AND OF THE SECOND LAW 37 



of itself to assume. An illustration will help to make this clear; 

 the irreversible case in which work (i.e., friction) is converted 

 into heat. " For example, the direct reversal of a frictional process 

 is impossible because this would presuppose the existence of an 

 elementary order among adjacent, mutually interacting molecules. 

 For then it must predominantly be the case that the collisions of 

 each pair of molecules must bear a certain distinguishable char- 

 acter inasmuch as the velocities of two colliding molecules must 

 always depend in a determinate manner on the place where they 

 meet. Only thereby can it be attained that there will result 

 from the collisions predominantly like directed velocities." 



The outcome of the whole study of irreversibility results in 

 the briefly stated law : " There exists in Nature a quantity which 

 changes always in the same sense in all natural processes" 



This boldly asserts the essential one-sidedness of Nature. 

 The proposition stated in this general form may be correct or 

 incorrect; but whichever it may be it will remain so independently 

 of human experimental skill. 



SECTION D 



(i) The Gradual Development of the Idea that Entropy Depends 



on Probability 



Entropy is difficult to conceive, in that, as it does not directly 

 affect the senses, there is nothing physical to represent it; it 

 cannot be felt like temperature. It has no analogue in the whole 

 of Physics; Zeuner's heat weight will perhaps serve as such for 

 reversible states, but is inadequate for irreversible ones. This 

 is not surprising when we consider the outcome, namely, that it 

 depends on probability considerations. 



CLAUSIUS coined the term Entropy from the Greek, from a 

 word meaning transformation; with him the transformation value 

 was equal to the difference between the entropy of the final and 

 initial states. As there is a general expression for entropy, we 



