LXVII. Pleochroic Halos

Philosophical Magazine Series 6

ISSN: 1941-5982 (Print) 1941-5990 (Online) Journal homepage: http://www.tandfonline.com/loi/tphm17

LXVII. Pleochroic halos

J. Joly F.R.S. & Arnold L. Fletcher B.A.I.

To cite this article: J. Joly F.R.S. & Arnold L. Fletcher B.A.I. (1910) LXVII. Pleochroic halos , Philosophical Magazine Series 6, 19:112, 630-648, DOI: 10.1080/14786440408636842

To link to this article: http://dx.doi.org/10.1080/14786440408636842

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Download by: [University of Arizona] Date: 06 June 2016, At: 16:15 ~30 Prof. J. Joly aJ~d Mr. A. L. Fletcher : the reflected snnlight arc separ~lted, there is not a largo p.rtion of unpolarized light in the latter, a serious objection to a radiation pressure theory of t)le tails al,pears at once. When a comet has a dense bright nucleus, a certain small proportion of fis intrins;c light must show evidence of polarization, due to scatterhag by the tail, of light emanatin V t~om the nucleus. But this effec~ would probably be t,o small to be observed in any case. Radiation from the nucleus may also, in some cases, give a radiation pressure assisting: that of the sun, although the gravitational actioll of the nucleus can never be important. All observations r~lating to polarization would be liable to be affected by the presence of fluorescence, and in view of the results (~btained by Wood *, deductions from them demand as much caution as those fi'om the observed brightness.

LXVII. _Pleochrolc tildes. HT/ J. JoLv, F.R.S., and ARI~'OLD L. FLETCHER, B.A.I.t [Plates VIII. & JX.] INCE the date of an earlier paper on the subject of S Pleochroic Halos:~ the examination of many halos of special interest has elicited points of sufficient importance to justify fuller con.~ideration o[ the subject. In the paper referred to it was shown from the measurement o[ halos in a Greisen that the radial dimension of the corona, or outer halo, when compared with that of the darker central area was such as to support the conclusion that the corona represents a shell of less complete ionization (or other effect) due to the a rays of radium C only ; while the more intensely darkened centre was ascribable to the influence of all those rays which range no further than those of RaA. It was also found that calculation of these radii, according to Bragg's law connecting the ionization range

Downloaded by [University of Arizona] at 16:15 06 June 2016 with the square root of the molecular weight, gave distances closely corresponding with those observed. The mica in which these halos occurred was subsequently found to be a lithia-bearing variety--the somewhat variable species classed as zinnwaldite. But, as will be seen further on, this emen- dation introduces no contradiction to the views originally expressed. In a letter to 'Nature' of February 10th the fact of the existence, in some cases, of an accentuated outer border to the corona, in accordance with the increased effect ~ Phil. Mag. July 1908, p. ] 84. CommunieA,tedby the Authors. ** Joly, Phil. M~g. February 1910, Pleocl~'oic Halos. 6;41 of the :z ray towards the end of its hinge, "~s shown in tile Bragg curves, was confirmed; and the identification of thorium halos recorded. As the number of measurements multiplied it appeared as if the calculated or theoretic values tended to range a very little above those determined by observation. The source of this has been found to lie in tl~e mode in which Bragg's law was being applied. The method of applying this law had in- vited the comparison of the average square root of the atomic weight of the mineral containing the halo with that of air. Now Bragg and Kleeman's figures and results do not bring air quite harmoniously into llne with the solid substances examined (Phil. Mag. Sept. 1905), and it seemed a more correct procedure to calculate the range in the mineral by comparison with the square root of the atomic weight of a substance more generally in agreement with the law and similar to the mineral in 1;hysicai-state. Choosing aluminium tbr the purpose, the table given further on for the ranges in some important halo-bearing minerals, has been calculated both for the ~t rays of RaC and RaA and for ThC and ThX. The calculation is most readily made as follows. Bragg and Kleeman found that if the products of range and density in a number of different substances are compared with the corresponding products in ~he case of air, the several quotients obtained stand in the same relation one with another as the square roots of the atomic weights of the several substances. The quotient for aluminium has the value 1"23. If we assume the range in air to be one centimetre, then the A x 1"23 corresponding range in aluminium is ~'= ~, where A and $ are the densities of air and aluminium. Also we have ~r?.r a t Sr =a; ~here ~:, r' are the density and range in any particular mineral; a' is the average square root of the Downloaded by [University of Arizona] at 16:15 06 June 2016 atomic weight of the mineral, and a tim square root of the atomic weight of almninium. Substituting the expression for r and taking the density of air as 0"0012; and writing ! 5 15 for a ; we have d=0"000287~p ; which gives the range in the mineral corresponding to one centimetre in air. Thus for certain biotites a ~ is found to be 4"5 and ~ 2"8, hence ~=0"000461. Then for the rays of RaC, having a range in air of 7"06 cm., we find the range in this biotite to be 0"0326 mm. The quantity a: may be determined either from 632 Prof. J. Joly a~zd Mr. A. L. Fletcher : a molecular formula or fl'om a chemical analysis. In the first case we multiply the square root of each atomic weight by the number of such atoms present; sum the values so found ; and finally divide by the number of atoms entering into the formula. In the second case we deal with each molecule separately, adding the square roots of the component atomic weights and dividing' by the number of atoms in the molecule. The number obtained is the average square root of the atomic weights entering into the molecule. It is to be multiplied by the percentage quantity of the molecule present, as given in the analysis. The numbers obtained are to be added and their sum divided by 100. The following are the ranges in air of the a rays of the uranium, actinium, and fhorimn families of elelnents :--

I. II. IIl'. Iv.

em. cm. Olli, OlD, Uranium ... 3'5 2"8 t~adioactinium. 4"8 Thorium 3"5 Ionium 2"8 2"1 Act X 6"55 Radiothorinm 3"9 Radium 3"54 2'84 :Emanation. 5"8 Th X 5"7 Emanation... 4"23 3"60 Act B 5'50 Emanation. 5'5 l~aA 4"83 4'05 Th ]3 5"0 RaC 7"06 6"35 Th C 8"6 RaF 386 3"16

Column I. gives the lhniting ionization ranges in air at 760 ram. pressure, and Col. II. the approximate distances in air at which the ionization effect is a maximum in the case of the uranium family. The values in Col. lI. are deduced partly by scaling directly from the Bragg and Kleeman curves, partly by assuming that a deduction of 7 mm. from the limiting range ~ives the approximate position of most Downloaded by [University of Arizona] at 16:15 06 June 2016 intense ionization. Thesevalnes we shall require later. The distances in Cole. III. and IV. are the limiting ionization ranges in air in the case of the actinium and thorium families *. In a pleochroie halo we may have either the uranium derivatives acting or those of thorium ; or, again, a mixture of both. In the first case the radius of the halo is limited

* See Bragg and Kleeman, loc. cir. and Phil. Mag. June 1906 ; Allen, Phys. Rev. xxvii. 1908 ; Le Radium, Jan. ]910, p. 2 ; Levln, Phys. Zeit. 1906, p. 521 ; Ku~era and )Ia~ek, Pt, ys. Zeit. 1906, p. 339 ; Hahn, Phil. Mat. Sept. 1906. Pleoch roic Halos. 633 by the ionization range of ttaC in the particular mineral in which the halo is formed. In the second the radius is limited by the range of ThC. In the third it will again be limited by the range of ThC. It might, in general, be difficult to distinguish the last two cases from once another ; but if the halo is not over exposed we would expect in the Ra-Th halo a corona with inner and outer radii corresponding to the radii of RaC and ThC. Thus if formed in biotite the radius of the pupil would be about 0"033 mm. and of the corona or iris 0"040. There is no difficulty in distinguishing uranium from thorium halos, however blackened up they may be. In biotite the outer radius era good uranium halo is never very different fl'om 0'033 mm. and that of a thorimn halo from 0"040. The difference, mfless the originating radioactive particle is large enough to exaggerate the dimensions of the former, is such as cannot escape detection. In these cases if the halo is not over exposed the radius of the pupil will be respectively about 0"022 and 0"026; the former being due to I~aA, the latter to ThX. rr he table which...... follows gives the radn (m mflhmetres) m the case of the commoner halo-bearing nfinerals. Only some of the figures in the above table are based on an accurate knowledge of the density and the chemical composition. Both quantities require to be known for the particular mineral. The more reliable values will be recognized by the accurate figures given for the specific gravity.

U ranium, Thorium.

RaC. l~aA. C. X.

Common J 3"0 0"0346 0"023~ 0"0422 0"028( (MgFe)2Si~O6(A1Fe)~O 6. Hornblende 3"1 0"0336 0'023( 0-0409 0"0272

Downloaded by [University of Arizona] at 16:15 06 June 2016 Diopside ... 3'492 0'0281 00193 0"0343 0"0227 tledenbergite.

:Biotite 2"8 0"0"~260'0222 0'0397 0"0263 (tIK)2( Mg Fo)~At~(SiO4) ~ Anomite ...... 2"846 0'0321 0'0219 0"0390 0"0259 Analysis selected by Tsehermak. Haughtonite. 2'934 0'0334 0"0229 00407 0"0270 ,, see p. 638.

),~uscovite ... 2'8 0"0304 00208 0"0370 0"0242 A I:~KIt2Si.~O ~2. 2"830 0'0310 0-0212 0"0378 00250 Analysis selected by Tsehermak.

Lepidolito ...' 2"885 0"0307 0'0210 0"O374 0"0248 Analysis selected by T~chermak. 2"8 0"027~ 0 0188 0'0335 0"0222 KLiA12F%HSi30~o.

Zianwaldite.. 2'9 0"032~ 0"0221 0'0391 0'0260 Analysis selected by Tsehermak 2"9 0"032r 0"0225 00400 0'0265 20 Li, 20 A1, 34 Si, 99 O, 20 Fe. (;3~ Prof. J. Joly a~ul Mr. A. L. F:cWher : In selecting tmlos for investigation several sources of error must be borne in mind, the ne~'lect of which has doubtless been largely responsible for the general failure of petro- logists to recognize their wonderful obedience to law. In the first place it is apparent that only a central section of the sphere can give the true radios or, if there is differentiation, a correct ratio cf the radii. Thus in complex halos a section off the centre tends to exagger~,te tho dimensions of the iris compared with that cf the pupil ; and in simple or structureless halos the radius of' an excentric section of the halo mus~ be misleading. With respect to this source of error the only safeguard is the certain presence of the originating radioactive particle. When this is plainly defined we cannot be i~r froin a central section. A second source of error arises when the originating radioactive mineral has dimensions which are not negligibly small compared with the radius of the halo. When the radius of the nucleus is, relatively, not negli~'ible, its mean diameter should, in general, be deducted from the di:~meter of the halo. This is so because in many cases it is demon- strable that rays leaving the surface--or from points near the surface~of the nucleus have determined the boundar y. Thus a large square zircon-section may show a sub-circular halo enclosing its sides ; the measurements from the bonn- daries of the zircon being correct for radium C. In general halos occasioned by large nuclei are badly defined and h~lzy upon the edge owing to rays coming from different distances in the nucleus spoiling the boundary definition. In very small nuclei there is but little error due to the passage of central rays through the nucleus. Thus for zircon the quantity ~/atomic weight density is 1"1; in the case of thorite it is 1"2; in uranlnite about 1; and in the cases of hornblende and biotite about 1"8 and 1"6 respectively. There is, there- Downloaded by [University of Arizona] at 16:15 06 June 2016 fore, some retardation in nuclei formed of the radioactive minerals ; but it is easy to see that the error introduced by neglecting it is of quite a different order from that which arises when the ranges in air of rays proceeding from beneath the surface of a solid fihn are being observed. The correction, such as it is, seems to be in most cases sobtractive. Thus in the case of a nucleus having a radius of, say, 0"0021 ram. we cannot well assume that the outer boundaries arc defined either by rays proceeding frcm the very surface, ncr yet from the very centre of the radioactive particle. If we Pleochroic II~dos. 635 ,ssume that their average point of de~ aTture is about 2/3 the radiu~ measured i'ronl the centre, we find that the distance, 0"0007 mm., traversed ill tile denser mineral (we will suppose uraninite) is equivalent to 0'0011 in biotite, say. So far there should be an additive correction of 0"0004 nun. But the point of departure not being at the centre, but 0"0014 ram. away from it, the subtractive correction exceeds the additive, leaving finally a subtractive allowance of 0"0010 mm. The correction is, probably, of approximately this amount. The effect on measurements of outer radii is small. On inner and much lesser radii it may be more serious. In certain minerals the nucleus itself takes a roughly circular form and is opaque, and when the whole is blackened up quite erroneous conclusions may be arrived at. Exami- nation by reflected light will, in such cases, generally reveal the central nucleus. Indeed, it is well to examine all much darkened halos by reflected light before forming conclusions as to their radius. So far as our experience goes halos in eordierite or in andalusite cannot he used for accurate measurements. Ap- proximate measurements quite in accord with calculated values can be obtained, but the boundary of the pale yellow halos is too indefinite to permit of accuracy. Structural features we have not traced in such halos. Cordierite is much more sensitive to the a radiation than biotite*. It is possible that even a lesser quantity of a radioactive substance could be detected in this medium than in biotite. The ~reater sensitiveness probably leads to more rapid over exposure ~,nd loss of detail. As an instance of the necessity of making allowance tbr a relatively large nucleus we ma37 refer to the photograph of a halo in biotite illustrating an earlier paper on this subjeett. The measured diameter of this halo is 0"096 nnn. The radius, 0"048, however, will not agree with any calculated radius in

Downloaded by [University of Arizona] at 16:15 06 June 2016 biotite, but it is found that the central mineral, in this case zircon, is too large to be neglected. A number of observations in different azimuths from the boundary of the zircon to the periphery gives the radius as 0"039. This is quite in harmony with the view that this is a thorimn halo. In the next table we have collected some Iueasurements of halos in various minerals.

* ~-YIii,~e. CentrbLfi Min. 1909, p. 66. t Joly, t hil. Mug. March 1907. (;36 Prof. J. Joly and Mr. A. L. Fletcher:

Containing 1%. r. gadioactive Rock. mineral. nlnl. nlnl. substance.

:Bictite ...... 0"023 ... ~a Durbaehite, Sehwartzwald...... 0"039 ... Th ...... 0"039 0"023 Th Diorite, 1Redwitz...... 0"032 ... Ra ~J ,, ...... 0"033 3"021 Ra Granite, Freiberg...... 0"0393 3"0244 Th ,, Oehsenkopf...... 0'0319 3"0191 Ra ...... 0"033 i 3"022 Ra Leinster. Zin'nwaldite.. 0'0312 ... Ra Greisen, Altenberg...... 0'0305 3"0199 Ra ..... 0"0321 C)'0211 Ra ..... 0"0323 C)'0202 .... Corona detached...... 0'0326 ,, 0'0317 0.0"1"98 .... Corona detached...... o.o3o~ ~.o191 I{;L ~p ... 0"0316 0"0190 Lepidolite ... 0"028 ... Lourdalite, Langesundfiord. Cordierito ... 0"031 .. :Hornblende.. 0"040 0"028 Syenite, Knorrc. ,, ... 0033 ... ,, St. Main'ice.

One of the thorium halos in the durbachite is that of which we have just given details, and is illustrated in the paper referred to. We reproduce here (P1. VIII.) a photograph of a thorium and a uranium halo (contained in the one crystal of mica from a granite of Ochsenkopf in tile Fiehtelgeb{rge) the dimensions of which are given in the table. The uranium halo (in lower part of figure) is too dense to pernfit of clear photographic reproduction of the pupil; but the latter can be seen and measured in the microscope. The corona of the thorium halo is not strongly developed, hut is quite distinct. The nuclei in the case of both these halos are small and practically negligible. From the measurelnents given in the table it will be seen that halos developed in biotite are somewhat greater in radius

Downloaded by [University of Arizona] at 16:15 06 June 2016 than those in zinnwaldite or lepidolite. This agrees with lhe theoretic radii in these minerals as already deduced. The fact that the radius of the pupil is not in every case quite up to the theoretic size is, we think, explicable on the view that these halos are not always fully developed. It will be seen further on that the corona appears before the pupil reaches its limits. Thorium halos are not so frequently met with as uranium halos. ~he thorium halo observedin the syenitic hornblende of Knorre is a very beautifully coloured object when seen between crossed nicols. The nucleus in this case is pre- sumably thorite. It is black and nearly opaque and aJmost Pleoch~'oic llalos. 637 square in cross-section. Thorite is isomorphous with zircon, and its.presence in syenites has been denlons~rated before now. The inner radius is not sharply defined in this ease ; the measurement given is approximate only. Reference to the table of calculated radii will show that the agreement between observed and calculated values is in general very striking.

Stages in tlle Development of Pleochroic ttalos. We have included in the preceding table of observations only those h~dos whose development appears to be nearly or quite perfect. In some cases these halos may be described as " over-exposed"; that is, the detail within the halo is obliterated by excessive darkening of' the medium. [n others we see the halo at an earlier stage; corona and pupil being distinct, or the former even detached, froln the latter and encircling it as a delicate ring. Plainly this last is an earlier stage of development. But halos may be observed in still earlier stages. We have been so fortunate as to find in a single large crystal of biotite (var. haughtonite) from the Leinster granite, uranium halos in every stage of development. Those stages we shall presently describe ; but, first, some account, of the conditions attending the occurrence of these halos will be of interest. The crystal of biotite, which measures about 2'5 cm. in its greatest dimension, is enclosed within a fringe of silvery white mica, a relation not uncommon in the Leinster granite. This appears to be an original association, the muscovite crystallizing after and in ('ontinuation of the biotite. But the same association is believed to occur as the result of met;t- nmrphic action in certain rocks. The boundary is abrupt ;m ~, even in the microscope, the demarcation is sharply defined. Halos are developed only in the biotite, 'rod principally in the vicinity of the junction of the micas. The central Downloaded by [University of Arizona] at 16:15 06 June 2016 parts of the biotite crystal are halo-free and clear of all inclusions. The radioactive substance is often found extending along cracks in the biotite, which then are intensely dark, and may carry strings of halos, their nuclei contained in the cracks, their spherical shells extending into the clear biotite. In other places the halos are crowded in clusters or, again~ setttered without apparent connexion. Much radioactive blackening has also occurred around elongated or shapeless inclusions, or in stains and blotches. The radioactive darkening never extends into the limpid muscovite; a halo often appearing as bisected where it meets the colour- 638 Prof. J. Joly a~d Mr. A. L. Fletcher : less mica. The diverghlg arrangement of the darkened cn~cks from largo are,as of blackened biotite situated near the margin or' the crystal strongly suggest~ the idea that a radioactive solution has at some time penetrated the biotite. An occasional yellowish staining' of the muscovite may possibly indicate the presence of this sohltion.. Zircons, not present in the muscovite, ofteu act as radioactive nuclei in the biotite, but some of the smallest nuclei are, apparently, not zircon. They are, possibly, uraninite or some allied mineral. The following analysis of this mica shows it to be of the variety called haughtonite. The iron is nearly all in the ferrous state and is calculated as such. SiO._, ...... 34"12 FeO ...... 2t"12 AI:O3 ...... 25"62 M~(9 ...... 0"48 K oO ...... 13"40

97"74 The small quantity of magnesia is remarkable. The mica is of a deep reddish brown colour with small optic-axlal angle. A careful experiment gave the density as 2"934. From these data we find the average square root of the molecular weight to be 4"84, and the quotient ~ to be 1"65. Hence / has the value 0"000473; i. e. the range in this mica corresponding to one centime(re in air. A determination of ~he radioactivity of the mica was re:Me at a sacrifice of one gram of material. The experiment was made in the new Physical Laboratory (by kind permission of Professor W. E. Thrift) under circumstances precludil)g any probability of contamination. The result arrived at, on t~o identical determinations, was 11"87x 10 -~ gram per gram. Downloaded by [University of Arizona] at 16:15 06 June 2016 This result gives, on the face of it~ an idea of the extremely small quanti W of radioactive material sufficing to originate a halo. It is not too much to say that some thousands of halos, or equivalent tad oactive staining, occur in a gram of this mica. Au endeavour to observe any local radioactive intensity by placing halo-rich parts of the mica beneath and in contact With a screen of sensitive zincblende gave a negalive result. Using the value obtained above for the range-equivalent of 1 cm. in air, and the measuremen*s given in the table p. 632, tim following are the limiting ranges and the ranges .Pl.ochroic Halos. 639 of maximum ionization of the various a rays of the uranium series in this biotite. Maximum Limit l]lnl, 111112. R~,(] ...... 0"030t 0"033~ RaA ...... 0"0201 0"0229 Emanation ...... 0"0170 0"0200 RaF ...... 0'0150 0"0183 ]~a and Ur ...... 0"0135 0"0168 Ionium ...... 0"0100 0'0133

These ranges appear to be concerned in the genesis of the lmlo, but in a manner which can only be completely under- stood by reference to the Bragg curves showing the ionization at every point of the path of an r particle in air. For better convenience we reproduce here the curves (I. 640) as given by Bragg and Kleemau in the paper which has been the starting point of all subsequent work of the kind* (loc. cit.). Ioniza- tion being plotted as abscissse, and distance from the origin as ordinates, the complete curve for the a rays expelled by RaC (curve chad) shows that the ionization due to the a particle increases slowly at first, but augments very rapidly towards the end of the ionization range. At a distance of about 6"3 cm. the intensity of ionization is a maximum, and at 7"06 cm. the ray no longer ionizes. Its presence beyond this distance-- if it penetrates further--could only be detected by a cumula- tive experiment and spectroscopic analysis; for it will neither affect a photographic plate nor a sensitive zinc sulphide screen, m,r will it ionize a gas. The rec.cnt experiments of Geiger ('Nature,' Feb. 24, 1.~10, p. 508) support the view that the ot ray then comes to rest, and, probably, losing its charge, takes on the character of an atom or' helium (Ruther- ford and Geiger, Prec. R. S. ]xxxi. A. p. 162). The ibrm of the ionization curve of RaC is found to apply Downloaded by [University of Arizona] at 16:15 06 June 2016 to the other rays, so that the effects attending the motion of each particle in the medium are defined by transporting the curve for the rays of RaC parallel to itself, and so as to suit the ranges of other rays. Each ray has therefore the same intensification of its effects just before it ceases to ionize: the initial velocities and ranges of lhe rays alone vary. A radioactive nucleus giving out rays in all directions fi'om the several a-ray-producing constituents is, therefore, sur- rounded by successive shells of maximum ionization. That * We desire to thank Professor Bragg for his permission to reproduce these curves. r Pro~. J. Joly and 5tr. A. L. Fletcher : due tr ionium lies nearest to the centr~, in air at about 2"1 cm., in biotite at about 0"01 ram. After this the shells o[ the several maxi,na crowd together, merging one into the other, as the table of ranges, given above, shows. The complex curve, given by Bragg and Kleeman for radium which

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! ~ ,-~

/ ~,,,~ ~ .,

! *

1 z! i "',

: le-~

t j i ;[' Downloaded by [University of Arizona] at 16:15 06 June 2016

t !S .... --~ i [ I _4~J

has developed emanation and the .nsuing products of rapid change, illustrates this eff~.ct; the line of" ionization sloping, with perturbations, upwards from right to Ief~, the slope ending with the effects due to RaA. In the halo we mus~ Pleochroic Halos. 641 imagine additional rays at work which would prolong the Bragg-Kleeman curve further to the right, and also bring it downwards nearer to the origin. Here we may recall Rutherford's suggestion, based on Boltwood's work, that actinium is probably a collateral member of the uranium-radium series ; certain of the unstable atoms disintegrating according to a different plan and determining a separate line of descent. As such atoms are known to be subordinate in number it is probable that halos generated by actinium, or even definitely revealing its presence, will never he observed; its effects being always associated with and, as it were, submerged in, those of the preponderating radium series. There is som'~ reason to believe that the fastest moving ~ ray products of actinium, ranging from 5"5 to 6"5 cm. in air, are concerned in finally blackening' up the uranium-radium halo; but so far as our observations have gone no halos referable solely to this radioactive series occur. The extension of the work of Bragg and Kleeman has re- sulted in showing that the laws discovered by these investi- gators apply to ~t rays however originating, ~vhether from actinium or thorium. In the Leinster haughtonite we find that a considerable number of small halos exist~ possessing a radius of about 0'013 ram. These are simple, generally structureless, halos, but occasionally showing a darkened peripheral border. They mayvary much in intensity, the fainter being associated usually with the smallest nuclei. In most cases the nucleus is clearly and sharply defined as a minute black or nearly opaque speck, which may look square or angular. The colour of these halos varies from that of a faint smoky grey to a deep brown or even black. Their form is perfectly circular. The following are some radial dimensions in millimetres read in cleavage ~lakes of the mica :--0"0129, 0"0147, 0"0125, 0"0132, 0"0128, 0"0128, 0"0135, 0"0137, 0"0137, 0"0111, 0'0141, 0"0135, 0"0132,

Downloaded by [University of Arizona] at 16:15 06 June 2016 0"0138, 0"0122. We might add many scores of such determinations. These halos only very rarely reveal any trace of an outer corona or ring. Thus two of the above, h~ving radii in each case of 0"0137 mm., showed a first very faint trace of the corona. Such coronas lie at the correct radial distance for RaG as given in measurements cited further on. It is remarkable that halos at this stage may be much darkened and yet fail to show any trace of the action either of RaC or other of the more penetrating rays, while those showing ~races of a corona may he only fairly well defi~ed. As we may assume that the time of accumulation of the radioactive Phil. Mag. S. 6. Vol. 19. No. 112. April 1910. 2 T 6~2 Prof. J. Joiy and Mr. A. L. Fletcher : effect (whatever its nature may be) is the same for all, the differences in de~,elopment muse simply depend upon the radioactivity of the central originating substance. The larger nuclei are, accordingly, generally found to be attended with the best developed halos. It seems certain that these embryonic halos owe their radial dimensions to the effect of the slower moving a rays; those of ionium having their maximum effect at 0'010 ram. defining the smallest, and the maxima of radium and uranium lying at 0"013 nun. defining the larger. Some allowance for the radius of the nucleus, which very often has been found to be about 0"0022 mm., would refer some of these halos solely to the effects of ionium. But none are ever found of radial dimensions less than can be accounted for by ionium. This seems additional evidence that ionizing a rays of less velocity than those given out by ionium do not exist in the uranium-radium series. The influence of the radium and uranium a rays is, probably, an early one ; the latter element being known to give two ~ rays upon disintegration. At the radial distance of 0"0135, therefore, three a rays out of the total eight which are evolved in the entire sequence of changes, exert their greatest effect. It is, therefore, not to be wondered at that the number of embryonic halos having a radius ascribable to these influences is very great, but owing to the uncertain allowance for the nucleus and the gradation of the observed dimensions it is impossible to clearly define those due to any particular ray at this stage of development. In agreement with this, simple halos of varying dimensions somewhat greater than those cited above are found. This, too, is quite in accordance with the Bragg curve. If this curve were prolonged to the right so as to bring in the uranium, and the slower moving ionium, ray, it is evident that while halos would not be expected to show a lesser radius than is due to

Downloaded by [University of Arizona] at 16:15 06 June 2016 ~he ionium ray, the enlarging of the radius under the intense ionization o~ uranium attd radium should be rapid. It should be noted as generally true of the initiation of halos that the first beginning is not from within outwards. The halo suddenly presents itself' as a very faint spherical darkening of the mica, which if differentiated at all is darkest at or near the surface. Any greater darkening occasionally observed near the nucleus is more or less irregular and fuzzy, is confined generally to small radial distances, and may be due either to the convergent concentration of the rays or to some escape of" emanation. This mode of initiation is, of course, in keeping with Bragg's observations. Pleocltro~c Halos. 643 Two shells of m~ximum ionization succeed closely tbo,e due to the radium and uranium rays; that due to RaF at 0"0150 ram., that due to emanation at 0"0170 and extending to 0"0200 ram., about. Just beyond this limit should lie the maximum effec~ of RaA. The following are typical measurements of halos whose limitations may be ascribed to RaF or to emanation:--0"0158, 0"0167, 0"0165, 0"0170, 0"0181, 0"0182, 0"0184, 0"0188. We might quote very largo numbers of such measurements. Such halos are very often, if not most generally, still without a corona. These simple halos may even be quite blackened up and yet show no signs of the effects of RaA or RaC. The dimensions of the pupil at which RaC first makes its appearance are, typically, somewhat greater than the above, but not such as can well be ascribed to RaA. This is one of the most remarkable points about the development of these llranium-radium halos. RaC seems to create an effect in advance of RaA, although the density of the former is less owing to the greater radius, and although RaA is somewhat strengthened by a coincident actinium ray. The radius of the faint, early, corona is never less than can be ascribed to RaC, as will be seen from the list of dimensions which follows. R is measured to external radius of corona, R 1 to its inner radius.

~tlc]e~tr r. I~'. R. rmlius.

0'0165 OO297 0"0169 0'030~} 0"0170 O'O3OO 00171 0"0318 0"0174 0"0328 0"0020 0'0176 0'0314 0 0029 0"0174 0'0319 0"0019 0"0177 0"0306 0'0179 0'0323 Downloaded by [University of Arizona] at 16:15 06 June 2016 0'0180 0"0328 0'0183 o.62~o 0'0323 0"0025 0"0184 0"0267 0"0323 0"0029 0'0185 0"0286 0-0323 0"0185 0 O284 00317

Such halos as the above are not, of course, fully developed. The corona changes indeed but little in its subsequent dimensions, but the pupil expands considerably. The next feature observed on close examination is sometimes the appearance of a very fine and faint ring lying outside and close to the pupil. It is, indeed, probably in all cases connected to the 2T2 644 Prof. J. Joly and Mr. A. L. Fletcher : pupil by intermediate shading 1,.ss intense than that which makes the ring visible. This ring is very probably repre- sentative of the effects of RaA. The following are some nleasnrements :~

R,IA. a r"

0'0103 0 0169 0'0228 0-0295 0"0333 0"0187 0'0227 0'0327 0 0186 0"0229 0 0284 0'0339 0'0192 00233 00280 0'0338 0"0201 0"0219 0'0283 00323

In the case of' the 5rst halo the inner radius, v r, of a dark band edging the pupil is recorded. It will be seen that there is a very elo~e agreement amon~ the measured radu of the ring ascribed to RaA. Ihe radn are measured to the outside of the ring. Allowing for some effects due to the central nucleus the coincidence between the measured and calculated ranges (the calculated maximum is 0"0201 and limit 0"0229) is sumcient to leave hardly any doubt as to the influence of RaA in forming this ring. In PI. IX fig. 2 we reproduce photographically a field of halos in various stages o[ de~'elopment, one showing the pupil well developed and three others in various stages of incom- pleteness. In the more rudimentary ones the appearance of the pupil will serve to convey the form of the simple halos which precede the advent of the corona. In P1. IX. fig. 3, a very much enlarged halo is shown bearing the ring due to the" rays of RaA. This halo is 0"033 ram. in diameter, and as it scales here about 3 cm. the linear magnification is Downloaded by [University of Arizona] at 16:15 06 June 2016 about 450 times. It is difficult to offer any explanation of the relatively backward effects of RaA. If there was "initial recombination" progressing and such recombination was stimulated by the shock attending the passage of the fast rays of RaC, such an effect might be more marked here then elsewhere, and so the absence of the effects of RaA be explained upon their elimination. It must be borne in mind that ~he changes are progressing in a crystalline medium, and there may be threes tending to restore the original molecular grouping which may be stimulated into action where they are not Pleochroic ItaIos. 645 permanently overcome. The true explanation may, on the other hand, lie in effects not yet suspected. The further development of halos is marked bv widening of the corona and advance outwards of the pupil. "In general there seems to be a pause after the effects of RaA have bronght the pupil out to such dimensions as are given in the foregoing table. In many old halos, as described in the first part el this paper, the differentiation is limited to the pupil and corona--the latter a more lightly shaded continuation of the former. But for some reason the development does not always progress as if leading to this result. Thu~ in the mica we are dealing with we have found halos with a fMrly uniform clark pupil brought out to a radial distance of 0"027b, and still separate from the corona. That this is not due to a large central nucleus is shown by the outside radius of the corona--0"0335 mm.--and can perhaps best be explained on the activity of several nearly coincident actinium rays ranging over 5 cm. in air. If this is tile normal process of develop- recur--that is by the gradual advance of the pupil--the slate of simple differentiation into pupil and iris must be explained on the subsequent ~reater accumulation of effects within the limits reached by RaA. But it must be admitted that while the general features ot~ the development seem [ree from any obscurity, the relative intensity and order of the events ~re not wholly explicahle. We arc, however, in total ignorance of what really takes place in the crystalline medimn. The highest magnification applied to halos does not appear to throw light upon the latter point. A fidntly radiate structure is sometimes seen, ill particular lighting, in halos which are under-exposed. This may sometimes appear to be confined to the corona. Its significance, if it has any, is unknown. Doubtless helium is actually stored in the successive shells which mark the limits of each ray. Under what conditions, whether free or possibly in synthetic com-

Downloaded by [University of Arizona] at 16:15 06 June 2016 bination with atoms into which it has penetrated, is not known. Strutt's results on the helium stored in various rocks and minerals point to a mere mechanical storage or' the heli ran. It seems very improbable that the presence of the helium occasions the darkening observed. There is really very little of it. As will be seen later there is, probably, not sufficient to create an atmosphere pressure if distributed throughout the halo. Again, if the helium is the cause of the darkening, why are halos confined to certain minerals ? The restrictions are not due to want of radioattive nuclei, for zircon ,nd ~q)atite, which in some minerals give rise to halos, in others in the same rock-section are without effect; and, as observed, 646 Prof. J. Joly a~d Mr. A. L. Fletcher: the halos in this Leinster biotite may be abruptly bisected where they abut against the muscovite. This~ too, is a frequently observed appearance in granites, syenites, &c. The halo is not continued into quartz or felspar adjacent to the biotite or hornblende, in which it is developed. If the appearance is due to a mere storage of helimn this should not be. On the other hand, the view that the molecular structure is broken down by the passage of the positively charged ray seems in harmony with the photographic and other chemical effects which have been shown to attend the passage of the ray. The nature of the changes produced is as yet unknown, or whether the crystallographic structure influences them. There might be phenomena resembling "reversal " in photographic films. We are really in very complete ignorance of the nature of the wonderfully graphic yet minute phenomena o t the halo, as the number of speculative possibilities associated therewith demonstrates. It is of interest to note the extremely minute quantity of radioactive material which suffices to make a halo. We have already suggested that the nuclei are in some cases possibly uraninite or some related substance. For they may be blaclr and opaque, and, when their lustre can be observed by reflected light, show the coaly appearance of pitchblende. The assump- tion does not involve an amount of radioactive matter greater than the ascertained gross amount per gram of nlica might indicate. The diameter of a nucleus sufficing to evolve all the characters of the halo, pupil and corona, is very often less than 5 • 10-%m. The correspondingvolumeis about65 x 10-1~cm. If composed of uraninite having the density 8 the mass would be approximately 5 • 10 -1~ gram, and the associated radium in each nucleus about 10 -16 gram. As one gram of the mica contained nearly 12 x 10 -1~ gram radium tllere is sufficient to build 100,000 such halos per gram. We may,

Downloaded by [University of Arizona] at 16:15 06 June 2016 therefore, without unduly taxing the resources available, suppose uraninite to be one of the radioactive substances present. The quantity of helium stored in a halo must be small on any probable estimate of the time which has elapsed since its initiation. On Rutherford and Geiger's estimate that 3"4 • 10 lo helium atoms are expelled per second per gram of radium, the qnantit,y 10 -~6 gram radium and the 8 ~ ray producing elements in equilibrium with it evolve only about 850 helium atoms in a year. If the time was 10 s years the number of atoms is 85 x 109. Rutherford and Geiger have calculated that there are in one cubic centimetre of a gas at ~tandard pressure and temperature 2"72• 1019 molecules. Pleoch~ oic tlalos. 647 The volume of helium is, therefore, only about 3 x 10 -I9 c.c. If this is distributed uniformly throughout a halo having the radius 0"033 mm. the pressure is less than the forty-sixth part of an atmosphere. A nucleus of ten times the mass would still yield a much less amount of helimn than would fill the halo-sphere at atmospheric pressure. Nothing is so surprising in connexion with the pleochroic halo than the extremely minute effects which suffice to give rise to it. The calculation given above shows that nuclei of the size repeatedly measured contain but 10 -16 of a gram of radium, even if we suppose them composed of the most ~'adioactive of naturally occur~'ing minerals. Th~s quantity gives of[ but one a ray in about ten hours. But even smaller nuclei suffice to produce small simple halos whose radioactive character is still quite unmistakable. A series of measurements, using a high magnification, gave the following readings in millimetres for the nuclear diameters of embryonic halos previously described as averaging 0"013 mm. in radial dimension :--0"00195, 0"0024:3, 0"0014:4:, 0"00243: and for two such embryos showing very faint coronas 0"00292, 0'00316. Calculating the volmne corresponding to the diameter 0"002 mm. we have, finally, a quantity of uraninite less than one-sixteenth the amount giving rise to the more developed halos. We are recognizing, therefore, by mere inspection, the presence of considerably less than 10 -57 of a gram of radium; a quantity expelling about. 80 helium atoms in a year. It is to be added that many of the smallest nuclei are probably zircon, which would lead to still lower estimates of the amount of radium involved. We are in some hope that experiments now in progress upon the rate of growth of artificial halos in this biotite may enable us to arrive at aa estimate of the time required for the formation of these halos. Here we would remark upon an aspect of the study of pleochroic halos which may, in the future, prove of im-

Downloaded by [University of Arizona] at 16:15 06 June 2016 portance. We are dealing with quantities of radioactive substances many thousands of times less than can be measured by any known method, and not only so, but the means of discriminating between one radioactive family and another is given to us. Even more, ,~ certain number of the specific a rays can, under favourable conditions, be inferred, and the ranges of some accurately measured. Thus in the uranium-radium halos of the Leinster biotite we might, if starting the research in ignorance of the specific results arrived at by Bragg and his successors, have inferred the presence of the ionium ray : of that ~'ery vigorous and probably, therefore, complex group of rays which enlarge the 648 Mr. G. H. Berry o~ tl~e primary halo and darken its border: of the ray having the definite range of RaA; and, finally, of the very penetrating ray giving rise to the corona. It would seem, therefore, that in the pleochroic halo we possess a method of search for new and rare radioactive substances as yet unapproached for sensitiveness. This is due plainly to conditions analogous to those which enable the photographic plate to reveal stars which are too faint to be visually detected, even in the most powerful telescopes. The halo, however, not onlvlntegrates the radioactive effects of ages, but presents tl~em to us sorted out according to the laws which govern its formation. But there is another aspect of these conditions. The fact that mi(.a, sensitive to the ~ radiation and capable of inte- grating its effects over geological time, is found unaffected and unaltered in association with many elements in the rocks --elements present in quantities which are enormous relatively to those we have been considering--appears to show con- elusively tha~ these elements do not expel any ionizing a rays even at very prolonged intervals of time. This we appear entitled to accept as testimony of the high stability of many rare and common elements throughout geological time.

LXVII[. The StrikiT~g Point of Piano/brte Strings. By G. !-I. BERRY * [Plates X. & XI.] T is the practice of all pianoforte makers to construct I their instruments so that the hammers shall strike the ~trings near the fixed bridge. The well-known explanation of this, first given by Helmholtz, is that striking the string near its end tends to eliminate the upper partials which are dissonant with the fundamental. Experiments carried out by Hipkins t ~howed that these upper partials, having a node at the point struck, could still be plainly heard, and it would Downloaded by [University of Arizona] at 16:15 06 June 2016 seem that this explanation of Hehnholtz is not a sufficient one. The object of this paper is to show that an entirely different reason can be given/'or this preference of striking- place. .Apparatus. Dr. Barton has published J/ the results of research work on the vibrations of the various parts of the monochord and Communicated by Prof. E. I-I. Burton, :D.Sc., F.R.S.E. + ' Sensations of Tone,' translated by Ellis, 3rd ed. p. 77. :t :Phil. Nag. July 1905~ p. 149; :Dec. 1906, p. 576; April 1907~ p. 446 ; Aug. 1909, p. 233. Jo~ & rL~cnE~. Phil. Mag. Ser. 6, u 19, PI. VIII.

_[710. ].

Downloaded by [University of Arizona] at 16:15 06 June 2016 Thorium and .Radium Halos in Biotite. • 150 diams. JOLY ~ ~LETCHER. Phil, Mag. 8er. 6, Fol, 19, P1. IX.

FI~. 2.

Group of four developing Halos in Biotite. • 75 diams.

FIG. 3. Downloaded by [University of Arizona] at 16:15 06 June 2016

Halo in Biotite. • 450 diams. Showing ring due to Ra A.