the consequent necessity of waiting for a fraction of a second or a few seconds of time (with apparatus of ordinary experimental magnitude), to see a subsidence from a larger change of pressure down to the amount of change that verifies Boyle’s law.  The consideration of these phenomena forty years ago by Joule, in connection with Bernoulli’s original conception, formed the foundation of the kinetic theory of gases as we now have it.  But what a splendid and useful building has been placed on this foundation 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 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).]