﻿146 Prof. A. W. Porter : Variation of Triple-Point 



the value of this latter quantity for each stage. We will call 

 the separate values Si, S 2 , S 3 , S 4 . 



i. Move the pistons A and B from left to right so that 

 one gram of liquid evaporates at constant pressure. 



Si=0 because the pressures p and ir keep constant, 

 ii. Raise the temperature an amount ^T. This change 

 increases the pressure of liquid and vapour bv ^LdT and 

 ^fdT respectively. 



[Note. Part of the change in it will be consequent on the 

 change in temperature (p constant) and part on the change 

 of p itself.] 



If U and V were the initial volumes of the liquid and 

 vapour, and u and i\ their specific volumes, the volumes 

 during stage ii. are U— u and Y-j-v w respectively. 



We have therefore 



1 v v p) ^>j< v J ~>fY. 



iii. Push the vapour back into the liquid at the constant 

 temperature T + <iT and the constant pressures it + S. dT 

 and^+ <£n^T respectively. 



S 3 =0 because the pressures keep constant. 



iv. Lower the temperature by the amount dT. 



Now the heat given out at the higher temperature is L, 

 the latent heat of vaporization. Hence 



%(pdv) _ dT 

 -L ~ T 



or 





The cycle here employed can of course be exhibited 

 graphically. It is best to employ two diagrams — one for 

 the liquid and the other for the vapour. 



