Latent Heat and Surface Energy. 33 



Testing this expression on the assumption that — =K is 



a 

 identical with van der Waals' — 2 , and using values of a e 



derived in accordance with van der Waals' equation from 

 the critical data of the vapour, Bakker found values for A; 

 at the boiling-point of about two-thirds of the experimental 

 figures. He drew the conclusion (Zeit. physik. Chem. xii. 

 p. 670 (1893)) that a is not independent of the temperature. 

 Traube (Ann. der Physik, v. p. 555 (1901) ; viii. p. 300 

 (1902)) has attempted to calculate van der Waals'' " b " for 

 liquids by the use of the equation given by van t'Hoff 

 (Vorles. fiber Theor. u. Phys. Client, iii. p. 30) 



273 % 



^T — ^T T 1 ' 



(T c being the critical temperature) together with his theory 

 of " liquidogenic " and ; ' gasogenic " molecules. Values of 

 a T derived from b T give no better results than the ordinary 

 " critical " value a c when used in Bakker's equation. By 



combining, however, the author's equation, -~- . -j = L u 



with Bakker's equation, it is possible to calculate a value 

 for a T which can be shown to vary linearly with temperature 

 up to the critical temperature. From a knowledge of a T and 

 its temperature coefficient it becomes possible to calculate 

 latent heats with very fair accuracy right up to the critical 

 temperature. 



Referring to Bakker's equation, it is obvious that at low 

 temperatures v 2 , the volume of the vapour, is very great 

 compared with v 1? the volume of 1 gramme of liquid. We 

 may write, therefore, 



From the author's equation we have, for the latent heat 

 of vaporization of 1 gramme, 



_ 6>i 



**— S~" 



Hence a T = "^ (1) 



External latent heats of vaporization per gramme will 

 now be calculated for several liquids by the following 

 procedure : — 



(i.) A value of a T will be derived from equation (1) at 

 low temperatures. 

 Phil. Man. S. 6. Vol. 39. No. 229. Jan. 1920. D 



