AND OF THE SECOND LAW 75 



Assuming that an ideally perfect gas was employed in the 

 experiment, and that the final state for our consideration is that 

 corresponding to its attainment of thermal equilibrium, we see 

 that because of the unchanged temperature there is here no loss 

 of internal energy, for the work consumed by the friction of the 

 porous plug is all returned to the gas by the heat developed by 

 said friction. Moreover, the + and external work in this 

 experiment also balance. Now although there has been no loss 

 of energy there has been a growth of entropy corresponding to 

 the evident increase in the number of complexions. This increase 

 is exactly equal to that found for reversible isothermal change 

 of state when accompanied by an increase in volume, and the dis- 

 cussion is therefore not repeated here. 



One phase of the above process is the conversion of mechanical 

 work into heat through the medium of friction. 



INCREASE OF VOLUME WITHOUT PERFORMANCE OF EXTERNAL 

 WORK BY ELASTIC FORCES OF THE GAS 



This case of an irreversible process comes into group c. We 

 will consider here JOULE'S well-known experiment with the air 

 tanks, in which the compressed air, initially stored in the one 

 tank, was allowed to discharge into the other tank which, at the 

 start, contained only a vacuum. At the end of the experiment, 

 when thermal equilibrium obtained, the temperature in the two, 

 now connected, tanks was the same as originally existed in the 

 compressed-air tank. Here of course it is assumed that the air 

 exchanged no heat whatever with the outside. 



As the final state, like the initial state, is in thermal equilibrium, 

 and possesses the same temperature, we can ascertain the total 

 change in the number of complexions as we did when discussing 

 isothermal and reversible changes and because of the accompany- 

 ing increase in the volume of the air, find that here as there the 

 number of complexions has increased and that therefore the 

 entropy of the air has increased in this case. 



