16 THE PRESIDENTIAL ADDRESS. 
of the essential unity of matter and the mode of genesis of the elements. 
Members of the British Association may recall the suggestive address on 
this subject of the late Sir William Crookes, delivered to the Chemical 
Section at the Birmingham meeting of 1886, in which he questioned 
whether there is absolute uniformity in the mass of the atoms of a 
chemical element, as postulated by Dalton. He thought, with 
Marignac and Schutzenberger, who had previously raised the same 
doubt, that it was not improbable that what we term an atomic weight 
merely represents a mean value around which the actual weights of the 
atoms vary within narrow limits, or, in other words, that the mean 
mass is ‘ a statistical constant of great stability.’ No valid experi- 
mental evidence in support of this surmise was or could be offered at 
the time it was uttered. Maxwell pointed out that the phenomena of 
gaseous diffusion, as then ascertained, would seem to negative the 
supposition. If hydrogen, for example, were composed of atoms of 
varying mass it should be possible to separate the lighter from the 
heavier atoms by diffusion through a porous septum. ‘As no chemist,’ 
said Maxwell, ‘ has yet obtained specimens of hydrogen differing in 
this way from other specimens, we conclude that all the molecules of 
hydrogen are of sensibly the same mass, and not merely that their 
mean mass is a statistical constant of great stability." But against 
this it may be doubted whether any chemist had ever made experiments 
sufficiently precise to solve this point. 
The work of Sir Norman Lockyer on the spectroscopic evidence for 
the dissociation of ‘ elementary’ matter at transcendental tempera- 
tures, and the possible synthetic intro-stellar production of elements, 
through the helium of which he originally detected the existence, will 
also find its due place in the history of this new philosophy. 
Sir J. J. Thomson was the first to afford direct evidence that the 
atoms of an element, if not exactly of the same mass, were at least 
approximately so, by his method of analysis of positive rays. By an 
extension of this method Mr. F. W. Aston has succeeded in showing 
that a number of elements are in reality mixtures of isotopes. It 
has been proved, for example, that neon, which has a mean 
atomic weight of about 20.2, consists of two isotopes having the 
atomic weights respectively of 20 and 22, mixed in the proportion of 
90 per cent. of the former with 10 per cent. of the latter. By frac- 
tional diffusion through a porous septum an apparent difference of 
density of 0.7 per cent. between the lightest and heaviest fractions 
was obtained. The kind of experiment which Maxwell imagined proved 
the invariability of the hydrogen atom has sufficed to show the converse 
in the case of neon. 
' Clerk-Maxwell, Art. ‘Atom,’ Ency. Brit. 9th Ed. 
