CHAPTER XII. 



FREE PATH PHENOMENA (CONTINUED). 



VISCOSITY. 



293. WE have so far discussed only gases in which the molecular motion 

 at every point has been symmetrical with respect to every direction in space. 

 We now approach a much more complex class of problems for which this 

 property is not true. If we refer to the expression obtained for the law of 

 distribution of velocities at any point of a gas in the normal state, namely, 



we notice that there are five independent constants w , v , w , h and A. The 

 constancy of u , v , w indicates that the mass motion of the gas is the same 

 throughout the gas : if this mass motion varies from point to point in the 

 gas, the layers of gas move relatively to one another, and we have the 

 problem of determining the viscosity of the gas. Similarly the constancy 

 of h indicates the equality of temperature throughout the gas : if this varies 

 from point to point we have the problem of conduction of heat. Finally the 

 constancy of A indicates the mass-equilibrium of the gas : if this equilibrium 

 does not exist we have the problem of diffusion. These three problems of 

 viscosity, conduction, and diffusion have now to be dealt with. 



294. It may be said at once that there is no general method of the kind 

 which was available for the determination of the steady state. The method 



/ - 



which will be followed first in every instance is of the most primitive kind, 

 and the results obtained cannot claim any high degree of accuracy. On the 

 other hand the method in question possesses the great advantage of de- 

 pending only in a slight degree upon the actual structure of the molecules of 

 the gas. And as the exact nature of this structure is very largely a matter 

 of speculation, the comparative inaccuracy of the result obtained will be less 

 serious than it might have been if we had had fuller knowledge of the 

 molecular structure. 



We shall afterwards be able to correct the first general result so as to 

 obtain results of considerable accuracy upon the special supposition that the 

 molecules are elastic spheres. 



