Determining "a" of van der Waals' Equation 161 



The value of 21.92 thus obtained for P C V C /T C corresponds 

 with the value determined from Young's very careful obser- 

 vations on the critical data of some non-associating substances. 



The values obtained for M 2 K by the use of the foregoing 

 formulas, the surface tension formula, and the ordinary for- 

 mula, when applied to the critical data of the substances so 

 carefully investigated by Young 1 are given in Table I. 



The value of "a" in atmospheres for i cc of a gas under 

 standard conditions of pressure and temperature may be 

 obtained from any of the values in this table by multiplying 

 by 7-573 X io 32 . 



A comparison of columns 3 and 4 of Table I will show 

 that the values obtained by the surface tension formula, while 

 on the whole closely similar to those obtained by the ordinary 

 formula given in column 4, differ, nevertheless, in some in- 

 stances quite markedly from them. The values of column 3, 

 on the other hand, are almost identical with those of column 

 i. Owing to the fact that there is less uncertainty about the 

 factors used in computing "a" from the surface tension than 

 from the usual formula, I believe the surface tension values 

 are to be preferred. If the coefficient in the surface tension 

 formula had been taken as 2.12 instead of 2.19, all the values 

 of column 3 would need to be reduced proportionally, and the 

 values in columns i and 2 would also be lower. That the values 

 of "a" computed in this way from the surface tension are the 

 more accurate is indicated also by the fact that these values 

 exhibit most clearly and with fewest exceptions the relation 

 of cohesion to molecular weight and the number of valences 

 in the molecule, as is set forth in a subsequent paper. 



University of Chicago 



1 Young: "The Vapor Pressures, Specific Volumes, Heats of Vaporiza- 

 tion and Critical Constants of 30 Pure Substances," Proc. Roy. Dublin Soc., 

 12, 374-443 (1910). 



