72 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



simultaneously therefore tend to neutralize each other and if 

 they exist in the proper ratio, derivable from the bracketed quantity 

 in Eq. (31), they will completely balance each other and produce 

 no change whatever in the number of complexions while passing 

 from the initial to the final state of equilibrium, i.e., will produce 

 no change whatever in the entropy of the gas under consideration. 

 In isentropic change Nature has no preference for its various 

 states. 



The temperature-entropy diagram considers mainly thermal 

 changes, and as we have considered the influence of both of its 

 co-ordinates in the number of complexions, we can ascertain by 

 proper combination, for any reversible change of state, the corre- 

 sponding character of the change in the number of complexions. 

 It is evident, too, that in the diagram any reversible change of 

 state is equivalent, so far as the change of entropy in the one body 

 is concerned, to an isentropic change combined with an isothermal 

 change, the latter only affecting the result, so far as change in 

 nmmber of complexions is concerned. 



SECTION B 

 OF THE FUNDAMENTALLY IRREVERSIBLE PROCESSES 



If we consider only heat and mechanical phenomena and do 

 not include electrical occurrences, the irreversible processes may 

 be grouped in four classes: 



(a) The body whose changes of state are considered is in 

 contact with one or more bodies whose temperatures differ by a 

 finite amount from its own. There is here flow of heat from 

 hot to cold and the process is an irreversible one. 



(b) When the body experiences friction which develops heat 

 it is not possible to effect completely the opposite operation. 



(c) The third group includes those changes of state in which 

 a body expands without at the same time developing an amount 

 of external energy which is exactly equal to the work of its elastic 



