1904] Mills — Molecular Attraction. 161 



Joule-Thomson energy change is in reality a latent heat — 

 the very same effect of the Crompton equation, only the com- 

 pression is not carried to liquefaction. That effect has been 

 but little studied and is usually laid entirely upon cohesive 

 forces. This may not be the case, and certainly is not the 

 case when hydrogen, which gives negative results, is consid- 

 ered. *We here point out that if it is experimentally possible, 

 a continuation and extension of the experiments of Joule and 

 Lord Kelvin in connection with the theory of Crompton, should 

 enable Crompton 1 s theory to be understood and correctly 

 modified. 



If the PV curves, Diagrams 1 to 3, be examined in connec- 

 tion with Crompton's equation, it will be noted that Cromp- 

 ton's equation usually gives good agreement with the ther- 

 modynamical results at points corresponding to the descending 

 portions of those curves. 



THE VARIATION OF THE HEAT OF VAPORIZATION WITH THE 

 TEMPERATURE. 



The discussion in this and the previous paper of the data 

 bearing upon the latent heats of vaporization obtained for 

 the twenty-one substances examined cannot have failed to 

 impress one with the wonderful accuracy of the measurements 

 by Profs. Ramsay and Young and by Prof. Young, upon 

 which that data is based. The data upon heats of vaporiza- 

 tion here made available is therefore the most extensive and 

 the most accurate yet published. The variation of the latent 

 heat with the temperature has always been a question of 

 interest, and we therefore attempt to show most clearly the 

 manner of this variation. 



The function of a portion of the latent heat is well known. 

 It is expended in overcoming the external pressure. The por- 

 tion so expended can be calculated and neither theoretically 

 nor actually does it appear to be a simple function of the 

 temperature. We have called the energy so expended E, and 



