July 19, 19 1/] 



NATURE 



415 



merely that whereas in the latter the elements are 

 combined with other elements, in the so-called free 

 state they are combined with electricm-. I shall touch 

 but briefly on this first aspect, as in principle it is now 

 fairly well understood. But its consistent and de- 

 tailed application to the study of chemical character 

 is still lacking. 



The second sense in which the elements, or some ot 

 them at least, are known now to be complex has, in 

 sharp contrast to the first, developed suddenly and 

 startlingly from the recognition in radio-active 

 changes of different radio-elements, non-separable by 

 chemical means, now called isotopes. The natural 

 corollary of this is that the chemical element represents 

 rather a tvpe of element, the members of the type 

 being only chemically alike. Alike they are in most 

 of those properties which were studied prior to the last 

 decade of last centur>-, and probably due, as we now 

 think, to the outer shells of the atom— so alike that all 

 the criteria hitherto relied upon by the chemist as 

 being the most infallible and searching would declare 

 them to be identical. The apparent identity goes even 

 deeper into the region reached by X-ray spectrum 

 analysis, which fails to distinguish between them. 

 The 'difference is found only in that innermost region 

 of all, the nucleus of the atom, of which radio-active 

 phenomena first made us aware. 



But, though these phenomena pointed the way, and 

 easily showed to be different what the chemist and 

 spectroscopist would have decided to be identical, they 

 did more. They showed that although the finer and 

 newer criteria relied upon by the chemist in his 

 analysis of matter must of necessity fail in these 

 cases, being ultimately electrical in character, yet the 

 difference should be oDvious in that most studied and 

 distinctive characteristic of all — the criterion by which 

 Dalton first distinguished the different kinds of atoms 

 — the atomic weight. Those who have devoted them- 

 selves to the e.xact determination of these weights have 

 now confirmed the difference in two separate cases, 

 which, in the absence of what perhaps they might 

 regard as "preconceived notions," they were unable 

 to discover for themselves. This is The e-xperimental 

 development to which I wish more especially to direct 

 your attention It indicates that the chemical analysis 

 of matter is, even within its own province, superficial 

 rather than ultimate, and that there are indefinitely 

 more distinct elements than the ninety-two possible 

 types of element accommodated by the present periodic 

 system. 



The third sense in which the elements are known 

 to be complex is that which, in the form of philo- 

 sophical speculations, has come down to us from the 

 ancients, which inspired the labours of the alchemists 

 of the Middle Ages, and, in the form of Prout's 

 hypothesis, has reappeared in scientific chemistry. It 

 is the sense that denies to Nature the right to be 

 complex, and from the earliest times, faith outstripping 

 knowledge, has underlain the belief that all the elements 

 must be built up of the same primordial stuff. The 

 facts of radio-active phenomena have shown that all 

 the radio-elements are indeed made up out of lead and 

 helium, and this has definitely removed the question 

 from the region of pure speculation. We know that 

 helium is certainly a materfal constituent of the 

 elements in the Proutian sense, and it would be harm- 

 less, if probably fruitless, to anticipate the day of 

 fuller knowledge by atom building and unbuilding on 

 paper. Apart altogether from this, however, the exist- 

 ence of isotopes, the generalisation concerning the 

 periodic law that has arisen from the study of 

 radio-active change on one hand and the spectra of 

 X-rays on the other, and experiments on the scatter- 

 ing of a particles by matter do give us for the first 

 time a definite conception as to what constitutes the 

 NO. 2490. VOL. 99] 



difference between one element and another. We can 

 say how gold would result from lead or mercury, even 

 though the control of the processes necessary to effect 

 the change still eludes us. The nuclear atom proposed 

 by Sir Ernest Rutherford, even though, admittedly, it 

 is only a general and incomplete beginning to a com- 

 plete theory of atomic structure, enormously simplifies 

 the correlation of a large number of diverse facts. 

 This and what survives of the old electronic theory of 

 matter, in so far as it attempted to explain the periodic 

 law, will therefore be briefly referred to in conclusion. 



The Free Element a Compound of Matter and 

 Electricity. 



Although Davy and Faraday were the contempora- 

 ries of Dalton, it must be remembered that it tocfc 

 chemists fifty years to put the atomic theory on a 

 definite and unassailable basis, so that neither of these 

 investigators had the benefit of the very clear view we 

 hold to-day. Davy was the originator ot the first electro- 

 chemical theory of chemical combination, and Fara- 

 day's dictum, "'The forces of chemical affinity and elec- 

 tricity are one and the same," it is safe to say, inspires 

 all the modern attempts to reduce chemical character 

 to a science in the sense of something that can be 

 measured quantitatively, as well as expressed qualita- 

 tively. Faraday's work on the laws of electrolysis and 

 the discovery that followed from it when the atomic 

 theory came to be fully developed, that all monovalent 

 atoms or radicles carry the same charge, that divalent 

 atoms carry twice this charge, and so on, can be re- 

 garded to-day as a simple extension of the law of 

 multiple proportions from compounds between matter 

 and matter to compounds between matter and elec- 

 tricity. Long before the electric charge had been 

 isolated, or the properties of electricity divorced from 

 matter discovered, the same law of multiple propor- 

 tions which led, without any possibility of escape, to 

 an atomic theory of matter led, as Helmholtz pointed 

 out in his well-known Faraday lecture to the Chemical 

 Society in this theatre in 1881, to an atomic theory of 

 electricity. 



The work of Hittorf on the migration of ions, the 

 bold and upsetting conclusion of Arrhenius that in 

 solution many of the compounds hitherto regarded as 

 most stable exist dissociated into ions, the realisation 

 that most of the reactions that take place instantaneously 

 and are utilised for the identification of elements in 

 chemical analysis are reactions of ions rather than of 

 the element in question, made very familiar to chem- 

 ists the enormous difference between the properties of 

 the elements in the charged and in the electrically 

 neutral state. 



More slowly appreciated and not yet perhaps suffi- 

 ciently emphasised was the unparalleled intensity of 

 these charges in comparison with anything that elec- 

 trical science can show, which can be expressed tritely 

 by the statement that the charge on a milligram of 

 hydrogen ions would raise the potential of the world 

 to 100,000 volts. Or, if we consider another aspect, 

 and calculate how many free hydrogen ions you could 

 force into a bottle without bursting it, provided, of 

 course, that you could do so without discliarging the 

 ions, you would find that were the bottle of the 

 strongest steel^the breech of a gun, for example — it 

 would burst, by reason of the mutual repulsion of the 

 charges, before as much was put in as would, in the 

 form of hydrogen gas. show the spectrum of the 

 element in a vacuum tube. 



Then came the fundamental advances in our know- 

 ledge of the nature of electricity, its isolation as the 

 electron, or atom of negative electricity, the great 

 extension of the conception of ions to explain the' 

 conduction of electricity through gases, the theoretical 

 reasoning, due in part to Heaviside, that the electron 



