuing caution that the helium atom was but 'one of the secondary units with which the heavier atoms are built up,-324 is to be noted. So too is his newly circumspect statement that the 'atoms of all bodies are built up, in part at least, of electrons'.325 If he now felt concern about the problem of the positive charge326 then this was soon to be justified. In J.J.Thomson the persuasive force of Prout's hypothesis was manifested most clearly. Its effect on him dates back to the 1880's when Crookes was proclaiming as protyle the sun-element helium, then no more than a gleam in the spectroscopist's eye. And its influence runs through the entire first decade of radioactivity to Thomson's demolition in 1906 of his own all-electronic atom. 'The Number of Corpuscles in an Atom'327 turned out to be a mere one thousandth of the number required to account for its mass. At the opposite end of the discharge tube, whence he had in 1897 first extracted the corpuscular isubstanceq28 280 Thomson believed ho had found an alternative. The deflectable 'Rays of Positive Electricity'329 produced from a variety of different elemental gases were apparently mainly composed of streams of the alpha particles of radioactivity together with hydrogen atoms. Thomson's dualist Proutian interpretation of this particular result was unfortunately most impermanent.330 Yet the idea provides one thread by which to unravel the material upon which Lodge based his judgement: regarding Rutherford's Radio-activity he wrote 'Scarcely anything to be found in this book was known 10 years ago'.331 281 NOTES FOR CHAPTER 1 (pages 8-47)

1 W.C.D.Dampier-Whetham, A History of Science, London, 1929, 382; D.L.Anderson, The Discovery 6f-The Electron, Princeton, 1964, 16; L.Badash, RutherfoW:Tia Boitwood: Letters on Radioactivity, London, 1969, Introduct on, Trig-eTimond Soientific Revolution', p.lf.; M.P.Crosland, The Science of Matter, London, 1971, 32; M.J.Nye, BWIecular Reali15.71Bndon, 1972, p.x. 2 L.Badash, The Early Developments in Radioactivity, with Emphasis on Contributions from the United States, Dies., Yale Univ., 1964, p.xiii, citing Maxwell, Papers, 244. L.Badash, The Completeness of Nineteenth Century Science, Isis, 1972, 63, 48-58, has since expressed a modified -17174. 3 Maxwell, ibid. 4 Page v. 5 Page 303. 6 RI Lib.Sci., (1889), 3, 481-92, 492. 7 W.McGucken, Nineteenth:-Century Spectroscopy, Baltimore and London, 1969; S.G.Brush, The Development of the Kinetic Theory of Gases.VIII.Randomness and Irrevers- ibility, Arch.Hist.Exact Sci., 1974, 12, 1-88. 8 L.P.Williams the of VI7torian Science, Victorian Studies, 1966, 9, 197-204, 198-9. 9 Mme.S.Curie, Les Rayons de Becquerel et le Polonium, Rev.Ggn.des Sal., 1899, 10, 41f., Jan.; Oeuvres, 60-76. 10 Ibid., Oeuvres, 75. 11 RY-Eib.Sci., (1897), 5, 36-49, 30th Apr. 12 This. 787 13 D.M.Knight, Atoms and Elements, London, 1967, ch.3. 14 T.W.RichardsTriaTF-Weights, Chem.N., 1900, 81, 113. 15 W.V.Farrar, Nineteenth-century speculations on the complexity of the chemical elements, Brit.J.Hist.Sci., 1965, 2, 297-323, 303f. Also relevani7-2.11/4/6731,-THe theory of the elements and nucleosynthesis in 19th century, Ch ia, 1964, 9, 181-200, but this has some anachronistic n erpretations. 16 T.H.Levere, Affinity and Matter. Elements of Chemical Philoso 1600-1665, Lon:Eli77971. R.Pox Tscusses this in re a on to The caloric Theory of Gases from Lavoisier to Regnault, London, 1971, ch.4, 6E767-- 17 W.MoGuoken, Nineteenth-Century Spectroscopy, p.xi, lf. 18 Rep.Brit.Ass., 1666, 556-76. 19 IbidT7-561. 20 Ibid. 21 ysia. 22 Genesis of the Elements, RI Lib.Sci., (1887), 3, 403-26; Presidential Addressed to-MliEical Society of London, J.Chem.Soc., Trans., 1888, 53, 487f.; 1889, 55, 256f.; Maugura-Address as President of Institute c77 Electrical Engineers, delivered 15th Jan.1891. Notes for Chapter 1, p.8-47) 282

23 See e.g. E.von Meyer, A Histo of Chemistry, London, 1891, 349-50; also R. K. De kiisyk pecTF-oscopy and the Elements in the Late Nineteenth Century: the Work of Sir William Crookes, Brit.J.Hist.Sci., 1973, 6, 400-23, who discusses disagreements with French speotroscopists. 24 See A.E.Woodruff, William Crookes and the Radiometer, Isis, 1966, 58, 188-98; also G.G.Stokes, Mem.& Correa., 2, 8-408; S.G7Srush (ed.), L.Boltzmann, IdEfEiTtsWITTis Theo , London, 1964, 25. 25 On t e fractionation of yttria, Rep.Brit.Ass., 1886, 586-90; also Genesis of the Elements, loc.cit., 405-16. 26 Genesis of the Elements, loc.cit., 411=27 27 Rep.Brit.Ass., 1886, 586-90. 28 Genesis 63Whe Elements, 421. 29 W.V.Farrar, op.cit., 319. 30 Kelvin, in G.G.Stokes, Papers, 5, p.xxxi; R.T.Glazebrook, Report on Optical Theories, Rep.Brit.Ass., 1885, 157-261, 211. 31 Hereafter the name Kelvin is used, to avoid confusion with other scientists. 32 G.G.Stokes, Pa ers, 4, 373. 33 A.J.Meadows, Sc ence and Controversy. A biography of Sir Norman f..6CITre7717c -gon, 1972, 169. 34 Researches in spectrum analysis etc., Bakerian Lecture, Phil.Trans., 1874, 164, 479-94, 491. 35 =Stokes, Papers,-W7 365-6. 36 See e.g. On a certain reaction of Quinine, J.Chem.Soc., May 1869; Papers, 4, 327-33 37 On the Nature of the Röntgen Rays, Wilde Lecture 1897, Papers, 5, 273. 38 G.G.Stokes, Mem.& Correa., 1, 406-7. 39 W.H.Brock, Lociq'ei-FIETIThe Chemists: the First Dissoci- ation Hypothesis, Ambix, 1969, 16, 81-99. 40 W.McGucken, Nineteenth-Century Wectroscopy, 83-101. 41 A.J.Meadows, Science and Controversy 49tc.,ch.6; see also C.L.Maier, ThITRn6f—gpectroscopy iE7The acceptance of an internally structured atom, 1860-1920, Dies., Univ. Wisconsin, 1964, 186-202 on 'Reactions to TBWEyeris dissociation theory'. 42 W.McGucken, 22.oit., 98; G.D.Liveing and J.Dewar, Collected Papers on Spectroscopy, Cambridge, 1915, 79, 139. 43 Proc.Roy.Soc., 187, 61, 148-209; C.L.Maier, Diss., 202-37, Mrs-Fribes Lockyer's later dissociation theory put forward about this time. 'Enhanced lines' indicate dissociation, but not now into products common to different elements. 44 See above p.18. 45 Quoted in M.W.Travers, A Life of Sir William Ramsay, London, 1956, 100. 46 Ibid., 110, 154. 47 Phil.Mag., 1901, 1, 311-4; C.L.Maier, Dies., 105-6, 192, men ions the ProuTian expressions of tEi.:7-s-pectroscopist J.R.Rydberg about this time. 48 The Ultra-Violet Spectra of the Elements, RI Lib.Sci., (1883), 3, 257-67, 259. 49 Quoted in D.M.Knight, Atoms and Elements, 130. 50 RI Lib.Sci., (1888), 3, 472-5. Notes for Chapter 1, p.8-47) 283

51 W.V.Farrar, Nineteenth-century speculations etc., 317. 52 Proo.Camb.Phil.Soo., 1898, 10, 38-40, 28th Nov. 53 17RanTriiri317WaFEurg, Ueber die specifische Wgrme des Quecksilbergases, Ann.d.PhT, 1876, 157, 353-69. 54 Liveing, 22.cit., E752, 39- O. 55 G.J.Stoney, b?-the Kinetic Theory of Gas, regarded as illustrating Nature, Phil.Mag., 1895, 40, 362-83. 56 See Section 4 below, p.32-6. 57 Liveing, 22.cit., n.54. 58 Ibid. 59 kelvin, Nineteenth Century Clouds over the Dynamical Theory of Heat and Light, RI Lib.Sci., (1900-1), 5, 324-58, 335. 60 J.J.Thomson, RecollectIZEs and Reflections, Toronto, 1937, 341. 61 Lord Rayleigh, The Life of Sir J.J.Thomson, Cambridge, 1942, repr. London, 1969, 62 G.G.Stokes, Wilde Lecture 1897, Papers, 5, 257-8. 63 J.N.Lockyer, On the Chemistry of e otTest Stars, Proc.Roy.Soc., 1897, 61, 148-209. 64 W.H.Brock, Lockyer aria-the Chemists etc., 98-9. 65 A.Schuster, Note on the chemical constitution of the stars, Proc.Roy.Soc., 1897, 61, 209-13. Appended to Lockyer's paper, loc.cit., n.63. 66 A.J.Meeff&TrE; -ecience and Controversy etc., 152-3. 67 J.J.Thomson, TstoTirreciTims- and Reflecil7ns, 341. G.Fitzgerald, 0.Lodge, W.Sutherland, thought the effects purely electrical. 68 Electric Discharge through Gases, RI Lib.Sci., (1894), 4, 282-90. 69 R.F.Schaffner, Nineteenth-Century Aether Theories, Oxford, 1972, 76. 70 On these aspects of Faraday's work, see L.P.Williams, Michael Faraday, London, 1965. 71 Preface to the let ed., 3 ed., 1892, p.viii. 72 M.Faraday, Thoughts of Ray Vibrations, Phil.Mag., 1846, 28, 345. 73 Fe-scribed by K.F.Schaffner, 22.cit.; and E.T.Whittaker, History of the Theories of Aether and Electricity, 2 vols., London, 175111 1, ch.4f. /ST-Eiglyses of the latter are criticised by the former (p.viii). 74 Rep.Brit.Ass., 1885, 157-261. 75 Ibid. 260. 76 Ibid., 261. 77 Dated let Jan. 1885, Correspondence of J.J.Thomson, CUL. 78 Ibid., 1. 79 Phil.Trans.A., 1894, 185, 719-822. 80 Ibid., 719.- 81 Ibid., 806-22, dated 13th Aug. 1894. 82 H.A.Lorentz, La thOorie electromagn6tique de Maxwell et son application aux corps mouvants, Archives Neerlandaises des Sciences Exactes et Naturelles, 1892, 25, 363f.; Pa ers 164-343- 83 .H rosige, Electrodynamics before the Theory of Rela- tivity. 1890-1905, Jap.Stud.Hist.Sci., 1966, 5, 1-49; id., Origins of Lorentz' Theory °lc-Electrons and the Tincept of the Electromagnetic Field, Hist.Stud.Phys.Sci., 1969, 1, 151-209. Notes for Chapter 1, p.8-47) 284

84 R.McCormmach, H.A.Lorentz and the Electromagnetic View of Nature, Isis, 1970, 61, 459-97; id., Einstein, Lorentz, and the-EIT)(3tron Theory, Hist.STEd.Phys.Sci., 1970, 2, 41-87. 85 R.McCormmach, ibid., Isis, 1970, 463-4. 86 Versuch einer THI;7rie7d7gFt elektrischen and optischen Erscheini'ngen in bewegten Kbrpern, Leiden, 1895; Pa errs, 5, 1-137. 87 On the Influence of Magnetism on the Nature of the Light emitted by a Substance, Phil.Mag., 1897, 43, 226-39, section 17. 88 J.J.Thomson, Cathode Rays, RI Lib.Sci., (1897), 5, 36-49, 49. 89 R.Hertz, London, 1893; repr. New York, 1962, 20. 90 Stuttgart. 91 Curie papers, BN, dossier 9, contains her brief notes on the book. 92 CR, 1897, 125, 1165-9. 93 Mi;ude, op.-CIT., 589-90. 94 G.G.Stoki7-Un the Change of Refrangibility of Light, Pa ers, (1852), 3, 259-413, 388-97. 95 Drude, op.cit., p.vi. 96 L.Boltzmann, Vorlesungen liber Maxwell's Theorie der Elektrizitat und des Lichtes, 2 vols., Leipzig, 1891-3. 97 H.Pofncare, FIFitt,ETJE et Optique. Lee theories de Maxwell et la thoorie electromagnetique de la lumiare, 2 vols., Paris, 1890-1. 98 Stokes, Mem.& Corres., 1, 250-1. 99 RutherfoRT Papers, 25. 100 2 vols., Oxford, 1892. 101 Recent Researches in Electricity and Magnetism, Oxford, 1893. 102 Rutherford, Papers, 25, 26, 34, etc. 103 J.J.Thomson, Recent Researches etc., ch.1, 3-5. 104 Ibid., ch.2, 44-6. 105 Ibid., 5; see Section 4, p.38-44, below. 106 London, 1889; 2 ed., 1892. 107 Ibid., 184. 108 Later criticised by P.Duhem, see H.R.Post, Atomism 1900, P sics Education, 1968, 3, 1-13, 6. 109 Pa ers, . 110ee ection 4 below, p.32-3. 111 O.Lodge, Modern Views of Electricity, 1889, ch.10. 112 Ibid., 266-7. 113 TM., 267. 114 rua. 115 Ibid., 301-2. 116 Ibid., 250. 117 172TBrit.Ass., 1891, 574; quoted in J.T.Merz, A History European Scientific Thought in the Nineteenth Ugntu, (1904), repr. New York, 1965, 2, 193. 118 Gam. money, Of the 'Electron' or Atom o? Electricity, Phil.Mag., 1894, 38, 418-20. 119 b.145dge, Modern Views etc., 1889, 250. 120 Trans. Royal Dublin Society, 1891, 4, 585. 121 W.McGucken, Nineteenth-Century Spectroscopy, 110-6. 122 Ibid., 122-6. However, see Section 2 above, T7M-2, on Liveing. Notes for Chapter 1, p.8-47) 285

123 Rep.Brit.Ass., 1874, 22; title only. 124 Phil.Mag. 71881, 11, 381-90; read 16th Feb.1881 Ta-Rokir Dublin Society. 125 Ibid., 385. 126 TurI., 387. 127 W.McGucken, Nineteenth-Century Spectroscopy, 188-9, 202-3. 128 I.e. slowly exchanging energy on collision. 129 Ibid., 376. 130 Ibid., 377. 131 Ibid., 378-9 132 17Tehuster, The Kinetic Theory of Gases, Nature, 1895, 51, 293. 133 H.EberT7 Phil.Mag., 1894, 38, 332-6. 134 G.J.Stoney, Of the 'Electron' etc., Phil.Mag., 1894, 38, 418-20. 135 H.Eberi7 Electrische Schwingungen molecular Gebilde, Ann.d.Phys., 1893, 49, 651-72. 136 U77..Utoney, op.cit.-- 137 Id., Of the KfnliFfc Theory etc., 1895, 378-9 138 He Modern Development of Faraday's Conception of Electricity, J.Chem.Soc., 1881, 39, 277-304. 139 C.A.Russell, he HisiOTY of Valency, Leicester, 1971, ch.l3, 265; see J.R.Partington, Histo of Chemistry, London, 1964, 4, ch.21, for stud es of electrolysis during those years. 140 See above, p.34-6. 141 J.C.Maxwell, Treatise on Electricity and Magnetism, 3 ed., Oxford, 1892, 1, 380. 142 Ibid., 383. 143 Teliholtz, 22.cit., 1881, 302-3. 144 W.C.D.Whetham,17eatise on the Theo of Solution, Cambridge, 1902, provides many re erecens. 145 Rep.Brit.Ass., 1885, 723-72. 146 Ibid., 723. 147 }ep.Brit.Ass., 1894, 482-93. 148 Ibid.-7TO: 149 Hereafter referred to as Kelvin. 150 See R.H.Silliman, William Thomson: Smoke Rings and Nineteenth-Century Atomism, Isis, 1963, 54, 461-73. 151 Atom, Encyclopaedia Britannica, 1875; Papers, 2, 445-84, 473-6. 152 Elasticity viewed as possibly a mode of motion, RI Lib. Sci., (1881), 3, 136-7. 153 Kelvin, On the molecular dynamics of hydrogen gas etc., Papers, (1896), 5, 350-3. 154 London, 1883; repr., London, 1968. 155 Ibid., 1. 'Strength' = mean velocity of rotation x section area. 156 Ibid., 2. 157 Ibid., 109-14- 158 Ibid., 119. 159 Ibid., 108. 160 Cathode Rays, Phil.Mag., 1897, 44, 293-316, 313-4. 161 Shown by W.Mc6EFFen, Mineteenth7Uentury Spectroscopy, 174; and A.Romer, Experimental History etc., Isis, 1942, 34, 150-61, 151. Notes for Chapter 1, p.8-47) 286

162 J.J.Thomson, Treatise on the Motion of Vortex Rings, 1883, 120. 163 J.J.Thomson, On the Chemical Combination of Gases, Phil.Mag., 1884, 18, 231-67. 164 W7073twald, Lehrbuch der Allgemeinen Chemie, 2, 745; see J.J.Thomson, Reply to Prof. Wilhelm Ostwald's criticism on my paper etc., Phil.Mag., 1887, 23, 379-80. 165 J.J.Thomson, Applications of-DYFismics to P iras and Chemistry, London, 1888; from leo res-ae were the Cavendish. D.R.Topper, Commitment to mechanism: J.J.Thomson, the early years, Arch.Hist.Exact Sci., 1971, 7, 393-410, has discussed tharggig6TUf Thomson's work. 166 Phil.Mag., 1883, 15, 427-34. 167 Tura., 428. 168 Ibid., 432. 169 R-MSilliman, William Thomson: Smoke Rings etc., 472; also see above, p.39. 170 J.J.Thomson, On the illustration of the properties of the electric field by means of tubes of electrostatic induction, Phil.Mag., 1891, 31, 149-71. 171 Oxford. 172 Ibid., 3. 173 Ibid., 43. Different ether-motion theories were used 5T-Fthers, e.g. see Heaviside in Schaffner, 208-9; also Section 3 above. 174 Recent Researches etc., 5. 175 Ibid. 176 Ibid., 44. 177 Ibid., 45. 178 777Thomson, Phil.Mag., 1895, 40, 511-44. 179 Ibid., 513. ---- 180 37,(7i-A.Romer, Experimental History etc., 157. 181 See T.Hirosige, Electrodynamics etc.1890-1905, p.18-20; also A.Romer, op.cit., 156-7. 182 On the other hand, as early as May 1897 the physical chemist W.Nernst cited Wiechert's dicovery of subatomic charged particles and discussed its possible application to electrochemistry. G.V.Bykov, Historical Sketch of the Electron Theories of Organic Chemistry, Chymia , 1965, 10, 199-253, 200-1, considers that the app cation of ele-aron theories to chemistry began in 1897; we have seen that there were earlier attempts on these lines. 183 See W.McGucken, Nineteenth-Century Spectroscopy, 211-2; G.E.Owen, The discovery of the electron, Ann.Sci., 1955, 11, 173-82, 177-9. 184 7.J.Thomson, On the cathode rays, Proc.Camb.Phil.Soc., 1897, 9, 243-4. 185 J.J.Thomson, The Röntgen Rays, Nature, 1896, 53, 581-3. 186 W.C.R8ntgen, Ueber eine neue ArT-VaRStrahlen, Sitzungsberichte der physikal.-medicin. Gesellschaft, WUrzburg, 1895, 177=41, 134. 187 J.J.Thomson, op.cit., 1896, 581. 188 J.J.Thomson, TheOntgen Rays, Nature, 1896, 54, 302-6, lecture delivered 10th Jun. 189 Ibid., 304-5. Notes for Chapter 1, p.8-47) 287

190 J.J.Thomson, Longitudinal Electric Waves, and Rtintgen's X Rays, Proc.Camb.Phil.Soc., 1896, 9, 49-61; also E.Rutherford, On tEeTTlectriTICation of Cases Exposed to Röntgen Rays, Phil.Mag., 1897, 43, 241-55, Note by J.J.Thomson, 255. 191 J.J.Thomson, Cathode Rays, Phil.Mag., 1897, 44, 293-316. 192 Ibid., 310. The work of Lenard referred to was probably Te-Be'r die Absorption der Kathodenstrahien, Ann.d.Phys., 1895, 56, 255-75. 193 Thomson, 22.cit., n.191, 312. 194 Ibid., 313=47-- 195 Ibid., 312-3; on evidence of electric strength UrRases. 196 J.J.Thomson, A Treatise on the Motion of Vortex Rings, 1883, 1. 197 W.Kaufmann, Methode zur exacten Bestimmung von Ladung and Geschwindigkeit von Becquerelstrahlen, Phys.Z., 1901, 2, 602-3. J.J.Thomson was himself investigating whetheY. corpuscles 'have masses other than electrical' in 1901: letter to Rutherford dated 15th Feb.1901, A.S.Eve, Rutherford.Etc., 76. 198 A.S.Eve, Rutherford.7T6., Cambridge, 1939, 39; letter dated 30tEM=76. 199 In CUL, Add.mss.7653, and Royal Society Library, London; notebooks for the period 1896-1904, CUL, contain material generally similar to that published. 288 NOTES FOR CHAPTER 2 (pages 48-118)

1 W.C.R6ntgen Ueber eine neue Art von Strahlen, Dec.1895; G.Sarton, The discovery of X-rays, Isis, 1936-7, 26, 349-64. 2 T.41asser, Wilhelm Conrad Rönt en and the Early History of the Röntgen Rqys, Illino s, 377-3M. 3 W7CTRUntgen, op.cit., 139. 4 A.Romer, Acciairif-and Professor Röntgen, Amer.J.Phys., 1959, 27, 275-7. 5 A.H.Becquerel, Recherches sur une propriete nouvelle de la matiere etc., Paris, 1907:- 6 mar, 3. This account is repeated by O.Lodge, Becquerel Memorial Lecture, J.Chem.Soc. 1912, 101, 2005-42, 2032-8; also T.W.CHeIigis, A Liort Hil-517 of Radio- activity, pub. The Engineer, 1951, ; .BeTTianarSur l'origine de la decouverte de la radioactivity, CR, 1946, 223, 698-700, from personal memory support by his oWITTaboratory notes, dates Becquerel's interest in the photographic effects of pitchblende to 1893-4. 7 H.Poincare, Les rayons cathodiques et les rayons Röntgen, Rev.Gen.des.Sci., 1896, 7, 52-9, 56, 30th Jan. 8 S.P.Thompson,-agET7Vrable and Invisible, London, 1897, 260. 9 J.J.Thomson, Longitudinal Electric Waves etc., 1896, 60-1, 27-29th Jan. 10 A.Broca, L'Oeuvre d'Henri Becquerel, Rev.GOn.des Sci., 1908, 19, 803-13. 11 E.N.Harvey, A Histo of Luminescence from the Earliest Times until I9 , fgaelphia, 1957, 390-1. 12 7,7EaTikeWITle and G.F.Kunz, The Action of Radium, R6ntgen Rays and Ultra-violet light on minerals, Chem.N., 1904, 89, 1-6. 13 Tie above Chapter 1, Section 3, n.94. 14 E.N.Harvey, History of Luminescence, 363-4. 15 See above Chapter 1, Sections 1,4. 16 E.N.Harvey, 2.cit., 359. 17 Ibid., 364. 18 TRT-1885, 101, 1252-6. 19 711, 1891, 113, 618-23, 623. 20 VR, 1891, 112, 557-63. 21 TEid., 5637-- 22 17147Harvey, Historyof Luminescence, 364. 23 CR, 1896, 122, -1. A translation of this and three more "Fac BecquereiTs first papers, and others, with commentary, are provided in A.Romer, The Discovery of Radioactivity and Transmutation, New YoT.E7 1964. 24 .C.R7 1696, 122, 662, 695, 790, 791, Mar. Henry became arector 0-The Laboratory of Physiology of Sensations at the Sorbonne in 1897. 25 CR, 1896, 122, 321-4, 10th Feb. 26 Tad., 27 *are-above, p.48-9, n.6. Notes for Chapter 2, p.48-118) 289

28 G.H.Niewenglowski, Sur la propriete qu'ont les radiations emises par les corps phosphorescents de traverser certains corps opaques a la lumiere solaire, et sur lee experiences de M.G.Le Bon sur la lumiere noire, CR, 1896, 122, 385-6, 17th Feb. 29 CR, 1877 122, 420-1, 24th Feb. 30 E7Becquere17Sur les radiations invisibles emises par les corps phosphorescents, CR, 1896, 122, 501-3, 2nd Mar. 31 L.Badash, Becquerel's 'Unexposed' Photographic Plates, Isis, 1966, 57, 267-9, and A.Romer, Discovery of Tia-TOactivity, 9, have stressed the unusual aspects of developing unexposed plates, and the former, of working in the laboratory on a Sunday. It is to be noted however that Becquerel indicated that he worked on Sunday 29th Mar.1896, CR, 1896, 122, 762-7, 30th Mar. 32 H.Becquerel, Seances Soc.Fr.Phys., 1896, 88, 6th Mar., comment by M.de ChardonneT7 33 See e.g. C.Raveau, Les faits nouvellement acquis stir lee rayons de Roentgen, Rev.Gen.des Sci., 1896, 7, 251, 15th Mar. 34 H.Becquerel, Sur quelques proprietes nouvelles des radiations invisibles emises par divers corps phosphorescents, CR, 1896, 122, 559-64, 9th Mar. 35 I.e. Hertzian waves. 36 G.C.Schmidt, Ueber die von den Thorverbindungen and einigen anderen Substanzen ausgehende Strahlung, Ann. d.Phys., 1898, 65, 141-51; Schmidt concluded that Ehorium rays were reflected and refracted. 37 E.Rutherford, Uranium Radiation and the Electrical Conduction Produced by It, Phil.Mag., 1899, 47, 109-63, Jan.; Papers, 170-1; H.Becquerel, Note sur quelques proprietes du rayonnement de l'uranium et des corps radio-actifs, CR, 1899, 128, 771-7, 773, Mar. 38 L.Troost, Sur ITemploi blonde hexagonale artificielle pour remplacer lee ampoules de Crookes, CR, 1896, 122, 564-6, 9th Mar.; id., 694, 23rd Mar. 39 n7BecquereTTSur lee radiations Tvisibles emises par lee sels d'uranium, CR, 1896, 122, 689-94, 23rd Mar. 40 H.Becquerel, Seancesoc.Fr.Phys. 1896, 105, 20th Mar., Ch.-Ed. Guillaume, TEid., On Stokes' Law. 41 Op.cit., 23rd Mar.1896. 42 CR, 1596, 122, 762-7. 43 117Moissan,PF6paration et proprietes de l'uranium, CR, 1896, 122, 1088-93, 18th May; id., Le Four rlectrique, Paris, 1897. 44 CR, 1596, 122, 1086-8, 18th May. 45 Missan, Description d'un nouveau four electrique, CR, 1892, 115, 1031-3. 46 Mir la preparation de l'uranium a haute temperature, CR, 1893, 116, 347. 47 Etude du carbure de l'uranium, CR, 1896, 122, 274-80, 10th Feb. 48 Becquerel, Sur diverses proprietes des rayons uraniques, CR, 1896, 123, 855-8, 23rd Nov. 49 TEld., 8567-- 50 moura la loi de la decharge dans l'air de l'uranium electrise, CR, 1897, 124, 800-3. Notes for Chapter 2, p.48-118) 290

51 CR, 1896, 122, 689-94. 52 CR, 1896, I77, 762-7, 30th Mar. 53 Z7M.StewarT7Experiments on Beoquerel Rays, Physical Review, 1898, 6, 239-51. 54 J.J.Thomson, Tile Rantgen Rays, Nature, 1896, 22, 581-3, 23rd Apr. 55 Rayons cathodiques, rayons X et radiations analogues, Seances Soc.Fr.Phys., 1896, 121, 8th Apr. 56 G.G.Stokes, On the Nature of the Rbntgen Rays, Proc. Camb.Phil.Soc., 1896, 9, 215-6, 9th Nov.; id., Wilde Lecture, 2nd Jul.1897, Papers, 4, 256-77, Mb: the irregular impacts of 'cathode ray' particles produce a series of thin ether pulses, which constitute the X-rays; the irregularity of the sequence of pulses implies that the molecules of the glass of a prism cannot vibrate in harmony, thus the X-rays are not refracted; the thinness of the pulses implies absence of diffraction; both these properties of the pulses imply their penetrating nature. 57 See 0.M.Stewart, Becquerel Rays, A Resume, P sical Review, 1900, 11, 155-75, 175; R.H.Stuewer, W am H. Bragg's Corpuscular Theory of X-Rays and X-Rays, Brit.J.Hist.Sci., 1971, 5, 258-81, on corpuscular X-ray theories. 58 C.Henry, CR, 1896, 122, 312-4, 10th Feb. 59 S.P.Thompson, On Hyperphosphorescence, Phil.Mag., 1896, 42, 103-7, dated 6th Jun. 60 L.Badash, Radioactivity before the Curies, Amer.J.Phys., 1965, 33, 128-35; A.S.Russell, Madame Curie Memorial Lecture, J.Chem.Soc., 1935, 654-63; J.S. and H.G.Thompson, Silvanus Phillips Thompson his Life and Letters, London, 1920, 185-9. 61 G.G.Stokes, Mem.& Correa., 2, 495-6, letter from Thompson dafgY-2Uth Feb.189.b. 62 Ibid., letters from Stokes dated 29th Feb. and 2nd Mar.1896. 63 Op.cit., Phil.Mag., Jul.1897. 64 G.G7gTokes, Mem.& Correa., 2, 498, letter from Stokes dated 28th May. 65 Letter from W.Crookes to S.P.Thompson, dated 2nd Jun. 1896, Imperial College Archives. 66 W.Crookes, Rep.Brit.Ass., 1898, 23. 67 The Evolution of Mater, New York, 1907, 22-3. 68 as Johanniskifferlicht, Ann.d.Phys., 1896, 59, 773-81. 69 H.Muraoka and M.Kasuya, 7ohanniskeerliEHt und die Wirkung der Dampfe von festen und fliissigen Korpern auf photographischen Platten, Ann.d.Phys., 1898, 64, 186-92, received 24th Nov.1897. 70 Ueber Luminescenz, Ann.d.Phys., 1897, 61, 313-29. 71 A.F.McKissick, Becquerel Rays, ElectriETan, 1897, 38, 313. 72 Verh.phyp.Ges.Berlin, 1896, 15, 101. 73 Ann.dka771TeibI., 1897, 21, 366. 74 G.Le Bon, L'uranium, le radium et les emissions metalliques, Rev.Sc., 1900, 13, 548-52; id., The Evolution of Rater, 19-25, T9-28. 75 La radioaciTvite de la matiere et l'energie susceptible de se dovelopper A la surface des corps, Rev.Sc., 1901, 16, 161-70, 167. Notes for Chapter 2, p.48-118) 291

76 See E.Picard, Gustave Le Bon et son Oeuvre, Paris, 1909. 77 D.Martindale, The Nature aig '.hypes of Sociological Theo , London7-1961, 309717; R.A.Nye, The Origins of row Psychology. Gustave Le Bon and thii-Crisis of Mii-Democracy in the ThirZ-Republre,-ranM,97. 78 T.17 Son, La luia-fere n311757 CR, 1896, 122, 188-90, 27th Jan.; the name 'dark rays' had been used earlier by W.de W.Abney as synonymous with infra-red rays, it simply meant rays not visible to the human eye (Spectrum Analysis etc., RI Lib.Sci., (1882), 3, 207-15). 79 CR, 1896, 122, 75767 80 M, 1896, I22, 463-5. 81 M, 1897, T, 857-9. 82 M, 1897, Imo, 984-8. 83 M, 1896, 122, 233, 386, 462, 522, 1057, Feb. to May 1896; 'aimilar notes in Rev.Sc., 1896, Jan. to May, and 1897, Mar. to May; CR, 1697, 124, 755-8, 892-5; Sur lee propriotes de certaines radiations du spectre. Reponse aux objections de M.Becquerel, CR, 1897, 124, 1148-51. 84 CR, 1897, 124, 892-5. 85 Temarques W-firopos d'une Note recente de M.G.Le Bon, CR, 1900, 130, 1072. 86 G.Le Bon, Intra-Atomic Energy, R2p.Smithsonian Inst., 1904, 263-93; id., L'kvolution a la Me.tiere, par s, 1905; id., La Naissance et Logvanouissement de Ia Matiore, Wiris, 190d. On universal radioactivity see MUT"- Chapter 5, Section 3. 87 P.Curie, 22.cit., 1900; repeated by H.Becquerel, Recherches sur une propriete nouvelle etc., 1903, 5-6. 88 M.RutherforZTRaEro-activity, Cambridge, 1904, section 2; 1905, 4-5. 89 R.Colson, Wile des differentes formes de l'energie dans la photographie au travers de corps opaques, CR, 1896, 122, 598-600; id., Action du zinc sur la plaque photographique, CR1-1896, 123, 49-51; id., La Plaque Photo- graphique, Taris, 1897:- 90 R.Colson, 22.cit., CR, 1896, 123, 49-51. 91 W.J.Russell; Proc.ROT.Soc., 1877, 61, 424-33, received 13th May, he cis Colson. For his work in another area see J.R.Brown and J.L.Thornton, William James Russell (1830-1909) and investigations on London fog, Ann.Sci., 1955, 11, 331-6. 92 E.Rutherford, A Radioactive Substance emitted from Thorium Compounds, Phil.Mag., 1900, 49, 1-14; Papers, 226. 93 Russell, p.cit., 1897, 424. 94 Ibid., 425:- --- 95 Ibid., 427. Note the use of the terms 'active' and 'activity'; Marie Curie's use of the expressions in 1898 was thus not the first as has been supposed (A.Romer, Radiochemistry and the Discovery of Isotopes , New York, 1970, 64). C.T.R7WiIain too, Proc7Vamb.Phil. Soc., 1897, 9, 337, wrote of 'active' uranium salTat- 96 W77.Russell, 22.cit., 432-3. 97 Ibid., 433. 98 W.Crookes, Presidential Address, ap.Brit.Ass., 1898, 3-38, 26. 99 C.T.R.Wilson, On the Action of Uranium rays on the Condensation of Water Vapour, Proc.Cainb.Phil.Soc., 1897, 9, 333-8, 25th Oct. Notes for Chapter 2, P.48-118) 292

100 Proc.Camb.Phil.Soc., 1897, 9, 372, 22nd Nov. 101 1777Russell, further experiments on the action exerted by certain metals and other bodies on a photographic plate, Proc.Roy.Soc., 1898, 63, 102-12. 102 Id., On the Action of Certain Metals and Organic Bodies on a Photographic Plate, 222.Brit.Ass., 1898, 834-5 (Abstract). 103 Id., On hydrogen peroxide as the active agent in producing pictures on a photographic plate in the dark, Proc.Roy.Soc., 1899, 64, 409-19; id., On the Action of Wood on a Photographic Plate in the Dark, Chem.N., 1904, 90, 104-6. 104 G.L.Keenan, Substances which Affect Photographic Plates in the Dark, Chemical Reviews, 1926, 3, 95-111, 108. 105 G.C.Schmidt, Ueber die vom Thorium and den Thorver- bindungen ausgehende Strahlung, Verh.P:ys.Ges.Berlin 1898, 17, 14-16, 4th Feb., id., Ueber die vonen ThorveiTindungen and einigen anderen Substanzen ausgehende Strahlung, Ann.d.Phys., 1898, 65, 141-51. 106 Ibid., Verh.Phys.Ges.B-Olin, 16. 107 771TioquTigl, Sur diverses proprietes des rayons uraniques, CR, 1896, 123, 855-8, 23rd Nov.; id., Recherches sur les rayons uraniques, CR, 18977 124, 438-44; id., Sur la loi de la decharge dans l'aIr de l'uranium electrise, CR, 1897, 124, 800-3, 12th Apr. 108 E.Rutherford, The Velocity and Rate of Recombination of the Ions of Gases exposed to Röntgen Radiation, Phil.Mag., 1897, 44, 422-40, 440; Pa ers, 148. 109 H.Becqa5rel, CR, 1896, 55-8, 23rd Nov. 110 Id., CR, 1897, 124, 438-44, 443, 1st Mar. 111 U7Elster, Jahre -7d.Ver.f.Wiss.,Braunschweig, 1897, 10, 149-53, 10th Dec.189TT-J.EnTer and H.Geitel, Ann.d. P s.,Beibl., 1897, 21, 455. 112en .d.R.Acad.d.Scienze fis.e mat., 1897, 36, 178f. 113 kelvin, Papers, 6, 1; most oT'ITT) relevant papers are collected in this volume. 114 Cited in Kelvin, J.C.Beattie, M.S.de Smolan, On Apparent and Real Diselectrification of Solid Dielect- rics Produced by Röntgen Rays and by Flame, Edin.Roy. Soc.Proc., 1897, 21, 397-403; Kelvin, Paperi7-67 65. 115 RavIETJ.C.Beatfrg, M.S.de Smolan, Edin.Roy.Soc.Proc., 1897, 22, 131-3, 1st Mar.; Kelvin, Pa ers-,--6, 95-77 - 116 Id., Edin.Roy.Soc.Proc., 1897, 21, 1 - , Wth Apr.; re1viETPa ers7-6, 84-95. 117 J.C.Beattiet PhiT.Mag., 1897, 44, 102-7; Note by Kelvin, ibid., 107-8; read-1-6 Edin.Roy.Soc. 7th Jun.1897. 118 Citing J.J.Thomson and McClelland, Proc.Camb.Phil.Soc., Mar.1896, and E.Rutherford, Phil.Mag., ATi.71897; see Kelvin, Papers 6, 72-3, (1siMia-TTT ibid., 184, (17th Jun. 119 See J.J.Thomson, The Relation between the Atom and the Charge of Electricity carried by it, 1895, 537: 'contact electricity' due to oxide coatings; also Kelvin, Contact Electricity of Metals, Papers, (1898), 6, 110-47, 130, 138 etc.: true metallic contact electricity due to the affinity of differing metals. Notes for Chapter 2, p.48-118) 293

120 Kelvin and M.Maclean, On the Electrification of Air, Phil Mag., 1894, 38, 225-35; Kelvin, Papers, 6, 6-16, 6. 121 'N(7) -Chapter 1, Section 4, p.43. 122 Kelvin, Contact Electricity of Metals, RI Lib.Sci., (1897), 5, 50-83; Papers, 6, 110-47. 123 Id., Pa 4Fs, 6, 144-5. 124 ee apter T, Section 4, p.39: chemical H atom consists of two Boscovichean atoms. 125 Phil.Mag., 1899, 48, 97-106. 126 Ibid., 97. 127 E.V.Appleton, 'The Young Rutherford' in The Collected Papers of Lord Rutherford of Nelson, ed. J.Chadwick, 3 vole., London, 1962, 1, 17.T71171 of this work is hereafter referred to as E.RutherfUrd, Papers. 128 See Chapter 1. 129 Trans.New Zealand Institute, 1894, 27, 481-513; Papers, 25-55. 130 Ibid., Pa ere, 34. 131 Weabove, Chapter 1, Sections 3-4, p.31-2. 132 Rutherford, Pa ers, 25. 133 See Chapter , ection 2, p.133-4, 141-4. 134 Rutherford, Papers, 51. 135 Trans.New Zealand Institute, 1895, 28, 182-204; Papers, 55-79. 136 Rutherford, Pa ere, 69-70. 137 See below, C ap er 2, Section 3, p.95-6. 138 Refers most frequently to J.J.Thomson, Recent Researches in Electricity and Magnetism, 1893. 139 Ibid., 35: 140 Chapter 1, Sections 3-4, p.31-2. 141 Lord Rayleigh, The Life of Sir J.J.Thomson, Cambridge, 1942; repr. London, 1962, 62. 142 A.S.Eve, Rutherford.Being the Life and Letters of the Rt.Hon.Lord Rutherford,O.M., dgER67, 1939, 151 ITtTE57 to Mary Newton Oct.-1895. 143 N.Feather, Lord Rutherford, Glasgow, 1940; repr. London, 1973, 28-9. 144 R.Sviedrys, The Rise of Physical Science at Victorian Cambridge, Hist.Stud.Phys.Sci., 1970, 2, 127-51, 143; A History oTTHEUirrendish Laboratory,I871-1910, London, 9IU. 145 See the letter from Thomson, Cambridge, to Rutherford, London, dated 24th Sep.1895, in A.S.Eve, Rutherford. Etc., 13; also letter from Rutherford, Cambridge, to Mary Newton, N.Z., Oct.1895, ibid., 16. 146 A.S.Eve, Rutherford.Etc., 22-7. 147 Phil.Trans.A., 1697,19, 1-24; Rutherford, Papers, 80-10q7 -- 148 A.S.Eve, Rutherford.Etc., 34; letter to Mary Newton. 149 J.J.Thomson, Recent Researches etc., 53-207. 150 Ibid., 189. 151 Ibid. 152 Ibid., 119-27. 153 Ibid., 128. 154 TM., 45-7, 189-90. 155 Ibid., 193. 156 Ibid. Notes for Chapter 2, p.48-118) 294

157 Ibid., 45-6, 195-6. 158 TX Thomson, The Connection between Chemical Combin- ation and the Discharge of Electricity through Gases, Rep.Brit.Ass., 1894, 482-93. 159 Ibid., 489-92. 160 Ibid., 486. 161 Ibid., 487; also id., On the effect of electrification an chemical action on a steam jet etc., Phil.Mag., 1893, 36, 313-27. 162 Proc.RZY.Soc., 1895, 58, 244-57, received 17th Jun. 163 IETT.filag.77895, 40, 311-44. 164 Heber-die electrolTtische Leitung verdannter Gase, Ann.d.Phys., 1897, 61, 737-47, and references therein. 165 See above, Chapter IT Section 4, p.39-44. 166 A.S.Eve, Rutherford.Etc., 27. 167 See N.Feather, X-ra s and the electric conductivity of gases, Alembic u eTi;InTf2, Edinburgh, 1958, 16,-T1f. 168 Longitudinal Electric Waves, and Pecintgen's X Rays, Proc.Camb.Phil.Soc., 1896, 9, 49-61, 61; Kelvin, Papers, 6, 6571IsTg-geveral independent announcements of this, at about this date. 169 J.J.Thomson, On the Discharge of Electricity produced by the R8ntgen Rays, and the Effects produced by these Rays on Dielectrics through which they pass, Proc.Roy. Soc., 1896, 59, 274-6, received 7th Feb., assisted by J.A.McClellsEa. 170 Ibid., 275. 171 77.7ic.Camb.Phil.Soc., 1896, 9, 126-40, 9th Mar. 172 Ibid.7-170. 173 ITTa., 131-2. 174 Chapter 1, Section 4, p.42-6. 175 J.J.Thomson and J.A.McClelland, 22.cit., 132. 176 Of the order 0.001 cm./sec. per vofWm. 177 22.cit., Mar.1896, 128. 178 J.J7TEomson and E.Rutherford, On the Passage of Elec- tricity through Gases Exposed to Montgen Rays, Phil.Mag., 1896, 42, 392-407; read to British ssociation, Sep.I896; Rutherford, Papers, 105-18. 179 See N.Feather, Lord Rutherford, Chapter 2, 'Cambridge, the First Period, 1895-1898', 41. For his brief account N.Feather consulted the correspondence of Rutherford, used by A.S.Eve, also Rutherford's laboratory notebooks. 180 1896, 53, 581-3. 181 Ibid.,-583. 182 J.J.Thomson, The Rantgen Rays, Nature, 1896, 54, 302-6; 305-6; Rede Lecture, delivered 10th Sun. 183 Ibid., 304. 184 Ibid., 305. 185 Letter to Mary Newton, dated 18th Jun.1896, A.S.Eve, Rutherford.Etc., 36. 186 Read to BriiIih Association, Sep.1896; Rutherford, Papers, 105-18. 187 Similar to those of Kelvin, who is not cited; Kelvin, Papers, (1895), 6, 35, 51-2. 188 Thomson and Rutherford, op.cit., Rutherford, Papers, 106. 189 Ibid., 107. 190 TEid., 106. 191 Ibid. 192 Ibid., 117. Notes for Chapter 2, p.48-118) 295

193 Ibid., 107-8. 194 The-intermittency of the X-ray discharge affected the results, ibid., 109-10. 195 Rep.Brit.1713737, 1894, 482-93, 491-2. 196 ThomTEEailia-Rutherford, op.cit., 1896, 114. 197 See above Chapter 1, SecTfori, p.47. 198 Phil.Mag., 1897, 43, 241-55, Apr. issue, dated 28th Dec. 1896; Rutherford, Papers, 119-31. 199 Ibid., 128. 200 TM., 127. 201 Ibid., 119-22. 202 UtPirrin, Mecanisme de de-charge des corps olectrises par lee rayons de Ontgen, Seances Soc.Fr.Phys., 1896, 254-61, 261; id., Rayons cathodiques et rayons de Roentgen, Annales de Chimie et de Physique, 1897, 11, 496-554, Thesis Jun.1897. 203 THil.Mag., 1897, 44, 422-40; Papers, 132-48. 204 MIT.Mag., 1898, 7.L 120-54. 205 E.Rutherford, 22.-61t., Papers, 144-8. 206 Ibid., 148. 207 Nacre, 1896, 53, 581. 208 Nature, 1896, 5T, 304. 209 TeTTETtlow, Chapter 2, Section 4, p.114-5. 210 See above, Chapter 2, Section 1, p.63-5. 211 E.Rutherford, Papers, 148. 212 Proc.Camb.Phil.Soc., 1898, 9, 401-16; E.Rutherford, Pa ers, 149-62. 213 rbid., 149. 214 This was accepted by J.J.Thomson in 1893, Recent Researches etc., 54. 215 0.Lodge, Modern Views etc., 1889, 301-2. 216 E.Rutherford, Pa ers, 159. 217 Phil.n.E., 189 „ 109-63; Papers, 167-215. 218 mid., 214-5. -- 219 J.J.Thomson, Proc.Camb.Phil.Soc., 1898, 9, 393-7, 24th Jan. ---- - 220 See below, Chapter 2, Section 3, p.104-5. 221 J.J.Thomson, 22.cit., 1898, 397. 222 Ibid. 223 gig-below, Chapter 2, Section 3, p.100-1. 224 E.Rutherford, Uranium Radiation etc., Papers, 180. 225 Ibid., 214-5. 226 Ibid., 214. 227 TETU., 215. 228 Papers, 167-215; Phil.Mag., Jan.1899, dated 1st Sep.1898. 229 Ibid., 167, 170-1. 230 TWoquerel, CR, 1899, 128, 771-7, 772, 27th Mar. 231 E.Rutherford, Papers, 177=6. 232 Ibid., 185-6. 233 UTI7-1896, 122, 762-7, 765, 30th Mar. 234 Proc.Camb.f.al.Soc., 1896, 9, 126-40, 139-40, 9th Mar. 235 ConductioriOEM7tricity through Gases, Cambridge, 1903, 278. Rutherford's own resulfg-6Y-1898 (Pa ers, 178) showed that the alpha/beta ratio increase w h the thickness of the Ur layer; Thomson noted (loc.cit.) that if alpha rays were produced at the surfaci-Ey-Feta, then their ratio should be constant. New evidence of independence was produced by F.Soddy in 1902. Notes for Chapter 2, p.48-118) 296

236 Rutherford, Uranium Radiation etc., Papers, 180-1. 237 Ibid., 180. 238 Ibid., 178. 239 Ibis. 240 Ibid. 241 t177-1898, 126, 1101-3. 242 17ge e.g. M=Hesse, Forces and Fields, London, 1961, 2-3, 6. 243 J.C.Maxwell, Electricity and Magnetism, 1892, 2, 470. 244 H.R.Post, Atomism 1900, P sics Education, 196U, 3, 1-13, 5; discusses views of L. I.tzmann, E.Mach, - W.Ostwald. 245 Seven papers in CR, 1880-2; P.Curie, Oeuvres, 6-32. 246 M.Curie, Pierre Curie, trans., New York, 1923; repr., 1963, 20. 247 C.Friedel, Sur la pyroelectricite dans la topaze, la blends et le quartz, Neues Jahrb.Mineral., 1879, 585-6. 248 A.-C.Becquerel, De quTTTEFisTorigEomenes electriques produits par la pression et le clivage des cristaux, Annales de Chimie et de Physique, 1827, 36, 265-71. 249 M.Curie, Pierre Curie, N.Y., 21. 250 P. and J.Cur e, Dilatation electrique du quartz, Journal de P si ue, 1889; P.Curie, Oeuvres, 35-55. 251 P. and J.Tur e, R, 1881; P.Curie, Oeuvres, 18-21. 252 Ibid., 19. 253 157iahem crossed swords with P.Curie (Oeuvres, 33-4) in 1887 over the origin of piezo-electricity. In 1893 Kelvin corresponded with P.Curie (7 letters, Curie papers, dossier 32, BN) who provided him with a piezo-electric electroscope. 254 P.Curie, Sur les questions d'ordre: Repetitions, Oeuvres, (1884), 56-77; id., Sur la symetrie, Oeuvres, (1b84), 78-113. 255 E.g. Kelvin, Papers? 1 281, stated that 'Hall's recent great discovery' (186U) of the e.m.f. produced by a steady current in a constant magnetic field 'proves the rotatory quality to exist for electrical conduction through metals in the magnetic field'; but P.Curie was first to consider the symmetry of the Hall effect, Oeuvres, (1894), 137. 256 M.Curie, Pierre Curie, N.Y., 24-8, gives an account of this. 257 Oeuvres, (1894), 118-41. 258 Ibid., 141. 259 E.g. J.J.Thomson, Applications of Dynamics etc., 1888, ch.4, 32. 260 L.Rougier, En Marge de Curie de Carnot et d'Einstein, Paris, 1920, discusses 'Le prEiciTT-E sym6triet, ch.l. The principle is much used in modern electron theories of the chemical atom. 261 Oeuvres, (1895), 232-334. 262 Determined over a more limited temperature range by others, P.Curie, Oeuvres, 280-1; of present importance in electron theories of magnetism, this is now known as the 'Curie law'. 263 A and R are constants, different for each substance; T = temperature, H = magnetic field, I = magnetic intensity; D = gas density, P = gas pressure. Notes for Chapter 2, p.48-118) 297

264 P.Curie, Oeuvres, 331-2. 265 The I = f(H) and D = f(P) curves at constant T were dissimilar, ibid., 334. 266 Ibid., 333. ---- 267 157-Curie, Madame Curie, trans., London, 1938; repr. The Reprint Societ77EUndon, 1942, is a biography valuable for the non-scientific aspects of Marie Curie's life, much personal correspondence is published here. R.Reid, Marie Curie, London, 1974, provides a much improved account on similar lines. See also M.Curie, Pierre Curie, N.Y., 'Autobiographical Notes', 77-118. 268 'MentioElEtTes bien' and 'AB' respectively, Curie papers, dossier 29, BN. The Licence was of about the present masters or first degree standard. 269 Ibid 270 E.g., M.Curie, Proprietes magnetiques des aciers trempes, CR, 1897, 125, 1165-9. 271 P.Curie, Oeuvres, 277Y. 272 J.Hurwic,-MER-gSklodowska-Curie en tant que chimiste, Etudes d'Histoire de la Science et de la Technologie, Warsaw, 1966, 197-0'2. 273 See E.Curie, Madame Curie, Appendix: lists of Marie Curie's prizeT37als, decorations, honorary titles. 274 M. Curie, Pierre Curie, N.Y., 96-7. 275 Discussed in letti7F-fiom E.Rutherford, Manchester, to W.H.Bragg, dated 20th Dec.1911, CUL. Also R.Reid, Marie Curie, ch.17. 276 See M.Curie, La Radiologie et la Guerre, Paris, 1921; E.Curie, Madame Curie, ch.21, N:1779. 277 Irene Curie, lat1171Fene Joliot-Curie. 278 The film 'Madame Curie', M.G.M., U.S.A., 1943, re-shown occasionally to the present time by the British Broadcasting Company exemplifies one popular aspect. 279 I am indebted to L.Badash, who informed me of this claim. 280 M.Curie, Opening Lecture, Cours du physique gen6rale professe a la Sorbonne, Oeuvres, 322-35, 334-5. 281 Rev.Gen.des Sci., 1899, 10, 41i.; Oeuvres, 60-76. 282 See mow, Chapter 5, SeTiion 1, p.227-31, 245-6. 283 M.Curie, Pierre Curie, N.Y., 44-5, 89; her daughter Irene waiUOTE-in Sep.1897. 284 CR, 1897, 124, 800-3. 285 A.Romer, Radiochemistry, 6. 286 M.Curie, Pierre Curie, N.Y., 45, 89. 287 Ibid., 34. 288 77Terrin, Rayons cathodiques etc., Seances Soc.Fr.Phys., 1896, 121-9; G.Sagnac, Journal de Physique, 5, 193f. 289 M.Curie, Pierre Curie, N.Y., 40. 290 Seances Soc.Fr.Phys., 1896, 105, 20th Mar. 291 Laboratory notebooks of the Curies, 1897-9, comprise dossier 1, Curie papers, BN,but unfortunately these are radioactive, are undergoing treatment with some other items in the collection and are not available. They are described as: Uranium I, mainly Marie Curie's hand, pp.159, 1897-8; Uranium II, P. and M.Curie's hands, pp.143, 1898; Uranium III-Polonium, pp.126, P. and M.Curie's hands, 1898-9. Fortunately their Notes for Chapter 2, p.48-118) 298

291 contd.) previous owner , I.Joliot-Curie, has given a brief account of their contents in M.Curie, Pierre Curie, Paris, 1955, 103-20. See above also A.Romer, REITTOchemistry, 6-8, 64-75, who uses this source and gives translations of published papers. A fourth laboratory notebook of M.Curie, 1899-1902, is at the Wellcome Historical Institute and was made available to me. 292 J.Curie, Recherches sur le pouvoir inducteur specifique et la conductibilite des corps cristallises, Annales de Chimie et de P si ue, 1889, 17, 385-434. 293 M.Cur e, Oeuvres, 60-76,-71, Jan. 294 I.Joliot-Curiel in M.Curie, Pierre Curie, Paris, 103-7. 295 M.Curie, CR, 1898, 126, 1101-3, 12th. 296 G.C.Schmidt, CR, lags, 126, 1264. 297 E.Wiedemann aFa G.C.Schiaat, Ueber Lichtemission organischer Substanzen etc., Ann.d.Phys., 1895, 56, 18-26; id., Ueber Luminescenz von festen Wirpern und festen TUsungen, ibid., 201-54, 241-8; see above, Chapter 1, SectioriT p.30; on Wiedemann's earlier ether-envelope theory in spectroscopy see McGuoken, Nineteenth-Century Spectroscopy, 179-81. 298 G.C.Schmidt, Ueber die vom Thorium und den Thor- verbindungen ausgehende Strahlung, Verh.Phys.Ges. Berlin, 1898, 17, 14-16, 4th Feb.; see also Cat, Ueber die Jeziehung zwischen Fluorescenz und Actinoelek- tricitat, Ann.d.Phys., 1898, 64, 708-24. 299 J.Elster and H.Geitel, Ann.d.Phys.,Beibl., 1897, 21, 455, reviewed by G.C.Schmidt. 300 Ann.d.Phys., 1889, 60, 507f. 301 G75:chmidt, Ueber Tire von den Thorverbindungen etc., Ann.d.Phys., 1898, 65, 141-51. 302 G.C.Schmidt, op.cit., Verh.Phys.Ges.Berlin, 1898, 17, 16. 303 See above, p.aU. 304 CR, 1898, 126, 1101-3, 12th Apr.; Oeuvres, 43-5. 305 E.N.Harvey,Ei!toryofb Luminescence, 284; J.Elster and H.Geitel, Ani.T.aPhSis., 1890, 39, 321-31; S.Bidwell, Diselectriffe -aTion by Phosphorus, Nature, 1896, 55, 6; G.C.Schmidt, Ueber die Emanation des Phosphors, 7Hys.Z., 1902, 3, 475-81; F.Harms, phys.Z., 1902, 4, 111-3; J.J.Thomson, Conduction of Electricity thrau h Gases, Cambridge, 1903, 324; E. Rutherford, Rado-ac vity, Cambridge, 1905, 529-30. 306 These elements gave 1 to 10% of the uranium reading, which was 24 x 10' amps. All other substances, except phosphorus, gave less than 1% of this current: M.Curie, Oeuvres, 44. 307 See Section 2 above, p.90-1. 308 This consists in mixing a solution of uranium nitrate with one of copper phosphate in phosphoric acid then warming gently; crystals of chalcolite, copper uranyl phosphate, slowly separate. Although not mentioned in the note of 12th Apr., preliminary success in chemically concentrating the active ingredient may also have contributed to the evidence by this time. 309 E.Rutherford, Papers, 178. 310 P. and Mme.S.Curie, CR, 1898, 127, 175-8, 18th Jul. Notes for Chapter 2, p.48-118) 299

311 W.Crookes, Genesis of the Elements, 1887, 410-11; also W.N.Hartley, Opening Address to Brit.Ass. Chemistry Section, on spectroscopy, Nature, 1903, 68, 472-81, 481. 312 M.Curie, Oeuvres, (1898), 45, 12th Apr. 313 3 new elements were proposed in 1897, no less than 9 in 1898 (3 radioactive), and 2 in 1899, C.Baskerville, The Elements: Verified and Unverified, Chem.N., 1904, 89, 109-10, 121-3, 135-7, 150-1, 162-3, 170-1, 186-7, 194-5, 210. 314 G.Sagnac, Sur is mecanisme de la decharge des conducteurs frappes par lee rayons X, CR, 1898, 126, 36-40, 3rd Jan.; id., Transformation des rayons X par-Transmission, ibid., 4T7-70; id., Emission de rayons secondaires par l's17- sous l'iiTluenoe des rayons X, ibid., 521-3; id., Caracteres de la transformation des rayons X par la matiere, ibid., 887-90, 21st Mar. 315 J.Perrin,-13-6harge par les rayons de Röntgen. Role des surfaces frappoes, CR, 1897, 121, 455-8; L.Benoist and D.Hurmuzescu, CR, 1U76, 122, 779f., had expressed a similar view. -- 316 G.Sagnac, Sur la transformation des rayons X par les differents corps simples,Seances Soc.Fr.Phys., 1899 1*, 6th Jan. 317 See Section 2 above, p.89. 318 M.Curie, Oeuvres, (1898), 45, 12th Apr. 319 Ibid. 320 P. and Mme.S.Curie, CR, 1898, 127, 175-8, 18th Jul.; P.Curie, Oeuvres, 335-8. Active substances are here called 'radioactive' for the first time. 321 A.Romer, Radiochemistry, 80-105, gives a brief account of controversies concerning active bismuth, polonium and radiotellurium during 1899-1906 and provides translations of some papers. 322 P.Curie, M.Curie, G.Bemont, Sur une nouvelle substance fortement radio-active, contenue dans la pechblende, CR, 1898, 127, 1215-7, 26th Dec.; P.Curie, Oeuvres, 779-42. 323 Ibid., 340. 324 The authors thanked M.Suess, correspondent de l'Institut de France, Professeur a l'Universite de Vienne, for his request to the Austrian government, who donated the waste material freely, ibid., 342. All of the increasing amounts of material subsequently used by the Curies came from Joachimsthal, M.Curie, Pierre Curie, N.Y., 91. 325 Sur le spectre d'une substance radio-actiViTCR, 1898, 127, 1218; appended to the paper of Curies ana-Bemont. 326 P.Curie (with M.Curie and Bemont), Oeuvres, 341. 327 Atomic weights determined by Marie for increasingly concentrated radium were approximately as follows: M.Curie, Sur le poids atomique du metal dans le chlorure de baryum radifere, CR, 1899, 129, 760-2, atomic weight of Ba = 138, Ba-Ra = 140 to-175; id., Sur le poids atomique du baryum radifere, CR, 1700, 131, 382-4, Ba-Ra = 174; id., Sur le poids atomique du radium, CR, 1902, 135, 161-3, Ra = 225, the modern value. 328 P.Curie7Twith M.Curie and G.Bemont), Oeuvres, 340. 329 Rev.Gen.des Sci., 1899, 10, 41f., Jan.; M.Curie, Oeuvres,-0-75. Notes for Chapter 2, p.48-118) 300

330 Ibid., Oeuvres, 73. 331 Ibid., 71. 332 Ibid., 75. 333 Ibid. 334 Ibid., 72. 335 Ibid., 75. 336 Ibid., 76. 337 M.Curie remarked on this in a later footnote added during or after Dec.1898, Oeuvres, 76. 338 Re .Brit.Ass., 1898, 3-38. 339d., 26. 340 Ibid., 27. 341 J.Elster and H.Geitel, Versuche aber Hyperphosphor- escenz, Ann.d.Phys.,Beibl., 1897, 21, 455; J.Elster, Jahresb.17Ver.f.Wiss.,Braunschweig7-1897, 10, 149-53, 10th Dec.187'. 342 Verh.lys.Ges.Berlin, 1898, 17, 14-16. 343 1898, 8, W7U. 344 J.Elsf.E. and H.Geitel, Ann.d.Phys., 1898, 66, 735-40. 345 Ibid., 736. 346 77RUtherford, Pa ers, 169-215, dated 1st Sep.1898. 347 Sur la source e nergie dans lee corps radio-actifs, CR, 1899, 128, 176-8, 16th Jan., presented by H.Moissan. 348 1898, 739. 349 Tbia77 740. 350 L.Badash, Radioactivity before the Curies, Amer.J.Phys., 1965, 33, 128-35, 130, 134. 351 Nature, 1896, 53, 581; ibid., 54, 304. 352 See below, p.1.7-5. 353 Proc.Camb.Phil.Soc., 1897, 9, 372. 354 G.G.S=E6245s,- Mem.& Corres., "ff, 471. 355 W.Crookes, La7ratory Notebooks, vol.16, p.54-6, 3rd to 10th Aug.1897, RI. 356 Verh.Phys.Ges.Berlin,o 1898, 17, 14-16. 357 Proc.Cam . 5HIf7-87F7T 1898, 9, 393-7, 397. 358 LetteTWoET7P.Thompson to G.G.Stokes, dated 28th Feb. 1896; Stokes, Mem.& Corres., 2, 495. 359 Stokes to Thompson, dated 29th Feb.1896; ibid., 495-6. 360 Thompson had already been anticipated by Becquerel, see this Chapter, Section 1, p.59-60. • 361 Typescript letter from Stokes, correspondence of S.P.Thompson, Imperial College Archives; the inverted commas are omitted from Stokes, Mem.& Corres., 2, 495-6. 362 G.G.Stokes, Papers, 4, 256-77. 363 Ibid., 273-4. 364 ST61ies, Mem.& Corres., 1, 299. 365 Ibid., 294-7. 366 Saes, Mem.& Corres., 2, 478-83. 367 See Section above, p.'69. 368 E.Rutherford, Papers, 215; pub. Jan.1899. 369 R.B.Owens, Thorium Radiation, Phil.Mag., 1899, 48, 360-87, 361, pub. Oct. 370 J.Elster and H.Geitel, Weitere Versuche an Becquerel- strahlen, Ann.d.Phys., 1899, 69, 83-90, received 5th Aug.; p.83-7-g1so pub. as Ueber Becquerelstrahlen, Jahresb.d.Ver.f.Wiss.,Braunschweig, 1899, 11, 183, 271-6, IgtliTan.KGiesel, known as a cheast, Notes for Chapter 2, p.48-118) 301

370 contd.) joined in the discussion of the paper in Brunswick on 19th Jan., ibid., 183; his later publications on active substances and their rays are of importance. 371 Letter from J.Elster to E.Rutherford, dated 10th Feb. 1899, CUL. 372 Elster and Geitel, op.cit., Ann.d.Phys., 1899, 88. 373 E.Rutherford, Uranium TgaiatTUE etc., Papers, 215. 374 Having corresponded with Elster and Geitel n 1899 (letters from J.Elster to E.Rutherford, dated 10th Feb., 27th Jun.1899, CUL) and mentioned radioactivity, one would expect him to look out for their publications. Also, Elster in his letter of 10th Feb. promised to send Rutherford their paper on the subject. 302 NOTES FOR CHAPTER 3 (pages 119-178)

1 Jahrosb.d.Ver.f.Wiss.,Braunschweig, 1896, 10, 68, 73-7. 2 Ibid., 1899, 1T, 163; see above, Chapter 2, Section 4, n.370. 3 F.Giesel, Einiges fiber das Verhalten des radioactiven Baryts und tiller Polonium, Ann.d.Phys., 1899, 69, 91-4- 4 J.Elster and H.Geitel, Ann.d.Phys., 1899, 69,-8'3-90, 87. 5 F.Giesel, 6 See E.de Hadn, Ueber eine radioactive Substanz, Ann.d. Phys., 1899, 68, 902. 7 Letter from J.Elster and H.Geitel, in Elster's hand, to E.Rutherford, dated 27th Jun.1899, CUL. 8 J.Elster and H.Geitel, Ann.d.Phys., 1899, 69, 83-90; p.88-90, Ueber den EinflaTiis eines magnetie-CEen Feldes auf die durch die Becquerelstrahlen Bewirkte Leitfahig- keit der Luft, communicated to Deut. Phys. Gee., 5th May 1899. 9 Id., Ann.d.Phys., 1889, 38, 27-39; ibid., 1899, 69, F7-907-U8. 10 I.Joliot-Curie, in M.Curie, Pierre Curie, Paris, 1955, 110. 11 H.Becquerel, Influence d'un champ magnotique sur le rayonnement des corps radio-actifs, CR, 1899, 129, 996-1001, 11th Dec. 12 F.Giesel, Ueber die Ablenkbarkeit der Becquerelstrahlen im magnetische Felde, Ann.d.Phys., 1899, 69, 834-6, received 31st Oct. 13 L.Badash, An Elster and Geitel Failure: Magnetic Deflection of Beta Rays, Centaurus, 1966, 11, 236-40, has calculated the field required to defleFf the rays from radium and concludes that their magnet was too weak to give a noticeable effect in the phosphorescence experiment. A.Romer, Radiochemistry, 11, seems to consider that their positive air-conduction results were in fact due to deviation of the rays; the comment made here, that Elster and Geitel did not think in ionic terms, is debatable. 14 S.Meyer and E.von Schweidler, Tiber das Verhalten von Radium und Polonium im magnetischen Felde, Phys.Z., 1899, 1, 90-1, received 10th Nov., from Boltzmann's lab. 15 Ann.d.Phya., 1899, 69, 83-90, 90. 16 S.Meyer and E.von Schweidler, 22.cit., 91. 17 S.Meyer and E.von Schweidler, Ube-Faas Verhalten etc., P s.Z., 1899, 1, 113-4, received 18th Nov. Elster and e eT, ibid., D399, 1, 153, made it clear that this was Giese 'sdiscovery not their own; Meyer's paper did not. 18 See above, Chapter 1, Section 4, p.45-6. 19 W.Sutherland, Cathode, Lenard and Röntgen Rays, Phil.Mag., 1899, 47, 269-84; J.J.Thomson, Note on Sutherland's paper,-Ibid., 415-6. 20 J.J.Thomson, Phil.Mag., 1899, 48, 547-67, Dec. issue. 21 Ibid., 566-7. 22 J.J.Thomson, The Magnetic Properties of Systems of Corpuscles etc., Phil.Mag., 1903, 6, 673-93, 689. Notes for Chapter 3, p.119-178) 303

23 J.J.Thomeon, 22.cit., 1899, 565. 24 Rayleigh, J.J.Thbmson, 132-3. 25 Letter from 3.J.Thomson to E.Rutherford, dated 21st Dec. 1899, CUL. 26 Letter from Rutherford to Thomson, dated 9th Jan.1900, CUL. See Section 2 below, p.131f. for discussion of 'emanation'. 27 E.Rutherford, Pa ers, (1898), 180. 28 Electrisation negative des rayons secondaires produits au moyen des rayons Röntgen, CR, 1900, 130, 1013-6, 9th Apr.; P.Curie, Oeuvres, 37-62„ 362. 29 E.Rutherford, Energy of Rbntgen and Becquerel Rays etc., Pa ers, 260-95, 293, received Jun.1900. 30 295. 31 Ibid., 292-3; my stress. 32 H.Becquerel, Seances Soc.Fr.Phys., 1899, 71*-72*, 15th Dec. Mainly repeating 'Influence d'un champ magnetique etc.', CR, 1899, 129, 996-1001, 11th Dec. 33 H.Becquerel, Note our quelques proprietes du rayonnement de Puranium et des corps radio-actifs, CR, 1899, 128, 771-7, 27th Mar. 34 Id., Sur le rayonnement des corps radio-actifs, CR, 1899, 179, 1205-7, 26th Dec. 35 td., Contribution a l'etude du rayonnement du radium, Uff, 1900, 130, 206-11, 29th Jan. 36 PTCurie, Ac lion du champ magnetique sur lea rayons de Becquerel, CR, 1900, 130, 73-6, 8th Jan.; Oeuvres, 349-52. M.Curie, Sur la p6netration des rayons de Becquerel non deviables par le champ magnetique, CR, 1900, 130, 76-9; Oeuvres, 85-8. 37 :aTETT:: CR, 29th Jan.1900 38 R = radius of curvature of path of a particle, produced by magnetic field H; v = velocity of particle, m = its mass, e = its charge. 39 9.2.cit., n.35, 209. 40 H.Becquerel, CR, 1900, 130, 372-6, 12th Feb. 41 Letter from 147Urookes foiff.G.Stokes, dated 16th Dec.1900; Stokes, Mem.& Corres., 2, 484. 42 J.J.Thomson, Nature, 1876, 54, 302. 43 Id., Cathode Rays, Phil.Ma., 1897, 44, 293-316, 310. 44 -nem.N., 1900, 81, L45-6, 30th Mar.;-Trans. from Rev.Gn.des Sci.. 15th Mar.1900. 45 70707-617-539-40, 5th Apr. 46 P. and V.Curie, Sur la charge electrique des rayons doviables du radium, CR, 1900, 130, 647-50, 5th Mar.; P.Curie, Oeuvres, 353-7. 47 Ibid., 356. 48 tql7-1900, 130, 809-15, 26th Mar.; E.Dorn, CR, 1900, 130, TT26, 23rd Apr. wrote to claim priority for the qualitative electrostatic deviation, in February, of the rays from Giesel's active barium compound. 49 P. and M.Curie, CR, 1899, 129, 714-6, 6th Nov.; prior to the experimental deflection of the rays. 50 E.Dorn, Ueber die von den radioactiven Substanzen ausgesandte Emanation, Abh.der Naturf.Ges.zu Halle, 1901, 23, 1-15, Jun.1900. 51 PT and M.Curie, op.cit., 716. 52 CR, 1899, 129, 7]- 6,-----6-11 Nov., appended to paper of Curies. Notes for Chapter 3, p.119-178) 304

53 CR, 1899, 128, 771-7, 773. Refraction and polarisation were also now rejected, for different reasons. 54 1.1RR2ER, 169-215, 180-1, dated 1st Sep.1898; he cited only Schmidt on thorium. 55 Ibid, 56 E.Rutherford and R.B.Owens, Thorium and Uranium Radiation, Trans.Em.Soo.Canada, 1899, 2, 9-12, read 26th May; 1utherfor47-Pa ers, 216-9, The authors state that thorium nitrate gave a - a_rly constant radiation; we note that this contradicts the stated results of 1898. E.Rutherford, Notebook 3, CUL, contains experimental results of Owens and Rutherford. 57 Ibid., 218; E.Rutherford, Radio-activity, 1905, 238. TaTeter and H.Geitel, Phy2.2., 1899, 1, 11-14, (received 19th Aug.) in examining the conductivity of the ordinary air in the laboratory noted that this was markedly increased by a draught from the room containing Ra and Po samples, without at this stage stating any conclusions; they were soon to take up Rutherford's view. 58 E.Rutherford, Le2212, 218, May 1899. 59 R,B.Owens, Phil.Mag., 1899, 48, 360-87, Oct. issue, probably wrT117m—Vifore Jul. 60 E.Rutherford, Phil.ns.. 1900, 49, 1-14, Jan. issue, dated 13th Sep.1899; Papers, 2231. 61 Ibid., 220. 62 E.Rutherford, Some Remarks on Radioactivity, Phil.Mag., 1903, 5, 481-5; Viers, 578. 63 N.Feather, Lord Rutherford, 69-73, and A.Romor, The Restless Atom, 43-52, give brief accounts of the researches ofTaherford and others at about this time. 64 Letter from J.J.Thomson to E.Rutherford, dated 23rd Jul. 1899, CUL. The thorium oxide layer is designated AB. 65 Ibid.; question-mark omitted sic. 66 TT:Owens, Thorium Radiation, 1599, 366, 67 Ibid., 372-3. 68 Letter to Mary Newton, dated 2nd Dec.1899, A.S.Eve, Rutherford.Etc., 69. 69 E.Rutherford, A Radioactive Substance emitted from Thorium Compounds, Papers, 221. 70 Ibid., 222. 71 Ibid., 225. 72 min, Papers, (1894-7), 6, 17f., had found that gases could retain conductivity when bubbled through water; Rutherford made no mention of this; his experiments went much further. 73 E.Rutherford, Rp.cit., 224. 74 Id., Papers, (1902), 432. 75 17RutheaER, A Radioactive Substance etc., Papers, 227-9. 76 E.Rutherford, Uranium Radiation etc., Papers, (1898-9), 214-5. 77 Id., A Radioactive Substance etc., Papers, (1899-1900), 728. 78 1900, 62, 31-2, 10th May. Described more fully, and quantfT5.tively in 'Indications relatives ift la constitution do la matiere etc.', Rapports , Cong.Int.de Physique, 1900, 3, 138-51, Aug. Notes for Chapter 3, p.119-178) 305

79 Letter from Thomson to Rutherford, dated 23rd Jul. 1899, CUL. 80 J.J.Thomson, 22.cit., Nature, 10th May 1900. 81 A.S.Eve, RutherfOW.Et67767, letter dated Sep.1899. 82 Pa ere, 230. 83 Rutherford in 1905, Radio-activity, 239, mentioned only the method he used in 1t399. 84 E.Rutherford, Papers, (1899-1900), 230. 85 Ibid., 226. 86 Taa., 230. See below, Section 4, p.176. 87 Ibid., 231. 88 5' above Chapter 2, Section 2, p.81; E.Rutherford, Papleirs, 106. 89 Ruterford, Pa ere, 231. 90 Id., Phil.Mee., 1900, 49, 161-92; Papers, 232-59; Tited 22nd Nov.1899. -- 91 Papers, 255-7, Nov.1899. 92 .11, d., 256. 93 6W-above, Chapter 2, Section 2, p.71; Papers, 27-31. 94 E.Rutherford, Papers, (1897), 132f. 95 J.J.Thomson, Conduction of Electricity through Gases, 1903, 296. 96 E.Rutherford, Radioactivity Produced in Substances etc., Pa ere, 258. 97 Ibid., 98 17-Rutherford (and R.B.Owens), Pa ers (1899), 219. 99 P. and M. Curie, CR, 1899, 129, ; see above, p.129-30. 100 In fact it was Becquerel who had used the term phosphorescence, in his appended remarks; the Curies wrote 'rayons secondaires'. 101 E.Rutherford, Pa ere, 238. 102 Letter from J. ."homson to E.Rutherford, dated 21st Dec. 1899, CUL. 103 Letter, id., dated 22nd Nov.1898. 104 J.Zeleny, On the ratio of the velocities of the two ions produced in gases by Röntgen radiation etc., Phil.Mag., 1898, 46, 120-54. 105 Ibid., 134-5. 106 Letter from J.Zelony, Univ. Minnesota, to E.Rutherford, dated 25th Mar.1900, CUL; quoted in part, with a different interpretation, in N.Feather, Lord Rutherford, 73. 107 See above, Chapter 2, Section 4, p.117. 108 Ueber Luminescenz von festen Korpern and festen Losungen, Ann.d.PhT, 1895, 56, 201-54, 241-50. 109 Ann.d.Phys., 69, 220-35, Sep. issue; from Giittingen where Behrendsen (b.1850) was Professor at the Gymnasium. 110 Behrendsen, ibid., 234, refers to their paper read at Braunschweig-Tri-Jan., not the reprint in Ann.d.Phys., 1899, 69, 83-90, Sep. issue. 111 BehrenTgen, 22.cit., 233. 112 Only the German scientists had in fact said this; Marie Curie had speculated on the evolution of the elements, but to this Behrendsen made no reference. 113 Ibid., 235. 114 TEM. 115 T1F71.Z., 1900, 1, 476-8, Aug. issue. Notes for Chapter 3, p.119-178) 306

116 E.Rutherford, Uranium Radiation etc., Papers, (1898-9), 167-215, 215. 117 M.Curie, Les Rayons de Becquerel etc., Oeuvres, (1899), 60-76, 71. 118 G.G.Stokeo, Mem.& Corres., 1, 293-4, letter to Becquerel dated 16th Aug.1399; see above, Chapter 2, Section 4, P.115. 119 Stokes, Ibid., 294-7, letter dated 25th Aug.1899. 120 See e.g. E.Rutherford, Radio-activity, 1905, 210, 249, 391, and his references. 121 E.Rutherford, Pa era, (1900), 230, Jan. 122 Ann.d.Phys., 1900, 2, 335-7, dated May, pub. Jun. 123 T.-Elster and H.Geitel, Uber Becquerelstrahlen, Verh. Deut.Phys.Ges., 1900, 5-8, 5th Jan. meeting. 124 Read Jun. 1-970, see below, Section 4, p.172-3. 125 CR, 1899, 128, 771-7. 126 Md., 777. 127 Ibid. 128 G.G.Stokes, Mem.& Corres., 1, 297-9, letter from Stokes to Becquerel Tdied 4th Sep.1899. 129 H.Becquerel, CR, 1899, 129, 716, appended to the Curies' paper. 130 H.Becquerel, Deviation du rayonnement du radium dans un champ electrique, CR, 1900, 130, 809-15, 815; Curie Ra sample of unspecified T6Tivity. 131 Id., Sur le rayonnement de l'uranium etc., Rapports, anig.Int.de Plysique, 1900, 3, 47-78, 78. 132 ma. 133 7:75cquerell 1901, 63, 396-8. 134 Ibid., 398. 135 Tee-below, Chapter 4, Section 1, p.186-8. 136 Nature, 1900, 61, 443. 137 See e.g. 'S.W.', The Principle of Radium, Nature, 1903, 68, 496-7, who makes a similar point. 138 See above, Chapter 2, Section 4, p.113, n.355. 139 See Chapter 5, Section 1, p.230, on the emission of heat from radium. 140 CR, 1899, 129, 823-5. 141 a.cit.„ Nature, Mar.1900, 142 P.CaTIe, Oeuvres, 353-7; CR, 1900, 130, 647-50; see above, Section 1, p.127. 143 See above, Section 1, p.126-8. 144 Rev.Sc., 1900, 14, 65-71; M.Curie, Oeuvres, 95-105. 145 P7 aid M.Curie, Les nouvelles substances radioactives et lee rayons qu'elles omettent, EakvortE,Cona.Int.de P sis ue, 1900, 3, 79f., Aug.; P.Curie, Oeuvres, 3 409 146 Ibid., 409. 147 FTUarie and G.Sagnac, kectrisation negative des rayons secondaires produits au moyen des rayons Röntgen, CR, 1900, 130, 1013-6, 9th Apr.; P.Curie, Oeuvres, 358-62; E.Dorn, Abh.der Naturf.Ges.zu Halle, 1900, 22, 40-2. 148 See above, Chapter 2, sTaTion 4, p.112. 149 22.cit.; M.Curie, Oeuvres, 95-105. 150 mx7Fre, ibid., 104. 151 Ibid. 152 Nature, 1900, 62, 31-2, 3rd May issue. Notes for Chapter 3, p.119-178) 307

153 See above, Chapter 2, Section 4, p.117. 154 Mt. Curie, 92.cit., Oeuvres, 104-5. 155 See below, Chapter 4, Section 1, 156 See H.Becquerel, Sur le rayonnement des corps radio- actifs, CR, 1899, 129, 1205-7. 157 See below, Chaptei27 Section 1, p.189. 158 Phil.Trans.A., 1901, 196, 25-59, received 15th Jun.1900; E.RutherforU, Papers, 260-95. 159 Ibid., 292. He mentioned Becquerel and Giesel, not 7:754yer and Schweidler, Phys.Z., 1899, 1, 90-1, Nov., who first pointed out this di?ference. 160 H.Beoquerel, Note our le rayonnement de l'uranium, CR, 1900, 130, 1583-5, in fact announced a magnetic deflection, also in June; he was unsure of the uranium's purity. 161 Rutherford (and McClung), op.cit., Papers, 292. 162 ibid., 260. 163 Letter from E.Rutherford to J.J.Thomson, dated 9th Jan. 1900, CUL. 164 E, Rutherford, 22.cit., 268-70. 165 Ibid., 273-4; assumptions soon rejected - see below, this Section, n.I80. 166 ibid., 285. 167 See above, Chapter 1, Section 4, p.36-7. 168 E.Rutherford, 22.eit., Pte, 287. 169 Kelvin, RI Lib.8617, (1 J), 3, 227-56, 227; atomic or molecular diEneters ranged from 10-' to 10-6 cm. 170 E.Rutherford, op.cit., Papers, 294. 171 A.S.Eve, Rutherfo7Z-Etc., 172 E.Rutherford, Papers, 295. 173 Ibid., 294. 174 (7:7:Thomson, Radium, Nature, 1903, 67, 601-2; E.Rutherford, 22.cit., Papers, 287, 294; id., Radio- _._.qI121-±Y, 1905, 457. 175 -Elhirford, 22.oit., 2222E2, 294. 176 Ibid. 177 Ibid., 295; he had used the term 'atom' in 1898 rfaTers, 214-5). 178 For example, J.P.Cooke, The New Chemis_tEz, London, 11th ed., 1903, 72-7, wiTHreTirence fd-Keivin, states that 1 litre of any gas under standard conditions contains 61 x 10' molecules, and 1 litre of hydrogen weighs 0.09 gm. One can from this deduce the weight of a hydrogen atom to be 1046 gm., hence Ur = 2 x 10-/'4 gm. 179 E.Rutherford, op.cit., 0.._apers,. 293. 180 Ibid., 295. RaTrierford, Papers, (1903), 607, had to accept Townsend's commenin a letter from Oxford misdated 14th Jan.1900 (written in 1901), that his value for the ionisation energy was 'far too large' by a factor of at least twelve. Any difference this might have made to Rutherford's arguments soon vanished as the Curies reported radium specimens,of activities sufficiently high to compensate. The discrepancy was in the early assumption that all the radiated energy produced conductivity in the gas; it was later'impossible to estimate' how much was dissipated as heat(RUtherford, Radio-activity., 1905, 58-9). Notes for Chapter 3, p.119-178) 308

181 ibid., 294. 182 Ibid., 295. 183 X7g:Eve, Rutherford.Eto., 73. 184 Ibid. 185 reigon, 1900. 186 62, 525-6. Also, see the Proutian interpretations of 7Illard's work, below p.168, n.213. W.Kaufmann, Electrician, 1901, 48, 95-7, was to express ideas similar to those of Fitzgerald. 187 Chem.N., 1900, 81, 304-5; trans. from Ber.deut.chem.Ges. 188 7717tTica, Chem N., 1900, 82, 166-7; from Miliiik711"- Zeitung; he hadFeld such Views for twelve years or more. 189 E.E.Pournier d'Albe, The Life of Sir William Crookes, London, 1923, 372. 190 F.P.Venable, The Nature of Valence, J.Amer.Chem.Soc., 1899, 21, 192-200, 220-31, p.197. - 191 CR, 1899, 129, 593-5. 192 A.Debierne, CSR, 1900, 130, 906-8. 193 H.W.Kirby, T Discovery of Actinium, Isis, 1971, 62, 290-308, questions the identity of theEETerials described in Debierne's two papers and credits F.Giesel with the discovery in 1902 of 'emanium', the element now known as actinium. 194 P.Curie, Oeuvres, (1899), 345, Nov. 195 A.Debierne, p.cit., n.192, Apr.1900. 196 E.Rutherford (and ,.Soddy), The Radioactivity of Thorium Compounde.I, Papers, (1902), 376-402, 378, Jan. 197 Ann.d.Pplys., 1899, 69, 91-4; briefly in Phy.2.Z., 1899, . 198 The Curies appear to have observed this by July 1899, before Giesel's publication, see I.Joliot-Curie in M.Curie, Pierre Curie, Paris, p.120; but they accord priority to GiesCi1 TCongr6s 1900, P.Curie, Oeuvres, 388. The effect was later explained by an accumulation of 'emanation'. 199 F.Giesel, Einiges abor Radium-Baryum etc., Verh.Dout. Phvs.Gos., 1900, 2, 9-10, dated Dec.1899. 200 A.DebTeTne, CR, 1700, 131, 333-5, 30th Jul.; trans. Chem.N., 1907 82, 85. 201 751777 Chem.N. 202 Ibid., TN:- 203 FiTT:deuTTchem.Ges., 1900, 33, 1237-40; Chem.N., 1900, 82, 2576, dat:TiTiffay. 204 Ibid., 25. 205 J76T:deut,chem.Ges., 1900, 33, 1665-8, received 28th May. 206 F.Giesel, ibid., 1668; he sIso pointed out that his own and the Curies' polonium differed both in radiation type and rate of decay, to which the age of the specimens might be relevant. Marie Curie herself wavered towards the belief that polonium was merely induced bismuth, in 1902. 207 K.A.Hofmann and E.Strauae, Radioactives Bloi and radioactive seltene Erden, Ber.deut i chem.Ges., 1900, 33, 3126-31. Of interest are modern transformation series in which no less than four natural, active, true lead isotopes feature; their half-lives are about 27 min., 36 min., 11 hr., and 22 yr. It seems that Hofmann may have had any or all of these, followed by their active Notes for Chapter 3, p.119-178) 309

207 oontd.) decay products, Bi, Po, Tl, in his preparations from different minerals; even if he had avoided traces of other active elements. 208 P.Giesel l Ueber radioactive Stoffe, Ber.deut,chem.Ges., 1900, 33, 3569-71; Chem.N., 1901, 837-122-3. 209 Hofmann and StrausiTriiia., 1901, 7, 8-11, received 28th Dec.1900. 210 F.Giesel, ibid., 3772, thought this work unreliable. 211 Hofmann and A.Korn, ibid., 1901, 34, 407-9. 212 SOances Soc.Fr.TIcET.;7700, 59*, bth Jul. 213 In relatICTI T45 hIS G.Sagnac had written to J.Larmor (letter dated llth May, 1899, Royal Society Library) concerning types of vacuum tube. He suggested that Villard's idea that Whydrog6ne est indispensable a la formation dee ions cathodiques' would be unnecessary if all bodies were 'forme dune meme matiero simple qui serait la matiare radiante de Crookes'. Villard himself believed in the unity of radiant matter but explained this by the universal presence of the penetrating and chemically reducing element hydrogen, presumably as an impurity; see e.g. P.Villard, La formation des rayons cathodiques, Rev.Gon.des Sci., 1899, 10, 301-8; id., Les Rayons CathodUiaes, Rapports,Cong.int.de Physique, 1900, 3, 115-37/ 136-7. 214 See Stokes, Memac Correa., 2, 484. 215 J.C.McLennan, PhIl.Mag., 19u2, 3, 195-203. 216 Hofmann and Strauss, Leber das radioactive Blei.2. Mitteilung, Ber.deut.chem.Ges., 1901, 34, 907-13, 913. 217 Another feature was the dffiriirence between salts, id., 3.Mitteilung, Ibid., 3033-9. 218 F.Gieeel, Uebe7-717dleactive Stoffe, ibid., 3772-6; the modern theory Ewes only admixtures and does not admit induction; active lead is a 'transformation product' of radium. 219 K.A.Hofmann and Strauss, Ueber radioactive Stoffe, ibid., 3970-3, received 27th Nov.1901. 220 P,Giesel, On radio-active lead, Chem.N., 1902, 85, 89-90; from Ber.deut.chem.Ges., 7517, 35, 102f., Jan. 221 K.A.Hofmann 770, 1f1, radioacTIve Stoffe.l. Ueber radioactive° Blei, ibid., 1902, 35, 1453-7, Apr.; self-recovery shows the activity is not the induced kind. 222 Id., Chem.N., 1903, 87, 241-3; from Ber.deut.chem.Gos., 703,77; 7040f. 223 Proc.22y.Soc., 1900, 66, 409-22, read 10th May; reprinted WITFoai-iETTIrpretation in A.Romer, Discovery of Radio- activity, 70-84. 224 Crookes in Romer, 22.cit., 82. 225 G.G.Stokes, Memac.-Co7i-e-s., 2, 490. 226 Ibid., 490-2. 227 7/7nFookes, Notebook 16, pp.270, 305f., RI. 228 Ibid., Notebook 17, 102f. 229 Ibid., 106. 230 Crookes in Romer, opecit., 74. 231 Ibid., 71. 232 MU., 77. Becquerel was soon to deny this, with important riequences, see below, p.171; and Chapter 4, Section 1, p.187, n.28. Notes for Chapter 3, p.119-178) 310

233 Letter from W.Crookes to P.Curie, dated 13th Jul.1900, BR. 234 References were provided, namely Ber.deut.chem.Ges., 1900, 33, 1237-40 and ibid., 16657U7 Sae move, p.166-7. 235 TypescHpt translation-GT-letter from P.Curie to W.Crookes, dated 17th Jul.1900, appended at end of Crookes' Notebook 17, RI. 236 Letter from Crookes to P.Curie, dated 19th Jul.1900, BN. 237 W.Crookes, Radio-activity and the Electron Theory, Chem.N., 1902, 85, 109-12, 109. 238 Chem.7.1., 1901, UT, 179-81, 187-9; he referred to 737Riaunerts similar work; the latter, Chem.N., 1901, 84, 219, claimed priority; these researches did not extend beyond the field of inorganic chemistry. 239 Baskerville, ibid., 179. 240 CR, 1900, 130, 1583-5. 241 H.Becquerel, Sur is rayonnement de l'uranium, ibid., 1900, 131, 137-8, 16th Jul. 242 BecqueI, Sur le rayonnement de l'uranium etc., 11122ports,241264.Int.do Pbrsi ue, 1900, 3, 47-78, 74. 243 P. aT7M.(:urieTres nouvo lea substances radioactives etc., ibid., 79f.; P.Curie, Oeuvres, 374-409. 244 ibid.,qm 245 YETa., 407. 246 See above, Section 3, p.155-6. 247 P.Curie, 22.cit., Oeuvres, 384. 248 Ibid., 379-80, 404-b. 249 Abh.der Naturf.Ges.zu Halle, 1901, 23, 1-15, read 7aii.1900. 250 Ibid., 1. 251 YETU., 11-12. 252 Ibid., 13. 253 TUTU., 15. 254 77gnning, Ueber radioactive Substanzen, Ann.d.Phys., 1902, 7, 562-75. 255 Ibid., 569. 256 Letter from J.J.Thomson to E.Rutherford, dated 15th Feb.1901; A.S.Eve, Rutherford.Etc., 76. 257 E.Rutherford, Pa ers, 261. 258 E,Dorn, Phys.Z., 1 1, 2, 218, received 24th Dec.1900. 259 Rutherford, PEyq.Z., 19a, 2, 429-31; Papers, 296-300. 260 Ibid., Papers, 230. 261 Rutherfor op.cit., (1901), 300. 262 A.S.Eve, Rutherford.Etc., 77. 263 Ibid., 78. Letter dET-Jd 12th Apr.1901. 264 Above, Section 2, p.145. 265 Letter from J.J.Thomson to E.Rutherford, dated 25th Apr., CUL. 266 E.Rutherford, Papers, (1901), 325, 358-9, Dec.; see below, Chapter 4, Section 2, p.208, 223-4. 267 Id., Papers, 230. 268 TTss Brooks' first published research, performed with Rutherford's help and on one of his subjects, concerned 'Damping of the Oscillation in the Discharge of a Leyden-jar', Phil.Mag., 1901, 2, 92-108; she was B.A. Tutor in MathWffialics, Royal Victoria College for Women, Montreal, at the time. Notes for Chapter 3, p.119-178) 311

269 Trans.jya.Soc.Canada, 1901, 7, 21-5; Rutherford, 15656713, 301-5, 270 Nature, 1901, 64, 157-8; Papers, 306-8. 271 Ibid., 305, 3087 272 5-675-e.g. S.Glasstone, Sourcebook on Atomic Energy, London, 1950, 125. F.Soddy, Radio-activity, Electrician, 1904, 52, 681, deduced from the same results a doubled aTFinic weight of 160, at a time when theory demanded a value oloee to that of radium. 273 E.Rutherford, Papers, (100, 545. 274 Rutherford,227etT77- Papers, (1901), 305, 308, May. 275 Rutherford, op.-617., Nature, Papers, 308. 276 See below, CH-gpter 5, Section 1, p.233-6. 277 H.Geitel, Ueber die Elektrizitatszerstreuung in abgeschlossenen Luftmengen, 1900, 2, 116-9. 278 J.Elster and H.Geitel, Weiiire Versuche etc., ibid., 1901, 2, 560-3. 279 Id., Leber eine fernere Analogie in dem elektrischen Wrhalten der naturlichen and der durch Becquerel- strahlen abnormleitend gemachten Luft, ibid., 590-3. 280 H.Geitel, Ueber die durch atmospharischirrift induzierte Radioaktivitat, ibid., 1901, 3, 76-9. 281 Id., Archives dee Sciences, 02,S 13, 113-28, 122; 271-Fie the followIE7g Section. 282 Ibid., 124. 312 NOTES FOR CHAPTER 4 (pages 179-225)

1 H.Geitel, Archives des Sciences, 1902, 13, 113-28, dated Dec.190i. 2 Ibid., 127. 3 Ibid., 126-7. 4 Ibid., 117. 5 Mister and H.Geitel, Vereuche fiber induzierte Radio- activitAt der atmosphilrischen Luft durch positive Potentiale, Phys.Z., 1902, 4, 97. 6 Id., On the radio-active emanation in the atmospheric air, Chem.N., 1903, 88, 29-32, 52-4, received by P s:277-6-Th Jun. 7 er .Deut.Ges.Natf., 1902, 73, 83-99, 98; read ME 76571901.-- 8 CR, 1901, 132, 548-51; P.Curie, Oeuvres, 410-13. 9 UR, 1901, 137, 768-9; P.Curie, Oeuvres, 414-6; read 25th /5.7.1901. 10 Paers, 301-5; with Miss H.T.Brooke. See above, apter 3, Section 4, p.176. 11 P.Curie, 2p.cit., Papers, 414-6. 12 Ibid., 41 13 T.77 at the time of the Congress in Aug.1900, P.Curie, Oeuvres, 407-8. 14 1'. (,curie, Oeuvres, 412, 5th Mar.1901. 15 Notebook, pp.125 plus pp.18 in reverse direction, written by P. and M.Curie; records experiments on induced activity from 5th Dec.1900 to mid-1902; also various radiation and chemical studies from May 1899 to late 1902; held at Wellcome Historical Institute. 16 E.g., ibid., p.72; mouvement propre: 10th Jan., 90 gm. in 40 sec.;11th Jan., 90 in 36; 12th Jan. worsening to 90 in 11. 17 Ibid., p.101, 10th Jul.1901, action of the extreme cold of liquid oxygen upon radium and thorium: radium's activity fell from 2000 gm. in 20 sec. to 200 in 12; thorium gave initial readings of 50 gm. in 30, then 23, then 37 sec. But mouvement propre was 50 in 23, consequently 'c'est mt. propre pas action'. 18 CR, 1901, 133, 276-9, 29th Jul.; P.Curie, Oeuvres, 420-3. 19 Tad., 421. 20 CR, 25th Mar.1901; P.Curie, Oeuvres, 416. 21 Mem.N., 1901, 84, 88-9. Perhaps P.de Heen's publication TO-Tu-Li.1901 on 'La radioactivit4 de la matiere et l'energie susceptible de se developper e. la surface des corps' (Rev.Sc., 1901, 16, 161-70) should be mentioned here, formegave something of a mechanism for radioactive induction through gases: molecules irradiated by an active source themselves emitted rays or 'jets d'ether', which excited radiations in other molecules, and so on. No doubt P.Curie thought this work as weak as that of G.Le Bon which de Heen cited and which Curie had already criticised. The latter published no further comment on the researches of either of these obscure scientists, reserving his considerations for others now better known. Notes for Chapter 4, p.179-225) 313

22 P.Curie and A.Debierne, Sur la radioactivito induite provoquee par los sells de radium, CR, 1901, 133, 931-4, 2nd Dec.; P.Curie, Oeuvres, 424-7. 23 Letter from F.Giosei to P. and M.Curie dated 23rd Mar. 1902, BN; see below, Chapter 5, Section 1, p.232. 24 P.Curie, Oeuvres, 421. 25 E.g. Sur la radio-activite secondaire, CR, 1901, 132, 734-9, 25th Mar. 26 CR, 1901, 133, 977-80, 9th Dec. 27 On the Conductivity of Gases under the Becquerel Rays, Phil.Trans.&., 1901, 196, 507-27, 525. M. Curie, Sur la pbnetration dos rayons etc., CR, 1900, 130, 76-9, 8th Jan., had earlier likened these rays to 'projectiles'. 28 The statement flatly contradicted the comments published by Crookes on Ur nitrate in 1900; see above, Chapter 3, Section 4, n.232. 29 CR, 1900, 130, 1583-5, 1585. 30 -H.Becquerei;-Sur la radio-activitO de l'uranium, 978. 31 P. and M.Curie, CR, 1902, 134, 85-7, 13th Jan; P.Curie, Oeuvres, 428-30; A.Romer, Discovery of Radioactivity, 117-23, gives translations of this paper an of Becquerel's. 32 P.Curie, Oeuvres, (1900), 356. 33 M.Curie, Oeuvres, 104-5; see above, Chapter 3, Section 3, p.156-7. 34 Chem.N., 1902, 85, 169-72, read at RI on 7th Mar. 35 Rev.Sc., 1901, 15, 449-61, read Feb., pub. Apr. 36 Ibid., 460-1. 37 767above, Chapter 2, Section 4, p.110-1. 38 G.G.Stokes, Mem.& Corres., 2, 478-81; both hypotheses required an external supply. 39 See o.g. Phil.Trans.A., 1901, 196, 507. 40 Letter dated 16th Dec.1900 in reply to Stokes' of 15th; Stokes, Mem.& Correa., 2, 481-5. 41 Crookes, loc.cit. in Stokes, Mem.& Correa. The cited paper is W-P7VIllard, Sur la perm6EFI1ITe de la silice fondue pour l'hydroOne, CR, 1900, 130, 1752-3. 42 P.Villard, Les Rayons CaliRidiques, tiapportili,22EE.Int.de, Ph ai ue, 1900, 3, 115-37, 136-7, Aug. 43 Crook©s in Stokes, loc.cit., 489-90, letter dated 15th Jun,1901. 44 W.Crookes, Notebooks, 17, and 18, RI. 45 Ibid., e.g. 17, 308-69; 18, 149-85. 46 -076R.IT., 1907 85, 109-12, read to the Royal Society MeE. 47 1901, 83, 130, from Bristol; Martin gained his B.Sc. in that year. He studied at University College Bristol and several German universities before becoming Lecturer at University College Nottingham in 1907 and at Birkbeek College, London in 1910. He later held a variety of industrial posts and published prolifically on chemistry - pure, industrial, and popular. 48 Ibid., 141, 22nd Mar.; E.Booth, ibid., 262-3, discussed Ts further. 49 Chem.N., 1902, 85, 205-6, dated 26th Mar., Berlin University. Notes for Chapter 4, p.179-225) 314

50 L.Boltzmann, Lectures on Gas TheoriL, (1898), 377, dopioted the chemical.- YOna"-Tis an overlap of supposed sensitive regions of blank material atoms. VI.Sutherland, The Cause of the Structure of Spectra, Phil.Mm., 1901, 2, 245-74, 269, illustrated his spherical material atom as furnished with a few electrons, some of whose orbits collided with the atomic surface. See Chapter 1 above and Chapter 5, Section 2 below for further discussions of theories of atomic structure. 51 Chem.N., 1911, 103, 169. 52 Leipzig, 1902; pref. dated Easter. 53 E.g. see W.Kaufmann, The Development of the Electron Idea, Electrician, 1901, 48, 95-7; see the preceding Section, p.163T-YOr Fitzgerald's similar speculations in 1900. 54 J.Stark, 22.cit., 93-4. 55 Ibid., 34. 56 Tail., 35. 57 Letter dated 5th Jan.1902, A.S.Eve, Rutherford.Eto., 80-1. 58 Letter to Mary Newton, dated 25th Jan.1896, ibid., 23-6,26. 59 Letter from F.Soddy to E.Rutherford, dated 17TE-Dec.1903, CUL, concerning among other items Becquerel's book of 1903. 60 N.Foather, Lord Rutherford, 1940, 78-90, describes some of the poin1 A.Romer, The Transformation Theory of Radioactivity, Isis, 1958, 49, 3-12; id., The Restless Atom. The Awakening of Nuclear Physics, NeW-York, 1960, 591, outlines ithe stages in clear and simplified form. T.J.Trenn, Rutherford and Soddy: from a search for radioactive constituents to the disintegration theory of radioactivity, Rote, 1971, 1, 51-70; id., The rise and early development of the disintegration theory of radioactivity, Dies., Univ. Wisconsin, 1972, gives more detailed but sometimes less clear descriptions specifically limited to the Rutherford-Soddy experimental collaboration of Sep.1901 to May 1903. 61 E.Rutherford, Pa ere, 305, 308; see above, Chapter 3, Section 4, P-1 62 T.J.Trenn, Dips., 14-15, 60, states that there is no evidence for the assumption that the collaboration began before Sep.1901. 63 A.S.Eve, Rutherford.Etc., 77. 64 M.HoworthTPIWEIFFResearch on the Atom ... The Life St2ry of Frederick Aaddz, London, 1958, 79-81; the Yaw mss. are in th-e-nddleian Library, Oxford, where they were placed by M.Howorth. Trenn, Dies., 60, has found Rutherford's brief notes on the 465Tings of the Physical Society from 1898 to 1907 in the McGill University Archives, and confirms that both parties refer to the same meeting. 65 'The Indivisibility of the Atom', pp. 23, typescript, Soddy-Howorth Collection, Bodleian Library. 66 M.Howorth, Pioneer Research.Soddy, 81; id., Atomic Transmutation. The Greatest Iiiadvery Ever Made, London, 17577617---- 67 J.J.Thomson, On Bodies Smaller than Atoms, pop.Sci. Monthly, 1901, 59, 323-35, Aug.; similar to R1Lecture o 19th Apr.19017 68 M.Howorth, Pioneer Research.Soddy, 65; Soddy's own comment. Notes for Chapter 4, p.179-225) 315

69 Ibid., 64; six lectures are preserved. 70 Tura., 85. 71 E.Rutherford and F.Soddy, J.Chem.Soc., 1902, 81, 321-50; Rutherford, aTers, 376-407. 72 E.Rutherford an .Soddy, An Investigation of the Radio- active Emanation produced by Thorium Compounds.I , Proc. Chem.Soc., 1902, 18, 2-5; Chom.N., 1902, 85, 55-6. 73 E70treFford (and-P.Soddy)77a.at., Papers, 381. 74 Ibid., 385-7. 75 1:.g. at that time G.Martin asked 'Is Argon an Elementary Substance?', Chom.N., 1902, 85, 9, 3rd Jan., but only to suggest it m3.ghh be a mixare of several inert gases. H.E.Armetrong, who attacked W.Ramsay's conclusions regarding these gases, was in this case an exception. 76 E.Rutherford (and F.Soddy), 22.eit., Papers, 395-6. 77 Ibid., 395-6. 78 YTT3., 388-9. 79 170., 392-4. 80 nu., 390. 81 TEM., 391. 82 TE,frOworth, Atomic Transmutation, 44; id., Pioneer Research.Soddy, 82-3. 83 E.RutherfordCand F.Soddy), ap.eit., Papers, 396. The other possibility was that thiiiraR induced activity upon one of the atmospheric inert gases; concerning which see below. 84 E.Rutherford, Early days in radio-activity, J.Franklin Inotitute, 1924, 198, 281-9, 285. 85 T-Fullord, Papers_, 396. 86 Ibid., 391. 87 Tailierford had seen J.J.Thomson's paper to that Congress by March 1901; letter from Rutherford to Thomson, dated 26th Mar.1901, A.S.Eve, Rutherford.Etc., 77. 88 J.Amer.Chem.Soc., 1901, 2:3, 761f., presented 27th Aug.; aem.N., 1961784, 179-817 187-9, p.181. 11th Oct. 89 TETEerford, 2p.at., Pa ers, 379-80. 90 Letter from Ur-o-61-gs to z erford, dated 18th Dec.1901; A.S.EVe, Rutherford.Etc., 79. 91 E.Rutherford, op.eit., The Radioactivity of Thorium Compounds.I.Etc., Papers, 389. 92 Ibid., 396-7. 93 1.-15ra. 94 ibid., 397. 95 mu. 96 ibid., 398. 97 1.RUmer, Restless Atom, 61; T.J.Trenn, Dies., 94. 98 E.RuthernTITTEHff IF:Soddy), 22.cit., 398, 402; they confirmed this dismissal with the German nitrate from which the impurity was absent, as noted in their 2nd publication of May 1902, Pa ers, 435. 99 Ibid., 398; the 'two directions' are probably the precipitation and washing methods of removing ThI from Th, see below. 100 See the preceding Section, p.184-5. 101 Op.cit., Rutherford, faaE2, 402. 102 ma 390. 103 Ibid., 399-400. 104 R.Rutherford (and F.Soddy), Papers, 447. Niates for Chapter 4, p.179-225) 316

105 Id., The Radioactivity of Thorium Compounds.I.Etc., Pa ern, 399. 106 Barramer.Ehys.Sac., 1901, 2, 37-43; E.Rutherford, Ti-ir-67525-c30",-7351-9, &I-tea 15th Dec. 107 / .J.Allen, rhys.Z.. 1902, 3, 225-30, dated 20th Dec.1901; RaTher?ord, pppprs, 360-9. 108 E.Rutherford, Transmission off-Excited Radioactivity, DIT2TP, 329. 109 Td:,-Excited Radioactivity etc., Pa ers, 367. 110 LIR, 1901, 133, 977-80, 9th Dec. Ruvher±ord, whose paper was dated I5Th Dec., received by Ehys.Z., 22nd Jan.1902 (Pa ors, 359) may just possibly have derived the recoil idea rom Becquerel; the Abstract of Rutherford's similar paper to the American Physical Society, dated 14th Dec.(Papere 330) does not mention it. Becquerel, however, u-sedTHE)' notion in a slightly different way. 111 E.Rutherford, ap..cit., Papers, 368. 112 E.Rutherford (and F.Soddy), the Radioactivity of Thorium Compounds.I.Etc., IlusEE, 378; perhaps Soddy influenced the brief statement given there. 113 E.Rutherford, op.cit., Papers, 368. 114 Id., (with S.J.Afien), Papers? 509, dated Jun.1902. 115 13Toc.Rox.Soc., 1902, 6971-77:82, Dec.1901. 116 E.Rutherford, Excited -Radioactivity etc., Pa ers, 369. 117 E.Rutherford and F.Soddy, Chem.N., 1902, 8,-6.-6. 118 Ibid., 56. 119 771athorford (and F.Soddy), Radioactivity of Thorium Compounds.I.Etc., Pates, 396. 120 E.Rutherford, Exciid Radioactivity etc.. Papers, 368. 121 Id., Phys.Z., 1902, 3, 254-7, Papers, 376-5. There are apparently no surviving English versions of several of Rutherford's publications. 122 Made by early December, 1901; Rutherford, Papers, 327, 371. 123 ibid., 372-3. 124 E.Rutherford, Papers, 436. 125 CR, 1901, 133.-0778-0; see the previous Section, p.187. 126 rffiomson's letter of 2nd May 1902 to Rutherford (CUL) may modify the view (A.Romer, Isis, 1958, 3) that the election of this candidate was not to be expected at the first attempt. The former regarded the election as certain, thought the result 'a great scandal', and accused the new Secretary of bias in favour of his 'fellow townsmen' of Belfast. Rutherford at 31 was not particularly young for those days; C.T.R.Wilson at that same age had been one of the fifteen out of sixty candidates to be selected in May 1900. Rutherford, however, along with J.S.Townsend had only a year to wait for the honour, whilst Pierre Curie was experiencing worse problems with the Acadomie des Sciences in Paris. 127 J.Chem.Soc., 1902, 81, 837-60, 15th May meeting; .Rutherford, Papers, 435-56. 128 Ibid., 436. 129 Ma., 438-40. 130 UAW., 440. 131 Mid., 441-2; the emanation was as usual prevented from interfering by means of a draught of air, ibid., 436. Notes for Chapter 4, p.179-225) 317 132 Ibid., 449. They did not make it clear that this was E3T—a case of direct induced activity, and in one place referred to excited activity as a 'secondary radiation' (ibid., 450). A year earlier Rutherford (Pa ers, 305, 308) wrote of the production from radium emanation of 'a positively charged substance which ... becomes a source of secondary radio-activity'. The terminology was sometimes ambiguous but was explicitly clarified shortly afterwards as discussed below. 133 Ibid., 451. 134 7737., 447. 135 151a., 442-4, 448. 136 mod., 444-5. 137 Ibid., 455. 138 Ibid., 440-1. 139 TM. 140 !Era., 379. 141 See above, Chapter 3, Section 4, p.167-9. 142 K.A.Hofmann and F.Zerban, Ueber radioactive° Thor, Ber.deut.chem.Gos., 1902, 35, 531-3, received 23rd Jan. 143 Ylj.561f-discliTa7ge in aboUT 4 mins. 144 Rutherford, 22.cit., 452. 145 Ibid. 146 mod., 454. 147 Ibid., 452. 148 =em.Soc., 1902, 81, 860-5, presented (not read) 15TE-May; also Chem.N., 1902, 86, 199-200. 149 Ibid., 863. Within weeks he hathe modern answer: the 7TEdual activity belongs neither to 1. nor 2. but to Ur itself; it declines immeasurably slowly. The path to that interpretation was not simple; see below. 150 Ibid., 864. 151 F.Giesel, On Radio-active Lead, Chem.N., 1902, 85, 89-90; from Ber.deut.chem.Ges., 18th 7- an. 152 F.Soddy, RadioactivitTU? Uranium, 864. 153 P.and M.Curio, Sur lea corps radioactifs, Jan.1902; P.Curie, Oeuvres, 429. Rutherfores cautious description at this time was 'polonium (radioactive bismuth)', Deviable Rays etc., Pa ere, 470, dated 7th May 1902. 154 Ber.deut.chem.Ges., 02, 35, 2285-8, presented 9th Jun.; Chem.N., 1g57,-166, 52-3. 155 P. Soddy wrote from Montreal to E.Rutherford, who was on vacation, explaining the paper's contents and stressing its importance; letter dated 12th Jul.1902, CUL. 156 F.Soddy, Radioactivity of Uranium, 864- 157 Dated 29th Apr.1902, CUL. 158 Proc.Roy.Soc., 1902, 69, 413-22, discussed above, p.191-3. 159 ma., pr(567Roy.soc.,-799-413. 160 ibid., 410-3. 161 N.Feather, Lord Rutherford, 88; L.Badash, How the 'Newer Alchemy' Was Received, Sci.Amer., 1966, 215, 88-95. 162 E.Rutherford (and F.SodiTYT, Radioactivity of Thorium Compounds.II, Papers, 454-6. 163 E.Rutherford, The Existence of Bodies Smaller than Atoms, Papers, 403-9, 409; read to the Royal Society of Canada, 2/th May 1902. Notes for Chapter 4, p.179-225) 318

164 E.Rutherford, Radioaotivity of Thorium Compaunds.II, loo.cit., 452, 455. 165 PEI17E., 1902, 4, 569-85; Rutherford, Papers, 494-508. 166 WY:Trann, Diss.. 316-22, suggests a mid-June submission since they did not cite Marckwald on Po, and the delay for some other papers at this time was around 5 to 6 months. 167 E.Rutherford, op.cit., Pa ers, 508; the phrase 'accompaniment of a chemical change -used in the previous paper written in April (id., Papers, 455) evidently had no specific temporal meaning. 168 Ibid., 508. 169 THU. 170 Radioactive Change, Phil.Mag., 1903, 5, 576-91; Rutherford, Pa ere, 3W608, 603;•proNlably submitted in about Mar. 1 . 171 Letter from P.Soddy to E.Rutherford, dated 26th Sep. 1903, CUL; any reply appears to be lost. 172 Ibid. 173 E.Rutherford, Phil.Mag., 1903, 5, 95-117, dated 29th Jul. 1902; Papers, 377:0, 529. 174 Ibid., 539-41. 175 IFItE, 358-9. 176 IcRatherford, Pa ors, 415-34, 421. 177 E.Rutherford (an .J.Allen), Papers, 509-27. 178 Ibid., 517. 179 ITRUtherford, Excited Radioactivity etc., Papers, 545-6. 180 Ibid., 546. 181 It is interesting that Rutherford at this point had adopted a completely particulate or non-vibrational view of all radioactive radiations including, temporarily, the penetrating gamma rays which he thought were high velocity electrons; E.Rutherford, Penetrating Rays from Radio-active Substances, Nature, 1902, 66, 318-9, 6th Jul.; faa.2, 410-4, 413. 182 Rutherford, 22.cit., Pa ere, 546. 183 Ibid., 544-7; atom and molecule of an inert gas were Fin to bo identical. 184 E.Rutherford, Phil.Mag., 1903, 5, 177-87, dated Nov.1902; id., Papers, 1.ggl-57; T.J.Trenn, Dies., 209-31; A. Romer, Tistless Atom, 71-84. 185 TTUTEriford and F.Soddy, The Radioactivity of Uranium, Phil.Ma .1 1903, 5, 441-5; id., A Comparative Study etc. THU., 45-57; Rutherford, Papers, 561-75, 564, 575. 319 NOTES FOR CHAPTER 5 (pages 226-280)

1 M.Curie, Pierre Curie, N.Y., 57. 2 P.Curie, URT-Tg6277154, 420-3; Oeuvres, 431-4. 3 P.Curie, MI, 1902, 175, 857-9, I7TEYE.; Oeuvres, 435-8. 4 Seances Soc.Fr.phyp., 1902, 60*, 19th Dec. 5 Pieuriel7174-7903-,-- 136, 223-6; Oeuvres, 440-3. 6 Ibid., 4477 7 77RUtherford, Papers, 508. 8 Ibid., 498. 9 TTOTirie, 22.cit., Oeuvres, 442. 10 Id., 443. 11 E.Rutherford, 2E12E2, 507-8, Nov.1902; the previous publication in Phil.M2a., Sep.1902, had made the point fairly clear however; and an abstract of the earlier paper of May 1902 in Rev.Gen.des Sci., 1902, 592, 30th Jun. ended 'La radioactivitb serai7Th manifestation d'un changement chimiqus sous-atomique'; whether Curie saw or understood this one cannot say. 12 E.Rutherford, Phil.Mea., 1903, 5, 481-5, dated 28th Feb., Apr. issue; Pa ere, 576-9. 13 P.Curie, Sur la r eparition de la radioactivite induite par le radium eur les corps solides, CR, 1903, 136, 364-6; Oeuvres, 444-7. 14 CR, 19077 T77 673-5. 16th Mar.; P.Curie, Oeuvres, 448-51. 15 A.Laborde,-fferre Curie dans son Laboratoire, Univ. do Paris, 1956, 5773. 16 P.Curie, 22.cit., Oeuvres, 450. 17 E.Rutherford, Papers, 607, probably written Mar.; pub. May 1903. 18 H.Becquerel, Recherches sur une propriet6 nouvelle etc., pref. dated Aug.1903. 333, seems to have thought that Curie had actually adopted a slow atomic transformation theory. 19 P.Curie (and J.Danne), Sur l' emanation du radium et son coefficient de diffusion dane lair, CR, 1903, 136, 1314-6, 2nd Jun.; Oeuvres, 452-5. 20 Proo.RI, 1903, 17, 389-402. 21 Letter from Soddy to Rutherford, dated 31st Mar.1903,CUL. 22 Trans. in Chem.N., 1903, 88, 85f., in several instalments; contains no menTion of the condensation and diffusion experiments of P.Curie on the emanation. 23 See Chapter 3, Section 4, p.169. 24 Letter from F.Giesel to P. and M.Curie, dated 23rd Mar. 1902, from Braunschweig, pp.4, BN. 25 Ibid. 26 V.Giesel, Zeit.f.Elektrochemie, 1902, 8, 579-85, pub. Aug. 27 F.Giesel, Neues fiber Radium and radioaEtive Substanzen, Jahresb.d.Ver.f.Wiss.,BraunschweiE, (1902), 13, 43-5, 43; 30th Oct. meeting, pub.1904. 28 Ibid., 45. 29 Ti7e7k)1, ok.cit., n.26. 30 E.Rutherford, Pa ere, (1904), 706. Also, early in 1903 Giesel criticise e chemical evidence for the transformation of thorium: this element, he thought, owed its activity to the 'permanently' active constituent which he had extracted from pitchblende, Chem.N., 1903, 87, 97-8. Notes for Chapter 5, p.226-280) 320

31 E.Rutherford, Loos the Radio-aotivity of Radium depend upon its Concentration?,Nature 1904, 69, 222, dated 18th Dec .1904; Id., Papers, 6.18-9. 32 Radium, Nature,-T903.677601-2, 33 F.Giesel snot© equivoCgIly on the matter, as discussed above; E.Dorn, discoverer of radium emanation, remained silent. 34 Letter from Thomson to Rutherford, dated 13th May 1902; A.S.Eve, Ruthorford.Etc., 82. 35 J.J.Thoms7,7767 755ffEiises of the Ions etc., 1899, 558-9 refers to this result of Wilson; see C.T.R.Wilson, On the condensation nuclei produced in gases by the action of Röntgen rays, uranium rays, ultra-violet light, and other agents, Phil.Trans.A., 1899, 192, 403-53. 36 C.T.R.Wilson, P166.Roy.goc., 1902, & 277-82. 37 Phil.ym., 1902, 3,-I95:2-0-3, dated Mic.1901; presented by Thomson. 33 Proe.Camb.Phil.Soc., 1902, 11, 504. 39 ITATDDFCTIvity PY-auced by Action of Thorium Compounds, papers, 259. 40 T:7.Thomson, Proc.Camb.Phil.Soc., 1902, 11, 505. 41 Letter from ThomsoirTiT RutheTTTIrd, dated-7nd May 1902, (JUL. 42 Ibid. 43 17:17-iutherford, 2.E.p.frs. 378, read Jan., pub. Apr.1902. 44 Ibid., 368-9, dated Dec.1901. 45 175TTer from Thomson to Rutherford, dated 13th May 1902; A.S.Eve, Rutherford.Etc., 82. 46 Phil4ag., 1902, 4, 35Z-671 dated Jun., Sep. issue. 47 TEIL9-353. 48 'o. d. 357. 49 See above. Chapter 4, Section 1, p.179-80. 50 Thomson, op.cit.. 360. 51 Ibid., 364--57-- 52 17Etherford, papp_KR, 455, pub. Jun.1902. 53 E.Rutherford (iiidS-;J.Allen), Excited Radioactivity and Ionization of the Atmosphere, Phil.Maa., Dec.1902; Papers, 509-27, 513. 54 II:Rutherford (and F.Soddy), Phil.Mag., Sep., Nov., 1902. Paers, 472-508. 55 1903, 106, 289-93. 56 Ibid., 291. 57 I:FIT..., 292. 58 TBIJ., 292-3. 59 Ibid., 293; my emphasis. 60 Described in a Note read on 19th Nov., Proc.Chem.Soc., 1902, 219-20; E.Rutherford (and F.Soddy77-Papers, 528. First achieved in Oct. when a liquid air plant was installed, A.S.Eve, E.Rutherford.Etc., 89. 61 Letter from E.Rutherford to J.J.Thomson, dated 26th Dec. 1902, CUL; also reporting the as yet unpublished magnetic and electric deviation of alpha rays; Thomson's article published shortly afterwards in Harpers, 22.eit., describes them still as X-rays. 62 Phil.Mag., 1903, 5, 419-28, Apr. issue. 63 L. 64 Letter from Thomson to Rutherford, dated 14th Apr.1903, A.S.Eve, Rutherford.Etc., 94; followed by Thomson's paper 'On the existence of a radio-active gas in the Cambridge tapwater', Proc.Camb.Phil.Soc., 1903, 12, 172-4, read Notes for Chapter 5, p.226-280) 321

64 contd.) 4th May; also Nature, 1903, 68, 90-1. 65 J.J.Thomson, Radium, Nature, 1903, 677 601-2, Apr. 66 See below. Section 3, p.276-9. 67 The Disintegration Theory of Radioactivity, Times Lit. Suna., 1903, p.201, 26th Jun. 68 T.-J.:Thomson, Radium, Nature, 1903, 67, 602. J.Stark (Nature, 1903, 68, 230, 9th Jul.) then claimed the prior expression in 192 of this idea (see above Chapter 4, Section 1, p.195-6). Thomson's early statement on uranium was not brought up. The latter (Conduction of Electricity through Gases, 552) had accepted the later atomic-expulsion version of the disintegration theory by Aug.1903, citing Rutherford and Soddy's general statement made in Phil.Mag., May 1903. 69 Letter f176E-S'6d4y to Rutherford, dated 31st Mar.1903,CUL. 70 Ibid. 71 W7Ti'ookes, The Emanations of Radium, Chem.N., 1903, 87, 157-8, 158; read to the Royal Society, 19th Mar.; the scintillation effect was noticed independently by Elster and Geitel, and Becquerel, and given different interpretations: respectively, release of electrons, Chem.N., 1903, 88, 37; and crystal fracture, CR, 1903, 137762.g-34. 72 Times,-75th Mar.1903, p.10,d. 73 MiTE's suggestion of a possible atomic transformation was not mentioned, 74 In a Minor Key, Punch, 1903, 124, 214, 25th Mar. 75 Times, 28th Mar.TUUTT p.14,f.--- 76 30th Mar.1903, p.12,f; dated 26th Mar., from 30 Ledbury Rd., Notting Hill, near Crookes' address; referred to by Soddy, see above. 77 Phil.M2c., 1893, 35, 389-92. 78 YETU., 392. 79 TN5otator'. Times, 13th Apr.1903, p.6,d. 80 W.Crookes, The Mystery of Radium, Times, 14th Apr.1903, P,5,a- 81 Id., 7th Apr..1903, p.10,b: Chem.N, 1903, 87, 184. 82 TTCrookes and J. Dewar, Chem.N., 1903, 88,-75-6, read 28th May. These two chemists also collaborated e.g. in examining the 'London Water Supply', ibid, 40. 83 W.Crookes, Certain properties of the emanations of radium, Chem.N., 1903, 87, 241, 22nd May. 84 Crookes and Dewar, On tF6 effect of extreme cold etc., loo.olt. 85 UEWm7,77, 1903, 87, 277-81, 12th Jun.; delivered to Congress of Applied Chemistry in Berlin on 5th Jun. 86 Ibid., 279. 87 W.Crookes, The Stratifications of Hydrogen, 410-3; see the previous Section, p.220. 88 Modern Views, 2E.eit., 280. 89 Ibid., 278. 90 7057., 281. 91 The Position of Radium in the Periodic Table as indicated by its Spectrum, Chem.N., 1903, 87, 145-6; 2102.Z., 1903, 4, 285-7. 92 W.Crookes, Modern Views, loc.cit., 278. 93 0.Lodge, Pop.Sci.Monthly, 1903758, 289-303; delivered 12th Jun. at baroxd. 94 Ibid., 294-5. Nbtos for Chapter 5, p.226-280) 322

95 Lodge made this second point most plainly in a Note to Nature, 1903, 68, 128-9, 11th Jun. 96 E.RIITE6Word, Tapers, 596-608. 97 0.Lodge, Modern Views etc., loo.cit., 299. 98 0.Lodge, On Electron°, ElectFIFian, 1903, 51, 286. 99 Lodge, Modern Views etc77-7778766tion 2 ViiTow contains aocounts of the related views of Lodge and Thomson on the mechanism of radioactivity during 1903-4. 100 M.W.Travers, Life of Sir William Ramsay, ch.8, 133-54. 101 Ibid., ch.7, 100f., 170T. 102 77Titimsay and P. Soddy, Nature, 1903, 68, 246, 16th Jul. First observed 8th Jul., Travers, RE.cit., 212-5. 103 Ramsay and Soddy, Chem.N., 1903, 100-1; communicated to Royal Society, OTE Jul. 104 Letter from P.Curie to J.Dewar, dated 22nd Jul.1903, RI. 105 P.Curie, J.de Chimie Physique, 1903, 1, 409f.; Oeuvres, 456-90, 4189. 106 Production of helium from salts of radium (not from its emanation) confirmed by about Nov.1903 by Curie and Dewar, CR, 1904, 138, 190f., Jan.; P.Curie, Oeuvres, 491=3. 107 P,Curie, Recherches rocentes, Oeuvres, 471-2. 108 Ibid., 489. 109 TFIJ., 463. 110 137.-Caric (and J.Danne), CR, 1904, 138, 683f., 14th Mar.; Oeuvres, 494-7. 111 (75iii57.Med- after mid-Sep.1903, pub.1904, in Oeuvres. 112 M.Curie, ibid., Oeuvres, 219-21. 113 Ibid., 239. 114 75171. r 238; the adjective refers to 'hypothbse'. 115 See above Chapter 2, Section 3, p.98; M.Curie, Oeuvres, 334-5. 116 Chem.N., 1903, 88, 40; at Bradford on 16th Jul. 117 TEa.V., 1903, 8d, 39-40; from Times, 20th Jul. 118 Proc.Roy,Soc., 1903, 72, 196-9, received 17th Jul. 119 Letter from Soddy to Rutherford, dated 7th Aug.1903, CUL, concerning comments by a referee on E.Baly's paper on xenon. 120 Sir W. and Lady Huggins, 22.cit., Proc.RF.Soc., 1903, 72, 198-9, addition received-5Th Aug.; discussion continues in 'Further Observations etc.', ibid., 409-13, Oct. 121 Letter from Soddy to Rutherford, dated 28th Aug.1903, CUL; the cutting is now lost. 122 Punch, 1903, 125, 139, 26th Aug. 123 17th Jul.1903, 225-6. 124 Punch, 1903, 125, 133, 26th Aug. 125 ?(he U.S.A. see L.Badaeh, Dias., 174-82, ch.'Popular- isation for the Public, 1900-1703'. 126 Lotter from P.Curie to W.Ramsay, dated 14th Feb.1904; Ramsay, Letters and Papers, 13, p.85a, UC. 127 Letter from F. Soddy to E.Rutherford, dated 12th Dec. 1903, CUL. 128 Letter from Rutherford to his mother, dated 10th Aug. 1904; A.S.Eve, Rutherford.Etc., 118. Notes for Chapter 5, p.226-280) 323

129 Soddy, loc.cit., 12th Dec., thought Ramsay might claim the entire TEWory as his own; J.J.Thomson wrote in similar vein to Rutherford about Ramsay (letter dated 4th Feb,1904, CUL). Further unpleasant priority disputes developed around mid-1903 involving e.g. Becquerel; Lodge thought he should be 'rapped over the knuckles for it' (letter to Rutherford, dated 11th Dec.1903, CUL). P.Curie's comments to Dewar about Rutherford and Dorn have been noted above; so too has Marie Curie's claim. A difference also arose, to be quickly settled, between Rutherford and Soddy concerning the publication of books. 130 P.Soddy, loc.cit., letter of 12th Dec.1903. 131 Letter dated 7fq Apr.1903, CUL. 132 Electrician, 1903, 51, 210-11, 22nd May; contains the incorrect statement that excited activity could be produced directly by the rays. 133 Ibid., 314, 12th Jun. issue. 134 "TETE1-J.D.Everett, Analogue to the Action of Radium, Nature, 1903, 67, 535-6, 9th Apr. 135 MTUUTHerford, Radioactive Processes, Proc.Physical 112121y, 1903, 18, 595-7, abstract and discussion; id., Pa ers, 614-7. 136 Letter from F.Soddy to E.Rutherford, dated 7th Aug. 1903, CUL. 137 J.V.Eyre, Henry Edward Armstrong, London, 1958, 125. See also W. rock, 4.1.E,A7EiTiron and the Teaching of Science, 1880-1930. Cam r dge, 173. 138 Letter from Soddy to Rutherford, dated 28th Aug.1903,CUL, 139 H.E.Armetrong and T.M.Lowry, Chem.N., 1903, 88, 89-91, 21st Aug.; read to Royal SociWiT IBth Jun. 140 ibid., 91. 141 TTOddy, loc.cit., letter of 28th Aug. 142 W.Crookes, Modern Views etc., Chem.N., 1903, 87, 281; 12th Jun. issue. 143 W.Brock, Lockyer and the Chemists etc., 93, 95. 144 H.E.Armstrong, Presidential Address, Rep.Brit.Ass., 1885, 945-64, 961; id., Osmotic Pressure and Ionic Dissociation, Nature 1896, 55, 78-9. For his alternative IresiduTIEYrinityl view of valency see C,A.Russell, Histo., of Valenc , 205-13. 145 E.g. Report on p yss c a he rit.Ass., Nature, 1900, 62, 564. 146 Proc.E2y.Soc., 1902, 70, 99-109, 102. 147 H.E.Armstrong, Chem.N, 1902, 85, 86-8, 103-6, p.86. 148 In Chem.N.; mentioned by W.H.Brock, H.E.Armstrong etc.,36. 149 This marbe related to his adoption in 1903 of the view that weak radioactivity might in fact be due to 'a minute amount of chemical change of an ordinary character ... a sort of Russell effect', H.E.Armstrong, The Assumed Radio-activity of Ordinary Materials, Nature, 1903, 67, 414, 5th Mar. 150 Summarised in Electrician, 1903, 51, 880; not contained in Rutherford, 2222E2, 151 L.Badash, Sci.Amer., 1966, 215, 88-95, 93; no sources given. There are valuable reports of the meeting in Electrician, 1903, 51, 880-1, 892-3. Notes for Chapter 5, p.226-280) 324

152 In 1906 Armstrong again expressed sceptical views; see F.Soddy, The recent controversy on radium, Nature, 1906, 74, 516-8, 516, 153 Letterrom F.Soddy to E.Rutherford, dated 12th Dec. 1903, GUL. 154 Electrician, 1903, 51, 800, 4th Sep.; refers to letter in The Times. 155 L.Koenigeberger, Hermann von Helmholtz, (1906), London, 1965, 438. 156 F.Richarz and R.Schenck, Sitzber.Akad.Wiss.,Berlin, 1903, 1102-6; R.Schenck, ibid., 17547 77F7; mentioned by Rutherford, Radio--activity, 1905, 441. 157 But for Kelvin's views see Section 2 below, p.266. 158 Electrician, 1903, 51, 800. 159 11765ITEletEtrioian, 1903, 51, 835, 11th Sep. 160 W.Ackroyd, Experiments and-6bservations with Radium Compounds, Chem.N., 1903, 88, 205-6, read at Brit.Ass. Chemistry (TT-Se-6-tion, Sep.1903; id., The Source of the Energy of Radium Compounds, Nature, 1904, 69, 295, 28th Jan.; id., On the Bearing of the Colour Phenomena presented by Radium Compounds, Chem.N., 1904, 90, 157, read at Brit.Ass. Chemistry (B) Section, Sep.1754. See also, C.Winkler, Radio-activity and Matter, Chem.N., 1904, 89, 289-91, who advocated a magnetic analogy for the energy source and accepted radioactive induction; he appears as a standard 'unconvinced chemist', see p.251 above. 161 M.Berthelot, CR, 1901, 133, 973-6; id., Essais etc., ibid., 659-64:- 162 M., CR, 1904, 138, 1553-5; stresses the effects of traoes of vapours of chemical substances. 163 See e.g. F.Giesel, Emanium, Chem.N., 1904, 90, 259-60, who exhibits some confusion; and W.Marckwald, Heber das Radiotellur, Ber.deut.chem.Ges., 1905, 38, 591-4, who does not. 164 Debierne, to radium et la radio-activity, Rev.Gon.des Sol., 1904, 15, 11-22, 60-71, 69-71, adoptiThle compromise (like M.Curie) of Ra as a catalyst for atomic transformations; then, CR, 1905, 141, 383-5, the atomic disintegration theory. 165 E.Rutherford, Magnetic and Electric Deviation etc., Pa ere, 557= 166 E,Ru herford (and F.Soddy), Thorium II, Papers, 456, read May 1902. 167 Mentioned e.g. by E.Rutherford and A.G.Grier, Deviable Rays of Radioactive Substances, ibid., 457, dated 7th May 1902. 168 E.Rutherford, Magnetic and Electric Deviation etc., Pa ers, 557; Phil.Mag., Feb.1903, dated Nov.1902. 169 o . 170 1711772.22.p7og.Chem., 1904, 1, 30-54, 30-2. 171 Ti.ilgns.,c London, 1904; pref: dated Oct.1902. 172 See 'Professor Mendereeff on Argon', Nature, 1895, 51, 543, for his initial reaction. 173 Chemical Conception of the Ether, 44-51. 174 Ibid., 45. 175 Tr:F.:Brush, Chem.N., 1898, 78, 197-8; Science, 1898, 8, 485-94. Notes for Chapter 5, p.226-280) 325

176 Chem.P., 1898, 78, 221-2. 177 Verh.ply2-Ges.B-6-ilin, 1898, 17, 135-7. 178 Opening address by President of Chemistry (B) Section of Brit-Ass., Nature, 1903, 58, 472-81. 179 Ibid., 479. 180 Te-W-tAe preceding Section, p.246; M.Curie, Thesis, 2 ed., 1904; id., Oeuvres, 239. 181 Notices of Books, Chem.N., 1904, 90, 326. 182 P.P.Bedson, op-cit., Anii.liep.prog:Uhem., 1904, 1, 32. 183 Chemical Conception o1 the Ethii; 6; D.MendeleeT, An attempt to apply to the tryone of the principles of Newton's natural philosophy, RI Lib.Sci., (1889), 3, 540-59. 184 D.Mendeleef, The periodic law of the chemioal elements, J.Chem.Soc., 1889, 55, 634-56, 641-7. 185 Td77-MIWiacal Conce15Tion of the Ether, 14. 186 Tad-, 44-5, 47. 187 :e vinretained that notion at this time, Papers, (1905) 6, 223. 188 W.Ostwald, J.Chem.Soc., 1904, 85, 506-22; reprinted in D.M.Knight, Classical Scientific Papers.Chemistry, London, 1968, 354-70. 189 Ibid., 356-7. 190 Ibid., 369. 191 -SW-e—e.g. S.Glaestone, Sourcebook on Atomic Ener . London, 1950, 357-61, on liquid-drop models of nuclear fission in the 1930's. 192 He later appreciated the success of the nuclear atom, Ann.Rep.Prog.Chem., 1913, 10, 262-88, 271-2. 193 S(79r above, Chapter4, SectrOn 2, p.198-9. 194 In 19 parts in Electrician, 1903-4, 52, 7-10 etc., pub. Oct.1903 to Feb.190/f. 195 Ibid., 163. 196 1570-6-ddy, Radio-activity: an Elementary Treatise from the Stand,oint of the Didategration Theory, London, 774, pref. datWU 5TE May. 197 Ibid., 55. 198 fbid., 164, 199 U7E.Vincent, Phil.Mag., 1902, 4, 103-15. 200 Soon to be moderated ( F.Soddy, Radioactivity, Ann.Rep. pERg.Chem., 1904, 1, 244-80, 276) by one who a decade a or paced atomic disintegrations within the Periodic Table by means of the Displacement Law. 201 F.Soddy, Radio-activtT, 178. 202 Pa ere, 73; see thiT011owing Section, p.272-4. 203 ..Soddy, Radio-activity; 125, 176-8, citing only Lodge; see belowTT5726I-.37f61; this physicist's suggestions. 204 Ibid., 178. 205 Ibid., 178-80. 206 fbid., 125. 207 "A-i-J,Walker on 'Time Conception of Minute Concentrations' in 'General and Physical Chemistry', Ann.Rep.Prog.Chem., 1904, 1, 1-29, 25-6. Also C.A.Russell on 'The Oscillation Theory' in History of Valens , 254-6. 208 W.H.Bragg, On the Absorption ofc(Rays, and on the Class- ification of theotRays from Radium, Phil-Mag.1904, 8, 719-25; id. and R.Kleeman, On the Iorirgation vesCur of RadiuiliT ibid., 726-38. Notes for Chapter 5, p.226-280) 326

209 Letter from E, Soddy to W.H.Bragg in Adelaide, dated 12th Jan.1905, RI. 210 F. Soddy, Radio-aotivitz, 1904, 178. 211 See e.g. the FtiView 'by J.A.Fleming, The electronic theory of electricity, RI Lib.Sci., (1902), 5, 551-69. 212 E.von Sohweldler, Dmt.-riit.poluTET•21-ude de la Radiologie etc., 1905,- I, dated Jun.175. 213 E.g., L.BoliFiann, Lectures on Gas Theo , (1896, 1898). 214 E.Rutherford, Papers, (1908), 2710 , provides references for 1905731-T5th electrical and scintillation methods were employed by him. 215 0.Lodge, On Electrons, Electrician, 1903, 51, 123-5, 125. 216 Nature, 1903, 67, 511, dated 28th Mar. 217 J,J.Thomson, Radium, Nature, 1903, 67, 601-2. 218 Ibid., 601. 219 0.Lodge, Nature, 1903, 68, 128-9, 11th Jun. 220 Reported in Electrician, 1903, 51, 417-9; see also E.Rutherford, Pa e737616. 221 J.Larmor, Aether anti Matter, 227-32; 0.Lodge, On Electrons, Electrician, 1903, 51, 286. 222 Lodge, On EITEIi5iiii,Toc.cit.,-125. 223 Lodge, Note on the pro-E11517occasional instability of all matter, 128-9. 224 Ibid. 225 .I Jeans, Phil.Mag., 1901, 2, 421-55. 226 J.Heilbron,-15170., 137. 227 Phil.Mr3z., 1777 7, 445-55, paper read 5th Dec.1903 in Tokyo. 228 Ibid., 455. 229 Phil.Meo., 1904, 7, 237-65; see below, p.268-70. 230 IT5.--ft.58, Electrician, 1904, 52, 805; see also abstract, ibid., 823. 231 laa., 805. 232 aaro-activity, 1905, 488; unchanged comment from 1904 ed.; see the following Section, p.272f. 233 Kelvin, Papers, 6, 206-9. 234 See e.g. Editorial note, 'Explanations of Radio- activity', Electrician, 1903, 51, 892-3. 235 Perhaps referring io A.HeydweiTler, Phys.Z., 1902, 4, 81-2, and/or R.Geigel, Ueber Absorption von Gravitationsenergie lurch radioactive Substanz, Ann.d.Phys., 1903, 10, 429-35. But e.g. C.Forch, lEY:27Z., 1903, 4, 315.-9, 443-5, citing W.Kaufmann, Ann.d7Phy.z., 1903, 10, 894, had by autumn 1903 published experimen al refutations of the apparent weight-loss. 236 Kelvin, Pa ers, 6, 208. 237 Letter da e 23rd Aug.1903, RI. 238 Letter dated 22nd Aug.1903, in M.W.Travers, A Life of Sir William Ramsay, 252. Travers here states that at a dinner which he attended in June 1903 Marie Curie attempted to 'convert' Kelvin to the disintegration theory. This however was before the announcement of the radium-helium transmutation in July. Also, a postcard from Soddy to Rutherford dated 22nd Jun.1903 (GuL) reporting Ramsay's visit to Marie Curie in Paris reads 'According to R., Curie thinks we are very "hardi" to put forward our hypothesis on such slight evidence. R. replied he thought there was a good deal of evidence'. Marie Curie may thus not have been a convert by that date. Notes for Chapter 5, p.226-280) 327

239 At the Brit.Ass. 1903, Kelvin, papers, 6, 208-9. 240 Rutherford.Etc., 109; see also WI07on-differences regarding geological-mineralogical time scales. 241 Pa err, 6, 216-22; Phil.Nas., 1904, 8, 528-34, Oct. - scUee 242 Ibid., 216. 243 Re Vins Phil.Mag., 1902, 3, 257-83, written in 1901. 244 C.f. L.BaTimann, Lectures on Gas Theory, 3-4, 376-9, who postulated overlapping moms to explain valency; mentioned above, Chapter 4, n.50. 245 Kelvin, Aepinus Atomized, loc.cit., 259; similar forces he conceived to act between atoms and the ether, Papers, 6, 237. 246 Ibid., 259. 247 Ibid.., 262. 248 RTIVin, Plan of a Combination etc., Papers, 6, 216. 249 Ibid. 250 TiiIVinv Pa ere, (1905), 6, 227-30. 251 Summarise by F.Soddy, THe recent controversy on radium, Nature, 1906, 74, 516-8. Soo Rayleigh, J.J,Thomson, 141-2 for KelvIri's criticisms in 1906 of Thomson's radiation-loss disintegration theory. 252 An attempt to explain the Radioactivity of Radium, Papers, (1907), 6, 231-4. 253 kelvin, On the Motions of Ether etc., Papers, (1907), 6, 235-43, 235-6, 25454R. Nagaoka, Kinetics of a system etc., 445. 255 Id., 454. 256 G.A.Schott, A dynamical System etc., Nature, 1904, 69, 437, from University College of Wales. J.Heilbron, Dies., 142-6, discusses the arguments of Schott and Nagaoka; those related largely to the above points but not to radioactivity, 257 H. Nagaoka, Kinetics of a system etc., 454. 258 Ibid., 454-5. 259 i.e. ultra-violet photoelectric action, the emission of electrons from an irradiated metal. 260 Phil.Mag., 1903, 6, 673-93. 261 Y.hia., 673. 262 Rayleigh, J.j,Thomson, 140, letter dated 11th Apr.1904. 263 J.j.Thomson, Magri 611c Properties etc., 678-81. 264 E.g. J.Larmor, On the Theory of the Magnetic Influence on Spectra; and on the Radiation from moving Ions, Phil.Mag., 1897, 44, 503-12, 512, had done so despite losses of the order 10-6 . 265 Thomson, Magnetic Properties etc., 682-5. 266 Ibid., 689. 267 TEIT.Mag., 1904, 7, 237-65, Mar. issue. 268 Ibid.7-255. 269 MU., 265. 270 T517. 271 Letter from Thomson to Rutherford, dated 18th Feb.1904, CUL, describing the above ideas shortly before their publication. 272 W.Kaufmann's results of 1901-2 were reported e.g. by E.Rutherford, Radio-activity, 1905, 127; 1904, section 76; by7„,7Trtrutt, The Becquerel Rays and the Praperties of Radium, London, 1904, 69; and by Notes for Chapter 5, p.226-280) 328

272 contd) Thomson himself, Conduction of Electricity throuEh Gases, 1903, 532-5. 273 17.757he0Fia-T,Radio-octivit 1904, 325, section 200. 274 Ibid„ 333, se-aion 'u3. 275 Ibid., 326, section 200. 276 YETU., 305, section 194. The point was noted by Soddy and Rutherford (Papers, 564, 599) without great emphasis early in 1903, and had possibly been anticipated by mid-1902 (121=2, 508). 277 Phil.Trans.A., 1904, 204, 169-219, ms. received 20th Aug.;-Pa-b-rs, 671-722. 278 ibid., 1'. 279 ibid. 280 TM., 712-3. 281 E.Rutherford, Radio-activity, 1905, 450. 282 Ibid., 456, 283 The apparent effect of temperature on the decay of RaC was the only exception; E.Rutherford, Bakerian Lecture, Papers, 713; Radio-activity, 1905, 390-1. 284 Ibid., 1903, 487-8; 1904, section 206. 285 2. Rutherford, Radioactive Transformations, (London, 1906), repr. Yale U.P., 1919; lectures delivered Mar.1905. pref. dated Jun.1906. 286 Ibid., 267. 287 17697., 268; there is no reference to Soddy. See Rutherford, Radio-activit , 1905, 446, for his own previous discussion regar ing the 'average life' of 'metabolons'. 288 E.Rutherford, Radioactive Transformations, 268. 289 Rutherford considered statistical aspects of radioactivity by 1908 (Ta.p_sE2, 2, 58, 69, 94, 106-8). However, in 1909 he reported results which made it seem 'probable that the atoms of emanation undergo a progressive change in properties before disintegration' (ibid., 168-9); some months later Thomson defended the theory of continuous atomic change by attributing a suitable distribution to the atoms themselves; these he supposed were of differing intrinsic strength when first formed (Rayleigh, J.J.Thomson, 142). But in 1910 Rutherford rejected the results on progressive change in the emanation (Pa ens, 2, 214-20) and intensified his statistical stud. es; e.g. the number of alphas he detected per minute fluctuated wildly and randomly between zero and twenty, in accord with probability laws. But regarding the cause of instability and disintegration, whether in nuclear or electronic (1912; ibid., 286-7) or quantum terms (1927: Papers, 3, 178-9, 183), the physicist still admitted ignorance. 290 E.Rutherford, Phil.Mag., 1907, 13, 110-17, dated 1st Nov.1906; Papers, 910-16,91-6. 291 Ibid., 916. 292 E.Rutherford, Radioactive Transformations, 276. 293 Radioactive Change, Papers, 596-608. 294 Ibid., 608, See Chapter 4, Section 1, p.195, for 77.7fark's similar suggestion in mid-1902. 295 1903, 68, 222, 9th Jul. Notes for Chapter 5, p.226-280) 329

296 G.H.Darviin, ibid., 496. 297 J.Joly, ibid, 7526. Rutherford, Pa ere, (1907), 926, later chid to have made calculations in 1902, unpublished, on the gao-thermal effects of active mineraln. 298 The problem that radioactive rays from the sun should be detectable on earth was raised by W.B.Hardy, 'Radium and the Cosmical Time Scale', ibid., 548, and disposed of by R.J.Strutt, 'Radium and the Sun's Heat', ibid., 572, 15th Oct. The lack of radium lines in the TaTir spectrum was partly eased by the abundance of helium, see e.g. E.Rutherford, Radio-activity, 1905, 492; 1904, section 207. 299 Radio-activit , 1905, 491-6; 1904, section 205. 300 !bid., 1 5, 459-66, and references there cited. 301 17,757therford, Pa era, (1907), 917-31, 930; earlier discussions, cting Ramsay and Soddy, appear in Rutherford, Radio-activit , 1905, 485-6, 554-8; also • Pa ere, 774-5, bop. n 1907 Rutherford left McGill to - ace A.Schuster's Chair at Manchester. 302 A.Sohuster, Rep.Brit.Aps., 1903, 538. 303 E.g. G.Le Bon, P,de Heen, G.Martin, D.Mendoleef, H. Wilde. During 1903-4 some connection was also seen, e.g. by Jean Becquerel, between radioactivity and the N-rays, which Blondlot imagined to be emitted by various materials. 304 P.Curie, Oeuvres, 378; Congr6s, 1900. 305 Radio-active Substances, Chem.N., 1903, 88, 99. 306 R.J.Strutt, Nature, 1903,-7,769-70, 19T Feb. See also J.J.Thomson, ibid., 391, who cited McLennan and Burton; and E.Ruther5ird, ibid., 511-2, 2nd Apr. citing Rutherford and H.L.Cooke. 307 Noted by Rutherford, ibid., also in Radio-activity, 1904, section 220. 308 E.Rutherford, Radio-activity, 1905, 487; 1904, section 206. 309 J.J.Thomson, Proc.Camb.Phil.Soc.7 1904, 12, 391-7. 310 Ibid., 397. ---- 311 TIETI.MaE., Mar.1904 issue; see the preceding Section, p.270. 312 N.R.Campbell, Phil.Mafc., 1905, 9, 531-44; id., 545-9; id., 1906, 11, 202-2 . See also J.J.Thomson, On the emission of negative corpuscles by the alkali metals, Phil.Ma., 1905, 10, 584-90. 313 E.Rutherford, Papers, 708, 775; Radio-activity, 1904, section 220. 314 Rutherford, Radio-activity, 1905, 539-42, section 286. 315 Rutherford, Radioactive Transformations, 217-8. 316 Summarised bYTTETIT6Word, Radio-activitq., 1905, 552-3. 317 See e.g. J.C.McLennan, On the Radio-activity of Potassium and other Alkali Metals, Nature, 1908, 78, 29-30; N.R.Campbell's defence, ibid., 55; then E.Rutherford, Radioactive Substances and their Radia- tions, Cambridge, 1913, 58U1:9, 596. Notes for Chapter 5, p.226-280) 330

318 E.Rutherford, Radio-aotivkIE, 1905, 483; 1904, section 201. 319 Ibid. 320 L. Rutherford, Baker-Lan Lecture, Pa ers, (1904), 712. 321 W.H.Bragg, Phil,Mag., 1904, 8, 7 -25,, 719-21, cited Rutherford's Bakerian 322 Letter from Bragg to Rutherford, dated 18th Dec.1904,CUL. 323 E.Rutherford, Radio-activity, 1905, 484, this edn. only. 324 Ibid., my emphases. 325 lbid., 1905, 77; again, my stress. 326 Men- ioned obliquely, ibid., 78. 327 J.J.Thomson, Phi1.n.A777906, 11, 769-81; discussed e.g. by 0.Lodge, Electrons etc., 1906, 146-51, 162, 192-4, 328 J.J.Thomson, Cathode Rays, 1897, 312; he estimated its rate of production at one three-millionth gm. per year. 329 Thomson, RI Lib.Sci„ (1907), 6, 232-47. 330 The general notion projected into the future. Rutherford in 1913, Radioactive Substance° etc., 621, speculated that atomic nucleT-Fonsist in pa of H and He atoms. 331 0.Lodge, Radio-Activity, Electrician, 1904, 53, 216-8. 331

BIBLIOGRAPHY

Note on manuscript sources p.332

Printed sources p.335

Abbreviations p.364 332

NOTE ON MANUSCRIPT SOURCES

Cambridge University Library

Correspondence and papers of E.Rutherford (Add.MSS 7653)

W.H.Bragg to Rutherford, 18th Dec.1904. Rutherford to Bragg, 20th Dec.1911. Rutherford to W.Crookes, 29th Apr.1902. J.Elster and H.Geitel (in Elster's hand) to Rutherford, 10th Feb.1899; 27th Jun.1899. J.Larmor to Rutherford, 3rd Apr.1903. O.Lodge to Rutherford, 11th Dec.1903. J.C.McLennan to Rutherford, 3rd Oct.1902. F.Soddy to Rutherford, 12th Jul.1902; 31st Mar.1903; 22nd Jun.1903 (postcard); 7th Aug.1903; 28th Aug.1903; 26th Sep.1903; 12th Dec.1903. J.J.Thomson to Rutherford, 22nd Nov.1898; 23rd Jul.1899; 21st Dec.1899; 25th Apr.1901; 2nd May 1902; 13th May 1902; 4th Feb.1904; 18th Feb.1904. J.S.Townsend to Rutherford, 14th Jan.1901 (misdated 14th Jan.1900 by the writer). J.Zeleny to Rutherford, 25th Mar.1900. Laboratory notebooks of Rutherford, 1896-1905, mainly in Rutherford's hand.

Correspondence of J.J.Thomson (Add.MSS 7654)

G.F.Fitzgerald to Thomson, 1st Jan.1885. E.Rutherford to Thomson, 9th Jan.1900; 26th Dec.1902. 333 Bibliotheque Nationale, Paris

Fonds Curie W.Crookes to P.Curie, 13th Jul.1900; 19th Jul.1900. F.Giesel to P.Curie, 3rd Oct.1901. F.Giesel to P. and M.Curie, 23rd Mar.1902; 25th Nov.1902. K.A.Hofmann to P.Curie, 16th Jan.1903, on 'inactive' Th. Kelvin to P.Curie, seven letters of 1893 on electroscopes, and symmetry. (Dossier 32) Notes of M.Curie, probably c.1896, on P.Drude's Physik des Aethers, 1894. (Dossier 9) M.Curie's certificates for the 'Licence es Sciences physiques', 1893, and 'Licence Os Sciences mathematiquest, 1894. (Dossier 29)

Academie des Sciences, Institut de France, Paris Rough draft of M.Potier's address to the Comito Secret in favour of P.Curie's candidature in 1902 for the Academie, pp.10; a section on symmetry studies, p.5-10, is apparently curtailed. (Dossier Curie)

Royal Institution of Great Britain, London Laboratory Notebooks of W.Crookes, vols. 6-20, 1881-1913; esp. vol.16, 1896-9; vol.17, 1899-19U1; vol.18, 1901-3; in the handwriting of Croaes and his assistants. Typescript translation of letter from P.Curie to W.Crookes, dated 17th Jul.1900; appended at end of Notebook 17. Correspondence of J.Dewar P.Curie to Dewar, 22nd Jul.1903. Kelvin to Dewar, 23rd Aug.1903. Correspondence of W.H.Bragg F.Soddy to Bragg, 12th Jan.1905. 334

Wellcome Institute for the History of Medicine, London Laboratory notebook of M. and P.Curie, 1899-1902.

Library of University College, London W.Ramsay, Letters and Papers, bound in 16 vols. P.Curie to Ramsay, 14th Feb.1904. (Vol.13, p.85a)

Library of the Royal Society, London Correspondence of J.Larmor G.Sagnac to Larmor, 11th May 1899.

Bodleian Library, Oxford Soddy-Howorth Collection F.Soddy, 'The Indivisibility of the Atom', 1901, draft of an unpublished address, pp.23. F.Soddy, 'Gas Analysis', six lectures, 1901.

Science Museum Library, London Correspondence of E.Rutherford, photocopies bound in 9 vols., ed. by E.Marsden, 1956. Contains most but not all of the known correspondence. The original materials are at Cambridge. Laboratory Notebooks of W.Crookes, vols. 1-6.

Imperial College Archives Correspondence and Papers of S.P.Thompson W.Crookes to Thompson, 2nd Jun.1896. G.G.Stokes to Thompson, 29th Feb.1896; 2nd Mar.1896; 28th May 1896. 335 PRINTED SOURCES

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Abh.der Naturf.Ges.zu Halle Abhaiialungen der naturforschenden Gesellschaft zu Halle. Amer.J.Phys. American Journal of Physics. Ann.d.Phys. Annalen der Physik (und Chemie), Leipzig. Also known as Wied.Ann., Wiedemann's Annalen. Beibl.= BeiblUtter. Ann.1122.212E.Chem. Annual Reports on the Progress of Chemistry, Chemical Society of London. Ann. Sci. Annals of Science. Arch.Hist.Exact Sci. AFFave for History of Exact Sciences. Ber.deut.chem.Ges. —Fgrichte der deutschen chemischen Gesellschaft, Berlin. BN Bibliotheque Nationale, Paris. Brit.J.Hist.Sci. British Journal for the History of Science. Bull.Amer.Phys.Soc. Bulletin of the American Physical Society. Chem. N. The Chemical News. CR Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, Paris. CUL Cambridge University Library. Edin.Roy.Soc.Proc. Proceedings of the Royal Society of Edinburgh. Hist.Stud.Phys.Sci. Tiritorical Studies in the Physical Sciences. J.Amer.Chem.Soc. Journal of the American Chemical Society.

365 J.Chem.Soc. Journal/Transactions of the Chemical Society of London. J.Franklin Inst. Journal of the Franklin Institute. Jahresb.d.Ver.f.Wiss.,Braunschweig riE7isbericht des Vereins fur Naturwissenschaft zu Braunschweig. Jap.Stud.Hist.Sci. Japanese Studies in the History of Science. Manchester Memoirs Memoirs of the Manchester Literary and Philosophical Society. Nature Nature, London. Phil.Mag. London, Edinburgh and Dublin Philosophical Magazine, and Journal of Science. Phil.Trans.A. Philosophical Transactions of the London Royal Society, series A. Phys.Z. Physikalische Zeitschrift, Leipzig. Pop.Sci.Monthly Popular Science Monthly, New York. Proc.Camb.Phil.Soc. Proceedings of the Cambridge Philosophical Society. Proc.Chem.Soc. Proceedings of the Chemical Society of London. Proc.RI Proceedings of the Royal Institution of Great Britain.

Proc.Roy.Soc. Proceedings of the London Royal Society. Rapports,Cong.Int.de Physique Rapports presenters au Congres international de Physique, Paris. Rep.Brit.Ass. Report of the British Association for the Advancement of Science. 366 Rep. Smithsonian In3t. Annual Report of the Smithsonian Institution. Rev.Gen.des Sci. Revue Generale des Sciences Pures et Appliquees. Rev. Sc. Revue Scientifique, also Revue Rose. RI Royal Institution of Great Britain. RI Lib.Sci. The Royal Institution Library of Science. Physical Sciences, 10 vols., ed. W.L.Bragg and G.Porter, London, 1970. Mainly reprinted from Proceedings of the Royal Institution, 1851-1939. Sci.Amer. Scientific American. Seances Soc.Fr.P s. ociete Franpaise de Physique. Seances, 1873-1901. Sitzber.Akad.Wiss.,Berlin Sitzungsberichte der preussischen Akademie der Wissenschaften, Berlin. Trans.Camb.Phil.Soc. Transactions of the Cambridge Philosophical Society. Trans.Roy.Soc.Canada Transactions of the Royal Society of Canada.

UC University College, London. Verh.Deut.Ges.Natf. Verhandlungen der Gesellschaft Deutscher Naturforscher and Aertze, Leipzig. Verh.Deut.Phys.Ges. Verhandlungen der Deutschen Physikalischen Gesellschaft, 1899f. Ver.Phys.Ges.Berlin Verhandlungen der Physikalischen Gesellschaft in Berlin, 1882-98. Zeit.f.Elektrochemie Zeitschrift fur Elektrochemie, Halle.