TRANSITION POINT 



135 



stable form, refers to the lower temperature. Thus, 

 e. g., benzophenone in the stable form melts at 48, in the 

 meta-stable at 26. 



C. Possibility of a Transition Temperature. 



As in all condensed systems, if a transition takes place 

 between polymorphic modifications it must be carried out 

 completely at one temperature. 

 The direction in which it takes 

 place may, however, depend 

 on temperature, as may be 

 seen from the vapour pressure 

 curves. If the curves I and 

 II for the two modifications 

 show a point of intersection 

 A (Fig. 26), then below the 

 temperature T A corresponding 

 to it the modification to 

 which II refers will be formed 



Fig. 25. 



from the other: above that temperature the state of 

 things is reversed, and consequently on passing that point 



Fig. 26. 



Fig. 27. 



a total conversion takes place, as is the case with 

 rhombic sulphur at 95-6, when it is converted into mono- 

 symmetric. Lehmann has proposed to call phases such 



