DIFFUSION. G31 



and the equation of motion becomes 



=p fd\ djj> _ dX~ciy 2 | d-v, _Xdo^ 

 l (dx* dt \ ~ d? dx j + ~da? ^ dx 



dxl 



dx dt dx 



If we add the corresponding equations together for all the gases, we find 

 that the terms in (7 12 destroy each other, and that if the medium is not 

 affected with sensible currents the first term of each equation may be neg- 

 lected. In ordinary experiments we may also neglect the effect of gravity, so 

 that we get 



or 



where p is the uniform pressure of the mixed medium. Hence 



dv 2 _ dv 1 , dv 2 _ dv t 



di = ~^t dx~ p ~~dx' 



and the equation becomes 



d\ _ Cu dVi 

 ~ 



an equation, the form of which is identical with the well-known equation for 

 the conduction of heat. We may write it 



D is called the coefficient of diffusion. It is equal to 



It therefore varies inversely as the total pressure of the medium, and if the 

 coefficient of resistance, <7 12 , is independent of the temperature, it varies directly 

 as the product kjc.,, i.e., as the square of the absolute temperature. It is 

 probable, however, that the effect of temperature is not so great as this would 

 make it. 



