Rich as it is in practical results, the kinetic theory of gases, as hitherto developed, stops absolutely short at the atom or molecule, and gives not even a suggestion toward explaining the properties in virtue of which the atoms or molecules mutually influence one another.  For some guidance toward a deeper and more comprehensive theory of matter, we may look back with advantage to the end of last century and beginning of this century, and find Rumford’s conclusion regarding the heat generated in boring a brass gun:  “It appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner the heat was excited and communicated in these experiments, except it be MOTION;” and Davy’s still more suggestive statements:  “The phenomena of repulsion are not dependent on a peculiar elastic fluid for their existence.” ...  “Heat may be defined 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 denser gas, if we assume the ordinary hypothesis regarding the temperature of a gas, according to which two gases are of equal temperatures [2] when the kinetic energies of their constituent molecules are of equal average amounts per molecule.