34 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



influences from without, we must regard the bodies exerting 

 such influences as incorporated with the original system and 

 then the statement will hold in the above given form." 



From what has gone before it is evident that the following 

 commonly drawn conclusions are correct: 



An irreversible process is a passage from a less probable to 

 a more probable state of the system. 



An irreversible process is a passage from a less stable to a 

 more stable state of the system. 



An irreversible process is essentially a spontaneous one, inas- 

 much as once started it will proceed without the help of any external 

 agency. 



We have in a general way reached the conclusion that entropy 

 is both the criterion and the measure of irreversibility. But now 

 let us become more specific and go more into certain details, 

 namely, the common features in all irreversibility. The property 

 of irreversibility is not inherent in the elementary occurrences 

 themselves, but only in their irregular arrangement. Irreversi- 

 bility depends only on the statistical property of a system possess- 

 ing many degrees of freedom, and is therefore essentially based 

 on mean values; in this connection we may repeat an earlier 

 statement, the individual motions of atoms are in themselves 

 reversible, but their result in the aggregate is not. 



(3) All the Irreversible Processes Stand or Fall Together 



This is proved with the help of the theorem (p. 30) which denies 

 the possibility of perpetual motion of the second kind. 1 The 

 argument is this: take any case in any one of the four classes of 

 irreversible processes given on p. 31. Now if this selected case 



1 At this stage we appreciate that any irreversible process is a passage from a 

 state A of low entropy to a state B of high entropy. We may simplify our proof 

 by considering the return passage from B to A to in part occur isothermal ly 

 and in part isentropically; then external agencies must produce work and absorb 

 an equivalent amount of heat. 



