Generalizations o/Van der Waals. 179 



Van der Wauls were strictly true. This relation may be 

 expressed by the equations 



v' V T p ' 



where v and v f are the molecular volumes of saturated vapour, 

 V and V those of liquid, and T and T' the boiling-points on 

 the absolute scale of any two substances at corresponding 

 pressures p and p' . 



When the critical pressures are equal p =p' and the equa- 

 tions become 



v V T 



v' V T ' 



or the molecular volumes, whether of liquid or saturated 

 vapour, at the boiling-points under equal pressure are pro- 

 portional to the boiling-points expressed on the absolute scale. 



That the relation cannot be strictly true when the critical 

 pressures are different is obvious, since it is not generally true 

 that the vapour-pressures at corresponding temperatures are 

 corresponding pressures. It is still possible, however, that 

 the relation may be true for the critical temperatures, pres- 

 sures, and volumes. Unfortunately, the critical volumes 

 have only been determined in a few cases, but it has been 

 shown that the ratios of the molecular volumes at correspond- 

 ing pressures do not vary within very wide limits ; and it is, 

 therefore, possible to test the relation in a limited form, taking 

 the critical temperatures and pressures and the mean ratios of 

 the molecular volumes at corresponding pressures. 



A preliminary comparison was given in the paper referred 

 to, but it was very incomplete, and there was a small error in 

 some of the calculations owing to the misplacement of a figure 

 in the critical pressure of fluorbenzene, which was given as 

 33190 instead of 33910. 



As the experimental results have been considerably extended 

 since that time and now include the molecular volumes of the 

 saturated vapours ; it is possible to give the Table in a com- 

 plete form. 



