The Internal Pressures of Liquids 621 



A = C(d 1/3 D!/ 3 ). The value of C was the mean C taken 

 from Mills' recent paper. These values of the internal latent 

 heat are very similar to those obtained by Dieterici's formula 

 >i = CRT/wd/D. If the internal latent heat is computed 

 using the vapor pressures computed by Biot's formula close 

 to the critical temperature too low values are obtained as 

 Mills has pointed out. 



The values for "a" for one gram mol computed from near 

 the critical temperature are given in Table 7. Column 2 of 

 that table shows how many degrees below the critical tempera- 

 ture data were taken for the computation. The nearer the 

 critical temperature the more reliable the data should be. 



(31) a = M(X fi)l(d r d v ). 



It is interesting now to see how large /* is relative to ^ at 

 different temperatures; that is, how much of the heat of vapor- 

 ization is rendered latent by the expansion of the molecules, 

 or by an increase in their rotatory energy. By formula (31) 

 a = M(J fi)/(d l d v )\ and by (29) a = (S 2 S + 2)P C V'A 

 (S 2) . Hence we have 



(32) v^ ft = (S 2 S + 2)P C VX ^)/M(S 2). 

 But it was found by Mills that X = C(d? /3 < /3 ) so that 



(33) fi = C(d 1 /* < /3 > (S 2 S + 2)PcVX dj /M(S 2) . 

 The calculation of / for one gram mol of pentane by formula 



(33) resulted as follows : 



At 30, therefore, the total latent heat for one gram is 

 85.76 cals; the total internal latent heat, or A, is 78.80 cals.; 



