THE PRESIDENTIAL ADDRESS. 18 
heat, light, elasticity, diffusion, electricity and magnetism, in gases, 
liquids and solids, and describing precisely the relations of these 
different states of matter to one another by statistics of great numbers 
of atoms when the properties of the atom itself are simply assumed. 
; 
When the theory, of which we have the first instalment in Clausius and 
Maxwell’s work, is complete, we are but brought face to face with a 
superlatively grand question: What is the inner mechanism of the 
atom ? ’ 
If the properties and affections of matter are dependent upon the 
inner mechanism of the atom, an atomic theory, to be valid, must 
comprehend and explain them all. There cannot be one kind of atom 
for the physicist and another for the chemist. The nature of chemical 
affinity and of valency, the modes of their action, the difference in 
characteristics of the chemical elements, even their number, internal 
constitution, periodic position, and possible isotopic rearrangements must 
be accounted for and explained by it. Fifty years ago chemists, for the 
most part, rested in the comfortable belief of the existence of atoms in 
the restricted sense in which Dalton, as a legacy from Newton, had 
imagined them. Lord Kelvin, unlike the chemists, had never been in 
the habit of ‘evading questions as to the hardness or indivisibility of 
atoms by virtually assuming them to be infinitely small and infinitely 
numerous.’ Nor, on the other hand, did he realise, with Boscovich, 
the atom ‘as a mystic point endowed with inertia and the attribute of 
attracting or repelling other such centres.’ Science advances not so 
much by fundamental alterations in its beliefs as by additions to them. 
Dalton would equally haye regarded the atom ‘as a piece of matter of 
measureable dimensions, with shape, motion, and laws of action, in- 
telligible subjects of scientific investigation.’ 
In spite of the fact that the atomic theory, as formulated by Dalton, 
has been generally accepted for nearly a century, it is only within the 
last few years that physicists have arrived at a conception of the 
structure of the atom sufficiently precise to be of service to chemists in 
connection with the relation between the properties of elements of 
different kinds, and in throwing light on the mechanism of chemical 
combination. 
This further investigation of the ‘ superlatively grand question—the 
inner mechanism of the atom ‘—has profoundly modified the basic 
conceptions of chemistry. It has led to a great extension of our views 
concerning the real nature of the chemical elements. The discovery 
of the electron, the production of helium in the radioactive disintegra- 
tion of atoms, the recognition of the existence of isotopes, the possibility 
that all elementary 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 particle of 
