fusivities of gases, calculated according
to Clausius' and Maxwell's kinetic theory of gases, are 0.089 for carbonic
acid, 0.16 for common air of other gases of nearly the same density, and
1.12 for hydrogen (all, both material and thermal, being reckoned in square
centimeters per second).]

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 asu