38 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



can readily write the equivalent of any transformation between 

 two particular states. 



Strictly speaking, however, entropy by itself depends only on 

 the state in question, not on any change it may experience, nor 

 on its past history before reaching the state contemplated. Of 

 course, this was appreciated by such a master mind as CLAUSIUS, 

 and, indeed, he defined the entropy as the algebraic sum of the 

 transformations necessary to bring a body into its existing state. 

 Moreover, as the formula for it was in terms of other more or 

 less sensible thermodynamic quantities, its relation to these was 

 at first more readily grasped, could be represented diagrammat- 

 ically, and had to do duty for the true, but still unknown, physical 

 idea of entropy itself. It was early understood, too, that growth 

 of entropy was closely connected with the degradation or waste 

 of energy; that it was identical with the Second Law. The fre- 

 quently given, but not always valid, relation, 



led to entropy being called a factor of energy. But all these 

 were change relations and did not go to the root of the difficulty, 

 as to what constituted the physical nature of unchanged entropy. 

 Quite early, too, there was a realization of the fact that 

 entropy had somehow a statistical character, that it had to do 

 with mean values only. This was well brought out by the long 

 known, and much quoted, " demon " experiment suggested by 

 Maxwell, in which a being of superhuman power separated, 

 without doing any work, the colder and hotter particles of a gas, 

 thus effecting an apparent violation of the Second Law. This, to 

 be sure, was getting close to the crux of the whole matter, but 

 still lacked much to give entropy a precise physical meaning. 

 Nevertheless, we see here a notable approach to the fundamental 



1 This relation is not a valid one, unless the external work performed by a gas 

 during its change is equal to pdV. 



