374 TRANSACTIONS OF SECTION A. 
over the Reports of this Association and to note how many addresses, either 
wholly or in part, have been devoted to a consideration of this subject. 
Amongst numerous examples I may instance the famous and oft-quoted 
lecture of Maxwell on Molecules, at Bradford in 1873; the discussion of 
the Kinetic Theory of Gases by Lord Kelvin, then Sir William Thomson, 
in Montreal in 1884; and the Presidential Address of Sir Arthur Ricker 
in 1901, which will be recalled by many here to-day. 
It is far from my intention to discuss, except with extreme brevity, the 
gradual rise and development of the atomic theory. From the point of 
view of modern science, the atomic theory dates from the work of Dalton 
about 1805, who put it forward as an explanation of the combination of 
elements in definite proportions. The simplicity of this explanation of 
the facts of chemistry led to the rapid adoption of the atomic theory as a 
very convenient and valuable working hypothesis. By the labour of the 
chemists matter was shown to be composed of a number of elementary 
substances which could not be further decomposed by laboratory agencies, 
and the relative weights of the atoms of the elements were determined. 
On the physical side, the mathematical development of the kinetic or 
dynamical theory of gases by the labours of Clausius and Clerk Maxwell 
enormously extended the utility of this conception. It was shown that the 
properties of gases could be satisfactorily explained on the assumption that 
a gas consisted of a great assemblage of minute particles or molecules in 
continuous agitation, colliding with each other and with the walls of the 
containing vessel. Between encounters the molecules travelled in straight 
lines, and the free path of the molecules between collisions was supposed to 
be large compared with the linear dimensions of the molecules themselves. 
One cannot but regard with admiration the remarkable success of this 
statistical theory in explaining the general properties of gases and even 
predicting unexpected relations. The strength and at the same time the 
limitations of the theory lie in the fact that it does not involve any definite 
conception of the nature of the molecules themselves or of the forces acting 
between them. The molecule, for example, may be considered as a perfectly 
elastic sphere or a Boscovitch centre of force, as Lord Kelvin preferred to 
regard it, and yet on suitable assumptions the gas would show the same 
general statistical properties. We are consequently unable, without the 
aid of special subsidiary hypotheses, to draw conclusions of value in regard 
to the nature of the molecules themselves. 
Towards the close of the last century the ideas of the atomic theory had 
impregnated a very large part of the domain of physics and chemistry. 
The conception of atoms became more and more concrete. The atom in 
imagination was endowed with size and shape, and unconsciously in many 
cases with colour. The simplicity and utility of atomic conceptions in 
explaining the most diverse phenomena of physics and chemistry naturally 
tended to enhance the importance of the theory in the eyes of the scientific 
worker. There was a tendency to regard the atomic theory as one of the 
established facts of nature, and not as a useful working hypothesis for 
which it was exceedingly difficult to obtain direct and convincing evidence. 
There were not wanting scientific men and philosophers to point out the 
uncertain foundations of the theory on which so much depended. Granting 
how useful molecular ideas were for the explanation of experimental facts, 
what evidence was there that the atoms were realities and not the figments 
of the imagination? It must be confessed that this lack of direct evidence 
did*not in any way detract from the strength of the belief of the great 
majority of scientific men in the discreteness of matter. It was not 
unnatural, however, that there should be a reaction in some quarters against 
the domination of the atomic theory in physics and in chemistry. A school 
of thought arose that wished to do away with the atomic theory as the 
basis of explanation of chemistry, and substitute as its equivalent the law 
