AND OF THE SECOND LAW 33 



excluded. But whether it is reversible or irreversible depends 

 solely and only on the constitution of the two states A and B, 

 not upon the other features of the course; after state B has been 

 attained, we must here simply answer the question whether the 

 complete return to A can or cannot be effected in any manner 

 whatsoever. Now if such complete return from B to A is not 

 possible then evidently state B in Nature is somehow distin- 

 guished from state A. Nature may be said to prefer state B to 

 state A. Reversible processes are a limiting case; here Nature 

 manifests no preference and the passage from the one to other 

 can take place at pleasure, in either direction. [In the common 

 case of isentropic expansion from A to 5, there is no exchange 

 of heat with the outside; external work is performed at the 

 expense of the inner energy of the expanding body. When 

 state B is attained we can effect a complete return to A by com- 

 pressing isentropically, thus consuming the external work per- 

 formed on the trip from A to B and restoring the internal energy 

 of the body.] 



"Now it becomes a question of finding a physical magnitude 

 whose amount will serve as a general measure of Nature's preference 

 for a state. This must be a magnitude which is directly determined 

 by the state of the contemplated system, without knowing any- 

 thing of the past history of the system, just as is the case when 

 we deal with the state's energy, volume, etc. This magnitude 

 would possess the property of growing in all irreversible processes, 

 while in all reversible processes it would remain unchanged. 

 The amount of its change in a process would furnish a general 

 measure for the irreversibility of the process." 



" Now R. CLAUSIUS really found such a magnitude and called 

 it entropy. Every bodily system possesses in every state a 

 particular entropy, and this entropy designates the preference 

 of nature for the state in question; in all the processes which 

 occur in the system, entropy can only grow, never diminish. 

 If we wish to consider a process in which said system is subject to 



