of the molecules, the diffusion of heat must be chiefly by interchange of
energies between the molecules, and should be, as experiment proves it is,
enormously more rapid than the diffusion of the molecules themselves, and
this again ought to be much less rapid than either the material or thermal
diffusivities of gases. Thus the diffusivity of common salt through water
was found by Fick to be as small as 0.0000112 square centimeter per second;
nearly 200 times as great as this is the diffusivity of heat through water,
which was found by J.T. Bottomley to be about 0.002 square centimeter per
second. The material diffusivities of gases, according to Loschmidt's
experiments, range from 0.98 (the interdiffusivity of carbonic acid and
nitrous oxide) to 0.642 (the interdiffusivity of carbonic oxide and
hydrogen), while the thermal diffusivities 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