38 MAXWELL'S LAW 



sides of the partition ; then between the values of N and 

 there subsists the relation 



We have then 

 and consequently 

 if 



or 



The flow of gas therefore proceeds in this case too from the 

 side where there are the more molecules, i.e. where the gas 

 is the denser, to the side where it is the less dense, just 

 as in ordinary effusion ; but while in this latter case the 

 rarefaction is produced by lowering of pressure, in the case 

 of thermal effusion just considered it is effected by warming. 

 Hence the flow of a gas from a colder region to a warmer is 

 a result of the theory no less than of experiment. 



The pressure therefore rises on the warmer side, and a 

 force opposing the motion is brought into play by which a 

 state of equilibrium is finally set up ; and we have now to 

 investigate under what circumstances this will happen. The 

 flow must cease when 



In this case, between the values of the pressure on the two 

 sides of the partition, 



p l = i-H-N^tl^ and p 2 = %7rN 2 ml 2 2 , 



the relation 



Pi = P* 



n; o 



must hold. Introducing into this equation the absolute 

 temperature defined in 15, and therefore putting 



n 2 = n 2 <H> 



in general, and in this particular case 



O2 _ O 2 (fi> O 2 - O 2 (H) 



j 1Z Q ffj, \L 2 1Z Q V_J 2 , 



we find as the condition of the final state of equilibrium 



