AND MODERN PHYSICS. 129 



However, a few years later Clausius criticised the 

 details of this part of the paper, and Maxwell, in his 

 memoir of 18G6, admits the calculation to have been 

 erroneous. The main principles remained unaffected, 

 the molecules pass from one gas to the other, and this 

 constitutes diffusion. 



Now, suppose wo have two sets of particles in 

 contact of such a nature that the mean kinetic 

 energy of the one set is different from that of the 

 other; the temperatures of the two will then bo dif- 

 ferent These two sets will diffuse into each other, and 

 the diffusing particles will carry with them their 

 kinetic energy, which will gradually pass from those 

 which have the greater energy to those which have 

 the less, until the average kinetic energy is equalised 

 throughout. But the kinetic energy of translation is 

 the heat of the particles. This diffusion of kinetic 

 energy is a diffusion of heat by conduction, and wo 

 have hero the mechanical theory of the conduction 

 of heat in a gas. 



Maxwell obtained an expression, which, however, 

 ho afterwards modified, for the conductivity of a gas 

 in terms of the mean free path. It followed from this 

 that the conductivity of air was only about of 

 that of copper. 



Thus the diffusion of gases, the viscosity of gases, 

 and the conduction of heat in gases, are all connected 

 with the diffusion of the particles carrying with them 

 their momenta and their energy ; while values of the 

 mean free path can bo obtained from observations on 

 any one of these properties. 



In the third part of his paper Maxwell considers 

 I 



