1904] Mills — Molecular Attraction. 159 



in temperature the results from Crompton's equation become 

 decidedly the lower, but near the critical temperature the 

 difference is not very marked. 



With acetic acid, Crompton's equation gives results more 

 than twice too large at the lowest temperature. As the tem- 

 perature is raised the disagreement becomes continually less. 

 It must be borne in mind that acetic acid vapor shows marked 

 association. 



Comparing- the three equations, the value of Crompton's 

 theory becomes doubly apparent. Crompton's equation does 

 not involve the vapor pressure and therefore, if trustworthy, 

 will act as a check upon the thermodynamical equation at the 

 end points of the curve, where, owing to the manner of 



S P 

 obtaining the y^-, the thermodynamical results are some- 

 what uncertain. On the other hand compared with equation 

 4, Crompton's theory does not depend upon the attraction and 

 would not be affected by a variation of the attraction with 

 the temperature. It should therefore furnish a clue to those 

 substances in which the molecular attraction does not remain 

 constant with increasing temperature. 



At low temperatures Crompton's equation gives values uni- 

 formly too high and it therefore cannot be used to check the 

 results of the other equations. But at the highest tempera- 

 tures the evidence obtained from the results is exceedingly 

 interesting. For ether, di-isopropryl, isopentane, normal 

 pentane, normal hexane, benzene, hexamethylene, fluo-ben- 

 zene, and carbon tetrachloride, Crompton's equation gives 

 results in better accord with the values obtained from equa- 

 tion 4 than with the thermodynamical results. For these 

 substances at the highest temperatures considered, Cromp- 

 ton's theory gives results differing in no case from those of 

 equation 4 by so much as 0.4 of a calorie, and in several cases 

 the agreement is almost exact. This is splendid confirmation 

 of our belief that in these cases the divergence in the con- 



