POLYMORPHISM 



547 



others the relative stability altered as a certain temperature (the 

 transition temperature) was passed through. The former class 

 he termed monotropic, the latter enantiotropic. 



Lehmann further recognised that the transition point of 

 enantiotropic polymorphs could be overstepped in either 

 direction, since the rate of change diminishes as that point is 

 approached. As the temperature falls below this, however, 

 the rate of change soon reaches a maximum value, after which 

 a diminution follows which can end in an apparently complete 



-Z 



Temperature 



Fig. i. 



cessation of transformation. This he connected with a diminu- 

 tion in the molecular mobility at the lower temperature. 



The explanation of monotropy and enantiotropy generally 

 accepted at the present time was given by Ostwald. He re- 

 garded both as due to the same cause, and concluded that in 

 enantiotropic modifications the melting points were above the 

 temperature of transformation and in monotropic modifications 

 below it, one modification being, therefore, always unstable, 

 since its region of stability lies above its melting-point, and so 

 cannot under ordinary conditions be realised . This is expressed 

 graphically in fig. i . 



