136 MOLECULAR GROUPING 



as those of sulphur, for which there is a transition tem- 

 perature, enantiotropic ; the others, for which the reaction 

 will go in one sense only, such as benzophenone, mono- 

 tropic. The latter phenomenon may obviously result from 

 the transition point lying above the melting points of 

 both modifications ; thus Fig. 27 represents enantiotropy, 

 Fig. 25 monotropy ; in each figure curves I and II refer 

 to the vapour pressure of the two solid phases, III to that 

 of the liquid. 



D. When there is a Transition Temperature, the Modi- 

 fication stable at Low Temperatures is formed from 

 the other with Evolution of Heat. 



The relations of vapour pressure, as shown in Fig. 26, 

 give this conclusion at once, with the aid of the second law, 

 in the form 



Here V is the increase of volume on evaporation, i. e. prac- 

 tically the volume of the vapour; at the transition point 

 this is the same for the two modifications, since they have 

 the same vapour pressure. Further, as the figure shows : 



dP n > dP l9 



where II is the modification stable at low temperatures ; 

 therefore 



q n > q l or q n - q, = R 

 is positive. 



But now, since q n and g x are the latent heats of evapo- 

 ration of the stable and meta-stable forms respectively, 

 q u q l is the heat evolved on formation of the former 

 from the latter ; consequently the form stable at low tem- 

 peratures is produced from the other with evolution 

 of heat. 



This law is very well illustrated by the researches of 



