70 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



Isobaric Change 



Next we interpret how the number of complexions are affected 

 by isobaric change during a reversible process, again assuming 

 that the temperature in the final state is greater than in the 

 initial one. Here the steps and the conclusion are exactly the 

 same as in the preceding case. In both cases just the opposite 

 result is reached when there is a fall in temperature. 



As the pu diagram contains the co-ordinates p, v, and repre- 

 sents mainly the mechanical changes in the body under considera- 

 tion, we can, by suitable combination, similarly interpret any 

 other reversible change of state represented in this pu diagram. 



Isothermal Change 



However, because of its general importance and because of 

 its bearing on the temperature-entropy diagram, we will here also 

 tell, in the same physical terms, what happens when our ideal 

 gas undergoes isothermal change with increase of volume. As 

 the temperature in the final state is equal to that in the initial 

 one, the quantity [w*]=%c 2 does not change and therefore C does 



not change nor (see Eq. (5) , p. 50) does the number - of mole- 

 cules possessing the velocity change. The variety of velocities 



c 



in the final state is therefore the same as in the initial state and 

 does not at all contribute to that necessary increase in the num- 

 ber of complexions (configurations) for which we are looking. 



The direction of the velocity of a molecule would be another 

 variety element, but as the final volume evidently possesses as 

 many velocity directions as the initial volume, this element or 

 co-ordinate will not contribute to increased complexity in the 

 final state. But, as the volume has increased, the final state 

 will contain more unit volumes (and these can be taken as small 



