se, say the particles are centres of force, of which
the action is insensible except at a certain very small distance, when
it suddenly appears as a repulsive force of very great intensity. It is
evident that either assumption will lead to the same results. For the
sake of avoiding the repetition of a long phrase about these repulsive
bodies, I shall proceed upon the assumption of perfectly elastic
spherical bodies. If we suppose those aggregate molecules which move
together to have a bounding surface which is not spherical, then the
rotatory motion of the system will close up a certain proportion of the
whole vis viva, as has been shown by Clausius, and in this way we may
account for the value of the specific heat being greater than on the
more simple hypothesis."(1)

The elaborate investigations of Clerk-Maxwell served not merely to
substantiate the doctrine, but threw a flood of light upon the entire
subject of molecular dynamics. Soon the physicists came to feel as
certain of the existence of these showers of flying molecules making up
a gas as if they could actually see and watch their individual actions.
Through study of the viscosity of gases--that is to say, of the degree
of frictional opposition they show to an object moving through them
or to another current of gas--an idea was gained, with the aid of
mathematics, of the rate of speed at which the particles of the gas are
moving, and the number of collisions which each particle must experience
in a given time, and of the length of the average free path traversed
by the molecule between collisions, These measurements were confirmed
by study of the rate of diffusion at which different gases mix together,
and also by the rate of diffusion of heat through a gas, both these
phenomena being chiefly due to the helter-skelter flight of the
molecules.

It is sufficiently astonishing to be told that such measurements as
these have been made at all, but the astonishment grows when one hears
the results. It appears from Clerk-Maxwell's calculations that the mean
free path, or distance traversed by the molecules between collisions in
ordinary air, is about one-half-millionth of an inch; while the speed of
the molecules is such that each one experiences about eight billions
of collisions per second! It would be hard, perhaps, to cite an
illustration showing the refinements of modern physics better than
this; unless, indeed, one other result that followed directly from th