example, the recent researches of M. Spring on the
limit of visibility of fluorescence), give this result:--that there
are, in this space, some twenty thousand millions of molecules. Each
of these must receive in the space of a millimetre about ten thousand
shocks, and be ten thousand times thrust out of its course. The free
path of a molecule is then very small, but it can be singularly
augmented by diminishing the number of them. Tait and Dewar have
calculated that, in a good modern vacuum, the length of the free path
of the remaining molecules not taken away by the air-pump easily
reaches a few centimetres.

By developing this theory, we come to consider that, for a given
temperature, every molecule (and even every individual particle, atom,
or ion) which takes part in the movement has, on the average, the same
kinetic energy in every body, and that this energy is proportional to
the absolute temperature; so that it is represented by this
temperature multiplied by a constant quantity which is a universal
constant.

This result is not an hypothesis but a very great probability. This
probability increases when it is noted that the same value for the
constant is met with in the study of very varied phenomena; for
example, in certain theories on radiation. Knowing the mass and energy
of a molecule, it is easy to calculate its speed; and we find that the
average speed is about 400 metres per second for carbonic anhydride,
500 for nitrogen, and 1850 for hydrogen at 0 deg. C. and at ordinary
pressure. I shall have occasion, later on, to speak of much more
considerable speeds than these as animating other particles.

The kinetic theory has permitted the diffusion of gases to be
explained, and the divers circumstances of the phenomenon to be
calculated. It has allowed us to show, as M. Brillouin has done, that
the coefficient of diffusion of two gases does not depend on the
proportion of the gases in the mixture; it gives a very striking image
of the phenomena of viscosity and conductivity; and it leads us to
think that the coefficients of friction and of conductivity are
independent of the density; while all these previsions have been
verified by experiment. It has also invaded optics; and by relying on
the principle of Doppler, Professor Michelson has succeeded in
obtaining from it an explanation of the length presented by the
spectral rays of even the most rarefied gases.

But however interesting are these results, t