September 23, 1921] 



SCIENCE 



259 



mands, yet very differently from that hitherto 

 required by the limitations of contempor- 

 ary chemical docti-ine. He conceives of 

 a lower order of atoms than the chemi- 

 cal 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 rela- 

 tive weight being one for hydrogen, sixteen 

 for oxygen, and so on with the other so-called 

 " elements." Graham, in fact, like Davy be- 

 fore 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 es- 

 sential unity of matter, has persisted through- 

 out the ages, and in spite of much experi- 

 mental work, some of it of the highest order, 

 which was thought to have demolished it, it 

 has survived, revivified and supported by 

 analogies and arguments drawn from every 

 field of natural inquiry. This idea of course 

 was at the basis of the hypothesis of Prout, 

 but which, even as modified by Dumas, 

 was held to be refuted by the monu- 

 mental work of Stag. But, as pointed out 

 by Marignac and Dumas, any one who will 

 impartially look at the facts can hardly escape 

 the feeling that there must be some reason 

 for the frequent recurrence of atomic weights 

 differing by so little from the numbers re- 

 quired by the law which the work of Stas was 

 supposed to disprove. The more exact study 

 within recent years of the methods of de- 

 termining atomic weights, the great improve- 

 ment in exiserimental appliances and tech- 

 nique, combined with a more rigorous standard 

 of accuracy demanded by a general recogni- 

 tion of the far-reaching importance of an 

 exact knowledge of these physical constants, 

 has resulted in intensifying the belief that 

 some natural law must be at the basis of the 

 fact that so many of the most carefully de- 

 termined atomic weights on the oxygen stand- 

 ard are whole numbers. Nevertheless there 

 were well authenticated exceptions which 

 seemed to invalidate its universality. The 

 proved fact that a so-called element 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 recognized that the fractional 

 values independently established in the case 

 of any one element by the most accurate ex- 

 perimental work of various investigators are, 

 in effect, " statistical quantities " dependent 

 upon a mixture of isotopes. This result, in- 

 deed, is a necessary corollary of modern con- 

 ceptions of the inner mechanism of the atom. 

 The theory that all elementary atoms are com- 

 posed of helium atoms, or of helium and 

 hydrogen atoms, may be regarded as an ex- 

 tension of Prout's hypothesis, with, however, 

 this important distinction, that) whereas 

 Prout's hyxwthesis 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 Hg, first detected by 

 J. J. Thomson, of which the existence has 

 been confirmed by Aston, would seem to be 

 an integral part of atomic structure. Ruther- 

 ford, by the disruption of oxygen and nitro- 

 gen has also isolated a substance of mass 3 

 which enters into the structure of atomic 

 nuclei, but which he regards as an isotope 

 of helium, which itself is built up of four 

 hydrogen nuclei together with two cementing 

 electrons. The atomic nuclei of elements of 

 even atomic number would appear to be com- 

 posed 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 to- 

 gether with cementing electrons. In the case 

 of the lighter elements of the latter class the 

 number of hydrogen nuclei associated with 

 the helium nuclei is invariably three, except 

 in that of nitrogen where it is two. The 

 frequent occurrence of this group of three 

 hydrogen nuclei indicates that it is structural- 

 ly an isotope of hydrogen with an atomic 

 weight of three and a nuclear charge of one. 

 It is surmised that it is identical with the 

 hypothetical " nebulium " from which our 

 " elements " are held by astro-physicists to 



