FUNDAMENTAL EQUATIONS OF IDEAL GASES 383 



and temperature which prompted the supposition of a change in 

 molecular species, and the measurements of density were then 

 used as confirmatory evidence to establish the fact of the con- 

 version of NO2 into colorless N2O4 as the pressure increased or 

 the temperature diminished. 



The assumption has often been made that the departure of 

 gases from the ideal state is to be ascribed generally to the 

 tendency to polymerization. The same idea appeared later in 

 modified form in the attempt to explain all departures from Van 

 der Waals' equation as due to an association collapse of the 

 molecular system, and again in the idea that the formation of 

 the liquid phase was conditioned upon such a collapse. It is 

 clear however that a distinct molecular species of the associated 

 type such as (N02)2 occurs comparatively rarely, and that the 

 formation of the liquid phase and the departure of gases from 

 the ideal state must in general be ascribed to quite different 

 causes. 



The case of convertible components offers one point of 

 contrast with that of chemically related components, for the 

 latter is as a rule subject to passive resistance (Gibbs, I, 58) 

 whereas the former appears not to be limited in the rapidity 

 with which the ratio of the molecular species can adjust itself to 

 follow the fluctuations of pressure and temperature.^'' 



The test, that equation [309] be applicable to the case of con- 

 vertible components, rests on its successful application in inter- 

 preting the densities of N2O4 observed under various conditions 

 of temperature and pressure. Admittedly the dissociation of 

 the latter substance into two molecules, and similar chemical 

 reactions, form ideal examples to which the thermodynamic 

 principles of chemical interaction may be expected to apply. 

 Reactions of this class in the gaseous phase appear to be free 

 from the effects of passive resistance and are subject unquestion- 

 ably to the conditions of equilibrium discussed by Gibbs from 

 page 56 on. They present a problem exemplifying a wide 

 range of the interpretative possibilities latent in thermody- 

 namics. 



Evidently it is difficult to provide specific heat data to use in 

 the reaction equation (114) [309] since the freedom of con- 



