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fluids against the sides of the vessels containing them arises 
from the rapid and ceaseless motions and collisions of the 
atoms, which in gases are least confined, and are allowed 
some length of free path without collision, in liquids are 
more confined, and in solid matter have veiy little motion 
indeed. 
These are the assumptions which, of course, must, as in the 
case of the law of gravitation, get their verification by experi- 
ment. The experiments which seem to have established the 
theory (which, however, we must consider to be yet in an in- 
fant state) are chiefly those of the rate of diffusion of gases, 
in connection with the laws of the assumed motions or vibra- 
tions of the molecules. And the facts which physicists of 
the highest reputation of the present day think they have in- 
disputably established are very wonderful indeed, and give us 
a much deeper insight into the mysterious workings employed 
in- the structure of the universe than we had before. For ex- 
ample, there have been found for the gases hydrogen, oxygen, 
carbonic oxide, and carbonic acid, and probably, by this time, 
for many others, the mean velocity for each molecule, and the 
relative mass, and with somewhat less degree of certainty the 
relative size, length of free path between collisions, and num- 
ber of collisions in a second; while conjecturally (that is, sub- 
ject to very great corrections from future observations) 
attempts have been made to determine the absolute masses of 
the molecules, and their number in a given space. To give 
some idea of the results, I may take the case of hydrogen, 
for whose atoms the mean velocity is 1,859 metres per second, 
and two millions of them in a row would occupy the length 
of a millemetre, and a million million million million of them 
would weigh between 4 and 5 grammes. Finally, in a cubic 
centimetre, at the standard pressure and temperature, there 
are about nineteen million million million atoms. Is not this 
wonderful ? Some of these results are only approximate, but 
they give an adequate idea of the correctness of the theory, 
and want only additional observations for their correction. 
And it must be borne in mind that the atomic theory is true 
for the whole universe. A molecule for example in Sirius or 
Arcturus executes its vibrations in precisely the same time as 
on the surface of our earth or our own sun. 
I will conclude this account of these marvellous elements in 
the excellent words of Professor Clerk-Maxwell at the end of his 
lecture delivered at Bradford inl873: “No thcorv of evolution/’ 
he says, “ can be formed to account for the similarity of mole- 
cules, for evolution necessarily implies continuous change, and 
