But some of these hypotheses have done yet further service. Combining
them with the mechanical theory of heat and the doctrine of the
conservation of energy, which are also products of our time,
physicists have arrived at an entirely new conception of the nature of
gaseous bodies and of the relation of the physico-chemical units of
matter to the different forms of energy. The conduct of gases under
varying pressure and temperature, their diffusibility, their relation
to radiant heat and to light, the evolution of heat when bodies
combine, the absorption of heat when they are dissociated, and a host
of other molecular phenomena, have been shown to be deducible from the
dynamical and statical principles which apply to molar motion and
rest; and the tendency of physico-chemical science is clearly towards
the reduction of the problems of the world of the infinitely little,
as it already has reduced those of the infinitely great world, to
questions of mechanics.[H]

In the meanwhile, the primitive atomic theory, which has served as the
scaffolding for the edifice of modern physics and chemistry, has been
quietly dismissed. I cannot discover that any contemporary physicist
or chemist believes in the real indivisibility of atoms, or in an
interatomic matterless vacuum. 'Atoms' appear to be used as mere
names for physico-chemical units which have not yet been subdivided,
and 'molecules' for physico-chemical units which are aggregates of the
former. And these individualised particles are supposed to move in an
endless ocean of a vastly more subtle matter--the ether. If this ether
is a continuous substance, therefore, we have got back from the
hypothesis of Dalton to that of Descartes. But there is much reason to
believe that science is going to make a still further journey, and, in
form, if not altogether in substance, to return to the point of view