186 Lord Kelvin on 



\ •§ 35. I see no explanation of this deviation from what 

 seems thoroughly correct theory. Accurate experimental 

 determinations of viscosities, whether of gases or liquids, are 

 easy by Graham's transpirational method. On the other 

 hand even roughly approximate experimental determinations 

 of thermal diffusivities are exceedingly difficult, and I believe 

 none, on correct experimental principles, have really been 

 made * ; certainly none un vitiated by currents of the gas 

 experimented upon, or accurate enough to give any good 

 test of the theoretical relation between thermal and material 

 diffusivities, expressed by the following equation, derived 

 from the preceding verbal statement regarding the three 

 diffusivities of a gas, 



P 



where 6 denotes the thermal conductivity, /jl the viscosity, 

 p the density, Kp the thermal capacity per unit bulk pressure 

 constant, K the thermal capacity per unit mass pressure 

 constant, c the thermal capacity per unit mass volume con- 

 stant, and k the ratio of the thermal capacity pressure constant 

 to the thermal capacity volume constant. It is interesting to 

 remark how nearly theoretical investigators f have come to 

 the relation 6 = hep; Clausius gave 6— %cyb ; 0. E. Meyer, 

 = 1'6027c/j,, and Maxwell, 0= J c/z. Maxwell's in fact is 

 = kcjub for the case of a monatomic gas. 



§ 36. To understand exactly what is meant by molecular 

 diffusivity consider a homogeneous gas between two infinite 

 parallel planes, GGG and RRR, distance a apart, and let it 

 be initially given in equilibrium ; that is to say, with equal 

 numbers of molecules and equal total kinetic energies in equal 

 volumes, and with integral of component momentum in any 

 and every direction, null. Let iV be the number of molecules 

 per unit volume. Let every one of the molecules be marked 

 either green or red, and whenever a red molecule strikes the 

 plane GGG, let its marking be altered to green, and, when- 

 ever a green molecule strikes RRR, let its marking be altered 

 to red. These markings are not to alter in the slightest 



* So far as I know, all attempts hitherto made to determine the 

 thermal conductivities of gases have been founded on observations of 

 rate of communication of heat between a thermometer-bulb, or a stretched 

 metallic wire constituting an electric resistance thermometer, and the 

 walls of the vessel enclosing it and the gas experimented upon. See 

 Wiedemann's Annalen (1888), vol. xxxiv. p. 623, and 1891, vol. xliv. 

 p. 177. For other references, see O. E. Meyer, § 107. 



t See the last ten lines of 0. E. Meyer's book. 



