as a peculiar motion, probably a vibration, of the corpuscles of bodies,
tending to separate them." ... "To distinguish this motion from others, and
to signify the causes of our sensations of heat, etc., the name _repulsive_
motion has been adopted." Here we have a most important idea. It would be
somewhat a bold figure of speech to say the earth and moon are kept apart
by a repulsive motion; and yet, after all, what is centrifugal force but a
repulsive motion, and may it not be that there is no such thing as
repulsion, and that it is solely by inertia that what seems to be repulsion
is produced? Two bodies fly together, and, accelerated by mutual
attraction, if they do not precisely hit one another, they cannot but
separate in virtue of the inertia of their masses. So, after dashing past
one another in sharply concave curves round their common center of gravity,
they fly asunder again. A careless onlooker might imagine they had repelled
one another, and might not notice the difference between what he actually
sees and what he would see if the two bodies had been projected with great
velocity toward one another, and either colliding and rebounding, or
repelling one another into sharply convex continuous curves, fly asunder
again.

Joule, Clausius, and Maxwell, and no doubt Daniel Bernoulli himself, and I
believe every one who has hitherto written or done anything very explicit
in the kinetic theory of gases, has taken the mutual action of molecules in
collision as repulsive. May it not after all be attractive? This idea has
never left my mind since I first read Davy's "Repulsive Motion," about
thirty-five years ago, and I never made anything of it, at all events have
not done so until to-day (June 16, 1884)--if this can be said to be making
anything of it--when, in endeavoring to prepare the present address, I
notice that Joule's and my own old experiments[1] on the thermal effect of
gases expanding from a high-pressure vessel through a porous plug, proves
the less dense gas to have greater intrinsic _potential_ energy than the