﻿of a Substance with Hydrostatic Pressure. 



147 



The cycles for the two will be traversed opposite ways 

 because when the total volume of the liquid diminishes 



Fijr. 2. 



Y 



u 



£. 



Liackici Vabocvr 



that of the vapour increases. The final equation can be 

 obtained almost by inspection. The sense in which the 

 cycles have been traversed happens to be that in which the 

 heat taken in at the higher temperature is — L. They could 

 equally well be taken with their directions both reversed, 

 the sign o£ the heat entry being reversed simultaneously. 

 The ordinary equation is obtained from this by taking p as 

 the pressure of the vapour alone, for it corresponds to the 

 case in which the liquid and vapour are in contact without 

 any intervening semi-permeable membrane. Both are then 

 at the same pressure, at any rate when capillary and other 

 special surface forces are absent. 



For this ordinary case ^.4 a ^ so ec l ua ls ^m, and con- 

 sequently 



This equation is of course exact for the special circum- 

 stances to which it relates; it is not the general equation, 

 however. 



Another special case is that in which the pressure p of the 

 liquid is kept constant throughout the cycle. This can be 

 done ; for the variations of p can be made independently of T, 

 In this special case we obtain 



The meaning of L in all these expressions is absolutely the 

 same ; but it is expressed in slightly different terms. 



Let now a diagram be made in which it is these values of tt, 

 corresponding to different temperatures but to precisely the 

 same hydrostatic pressure, which are plotted against the 

 temperature. There will be two such curves, one for the 



L2 



L = T- 



