The Internal Pressures of Liquids 619 



This formula gives remarkably constant results for the cal- 

 culation clear to the critical temperature from nearly the 

 point of solidification. The results are in agreement with 

 other methods of calculating "a" already given or to be de- 

 scribed. The density values for all except oxygen are those 

 of Timmermans or Young. 



3. Computation of "a" from van der Waals' Equation at 

 the Critical Temperature assuming that b c = 2V = 2Vc/S % 



The values for "a" computed by the preceding methods 

 from the surface tension agree with those computed from van 

 der Waals' equation at the critical temperature assuming that 

 b c , the co- volume, or the real volume of the molecules, is always 

 in all normal substances just twice the volume at absolute zero, 

 or that b c == 2V . As the volume at absolute zero is equal to 

 the critical volume divided by S, where S == RT C /V C P C , 

 b c = 2V C /S. Applying this to the equation, since V C P C S = 

 RT, we have 



(28) a = (S 2 S + 2)T'R 2 /(S 2 (S 2)P e ); or 



(29) a = (S 2 -S + 2)P c v;/(S-2). 



Formula (28) which corresponds to van der Waals' formula: 

 a --= 2yT'/(64 X 273* X P c ), has been used in computing 

 "a" in Table 8. The values thus obtained are practically 

 identical with those computed from the surface tension. 



Formula (29) has already been obtained as (18). 



4. The Computation of "a" from the Internal Latent Heat 



of Vaporization 



That the foregoing values of "a" are correct is proved by 

 their agreement with the values computed from the internal 

 latent heat of vaporization close to the critical temperature. 



If all the internal latent heat, I, was used in overcoming 

 molecular cohesion, then the equation should hold 



(30) X = L E = a(ijV,~ i/V w ), 



where L is the total latent heat and B the external work. 

 This equation does not hold except close to the critical tern- 



