AND OF THE SECOND LAW 71 



as we please) than the initial state. As it is here equally likely 

 that a particular molecule will be found in any one of these unit 

 volumes, it is evident that the increase of volume will add increased 

 variety to the location or configuration of the molecules and by 

 indulging in the swapping of places inherent in the production of 

 complexions, we see that said increment of volume will make 

 the number of complexions in the final state greater than in the 

 initial state, which in turn means that the entropy in the final 

 state is also greater. This accords with experience, but it can 

 also be seen from the formula 



Entropy S= constant +7Vflog f *7+log F], . . (31) 



derived by PLANCK (p. 63 of Vori. ii. Theor. Physik), from 

 probability considerations, for the state of thermal equilibrium. 

 Here k is the universal constant (see p. 66) and the other terms 

 have the same meaning as before. 



Isentropic Change 



The last reversible process, to be here physically interpreted, 

 is isentropic change from the initial state of thermal equilibrium 

 to its final state. Evidently only the physical elements under- 

 lying the bracketed term in Eq. (31) need to be considered. 



As we are considering isentropic change (dS=o), it does not 

 make any difference whether on the one hand we think of this 

 isentropic change as accompanied by an increase in temperature 

 and decrease in volume, or on the other hand think of said change 

 as taking place with decrease of temperature and increase of 

 volume. Suppose we assume the latter kind of change. Then 

 from what has preceded we know that increase of volume by 

 itself would increase the number of complexions of the final 

 state, also, from what has gone before, we know that the drop 

 in temperature by itself will lead to decrease in the number of 

 complexions in the final state. These two influences acting 



