58 PHYSICAL PROPERTIES AND COMPOSITION 



temperatures P' k P" k and T' k T" k , then for two correspond- 

 ing temperatures near together we have the following 

 states : 



For the first body 



al* k and 

 (a + da)P' k 



and for the second, corresponding : 

 aP" k and 



so that in each case 



dP m dT_ da m d,p 

 T* * T ~ * rt 



In so far as boiling points are corresponding tempera- 

 tures, according to Guldberg (p. 36) we have for them 



jL = constant, 



a rule long since given by Deprez and Trouton in the form, 

 that the quotient of the molecular latent heat by the abso- 

 lute boiling point is constant. 



The following numbers may serve as confirmation of 

 this : 



Substance 



The deviations for abnormal molecular weight are im- 

 portant. 



If, as with acetic acid, the saturated vapour is of ab- 

 normally high molecular weight, the relation 



