es a dynamical measure of the resistance to diffusion. It is to be
observed that, however small the relative velocity of the gases A and
B, it plays an all-important part in determining the coefficient of
resistance; for without such relative motion, and with the velocities
evenly distributed in all directions, no transference of momentum
could take place. The coefficient of resistance being found, the
motion of each of the two gases may be discussed separately.

One of the most important consequences of the kinetic theory is that if
the volume be kept constant the coefficient of diffusion varies as the
square root of the absolute temperature. To prove this, we merely have
to imagine the velocity of each molecule to be suddenly increased n
fold; the subsequent processes, including diffusion, will then go on n
times as fast; and the temperature T, being proportional to the kinetic
energy, and therefore to the square of the velocity, will be increased
n^2 fold. Thus K, the coefficient of diffusion, varies as [sqrt]T.

The relation of K to the density when the temperature remains constant
is more difficult to discuss, but it may be sufficient to notice that if
the number of molecules is increased n fold, the chances of a collision
are n times as great, and the distance traversed between collisions is
(not _therefore_ but as the result of more detailed reasoning) on the
average 1/n of what it was before. Thus the free path, and therefore the
coefficient of diffusion, varies inversely as the density, or directly
as the volume. If the pressure p and temperature T be taken as
variables, K varies inversely as p and directly as [sqrt]T^3.

Now according to the experiments first made by J. C. Maxwell and J.
Loschmidt, it appeared that with constant density K was proportional to
T more nearly than to [sqrt]T. The inference is that in this respect a
medium formed of colliding spheres fails to give a correct mechanical
model of gases. It has been found by L. Boltzmann, Maxwell and others
that a system of particles whose mutual actions vary according to the
inverse fifth power of the distance between them represents more
correctly the relation between the coefficient of diffusion and
temperature in actual gases. Other recent theories of diffusion have
been advanced by M. Thiesen, P. Langevin and W. Sutherland. On the other
hand, J. Thovert finds experimental evidence that the coefficient of
diffusion is proportional to m