4 THE PRESIDENTIAL ADDRESS. 
to have any detailed knowledge of the dimensions or structure of the 
atom. It was only necessary to assume that the atoms acted as indi- 
vidual units, and to know the relative masses of the atoms of the 
different elements. In the next stage, for example, in the kinetic 
theory of gases, it was possible to explain the main properties of gases 
by supposing that the atoms of the gas acted as minute perfectly elastic 
spheres. During this period, by the application of a variety of methods, 
many of which were due to Lord Kelvin, rough estimates had been 
obtained of the absolute dimensions and mass of the atoms. These 
brought out the minute size and mass of the atom and the enormous 
number of atoms necessary to produce a detectable effect in any kind 
of measurement. From this arose the general idea that the atomic 
theory must of necessity for ever remain unverifiable by direct experi- 
ment, and for this reason it was suggested by one school of thought 
that the atomic theory should be banished from the teaching of 
Chemistry, and that the law of multiple proportions should be accepted 
as the ultimate fact of Chemistry. 
While the vaguest ideas were held as to the possible structure of 
atoms, there was a general belief among the more philosophically 
minded that the atoms of the elements could not be regarded as simple 
unconnected units. The periodic variations of the properties of the 
elements brought out by Mendeléef were only explicable if atoms were 
similar structures in some way constructed of similar material. We 
shall see that the problem of the constitution of atoms is intimately 
connected with our conception of the nature of electricity. The 
wonderful success of the electromagnetic theory had concentrated atten- 
tion on the medium or ether surrounding the conductor of electricity, 
and little attention had been paid to the actual carriers of the electric 
current itself. At the same time the idea was generally gaining ground 
that an explanation of the results of Faraday’s experiments on electro- 
lysis was only possible on the assumption that electricity, like matter, 
was atomic in nature. The name ‘electron’ had even been given to 
this fundamental unit by Johnstone Stoney, and its magnitude roughly 
estimated, but the full recognition of the significance and importance 
of this conception belongs to the new epoch. 
For the clarifying of these somewhat vague ideas, the proof in 
1897 of the independent existence of the electron as a mobile electrified 
unit, of mass minute compared with that of the lightest atom, was of 
extraordinary importance. It was soon seen that the electron must 
be of a constituent of all the atoms of matter, and that optical 
spectra had their origin in their vibrations. The discovery of the 
electron and the proof of its liberation by a variety of methods from 
all the atoms of matter was of the utmost significance, for it strength- 
ened the view that the electron was probably the common unit in the 
