for 



Latent Heat of Vaporization of Liquids. 271 



in this case we are equating the expression T ^~ to the 



atent heat of vaporization per unit volume of vapour 

 (approximately), while in the former case the latent heat 

 referred to unit volume of liquid. The distinction really 

 hino-es on the fact that the system to which we have applied 

 van der Waals' equation is the liquid one alone, and the 



expression ^- refers to the liquid state. That this may be a 



magnitude of quite a different order from the corresponding 

 term for the saturated vapour is seen by a glance at the 

 p.v. diagram for the liquid vapour system in which the 

 dotted lines indicate the quantities referred to *. 



Liquid 



fa POUR 



In the following table the Trouton constant for the sub- 

 stances examined is also included, as there appears to be a 

 close connexion between the two expressions as regards their 

 range of applicability. 



* The connexion 



between P^*\ and fi P ™»<>A 



( C^ffvapour X v va 

 dT A'^k 



/5ff liquid \ 



v dT ; 



'liquid 



