AND OF THE SECOND LAW 35 



is in reality reversible, i.e., suppose a method were discovered 

 of completely reversing this process and thus leave no other change 

 whatsoever, then combining the direct course of the process with 

 this latter reversed process, they would together constitute a 

 cyclical process, which would effect nothing but the production 

 of work and the absorption of an equivalent amount of heat. 

 But this would be perpetual motion of the second kind, which 

 to be sure is denied by the empirical theorem on p. 30. But for 

 the sake of the argument we may just now waive said impossibility; 

 then we would have an engine which, co-operating with any 

 second (so-called), irreversible process, would completely restore 

 the initial state of the whole system without leaving any other 

 change whatsoever. Then under our definition on p. 30 this 

 second process ceases to be irreversible. The same result will 

 obtain for any third, fourth, etc. So that the above proposition 

 is established. " All the irreversible processes stand or fall together." 

 If any one of them is reversible all are reversible. 1 



(4) Convenience of the Fiction, the Reversible Processes 



A reversible process we have declared to be only an ideal case, 

 a convenient and fruitful fiction which we can imagine by elim- 

 inating from an irreversible process one or more of its inevitable 

 accompaniments like friction or heat conduction. But reversible 

 (as well as irreversible) processes have common features. "They 

 resemble each other more than they do any one irreversible process. 

 This is evident from an examination of the differential equations 

 which control them; the differential with respect to time is always 

 of an even order, because the essential sign of time can be reversed. 

 Then too they (in whatever domain of physics they may lie) 

 have the common property that the Principle of Least Action 



1 With the help of the preceding footnote this argument can be followed through 

 in detail for each of the cases enumerated on p. 31; only the complicated case of 

 diffusion presents any difficulty. 



