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pressed out between white filter papers and dried in the dark. Silk will be colored rose-red by the P-naphthol solution, orange-yellow b y the resorcinol, the wcol black by both, while the plant fibers will remain white. A portion of the work for this review was done in the library of the department of chemistry of Cornel1 University, through the courtesy of Professor I,. M. Dennis, for which the writer desires t o express his thanks, UNIVERSITY OF NEBRASKA,
LINCOLN.N E B .
INTER-RELATIONS OF THE ELEMENTS.' BY HERBERT N. McCoy. Received January I , 1908.
The history of chemistry from the first t o the sixth decade of the 19th century sliows all too clearly how unsuccessful were the attempts of Dalton and of his contemporaries and successors t o fix the atomic weights of the elements by means of arbitrary rules regarding the numbers of atoms which unite with one another, even when these rules were supplemented by knowledge of chemical behavior. Consistent results were obtained only by the aid of the auxiliary hypothesis of Avogadro; which, proposed in 181I , almost unappreciated until after its reanimation by Cannizzaro in 1858, to-day is the corner-stone of the magnificent structure of which the atomic hypothesis is the foundation. Between the physical and chemical properties of the elements and their atomic weights, fixed by means of the hypothesis of Avogadro, there are, as is well known t o every chemist, the most fundamental relationships. Of these relationships none is perhaps more significant than t h a t discovered in 1819 b y Dulong and Petit. This law supplemented by the relations discovered by Neurnann and by Kopp means, simply, t h a t for solid substances, the atoms of all elements, either free or combined, have approximately the same capacity for heat. h few months ago Lewis showed t h a t atomic heats a t constant volume are much more nearly constant than the ordinary values, which refer t o constant pressure. During the fifty years following Dulong and Petit's discovery many other relations, more or less quantitative, were found between the properties of elements and their atomic weights; all these relationships were correlated, in 1869, in the Periodic Law of Lothar Meyer and MendelCeff, which may be concisely summarized in the statement that, in general, the properties of elements are periodic functions of their atomic weights. The comprehensive nature of the law is best appreciated when we remember t h a t these properties include chemical nature, valence, atomic volume, hardness, thermal expansion, crystalline form, conductivity for heat and electricity, melting-point, boiling-point, spectral wavelength and ionic mobility, as well as other properties of less importance. What could illustrate more forcibly the validity of this great law than the startling coincidence in properties of the element, germanium, discovered by Winkler in 1886, and the hypothetical eka-silicon described by Mendeleeff sixteen years earlier, or the equally accurate predictions of the same famous chemist concerning eka-boron and eka-aluminium. Read at the Chicago meeting of the American Chemical Society.
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Very early in the history of ihe atornic hypothesis, in 181j! t o be e s act, Prout, a London physician and dilettantc in chmiistry, observed t h a t the currently accepted atoinic weights were, for iiiaiiy clerntants. exactly or very approsiriiately wliolc iiu:iibi.rs, the atomic wcight of hydrogen beiiig taken as unity. I'rout coiitvrided t h a t all atomic weights should be whole numbers aiid suggpstetl as tlic csplmation that t h i s elcnients were composed of hydrogen in various striqcs of contl.asation. These views were vigorously opposcd by iiiari;: who wvrc 1, t o judge ; Berzelius, cspccially stoutl!. denied thi1.t t h c fractio!Inl \-slues could be wholly due t o esperi:nental crror. Tat. r , Stas. i\.iiosc iiioIiutiicnt:tl work on atoxic weights was largely iiispired 11). a tlc'sirc t o svttlc this vital question, convincrd 1ii;imlf r m l d s o t h c!ic%wicalworld in general t h a t Prout's idea was fall.icious; a t k i s t as t o the atwiiic wviglits being whole numbers. , h d this conclusion is stroiigly coiifiriiictl by all the beautiful work of more rc.ceiit ?-e:trs upon atomic wciglits. \*ct the fact t h a t the values found 011 thc' I)asi.; of oxygen eclual 16, ;in>,i : ~SO inanv cases, so very near wliolc iiuiiilwrsj must liavc soiiic' f:tr-rcacliing significancc. Calculations by Strutt liased o:i the theory oi probaliility. have showii t h a t the chanccs arc' only ahout one iii a thousand t h a t an)set of numbers, nssigncd purely :it. r:inclo 11, wo:ild tli1Tvr from wliolc numbers I)>- so little as do the atoliiic wciqhts. From thc critical consideration of all tli-. c\Tidcnce of the sort t o whicli referciice had been rnndc, one niay safL,ly ;ny that thew was abundant reason for the belief that the elcriimt5 nrr h i t various iiiodifications of a primitive substance; hut, the c i i x wtz.7 i i ( d 2 rl fimxn. I:ar from being novel, the idea of thc' unity of iiiatter, with t h v attendant possibility of transrnutatipii, nntedntcs the wliolc science of chcniistry. 'I'ruc it is, of course. t h a t tiic a:iciciit ::iitl :iicdiev:d vicws liad othcr bases than the fiicts which wc' Iia\-c 1)vc.n consid~4iig;was riot t h e t r m s mutation of a base metal into gold ;L fLir si.nl)lcr change (apparently) than inany chemical and nietal1urgic:il t I-ansfoniintions n7itl1 which nicii werc cvc'n then faniiliar? To the alc!1.cLiiiiht tlic nrxuincnt TVRS convincing and the g0.d ;illurinq; and with vnthusius,!! \vori.liy of n twentiet 11 ccntur!, " pro:iioter" he often. in bo*iihastic and a3i;l)iguous tcr:iis, dvscribed a s :tcco.nplished t h a t which existed only ill hi.: i;:i;i$iiatioii. lkfore the close of the 18th centurv. 1iowcvc.r. :1t:iiinpts at transniutatioii were recognized RS futilc and onc. liiiiy :ippv:trcd ciartain, i'lvn :Lt that tiinc; the elements were not t o ljc di4iiL(>grxtcsclo r a l t c r d l)!- such chemical processx as thosf? ~vhicliservivl t o di.co.iipose their co.npounds. >lost of the fonsgoing facts a i d arguiiicwts arc ~ ~ linoivii ~ 1 to1 evc>rychc~mist, sincc they constitute import:int cliaptc.rs i i i t l i v history of our scic'ncc. I have nicrcBl?- touched upon t l i r : ~in~ ordi,r t o 1c.iid perspccti\*e t o tllat which is t o follow. During thc past elex-en years mornio1ts ndvaiiccs li:i\-c been iiiadc alonq entirely n e w lines, in thc' knowlctly: of thti interrclations of t!ic ?lenients and the nature of inzittcr. 'l'liis ii:w kiiowlcdgc Iiad its origin in tlic discovery by Tkcquerel. in 1 S q 6 . of th!. rxliations cmittcd b y urxniuin. I t waq found h y Becquc.rc'1 t l i n t all ur;iniii*i: co.npounds scnd out rLiy5 c:ipal)li. of :i!Tcctinq a. photographic pl:i.iLc tl:roa!;h light-proof pnper ant1 also of e!ior!nously iiicrensii;g t l i v c,lcctric:d voiicIncti\-ity oi xir, by ionizing it. Sch!nitlt, and iiidcpi.ndr.ntl!. 5Imi*. Curie, found t h a t all
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compounds of thorium produce similar rays. Scientists will never forget the intense interest taken in the discovery by the Curies, of radium, a substance which possessed the properties of uranium and of thorium augmented more than a million-fold. There were also new properties: powerful physiological effects, evolution of light and even of heat, it having been found by Curie and Laborde that the temperature of a tube of radium is always perceptibly above that of its surroundings. Here then was a most marvelous result ; the continuous and seemingly undiminished production of energy by a portion of matter, which appeared t o suffer no chemical change. It even seemed as if a source of perpetual motion had been found. It was soon clearly established that the activity of radioactive substances was not due t o the excitation of any known radiation. Some scientists, however, including Lord Kelvin, Becquerel and the Curies, imagined, as the source of the observed energy an unknown cosmic radiation, which was intercepted and transformed by the radioactive body: the elevation of temperature of radium above its surroundings being, according t o Kelvin, analogous t o that of a piece of black paper in a bottle exposed t o sunlight. The solution of the mystery of radium is largely due t o Rutherford and his colleagues. There were two main problems ( I ) the nature of the radiations, and (2) the nature and cause of radioactivity. The radiations were found t o be of three sorts, called the a-,,R-, and y-rays. The nature of the !-rays was first established. The photoactivity is chiefly due t o these rays; they readily penetrate light-proof paper; are easily deflected by a magnetic or an electric field, and in such a direction as t o show that they are negatively charged. In this respect, as well a s i n m a n y others, the P-rays closely resemble cathode rays, which latter have been shown t o consist of particles of negative electricity, called corpuscles. These corpuscles, which are shot out from the cathode with 1 / 1 0 of the velocity of light, have inertia, due t o the moving electric charge; and having inertia, they have mass. The mass of a corpuscle is very small; about ~jrooothat of an atom of hydrogen. The ,Brays seem t o differ fr0.n the cathode rays only in having greater velocity. The y-rays are apparently very penetrating X-rays; and just as the latter are produced by the cathode rays, the former are produced b y the p-rays. The a-rays are quite different: they are unable t o penetrate a single sheet of writing paper; but they are the chief cause of the ionization of the air and the elevation of temperature. Rutherford has shown that the a-rays consist of positively charged material particles, each having a mass 2 (or 4) times t h a t of an atom of hydrogen and moving with 113 the velocity of light. It was now readily understood t h a t the kinetic energy of the rays was the proximate cause of the observed evolution of heat by radium. The counterpart of the phenomena of the rays was found in the change which occurred a t the same time in the active substance itself. Crookes had observed that by treating uranium nitrate solution with an excess of a-nmonium carbonate a minute insoluble precipitate remained ; this precipitate, called uranium X , was found t o possess practically the whole of the photozctivity of the uranium from which it had been separated. Rutherford and Soddy precipitated thorium with ammonia and obtained
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from the filtrate a trifling residue called Thorium S ,which possessed a large share of the original activity. The activity of the 'I'hX decreased gradually with time; in 4 days the activity was reduced to 1 , 2 , in 8 days t o I 1'4.The loss of activity occurred according t o the exponential la%-, or, as the physical chemist would say, the change was t h a t of a first order reaction; in each unit of time a fixed fraction of the reiiiainiiig activity disappeared. 1' 0' some scientists it did not appear retilarkable t h a t the radioactive principle of uranium or thorium could be srparted by n chemical process: in fact the original activity of tlicse clemcmts 'ivas "explained" as duc t o the presence of these highly activi. itupuritic.s." Nor did the loss of activity by U X or ThS,considered by itself, sceni extraordinary; it might bc likened t o loss of tcniperaturc b y a cooling body. But a most extraordinary thing was discovercd by Rutherford and Soddy, in the further study of this same thorium expcrimcnt. The thorium from which 'I'hS had been removed had, a t first, only about I 13 of its original activity ; but in just thxt measure in which the scpara-ted T h X lost its activity, t h a t of the thoriuiii itself returned; until, aftcr a period of several weeks, the thorium had entirely rcgnincd its original activity, while the '1'hX had heco:nc inactive. A rcpctition ol thc treatment with aniinonia now yielded a new portion of ThS e q u d in aiiiount t o t h a t first extracted! The behavior of U S was siriiilar. Iiutherford and Soddy then announced their now cekbratcd Disintcsgr'L t'1011 Hypothesis: the radioactive atom is a coiiiplcs systciii. iiiadc u p of cor puscles and tr-particles. in rapid orbital inotion ; of such systciiis soinc. in the course of time, became unstabli. and disintcgrntc,, the corp'useles and tr-particles resulting froiii such atomic disintegrations constitutc the rays, the kinetic energy of which cxistcd previously iii thc atoiii. The residual portion of thc, atoin, after the cscapc of a corpusclc o r an tr-particle, constituted a11 atoil: of a new substance ( e . :'. 'I'hS), which atom iiiight h a w even less st ability than t h e original. In all. over 20 distinct radioactiT-c. substances aril now known. 'l'hesc constitute three series, in each of which each member produces, by its radioactive disintegration, the following inenher. The evidence in the case of each change is just as clear as in that of thorium and thorium X . One case iii particular, that of the forination of radium from uraniuni, is of especial interest. I t has bcen shown in two different wtys that radium and uranium arc. always associated iii minerals, a n d , iiiorc.o\v, in a perfc,ctlv fixed proportion. as deniandcd by the hypothesis t h a t the one is the product of the other. Furthcr. uraniuiii iiiincrals freed from radium by chemical processcs, slowly Iiut surely reproduce the latter substance. The chemical and phyaieal properties of radium and its salts clearly show it to be a member of thc alkaline earth family; its atomic weiqht is according t o the latest determination of Mnie. Curie.. 226.5, assuiiiing the elements to be bivalent. The forination of radium b y uranium was then a clear case of the transmutation of one element into another whatever might he thought of such charigcs a s that in\*olving the production of ThX. ;The properties of the wparticle led Kutherford and Soddy to suggest, in 1902, that it might he an atom of helium. In 1903 Rniiisay arid Soddy I'
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were able t o show t h a t radium emanation actually does produce helium in the course of its spontaneous disintegration; this observation has, since then, frequently been confirmed. The formation of helium seemed t o be another real transmutation ; since the element radium undoubtedly produced the emanation which yielded in turn another element, helium. If now uranium gives ultimately radium, what is the final product of the disintegration of the latter? Boltwood believes it t o be lead, since he finds that all uranium-radium ores contain notable amounts of lead. I n the disintegration of an atom of radium five distinct a-ray changes occur. If each produces one &-particle, with atomic weight of 4, the final product of radium should have anatomic weight of 2 2 6 . 5 - 20 = 206.5, whereas the atomic weight of lead is 206.9. There is now no shadow of doubt that the rate of radioactive change is entirely independent of the form of chemical combination of a radioactive substance ; temperature is also without influence ; a given transformation occurs a t precisely the same rate a t the temperature of liquid hydrogen and a t white heat. It thus appears t h a t radioactive change is a natural process which is entirely beyond man’s control, and import a n t a s is the establishment of this fact, the discovery falls far short of being a complete solution of the problem of the transmutation of the elements. A further step in this direction seems t o have been taken within the past few months, however, by Sir William Ramsay. The alchemist in his attempts to alter the elements, could apply only the forces of modera t e heat and chemical affinity. The chemist of the 19th century could command enormously greater ranges of temperature as well as powerful electrical forces; all these means were likewise without avail. And now Ramsay has tried a new force, that of radioactive radiations: corpuscles and particles of matter, both electrically charged, projected with almost inconceivable velocity. The results of the experiments of Ramsay and Cameron may be stated very briefly; radium emanation acting on water produces neon; while with a solution of copper nitrate it produces argon; and even the copper itself appeared to be attacked, for when the latter element was removed by means of hydrogen sulphide, the filtrate was found t o contain lithium. A blank experiment, in which the radium emanation was omitted but all other conditions remained precisely the same, yielded no trace of lithium. The formation of lithium from copper has been observed four times b y Ramsay and though, a s he writes in a private communication, “he was loth to believe,” yet he “had t o chronicle the results after four repetitions.” The importance of these facts, provided they are not due t o some spurious cause, needs no comment. I n this connection i t occurred to me t h a t radium emanation, or in general radioactive radiations, might act on copper in the solid state ; if so, uranium-radium minerals which contain copper should contain lithium also. For another purpose I had, more than a year earlier, separated a sample of pitchblende into its principal constituents. There was a considerable quantity of copper; there was also a solution which could contain only alkali and ammonium salts. It was but the work of an hour to examine this solution for lithium; in the spectrum the characteristic red line of lithium was next in brilliancy t o that of sodium! Later, from one to two grams of four other uranium minerals, including sam-
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ples of pitchblende, carnotite and gummite, from widely separated localities, were examined for copper and lithium : all contained lithium and all but one contained copper. The presence of lithium in a mineral free from copper may be explained upon the supposition th:it tlic tr-.insn;uta tion of the latter metal had been completed. ,, I he view that the atom is a complex system made up of negatively charged corpuscles and positively charged tr-particles has hcen Iliad(. the subject of extensive mathematical stridies by J. J . 'l'l!oiiipson. 'Chompson's hypothetical atom is a system composed of rings or shells of corpuscles in a uniform sphere of positive electrificatio!i. '!'he electropositive or negative nature of atoms, their chemical sffinit!- a s well as their dual valence toward hydrogen and oxygen arc all most bvautifully and a t the same time simply explained by tht. helia\~iorof suci. imaginary systems, the propertics of which vary periodically with ii1c:c;sing number of corpuscles and so fulfil another indispensable reyiiircment of any hypothetical atom. Nurilerotis other properties of mittcbr also receive a satisfactory explanation in tvrins of the new hypotlicsi such are the conductivity of irietals for heat and electricity, hr spectra and the Zeeman effect. That the corpusctilni. tlicwi->. is tlic last word on the nature of matter, none will contend : 'L'lio:lipsoi1 Iiiinself writes, '"She theory is not an ultimate one: its objcct i.; p1:ysic:tl rather than metaphysical. Froin the point of \+sv of tlic physicist,' a theory of matter is a policy rather than a creed; its object is to conticct and coordinate apparently diIwse phenomcnx, and a b o ~ - eall LO s:i qqcst , stimulate and direct experiment ." The view just given of tlic nature of the atoiii accounts also for its radioactivity. A system of reiTolving corpuscles of iicgative clectricity would sloivly radiate energy, a t an almost infinitesimal rat? it is true ; but the final result would be to produce in the systeii1 a condition of instability which would cause its partial disintegration and rurrangcuicnt. Is then all matter undergoing evolution? That is tlic ci:.!estion whicli cannot be positively answered a t present. rind yet tlierc is much evidence that many elements are faintly radioactive. Such activity may be either intrinsic or due to minute amounts of h o r n highly active substances. Radioactivc matter is easily extracted from the xi? by means of a negatively charged wire exposed oxt of doors, :is first shown in 1901 by Elster and Geitel. The rate of cliange of this kicti1-c i1i:ittc.r indicates that it is chiefly the excited activity, K a A , B ant1 C. r from the disintegration of radium einanatioii. -1 siinplr, i.spvriiiient which I recently tried shows the correctness of this view. Air was liquefied and about a liter of the liquid was allowed t o evaporatc, m d the last portion was examined for radioactive emanations. considerahle activity was found, the rate of change of ~vhichplainly indicated the p i - c ~ : i ence of radium emanation together with small amounts of otlicr acti1.e matter. The excited activity, xhich is deposited c\-cr?-ivhcre by the radium emanation contained in the air, is certainly chi. CBUSC of :I portion of the observed xctivity of common things. Ne\-ertheless, ~ 1 . t 1 e ; i K t one common element, potassiuiii, sconis from t h r .;i::idic-.s of C :~-i:!il)~~Il to possess slight intrinsic activity. On the other hand, it is found that a-partic1i.s lmving 1 i . s tli,in : t e
