been
caused to be denser. Now Joule's and my own old experiments on the efflux
of air prove that if the crowd be common air, or oxygen, or nitrogen, or
carbonic acid, the temperature is a little higher in the denser than in the
rarer condition when the energies are the same. By the hypothesis, equality
of temperature between two different gases or two portions of the same gas
at different densities means equality of kinetic energies in the same
number of molecules of the two. From our observations proving the
temperature to be higher, it therefore follows that the potential energy is
smaller in the condensed crowd. This--always, however, under protest as to
the temperature hypothesis--proves some degree of attraction among the
molecules, but it does not prove ultimate attraction between two molecules
in collision, or at distances much less than the average mutual distance of
nearest neighbors in the multitude. The collisional force might be
repulsive, as generally supposed hitherto, and yet attraction might
predominate in the whole reckoning of difference between the intrinsic
potential energies of the more dense and less dense multitudes.

It is however remarkable that the explanation of the propagation of sound
through gases, and even of the positive fluid pressure of a gas against the
sides of the containing vessel, according to the kinetic theory of gases,
is quite independent of the question whether the ultimate collisional force
is attractive or repulsive. Of course it must be understood that, if it is
attractive, the particles must, be so small that they hardly ever
meet--they would have to be infinitely small to _never_ meet--that, in
fact, they meet so seldom, in comparison with the number of times their
courses--are turned through large angles by attraction, that the influence
of these surely attractive collisions is preponderant over that of the
comparatively very rare impacts from actual contact. Thus, after all, the
train of speculation suggested by Davy's "Repulsive Motion" does not allow
us to escape from the idea of true repulsion, does not do more than let us
say it is of no consequence, nor even say this with truth, because, if
there are impacts at all, the nature of the force during the impact and the
effects of the mutual impacts, however rare, cannot be evaded in any
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 ma