4 
) THE PRESIDENTIAL ADDRESS. 15 
; 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, 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 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 
determined atomic weights on the oxygen standard 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 attri- 
: butes but of varying atomic weight—has thrown new light on the 
question. It is now recognised that the fractional values independently 
established in the case of any one element by the most accurate experi- 
_ mental work of various investigators are, in effect, ‘ statistical quanti- 
ties’ dependent upon a mixture of isotopes. This result, indeed, is a 
necessary corollary of modern conceptions of the inner mechanism 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 
extension of Prout’s hypothesis, with, however, this important distinc- 
tion, that whereas Prout’s hypothesis 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 H,, 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. Rutherford, by the disruption of oxygen and 
nitrogen, 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 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 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 structurally 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 be originally 
produced in the stars through hydrogen and helium. 
These results are of extraordinary interest as bearing on the question 
