September 8, 192 1] 



NATURE 



51 



grand question — the inner mechanism of the 

 atom "' — has profoundly modified the basic con- 

 ceptions of chemistry. It has led to a great ex- 

 tension of our views concerning- the real nature of 

 the chemical elements. The discovery of the elec- 

 tron, the production of helium in the radioactive 

 disintegration of atoms, the recognition of the 

 existence of isotopes, the possibility that all ele- 

 mentary atoms are composed either of helium 

 atoms or of atoms of hydrogen and helium, and 

 that these atoms, in their turn, are built up of two 

 constituents, one of which is the electron, a par- 

 ticle of negative electricity the mass of which is 

 only I / 1800 of that of an atom of hydrogen, and 

 the other a particle of positive electricity the mass 

 of which is practically identical with that of the 

 same atom — the outcome, in short, of the collective 

 work of Soddy, Rutherford, J. J. Thomson, Collie, 

 Moseley, and others — are pregnant facts which 

 have completely altered the fundamental aspects 

 of the science. Chemical philosophy has, in fact, 

 now definitely entered on a new phase. 



Looking back over the past, some indications of 

 the coming change might have been perceived 

 wholly unconnected, of course, with the recent ex- 

 perimental work which has served to ratify it. In 

 a short paper entitled, " Speculative Ideas respect- 

 ing the Constitution of Matter," originally pub- 

 lished in 1863, Graham conceived that the various 

 kinds of matter, now recognised as different ele- 

 mentary substances, may possess one and the same 

 ultimate or atomic molecule existing in different 

 conditions of movement. This idea, in its essence, 

 may be said to be as old as the time of Leucippus. 

 To Graham as to Leucippus, ** the action of the 

 atom as one substance taking various forms by 

 combinations unlimited, was enough to account for 

 all the phenomena of the world. By separation 

 and union with constant motion all things could 

 be done." But Graham developed the conception 

 by independent thought, and in the light of experi- 

 mentally ascertained knowledge which the world 

 owes to his labours. He might have been cog- 

 nisant of the speculations of the Greeks, but there 

 is no evidence that he was knowingly influenced 

 by them. In his paper, Graham uses the terms 

 " atom " and " molecule," if not exactly in the 

 same sense that modern teaching demands, yet 

 in a sense very different from that hitherto 

 required by the limitations of contemporary 

 chemical doctrine. He conceives of a lower 

 order of atoms than the chemical atom of 

 Dalton, and founds on his conception an 

 explanation of chemical combination based 

 upon a fixed combining measure, which he terms 

 the metron, its relative weight being one for 

 hydrogen, sixteen for oxygen, and so on with the 

 other so-called " elements." Graham, in fact, 

 like Davy before him, never committed himself 

 to a belief in the indivisibility of the Daltonian 

 atom. The original atom may, he thought, be far 

 down. 



The idea of a primordial yle, or of the essential 

 NO. 2706, VOL. I08I 



unity of matter, has persisted throughout the ages, 

 and, in spite of much experimental work, some of 

 it of the highest order, which was thought to have 

 demolished it, it has survived, revivified and sup- 

 ported by analogies and arguments drawn from 

 every field of natural inquiry. This idea, of course, 

 was at the basis of the hypothesis of Prout, 

 which, even as modified by Dumas, was held to be 

 refuted by the monumental work of Stas. But, as 

 pointed out by Marignac and Dumas, anyone who 

 will impartially look at the facts can scarcely 

 escape the feeling that there must be some reason 

 for the frequent recurrence of atomic weights dif- 

 fering by so little from the numbers required by 

 the law which the work of Stas was supposed to 

 disprove. The more exact study within recent years 

 of the methods of determining atomic weights 

 and the great improvement in experimental 

 appliances and technique, combined with a more 

 rigorous standard of accuracy demanded by a 

 general recognition of the far-reaching import- 

 ance of an exact know^ledge of these physical con- 

 stants, have resulted in intensifying the belief that 

 some natural law must be at the basis of the fact 

 that so many of the most carefully determined 

 atomic weights on the oxygen standard are whole 

 numbers. Nevertheless, there were well-authenti- 

 cated exceptions which seemed to invalidate its 

 universality. The proved fact that a so-called ele- 

 ment may be a mixture of isotopes — substances of 

 the same chemical attributes, but of varying 

 atomic weight — has thrown new light on the ques- 

 tion. It is now recognised that the fractional 

 values independently established in the case of any 

 one element by the most accurate experimental 

 work of various investigators are, in efi"ect, " sta- 

 tistical quantities " dependent upon a mixture of 

 isotopes. This result, indeed, is a necessary corol- 

 lary of modern conceptions of the inner mechan- 

 ism of the atom. The theory that all elementary 

 atoms are composed of helium atoms, or of helium 

 and hydrogen atoms, may be regarded as an exten- 

 sion of Prout's hypothesis, with, however, this 

 important distinction, that whereas Prout's hypo- 

 thesis was at best a surmise, with little, and that 

 little only weak, experimental evidence to support 

 it, the new theory is directly deduced from well- 

 established facts. The hydrogen isotope, H3, first 

 detected by Sir J. J. Thomson, of which the exist- 

 ence has been confirmed by Aston, would seem to 

 be an integral part of atomic structure. Rutherford, 

 by the disruption of oxvgen and nitrogen, has also 

 isolated a substance of mass 3 which enters into 

 the structure of atomic nuclei, but which he re- 

 gards as an isotope of helium, which itself is built 

 up of four hydrogen nuclei, together with two 

 cementing electrons. The atomic nuclei of ele- 

 ments of even atomic number would appear to be 

 composed of helium nuclei only, or of helium 

 nuclei with cementing electrons ; whereas those of 

 elements of odd atomic number are made up of 

 helium and hydrogen nuclei together with cement- 

 ing electrons. In the case of the lighter elements 



