A KINETIC THEORY OF MATTER. 221 



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, 1 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 



1 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 '0000116 square centimetres 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 '002 square 

 centimetres per second. The material diffusivities of gases, accord- 

 ing to Loschmidt's experiments, range from '098 (the inter- 

 diffusivity of carbonic acid and nitrous oxide) to '642 (the inter- 

 diffusivity of carbonic oxide and hydrogen) ; while the thermal 

 diffusivities of gases, calculated according to Clausius' and Maxwell's 

 kinetic theory of gases are '089 for carbonic acid, 'l6 for common air 

 or other gases of nearly the same density, and 1*12 for hydrogen (all, 

 both material and thermal, being reckoned in square cemtimetres 

 per second). 



