94 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



of entirely like molecules, and consequently it also does not here 

 matter whether in the course of the prospective changes of state 

 it experiences chemical transformation. . . . When the substance 

 is stationary the whole energy of the system will consist of the 

 so-called ' inner ' energy 27, which depends only on the mass and 

 inner constitution of the substance, which constitution is conditioned 

 by the temperature and density." 



(10) Let us suppose that with such a homogeneous body there 

 is conducted a certain reversible or irreversible cycle process which 

 therefore brings the body exactly back again to its initial con- 

 dition. Let the external influences on the body consist in the 

 performance of work and in heat supply or withdrawal, which 

 heat exchange is to be effected by any number of suitable heat 

 reservoirs. At the end of the process no changes remain in the 

 body itself, only the heat reservoirs have altered their state. Now 

 let us suppose the heat carriers in the reservoirs to be composed 

 of purely ideal gases, which may be kept at constant volume or 

 under constant pressure, at any rate only be subject to reversible 

 changes of volume. According to the last proved proposition, the 

 sum of the entropies of all the gases cannot have become smaller, 

 for at the end of the process no changes remain in any other body, 

 not even in the body which completed the cycle process. 



(n) Let d'Q be the heat gained by the body from some reser- 

 voir in an element of time and T the temperature of the reser- 

 voir 1 at the same moment, then the change of entropy experienced 

 by the reservoir at this instant will be 



_d'Q 



- T , 



and in the whole course of time all the reservoirs together will 

 experience the entropy change 



It does not here matter what the temperature of the body is at this instant. 



