attempt to realize a conception of the kinetic theory of gases. And in
fact, unless we are satisfied to imagine the atoms of a gas as mathematical
points endowed with inertia, and as, according to Boscovich, endowed with
forces of mutual, positive, and negative attraction, varying according to
some definite function of the distance, we cannot avoid the question of
impacts, and of vibrations and rotations of the molecules resulting from
impacts, and we must look distinctly on each molecule as being either a
little elastic solid or a configuration of motion in a continuous
all-pervading liquid. I do not myself see how we can ever permanently rest
anywhere short of this last view; but it would be a very pleasant temporary
resting-place on the way to it if we could, as it were, make a mechanical
model of a gas out of little pieces of round, perfectly elastic solid
matter, flying about through the space occupied by the gas, and colliding
with one another and against the sides of the containing vessel.

This is, in fact, all we have of the kinetic theory of gases up to the
present time, and this has done for us, in the hands of Clausius and
Maxwell, the great things which constitute our first step toward a
molecular theory of matter. Of course from it we should have to go on to
find an explanation of the elasticity and all the other properties of the
molecules themselves, a subject vastly more complex and difficult than the
gaseous properties, for the explanation of which we assume the elastic
molecule; but without any explanation of the properties of the molecule
itself, with merely the assumption that the molecule has the requisite
properties, we might rest happy for a while in the contemplation of the
kinetic theory of gases, and its explanation of the gaseous properties,
which is not only stupendously important as a step toward a more
thoroughgoing theory of matter, but is undoubtedly the expression of a
perfectly intelligible and definite set of facts in Nature.