Latent Heat of Vaporization of Liquids. 215 



of the liquid" minus the latent heat of polymerization of the 

 vapour is a negative quantity, i. e. heat is evolved in this 

 part of the process. Now as the temperature rises the degree 

 ol association in the liquid and vapour states will decrease 

 but not necessarily at the same rate. It is probable that the 

 vapour depolymerizes much more quickly than the liquid. 

 There must therefore be a temperature at which there is 

 equal polymerization in both the liquid and vapour phases. 



At this temperature therefore the expression T-^ 1 should hold 



good. This cannot be tested owing to the want of the 

 necessary data. 



The points which have been discussed above have consi- 

 derable bearing upon the Trouton constant itself. Thus, it 



ML 



would seem that if one wishes to make the expression -^p- 



really comparable for all substances, it should be made 

 independent of the heat required to cause depolymerization 

 of the molecules. One might perhaps accomplish this by 



employing the expression - 3- 1 instead of L, and write the 



Trouton constant in the form 



— — l = const., 



where M is the theoretical molecular weight and the other 

 quantities hold for the boiling-point of the liquid. In this 

 case, for normal substances the Trouton constant has its usual 

 value. For the alcohols, however, the value is only about 13, 

 and for acetic acid it is about 26. The constant is therefore 

 not any more general in its character than before ; and it is 

 doubtful if the above changes possess any advantage over 

 the usual method of stating this semi-empiric law. It should 

 be pointed out also that the actual values one obtains for the 

 compressibility or expansion of a polymerized liquid must 

 depend to a certain extent on the actual polymerization 

 existent in the liquid ; for it has been established that there 

 is definite equilibrium between simple and polymerized 

 molecules in the liquid state, this equilibrium being shifted 

 by external changes, such as temperature or pressure, in 

 accordance with the principle of Le Chatelier. Thus on 

 increasing the pressure the system will change in the 

 direction corresponding to decrease in volume, so that the 

 observed change of compressibilitv of a polymerized liquid 



T2 



