The Century's Progress in Chemistry

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THE CENTURY’S PROGRESS IN CHEMISTRY. 749

THE CENTURY’S PROGRESS IN CHEMISTRY. HENRY SMITH WILLIAMS, M.D.

tlie casual observer, it might .seem to beginnings have great end- have no alliance whatever. The won- SMALLings—sometimes. As a case in point, derful theory of atoms, on which the note what came of the small original ef- whole gigantic structure of modern chem- fort of a self-trained back-country Quak- istry is founded, was the logical out- er youth named John Dalton, who along growth, in the mind of John Dalton, of toward the close of the last century be- those early studies in meteorology. came interested in the weather, and was The way it happened was this; From led to construct and use a crude rain- studying the rainfall, Dalton turned nat- gauge to test the amount of the wa- urally to the complementary process of terfall. The simple experiments thus evaporation. He was soon led to believe inaugurated led to no fewer than two that vapor exists in the atmosphere as an hundred thousand recorded observations independent gas. But since two bodies regarding the weather, which formed the cannot occupy the same space at the same basis for some of the most epochal dis- time, this implies that the various atmos- coveries in meteorology, as we have seen. pheric gases are really composed of discrete But this w Tas only a beginning. The particles. These ultimate particles are so simple rain-gauge pointed the way to the small that we cannot see them—cannot, most important generalization of our cen- indeed, more than vaguely imagine them tury in a field of science with which, to —yet each particle of vapor, for example, 750 HARPER’S NEW MONTHLY MAGAZINE.

is just as mucli a portion of water as if 1803. I wonder if Dalton himself, great it were a drop out of the ocean, or, for and acute intellect though he had, sus- that matter, the ocean itself. But again, pected, when he read that paper, that he water is a compound substance, for it was inaugurating one of the most fertile may be separated, as Cavendish had movements ever entered on in the whole shown, into the two elementary sub- history of science? stances hydrogen and oxygen. Hence the atom of water must be composed of two lesser atoms joined together. Im- Be that as it may, it is certain enough agine an atom of hydrogen and one of that Dalton’s contemporaries were at first oxygen. Unite them, and we have an little impressed witli the novel atomic atom of water; sever them, and the wa- theory. Just at this time, as it chanced, ter no longer exists; but whether united a dispute was waging in the field of chem- or separate the atoms of hydrogen and istry regarding a matter of empirical fact of oxygen remain hydrogen and oxygen which must necessarily be settled before and nothing else. Differently mixed to- such a theory as that of Dalton could gether or united, atoms produce different even hope for a hearing. This vras the gross substances; but the elementary question whether or not chemical ele- atoms never change their chemical nature ments unite with one another always in —their distinct personality. definite proportions. Berthollet, the great It was about the year 1803 that Dalton co-worker with Lavoisier, and now the first gained a full grasp of the conception most authoritative of living chemists, of the chemical atom. At once he saw contended that substances combine in that the hypothesis, if true, furnished a almost indefinitely graded proportions be- marvellous key to secrets of matter hith- tween fixed extremes. He held that solu- erto insoluble—questions relating to the tion is really a form of chemical combi- relative proportions of the atoms them- nation—a position which, if accepted, left selves. It is known, for example, that a no room for argument. certain bulk of hydrogen gas unites with But this contention of the master -was a certain bulk of oxygen gas to form most actively disputed, in particular by water. If it be true that this combi- Louis Joseph Proust, and all chemists of nation consists essentially of the union repute were obliged to take sides with one of atoms one with another (each single or the other. For a time the authority of atom of hydrogen united to a single atom Berthollet held out against the facts,but at' of oxygen), then the relative weights of last accumulated evidence told for Proust the original masses of hydrogen and of and his followers, and toward the close of oxygen must be also the relative weights the first decade of our century it came to of each of their respective atoms. If one be generally conceded that chemical ele- pound of hydrogen unites with five and ments combine with one another in fixed one-half pounds of oxygen (as, according and definite proportions. to Dalton’s experiments, it did), then the More than that. As the analysts were weight of the oxygen atom must be five led to weigh carefully the quantities of and one-half times that of the hydrogen combining elements, it was observed that atom. Other compounds may plainly be the proportions are not only definite, but tested in the same way. Dalton made that they bear a very curious relation to numerous tests before he published his one another. If element A combines theory. He found that hydrogen enters with two different proportions of element into compounds in smaller proportions B toform two compounds, it appeared that than any other element known to him, the weight of the larger quantity of B is and so, for convenience, determined to an exact multiple of that of the smaller take the weight of the hydrogen atom as quantity. This curious relation w7 as no- unity. The atomic weight of oxygen then ticed by Dr. Wollaston, one of the most becomes (as given in Dalton’s first table accurate of observers, and a little later it of 1803) 5.5; that of water (hydrogen plus was confirmed by Johan Jakob Berzelius, oxygen) being of course 6.5. The atomic the great Swedish chemist, who was to be weights of about a score of substances are a dominating influence in the chemical given in Dalton’s first paper, which was world for a generation to come. But this read befoi’e the Literary and Philosoph- combination of elements in numerical ical Society of Manchester, October 21, proportions was exactly what Dalton had THE CENTURY’S PROGRESS IN CHEMISTRY. 751 noticed as early as 1803, and what had as regards the amount of space that sep- led him directly to the atomic weights. arates it from its fellows under given con- So the confirmation of this essential point ditions of pressure and temperature. The by chemists of such authority gave the compound atom,composed of two or more strongest confirmation to the atomic elementary atoms, Avogadro proposed to theory. distinguish, for purposes of convenience, During these same years the rising au- by the name molecule. It is to the mole- thority of the French chemical world, cule, considered as the unit of physical Joseph Louis Gay-Lussac, was conduct- structure, that Avogadro’s law applies. ing experiments with gases, which he had This vastly important distinction be- undertaken at first in conjunction with tween atoms and molecules, implied in Humboldt, but which later on were con- the law just expressed, was published in ducted independently. In 1809, the next 1811. Four years later, the famous year after the publication of the first vol- French physicist Ampere outlined a sim- ume of Dalton's New System of Chemical ilar theory, and utilized the law in his Philosophy, Gay-Lussac published the re- mathematical calculations. And with sults of his observations, and among other that the law of Avogadro dropped out of things brought out the remarkable fact sight for a full generation. Little sus- that gases, under the same conditions as pecting that it was the very hey to the to temperature and pressure, combine al- inner mysteries of the atoms for which ways in definite numerical proportions they were seeking, the chemists of the as to volume. Exactly two volumes of time cast it aside, and let it fade from the hydrogen, for example, combine with one memory of their science. volume of oxygen to form water. More- This, however, was not strange, for of over, the resulting compound gas always course the law of Avogadro is based on bears a simple relation to the combining the atomic theory, and in 1811 the atom- volumes. In the case just cited the un- ic theory was itself still being weighed ion of two volumes of hydrogen and one in the balance. The law of multiple proof oxygen results in precisely two vol- portions found general acceptance as an umes of water vapor. empirical fact; but many of the leading Naturally enough the champions of lights of chemistry still looked askance at the atomic theory seized upon these ob- Dalton’s explanation of this law. Thus servations of Gay-Lussac as lending strong Wollaston, though from the first he in- support to their hypothesis—all of them, clined to acceptance of the Daltonian view, that is, but the curiously self-reliant and cautiously suggested that it wr ould be well self-sufficient author of the atomic theory to use the non-committal word “equiva- ” himself,who declined to accept the obser- lent instead of ‘ ‘ atom ”; and Davy, for a vations of the French chemist as valid. similar reason, in his book of 1813, speaks Yet the observations of Gay-Lussac were only of “proportions,” binding himself correct, as countless chemists since then to no theory as to what might be the have demonstrated anew, and his theory nature of these proportions. of combination by volumes became one At least two gi'eat chemists of the time, of the foundation-stones of the atomic however, adopted the atomic view with theory, despite the opposition of the au- less reservation. One of these was Thomas thor of that theory. Thomson, professor at Edinburgh, who The true explanation of Gay-Lussac’s in 1807 had given an outline of Dalton’s law of combination by volumes was theory in a widely circulated book, which thought out almost immediately by an first brought the theory to the general at- Italian savant, Amadeo Avogadro, and tention of the chemical world. The other, expressed in terms of the atomic theory. and even more noted advocate of the The fact must be, said Avogadro, that un- atomic theory, Johan Jakob Berzelius. der similar physical conditions every This great Swedish chemist at once set form of gas contains exactly the same to work to put the atomic theory to number of ultimate particles in a given such tests as might be applied in the volume. Each of these ultimate physical laboratory. He was an analyst of the particles may be composed of two or more utmost skill, and for years he devoted atoms (as in the case of water vapor), but himself to the determination of the com- “ such a compound atom conducts itself as bining weights, “ equivalents,” or pro- if it were a simple and indivisible atom, portions ” of the different elements. These HARPER’S NEW MONTHLY MAGAZINE. determinations, in so far as they were ac- Here, then, were two utterly novel and curately made, were simple expressions of independent sets of empirical facts which empirical facts, independent of any the- harmonize strangely with the supposition ory; but gradually it became more and that substances are composed of chemical more plain that these facts all harmonize atoms of a determinate weight. This with the atomic theory of Dalton. So by surely could not be coincidence—it tells common consent the proportionate com- of law. And so as soon as the claims of bining weights of the elements came to Dulongand Petit and of Mitscherlich had be known as atomic weights—the name been substantiated by other observers, the Dalton had given them from the first—- laws of the specific heat of atoms, and of and the tangible conception of the chem- isomorphism, took their place as new le- ical atom as a body of definite constitution vers of chemical science. With the aid of and weight gained, steadily in favor. these new tools an impregnable breast- From the outset the idea had had the work of facts was soon piled about the utmost tangibility in the mind of Dalton. atomic theory. And John Dalton, the He had all along represented the different author of that theory, plain provincial atoms by geometrical symbols—as a circle Quaker, working on to the end in semi- for oxygen, a circle enclosing a dot for retirement, became known to all the hydrogen, and the like—and had repre- world and for all time as a master of sented compounds by placing these sym- masters. bols of the elements in juxtaposition. Berzelius proposed to improve upon this method by substituting for the geometri- During those early years of our cen- cal symbol the initial of the Latin name tury, when Dalton was grinding away at of the element represented—O for oxy- chemical fact and theory in his obscure gen, H for hydrogen, and so on—a nu- Manchester laboratory, another English- merical coefficient to follow the letter as man held the attention of the chemical an indication of the number of atoms world with a series of the most brilliant present in any given compound. This and widely heralded researches. Hum- simple system soon gained general ac- phry Davy had come to London in 1801, ceptance, and with slight modifications at the instance of Count Rumford, to as- it is still miiversally enjoyed. Every sume the chair of chemical philosophy schoolboy now is aware that H2 O is the in the Royal Institution, which the fa- chemical way of expressing the union of mous American had just founded. two atoms of hydrogen with one of oxy- Here, under Davy’s direction, the lar- gen to form a molecule of water. But gest voltaic battery yet constructed had. such a formula would have had no mean- been put in operation, and with its aid the ing for the wisest chemist before the day brilliant young experimenter was expect- of Berzelius. ed almost to perform miracles. And in- The universal fame of the great Swed- deed he scarcely disappointed the expec- ish authority served to give general cur- tation, for with the aid of his battery he rency to his symbols and atomic weights, transformed so familiar a substance as and the new point of view thus developed common potash into a metal which was led presently’ to two important discoveries, not only so light that it floated on water, which removed the last lingering doubts but possessed the seemingly miraculous as to the validity of the atomic theory. In property of bursting into flames as soon 1819 two French physicists, Dulong and as it came in contact with that fire-quench- Petit, while experimenting with heat, dis- ing liquid. If this were not a miracle, it covered that the specific heats of solids had. for the popular eye all the appear- (that is to say, the amount of heat re- ances of the miraculous. quired to raise the temperature of a given What Davy really had done was to mass to a given degree) vary inversely as decompose the potash, which hitherto had their atomic weights. In the same y7 ear been supposed to be elementary, liberat- Eilhard Mitscherlich, a German investi- ing its oxygen, and thus isolating its me- gator, observed that compounds having tallic base, wdiich he named potassium. the same number of atoms to the mole- The same thing was done with soda, and cule are disposed to form the same angles the closely similar metal sodium was dis- of crystallization —a property which he covered— metals of a unique pos- called isomorphism. sessed of a strange avidity for oxygen, THE CENTURY’S PROGRESS IN CHEMISTRY. 753 and capable of seizing on it even when selves as to their electrical properties, it is bound up in the molecules of water. some being positively, others negatively, Considered as mere curiosities, these dis- electrified. Electricity and “chemical coveries were interesting, but aside from affinity,” he said, apparently are mani- that they were of great theoretical im- festations of the same force, acting in the portance, because they showed the com- one case on masses, in the other on par- pound nature of some familiar chemicals ticles. Electro - positive particles unite that had been regarded as elements. Sev- with electro - negative particles to form eral other elementary earths met the chemical compounds, in virtue of the fa- same fate when subjected to the electrical miliar principle that opposite electricities influence, the metals barium, calcium, attract one another. When compounds and strontium being thus discovered. are decomposed by the battery, this mu- Thereafter Davy always referred to the tual attraction is overcome by the strong- supposed elementary substances (includ- er attraction of the poles of the battery ing oxygen, hydrogen, and the rest) as itself. “undecompounded” bodies. These re- This theory of binary composition of sist all present efforts to decompose them, all chemical compounds, through the but how can one know what might not union of electro - positive and electro- happen were they subjected to an influ- negative atoms or molecules, was extend- ence, perhaps some day to be discovered, ed by Berzelius, and made the basis of which exceeds the battery in power as his famous system of theoretical chem- the battery exceeds the blow-pipe? istry. This theory held that all inor- Another and even more important the- ganic compounds, however complex their oretical result that flowed from Davy’s composition, are essentially composed of experiments during this first decade of such binary combinations. For many the century was the proof that no ele- years this view enjoyed almost undis- mentary substances other than hydrogen puted sway. It received what seemed and oxygen are produced when pure wa- strong confirmation when Faraday show- ter is decomposed by the electric current. ed the definite connection between the It was early noticed by Davy and others amount of electricity employed and the that when a strong current is passed amount of decomposition produced in through water, alkalies appear at one the so-called electrolyte. But its claims pole of the battery and acids at the other, were really much too comprehensive, as and this though the water used were ab- subsequent discoveries proved. solutely pure. This seemingly told of the creation of elements—a transmuta- tion but one step removed from the crea- When Berzelius first promulgated his tion of matter itself—under the influence binary theory he was careful to restrict of the new “force.” It was one of Davy’s its unmodified application to the com- greatest triumphs to prove, in the series pounds of the inorganic world. At that of experiments recorded in his famous time, and for a long time thereafter, it Bakerian lecture of 1806, that the alleged was supposed that substances of or- creation of elements did not take place, ganic nature had some properties that the substances found at the poles of the kept them aloof from the domain of in- battery having been dissolved from the organic chemistry. It was little doubted walls of the vessels in which the water that a so-called “vital force” operated experimented upon had been placed. here, replacing or modifying the action Thus the same implement which had of ordinary “chemical affinity.” It w Tas, served to give a certain philosophical war- indeed, admitted that organic compounds rant to the fading dreams of alchemy are composed of familiar elements—chief- banished those dreams peremptorily from ly carbon, oxygen, hydrogen, and nitro- the domain of present science. gen—but these elements were supposed Even though the presence of the al- to be united in ways that could not be kalies and acids in the water was explain- imitated in the domain of the non-living. ed, however, their respective migrations It was regarded almost as an axiom of to the negative and positive poles of the chemistry that no organic compound battery remained to be accounted for. whatever could be put together from its Davy’s classical explanation assumed that elements—synthesized—in the laboratory. different elements differ among them- To effect the synthesis of even the simplest 754 HARPER’S NEW MONTHLY MAGAZINE.

organic compound it was thought that zelius had seized upon this discovery of the “ vital force” must be in operation. the compound radical, as it was called, Therefore a veritable sensation wr as because it seemed to lend aid to his dual- created in the chemical world when, in istic theory. He conceived the idea that the year 1828, it was announced that the all organic compounds are binary unions young German chemist Friedrich Woh- of various compound radicals with an ler, formerly pupil of Berzelius, and al- atom of announcing this theory ready known as a coming master, had in 1818. Ten years later, Liebig and actually synthesized the well-known or- Wohler undertook a joint investigation ganic product urea in his laboratory at which resulted in proving that compound Sacrow. The “exception which proves radicals are indeed very abundant among the rule ” is something never heard of in organic substances. Thus the theory of the domain of logical science. Natural Berzelius seemed to be substantiated, and law knows no exceptions. So the syn- organic chemistry came to be defined as thesis of a single organic compound suf- the chemistry of compound radicals. ficed at a blow to break down the chem- But even in the day of its seeming tri- ical barrier which the imagination of the umph the dualistic theory was destined fathers of the science had erected between to receive a rude shock. This came about animate and. inanimate nature. Thence- through the investigations of Dumas, wr ho forth the philosophical chemist would re- proved that in a certain organic substance gard the plant and animal organisms as an atom of hydrogen may be removed,and chemical laboratories in which conditions an atom of chlorine substituted in its are peculiarly favorable for building up place without destroying the integrity of complex compounds of a few familiar the original compound—much as a child elements, under the operation of univer- might substitute one block for another in sal chemical laws. The chimera “vital its play-house. Such a substitution would force” could no longer gain recognition be quite consistent with the dualistic the- in the domain of chemistry. ory, were it not for-the very essential fact Now a wave of interest in organic chem- that hydrogen is a powerfully electro- istry swept over the chemical world, and positive element, while chlorine is as soon the study of carbon compounds be- strongly electro - negative. Hence the came as much the fashion as electro- compound radical which united succes- chemistry had been in the preceding sively with these two elements must it- generation. self be at one time electro-positive, at Foremost among the workers who ren- another electro-negative—a seeming in- dered this epoch of organic chemistry consistency which threw the entire Ber- memorable were Justus Liebig in Ger- zelian theory into disfavor. many and JeanBaptiste Andre Dumas in In its place there was elaborated, chief- France, and theirrespective pupils, Charles ly through the efforts of Laurent and Frederic Gerhardt and Augustus Laurent. Gerhardt, a conception of the molecule Wohler, too, must be named in the same as a unitary structure, built up through breath, as also must Louis Pasteur, who, the aggregation of various atoms, in ac- though somewhat younger than the oth- cordance with “elective affinities” whose ers, came upon the scene in time to take nature is not yet understood. A doc- chief part in the most important of the trine of “nuclei” and a doctrine of controversies that grew out of their labors. “types” of molecular structure were Several years earlier than this the way much exploited, and, like the doctrine of had been paved for the study of organic compound radicals, became useful as aids substances by Gay - Lussac’s discovery, to memory and guides for the analyst, made in 1815, that a certain compound of indicating some of the plans of molecular carbon and niti’ogen, which he named construction, though by no means pene- cyanogen, has a peculiar degree of stabil- trating the mysteries of chemical affinity. ity which enables it to retain its iden- They ai’e classifications rather than ex- tity, and enter into chemical relations planations of chemical unions. But at after the manner of a simple body. A least they served an important purpose in year later Ampere discovered that nitro- giving definiteness to the idea of a molec- gen and hydrogen,when combined in cer- ular structure built of atoms as the basis tain proportions to form what he called of all substances. Now at last the word ammonium,have the same property. Ber- molecule came to have a distinct mean- THE CENTURY’S PROGRESS IN CHEMISTRY 755 ing, as distinct from “atom,” in the minds of the generality of chemists, as it had had for Avogadro a third of a century before. Avogadro’s hypothesis that there are equal numbers of these molecules in equal volumes of gases, under fixed conditions, was revived by Ger- hard t, and a little later, under the championship of Cannizza- ro, was exalted to the plane of a fixed law. Thenceforth the conception of the molecule was to be as dominant a thought in chemistry as the idea of the atom had become in a previous epoch.

Of course the atom itself was in no sense displaced, but Avo- gadro’s law soon made it plain that the atom had often usurped territory that did not really belong to it. In many cases the chemists had supposed themselves dealing with atoms as units where the true unit JOHN DALTON. was the molecule. In the case of elementary gases, such as hydrogen and oxygen, for example, the thing occurs among the hydrogen atoms. law of equal numbers of molecules in But now suppose the various pairs of oxy- equal spaces made it clear that the gen atoms come near other pairs of hydro- atoms do not exist isolated, as had been gen atoms (under proper conditions which supposed. Since two volumes of hydro- need not detain us here), then each oxygen unite with one volume of oxygen to gen atom loses its attachment for its fel- form two volumes of water vapor, the low, and flings itself madly into the cir- simplest mathematics shows, in the light cuit of one of the hydrogen couplets, and of Avogadro’s law, not only that each —presto!—there are only two molecules molecule of water must contain two for every three there were before, and hydrogen atoms (a point previously in free oxygen and hydrogen have become dispute), but that the original molecules water. The whole process, stated in chem- of hydrogen and oxygen must have been ical phraseology, is summed up in the composed in each case of two atoms— statement that under the given condi- else how could one volume of oxygen tions the oxygen atoms had a greater supply an atom for every molecule of affinity for the hydrogen atoms than for two volumes of water? one another. What, then, does this imply? Why, As chemists studied the actions of va- that the elementary atom has an avidity rious kinds of atoms, in regard to their for other atoms, a longing forcompanion- unions with one another to form mole- ship, an “ affinity”—call it what you will cules, it gradually dawned upon them —which is bound to be satisfied if other that not all elements are satisfied with the atoms are in the neighborhood. Placed same number of companions. Some ele- solely among atoms of its own kind, the ments ask only one, and I’efuse to take oxygen atom seizes on a fellow oxygen more; while others link themselves, when atom, and in all their mad dancings these occasion offers, with two, three, four, or two mates cling together—possibly re- more. Thus we saw that oxygen forsook volving about one another in miniature a single atom-of its own kind and linked planetary orbits. Precisely the same itself with two atoms of hydrogen. Clear- 756 HARPER’S NEW MONTHLY MAGAZINE. ly, then, the oxygen atom, like a creature hand with which to grasp—while oxygen with two hands, is able to clutch two has capacity for two bonds, nitrogen for other atoms. But we have no proof that three (possibly for five), and carbon for under any circumstances it could hold four. The words monovalent, divalent, more than two. Its affinities seem satis- trivalent, tetravalent, etc., were coined to fied when it has two bonds. But, on the express this most important fact, and the other hand, the atom of nitrogen is able various elements came to be known as to hold three atoms of hydrogen, and monads, diads, triads, etc. Just why dif- does so in the molecule of ammonium ferent elements should differ thus in va- can hold no one as it is (NH3); while the carbon atom lency yet knows; an empir- four atoms of hydrogen or two atoms of ical fact that they do. And once the na- oxygen. ture of any element has been determined Evidently, then, one atom is not al- as regards its valency, a most important ways equivalent to another atom of a dif- insight into the possible behavior of that ferent kind in combining powers. A re- element has been secured. Thus a con- cognition of this fact by Frankland about sideration of the fact that hydrogen is 1852, and its further investigation by monovalent, while oxygen is divalent, others (notably A. Kekule and A. S. Cou- makes it plain that we must expect to per), led to the introduction of the word find no more than three compounds of equivalent into chemical terminology in these two elements, namely, H—O— (written HO by the chemist, and

called hydroxyl) ; H O—H (H 2 O, or water), and H—O—- —O—H (H 20 2 , or hydrogen peroxide). It will be observed that in the first of these compounds the atom of oxygen stands, so to speak, with one of its hands free, eagerly reaching out, therefore, for another companion, and hence, in the lan- guage of chemistry, forming an unstable compound. Again in the third compound, though all hands are clasped, yet one pair links oxygen with oxygen ; and this also must be an unstable union, since the avidity of an atom for its own kind is relatively weak. Thus the well- known properties of hydrogen peroxide are explained, its easy decomposition, and the eager- ness with which it seizes upon the elements ofothercompounds. But the molecule of water, on the other hand, has its atoms ar- JOHAN JAKOB BERZELIUS. ranged in a state of stable equilibrium, all their affinities being satisfied. Each hydrogen atom a new sense, and in particular to an un- has satisfied its one affinity by clutching derstanding of the affinities or “valen- the oxygen atom; and the oxygen atom cy ” of different elements, which proved has both its bonds satisfied by clutch- of the most fundamental importance. ing back at the two hydrogen atoms. Thus it was shown that, of the four ele- Therefore the trio, linked in this close ments that enter most prominently into bond, have no tendency to reach out for organic compounds, hydrogen can link any other companion, nor, indeed, any itself with only a single bond to any other power to hold another should it thrust it- element—it has, so to speak, but a single self upon them. They form a “stable” THE CENTURY’S PROGRESS IN CHEMISTRY 757 compound, which under all ordinary circumstances will retain its identity as a molecule of water, even though the physical mass of which it is a part changes its condition from a solid to a gas—from ice to vapor. But a consideration of this condition of stable equilibrium in the molecule at once suggests a new question; How can an aggregation of atoms, having all their affinities satisfied, take any further part in chemical reactions? Seemingly such a molecule, whatever its physical properties, must be chemically inert, incapable of any atomic readjustments. And so in point of fact it is, so long as its component atoms cling to one another unremittingly. But this, it appears, is precisely what the atoms are little prone to do. It seems that they are fickle to the last degree in their individ- ual attachments,and are as prone to break away from bondage as JOSEPH LOUIS GAY-LUSSAC. they are to enter into it. Thus the oxygen atom which has just

“ ” flung- itself into tlie circuit of two hydro- point of view the saturated molecule—- gen atoms, the next moment flings itself that is, the molecule whose atoms have free again and seeks new companions. their valency all satisfied—may be thought It is for all the world like the incessant of as a relatively fixed or stable organism. change of partners in a rollicking dance. Even though it may presently be torn This incessant dissolution and reforma- down, it is for the time being a completed tion of molecules in a substance which structure; and a consideration of the va- as a whole remains apparently un- lency of its atoms gives the best clew that changed was first fully appreciated by has hitherto been obtainable as to the Ste.-Claire Deville, and by him named character of its architecture. How im- dissociation. It is a process which goes portant this matter of architecture of the on much more actively in some com- molecule —of space relations of the atoms pounds than in others, and very much —may be was demonstrated as long ago more actively under some physical condi- as 1823, when Liebig and Wohler proved, tions (such as increase of temperature) to the utter bewilderment of the chemical than under others. But apparently no world, that two substances may have substances at ordinary temperatures, and precisely the same chemical constitution no temperature above the absolute zero, —the same number and kind of atoms—- are absolutely free from its disturbing in- and yet differ utterly in physical proper- fluence. Hence it is that molecules hav- ties. The word isomerism was coined by ing all the valency of their atoms fully Berzelius to express this anomalous con- satisfied do not lose their chemical activ- dition of things, which seemed to negative ity—since each atom is momentarily free the most fundamental truths of chemistry. in the exchange of partners, and may Naming the condition by no means ex- seize upon different atoms from its former plained it, but the fact was made clear partners, if those it prefers are at hand. that something besides the mere number While, however, an appreciation of and kind of atoms is important in the this ceaseless activity of the atom is es- architecture of a molecule. It became sential to a proper understanding of its certain that atoms are not thrown to- chemical efficiency, yet from another gether haphazard to build a molecule, 758 HARPER’S NEW MONTHLY MAGAZINE

d iagrammatic pictures of the plan of architecture of any molecule whose composition is known. In the simple molecule of water two (H2O), for example, the hydrogen atoms must have re- leased one another before they could join the oxygen, and the manner of linking must apparently he that represented in the graphic formula H—O H. With molecules composed of a large number of atoms, such graphic representation of the scheme of linking is of course increasingly difficult, yet, with the affinities for a guide, it is always possible. Of course no one supposes that such a formula, written in a single plane, can possibly represent the true architecture of the molecule: it is at best suggestive or dia- grammatic rather than pictorial. Nevertheless it affords hints as to the structure of the molecule such as the fathers of chemistry would not have thought it pos- JUSTUS VON LIEBIG. sible ever to attain.

any more than bricks are thrown together These utterly novel studies of molec- at random to form a house. ular architecture may seem at first sight How delicate may be the gradations of to take from the atom much of its former architectural design in building a mole- prestige as the all-important personage cule was well illustrated about 1850,when of the chemical world. Since so much Pasteur discovered that some carbon com- depends upon the mere position of the pounds—as certain sugars—can only be atoms, it may appear that comparatively distinguished from one another, when in little depends upon the nature of the solution, by the fact of their twisting or atoms themselves. But such a view is polarizing a ray of light to the left or to incorrect, for on closer consideration the right, respectively. But no inkling of it will appear that at no time has the an explanation of these strange variations atom been seen to renounce its peculiar of molecular structure came until the dis- personality. Within certain limits the covery of the law of valency. Then much character of a molecule may be altered of the mystery was cleared away; for it by changing the positions of its atoms was plain that since each atom in a mole- (just as different buildings may be con- cule can hold to itself only a fixed num- structed of the same bricks), but these ber of other atoms, complex molecules limits are sharply defined, and it would must have their atoms linked in defini- be as impossible to exceed them as it tive chains or groups. And it is equally would be to build a stone building with plain that where the atoms are numerous, bricks. From first to last the brick re- the exact plan of grouping may sometimes mains a brick, whatever the style of ar- be susceptible of change without doing chitecture it helps to construct; it never violence to the law of valency. It is in becomes a stone. And just as closely such cases that isomerism is observed to does each atom retain its own peculiar occur. properties, regardless of its surroundings. By paying constant heed to this matter Thus, for example, the carbon atom of the affinities, chemists are able to make may take part in the formation at one THE CENTURY’S PROGRESS IN CHEMISTRY. 759 time of a diamond, again of a piece of 1815 called attention to a curious relation coal, and yet again of a particle of sugar, to be observed between the atomic weight of wood fibre, of animal tissue, or of a of the various elements. Accepting the gas in the atmosphere; but Rom first to figures given by the authorities of the last—from glass-cutting gem to intangi- time (notably Thomson and Berzelius), it ble gas—there is no demonstrable change appeared that a strikingly large propor- whatever in any single property of the tion of the atomic weights were exact atom itself. So far as we know, its size, multiples of the weight of hydrogen, and its weight, its capacity for vi- hration or rotation, and its in- herent affinities, remain absolutely unchanged throughout all these varying fortunes of position and association. And the same thing is true of every atom of all of the sixty-odd elementary substances with which the modern chemist is acquainted. Every one appears always to maintain its unique integrity, gaining nothing and losing nothing. All this being true, it would seem as if the position of the Daltonian atom as a primordial bit of matter, indestructible and non-transmutable, had been put to the test by the chemistry of our century, and not found wanting. Since those early days of the century when the electric batlery performed its miracles and seemingly reached its limitations in the hands of Davy, many new elementary substances have been discovered, but no single element has been displaced from its position as an undecomposable body. GUSTAV ROBERT KIRCHHOFF. Rather have the analyses of the chemist seemed to make it more and more certain that all elementary that others differed so slightly that errors atoms are in truth what John Herschel of observation might explain the discrep- called them, “manufactured articles”— ancy. Prout felt that this could not be primordial, changeless, indestructible. accidental, and he could think of no ten- And yet, oddly enough, it has chanced able explanation, unless it he that the that hand in hand with the experiments atoms of the various alleged elements are leading to such a goal have gone other made up of different fixed numbers of experiments and speculations of exactly hydrogen atoms. Could it he that the the opposite tenor. In each generation one true element—the one primal matter there have been chemists among the lead- —is hydrogen, and that all other forms ers of their science who have refused to of matter are but compounds of this ori- admit that the so-called elements are re- ginal substance? ally elements at all in any final sense, Prout advanced this startling idea at and who have sought eagerly for proof first tentatively, in an anonymous publi- which might warrant their scepticism. cation ; but afterward he espoused it open- The first bit of evidence tending to sup- ly and urged its ten ability. Coming port this view was furnished by an Eng- just after Davy’s dissociation of some lish physician, Dr. William Prout, who in supposed elements, the idea proved al- 760 HARPER'S NEW MONTHLY MAGAZINE. luring, and for a time gained such pop- the primordial element, not hydrogen it- ularity that chemists were disposed to self, but an atom half the weight, or round out the observed atomic weights of even one-fourth the weight of that of hy- all elements into whole numbers. But drogen, of which primordial atom the presently renewed determinations of the hydrogen atom itself is compounded. atomic weights seemed to discountenance But even in this modified form the hy- this practice, and Front's alleged law fell pothesis found great opposition from ex- into disrepute. It was revived, however, perimental observers. about 1840, by Dumas, whose great au- In 1864, however, a novel relation be- thority secured it a respectful hearing, tween the weights of the elements and and whose careful redetermination of the their other characteristics wr as called to weight of carbon,making it exactly twelve the attention of chemists by Professor times that of hydrogen, aided the cause. John A. R. Newlands, of London, who Subsequently Stas, the pupil of Dumas, had noticed that if the elements are ar- undertook a long series of determinations ranged serially in the numerical Older of of atomic weights, with the expectation their atomic weights, there is a curious of confirming the Proutian hypothesis. recurrence of similar properties at inter- But his results seemed to disprove the vals of eight elements. This so-called hypothesis, for the atomic weights of “law of octaves” attracted little imme- manyelements differed from whole num- diate attention, but the facts it connotes bers by more, it was thought, than the soon came under the observation of other limits of error of the experiments. It is chemists, notably of Professors Gustav noteworthy, however, that the confidence Hinrichs in America, Dmitri Mendeeleff of Dumas was not shaken, though he in Russia, and Lothar Meyer in Germany. was led to modify the hypothesis, and, in Mendeleeff gave the discovery fullest ex- accordance with previous suggestions of pression, expositing it in 1869, under the Clark and of Marignac, to recognize as title of “periodic law.” Though this early exposition of what has since been admitted to be a most important discovery was very fully outlined, the generality of chemists gave it little heed till a decade or so later, when three new elements, gallium, scan- dium, and germanium, were discovered, which, on being- analyzed, were quite unexpect- edly found to fit into three gaps which Mendeleeff had left in his periodic scale. In effect, the periodic law had enabled Mendeleeff to predi- cate the existence of the new elements years before they were discovered. Surely a system that leads to such results is no mere vagary. So very soon the periodic law took its place as one of the most important generalizations of chemical science. This law of periodicity was put forward as an expression of observed relations independent of hypothesis ; but of course the theoretical bear- ings of these facts could ROBERT WILHELM BUNSEN. not be overlooked. As Pro- LOUIS JACQUES MANDE DAGUERRE. From a daguerreotype made in Paris for Meade Brothers, New York, now in possession of Abraham Bogardus, New York. fessor J. H. Gladstone lias said, it forces which had been observed in the solar upon us “ the conviction that the elements spectrum by Fraunhofer as early as 1815, are not separate bodies created without and by Wollaston a decade earlier. It reference to one another, but that they was suspected by Stokes and by Fox Tal- have been originally fashioned, or have bot in England, but first brought to dem- been built up from one another, accord- onstration by Kircbboff and Bunsen, that ing to some general plan.” It is but a these lines, which were known to occupy short step from that proposition to the definite positions in the spectrum, are Proutian hypothesis. really indicative of particular elementary But the atomic weights are not alone substances. By means of the spectro- in suggesting the compound nature of scope, which is essentially a magnifying the alleged elements. Evidence of a to- lens attached to a prism of glass, it is pos- tally different kind has contributed to the sible to locate the lines with great accu- same end, from a source that could hard- racy, and it was soon shown that here ly have been imagined when the Proutian was a new means of chemical analysis of hypothesis was formulated, through the the most exquisite delicacy. It was found, addition of a novel weapon to the arma- for example, that the spectroscope could mentarium of the chemist—the spectro- detect the presence of a quantity of sodi- scope. The perfection of this instrument, um so infinitesimal as the one two-hun- in the hands of two German scientists, dred-thousandth of a grain. But what Gustav Robert Kircbboff and Robert Wil- was even more important, the spectro- helm Bunsen, came about through the scope put no limit upon the distance of investigation, toward the middle of the location of the substance it tested, pro- century, of the meaning of the dark lines vided only that sufficient light came from 762 HARPER’S NEW MONTHLY MAGAZINE. it. The experiments it recorded might philosophers contributed to the advance- be performed in the sun, or in the most ment of the new method. distant stars or nebulae; indeed, one of In 1843 Dr. John W. Draper, the fa- the earliest feats of the instrument was to mous English - American chemist and wT rench from the sun the secret of his physiologist, showed that by photography chemical constitution. the Fraunhofer lines in the solar spec- To render the utility of the spectro- trum might be mapped with absolute ac- scope complete, however, it was necessary curacy ; also proving that the silvered film revealed many lines invisi- ble to the unaided eye. The value of this method of observation was recognized at once, and as soon as the spectroscope was perfected, the photographic method, in conjunction with its use, became invaluable to the chemist. By this means com- parisons of spectra may be made with a degree of accuracy not otherwise obtainable ; and in case of the stars, whole clusters of spectra may be placed on rec- ord at a single observation. As tlie examination of the sun and stars proceeded, chemists were amazed or delighted, according to their various precon- ceptions, to witness the proof that many familiar terrestrial elements are to be found in the celestial bodies. But what perhaps surprised them most was to observe the enormous pre- ponderance in the sidereal bodies of the element hydrogen. Not only are there vast JOHX W. DRAPEE, quantities of this element in the sun’s atmosphere, but some other suns appeared 1o show to link with it another new chemical hydrogen lines almost exclusively in their agency, namely, photography. This now spectra. Presently it appeared that the familiar process is based on the property stars of which this is true are those white of light to decompose certain unstable stars, such as Sirius, which had been con- compounds of silver, and thus alter their jectured to be the hottest; whereas stars chemical composition. We have seen that are only red-hot, like our sun, show that Davy and Wedgwood barely es- also the vapors of many other elements, caped the discovery of the value of the including iron and other metals. photographic method. Their successors In 1878 Mr. J. Norman Lockyer, in a quite overlooked it until about 1826, when paper before the Royal Society, called at- Louis J. M. Daguerre, the French chem- tention to the possible significance of this ist, took the matter in hand, and after series of observations. He urged that the many years of experimentation brought fact of the sun showing fewer elements it to relative perfection in 1839, in which than are observed here on the cool earth, year the famous daguerreotype first while stars much hotter than the sun brought the matter to popular attention. show chiefly one element, and that one In the same year Mr. Pox Talbot read a hydrogen, the lightest of known ele- paper on the subject before the Royal So- ments, seemed to give color to the possi- ciety, and soon afterward the eflforLs of bility that our alleged elements are really Herschel and numerous other natural compounds, which at the temperature of 763 the hottest stars may be decomposed into tion of the facts he brings forward. As hydrogen, the latter “ element” itself be- yet it cannot be said that the controversy ing also doubtless a compound, which has been brought to final settlement. might be resolved under yet more trying Still, it is hardly to be doubted that now, conditions. since the periodic law has seemed to join Here, then, was what might be termed hands with the spectroscope, a belief in direct experimental evidence for the hy- the compound nature of the so-called ele- pothesis of Prout. Unfortunately, how- ments is rapidly gaining ground among ever, it is evidence of a kind which only chemists. More and more general be- a few experts are competent to discuss— comes the belief that the Daltonian atom so very delicate a matter is the spectral is really a compound radical, and that analysis of the stars. What is still more back of the seeming diversity of the al- unfortunate, the experts do not agree leged elements is a single unique form of among themselves as to the validity of primordial matter. But it should not be Mr. Lockyer’s conclusions. Some, like forgotten that this view, whatever its at- Professor Crookes, have accepted them tractiveness, still lurks in the domain of with acclaim, hailing Lockyer as “the theory. There is no proof that the Dal- Darwin of the inorganic world,” while tonian atom has yet been divided in the others have sought a different explana- laboratory.