30 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



a complete cycle, 1 and produce no effect except the raising of a weight 

 and the cooling of a heat reservoir" 2 



Now up to this time no natural event has contradicted this 

 theorem or its corollaries. The proof for it is cumulative, wholly 

 experiential and therefore exactly like that for the law of con- 

 servation of energy. 



Returning to irreversibility, the matter for immediate dis- 

 cussion, we premise that it will here clarify and simplify our 

 ideas if we consider all the participating bodies as parts of the 

 system experiencing the contemplated process. It is in this 

 sense that we must understand the statement: Every natural 

 event leaves the universe different from what it was before. 

 Speaking very generally, we may say that in this difference lies 

 what we call irreversibility. 



Now irreversibility is what really does exist, everywhere in 

 Nature, and our idea of reversibility is only a very convenient 

 and fruitful fiction; our conception of reversibility must, there- 

 fore, ultimately be derived from that of irreversibility. 



" A process which can in no way be completely reversed 

 is termed irreversible, all other processes reversible. That a 

 process may be irreversible, it is not sufficient that it cannot be 

 directly reversed. This is the case with many mechanical processes 

 which are not irreversible (See p. 32). The full requirement 

 is, that it be impossible, even with the assistance of all agents 

 in Nature, to restore everywhere the exact initial state when the 

 process has once taken place." 



Examples of irreversible processes, which involve only heat 

 and mechanical phenomena, may be grouped in four classes: 



(a) The body whose changes of state are considered is in 

 contact with bodies whose temperature differs by a finite amount 



1 Such an engine if it would work might be called " perpetual motion of the 

 second kind." 



2 The term perpetual is justified because such an engine would possess the most 

 esteemed feature of perpetual motion power production free of cost. 



