448 RICE ART. K 



latent heat of change of pressure at constant temperature. An- 

 other differential equation which we require now is that for 

 the f function of Gibbs (the "free energy at constant pressure") 



dUt, P) = -v{t, p)dt + v{t, p)dp, (47) 



where 



^ = € - tri -{- pv. 



From this we derive 



dy]{t, p) dv{t, p) 



But by (46) 



dp dt 



_ dv(t,p) 



In — I 



Therefore 



a well-known relation. 

 Also, from (46), 



dp df" 



(48) 



dp 



l.= -i'^^ (49) 



(50) 



There remains one more well-known relation. 



If an infinitesimal change takes place at constant pressure, 



