Law of the Rectilinear Diameter. 421 



This equation enables it to be calculated from the known 



values of u c , a, and R. Substituting for a its value ^ r 



we get " c *■ 



4R 

 7T=— (2T«-T) 



SRT 



?(-a • . . . w 



The value of ir at the absolute zero is thus given by 



£~RT 



-. If Young's value of the ratio of the ideal to the 



actual critical volume is taken this reduces at once £q 

 29'6P C . Apparently it should vanish at the temperature 

 T = 2T C . The formula can, however, only be approximate as 

 p approaches P c . An estimate of the error can be obtained 

 by placing T = T C and assuming it to include the external 

 pressure (which is not negligible under the conditions which 



TCrV, 



would then hold) . We then obtain TjTfr = 4, while the 

 experimental value is 3' 7. - c 



From (14) the temperature variation of it is at once 

 obtained. Thus 



dw 4R nKy 



= -f • • • • ? . (16) 



= -U'S^. (17) 



The relation expressed in (17) is obtained from (15) by means 

 of the equation (8). 



If (15) is divided by (13) we obtain 



-.^=-«. . . . . , . (18) 



This is a very simple relation. The above values of it and -~ 



ct_L 



are quite new and give the means cf calculating them for any 

 liquid for which the critical data. are available. In eases for 

 wj-rich v c is not given, equation (13) can, by using (8), be put 

 in the form 



^ = 1^S— p- ...... (19) 



PUl Mag, S. 6, Vol. 24. No. 141. Sept. 1912 ? 2 F 



