190 Albert P. Mathews 



I have also calculated the value of M 2 K and the quotient 

 C of a number of other substances from the surface tension 

 measurements of Ramsay and Shields. From the surface 

 tension, the critical temperature may be calculated approxi- 

 mately by the rule of Botvos, revised by Ramsay and Shields, 

 namely, K(T C T 6) =- S(M^) 2/3 , where S is the surface 

 tension in dynes at the absolute temperature T and Mi; the 

 molecular volume. Having thus found T the fraction T/T C 

 may be calculated for any temperature and by interpolation 

 from the curve expressing the relation between T/T C and V/V C , 

 V/V C may be found and from this V c may be calculated if 

 V is known. We thus obtain the theoretical critical volume. 

 From V c and T c the value of M 2 K may be calculated either 

 by the surface tension formula or the formula M 2 K * V C T C 

 3.594 X io~ 40 . I shall, in the first place, show how exactly 

 the law correlating M 2 K with weight and valence holds for 

 some substances which do not contain chlorine but contain 

 elements of which the valence is fairly certain. Some of 

 these substances associate slightly. The results are summar- 

 ized in Table VlX 



From Table VI it may be seen that the values obtained for 

 C, although less reliable than those from the other tables, 

 nevertheless agree well with the mean of about 3 X io~ 37 . 



The substances marked associating in the above table 

 were found to be so by Dutoit and Mojoiu, 1 who determined 

 the association and calculated the mean molecular weight at 

 various temperatures. 



There are two sources of uncertainty in computing "a" 

 and M 2 K for the simplest gases; the first is the uncertainty 

 of the critical data of some of them; and the second, the un- 

 certainty that the coefficients of the formulas hitherto used 

 for calculating these values remain the same for the gases. 

 If the usual formula for "a" be used, by which "a" is calcu- 

 lated from the pressure and temperature, the formula being 

 derived from certain assumptions as to the value of the crit- 



1 Dutoit and Mojoiu: Constante de capillarite et poids moleculaire. Jour. 

 Chim. Phys., 7, 169 (1909). 



