tion by Clausius and Maxwell, and what a
beautiful ornament we see on the top of it in the radiometer of Crookes,
securely attached to it by the happy discovery of Tait and Dewar,[2] that
the length of the free path of the residual molecules of air in a good
modern vacuum may amount to several inches! Clausius' and Maxwell's
explanations of the diffusion of gases, and of thermal conduction in gases,
their charmingly intelligible conclusion that in gases the diffusion of
heat is just a little more rapid than the diffusion of molecules, because
of the interchange of energy in collisions between molecules,[3] while the
chief transference of heat is by actual transport of the molecules
themselves, and Maxwell's explanation of the viscosity of gases, with the
absolute numerical relations which the work of those two great discoverers
found among the three properties of diffusion, thermal conduction, and
viscosity, have annexed to the domain of science a vast and ever growing
province.

[Footnote 1: "Preuve de la supposition que j'ay faite: Que la matiere
subtile ou etheree est necessairement composee de PETITS TOURBILLONS; et
qu'ils sont les causes naturelles de tous les changements qui arrivent a la
matiere; ce que je confirme par i'explication des effets les plus generaux
de la Physique, tels que sont la durete des corps, leur fluidite, leur
pesanteur, legerete, la lumiere et la refraction et reflexion de ses
rayons."--Malebranche, "Recherche de la Verite," 1712.]

[Footnote 2: Proc. R.S.E., March 2, 1874, and July 5, 1875.]

[Footnote 3: On the other hand, in liquids, on account of the crowdedness
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 dif