POLYMORPHISM 559 



particular modification into the super-cooled fusion always leads 

 to a complete solidification into that modification. In this way, 

 then, any one of the three acids can be transformed into either 

 of the remaining two. Transformation of the two lower melting 

 acids into that of higher melting-point may also be effected by 

 inoculation without previous melting, the progress of trans- 

 formation being visible by the growing opacity of the clear 

 crystals or under the polarising microscope by the change of 

 interference. The acid melting at 42 ° may be directly trans- 

 formed into the acid melting at 68° or indirectly by changing it 

 into the acid melting at 5 8°, and subsequently transforming this 

 so that the 68° acid may be regarded as stable, the 5 8° as meta- 

 stable, and the 42 ° labile. 



The three acids are therefore to be regarded as polymorphic 

 forms of the trimorphic cis. cinnamic acid. 



When a polymorphic compound, monotropic or enantio- 

 tropic, separates from solution, the unstable and not the stable 

 form almost invariably appears and subsequently transforms 

 more or less rapidly into the stable form. The frequency with 

 which such unstable forms appear when substances of the most 

 diverse compositions separate from solution leads to the con- 

 clusion that polymorphy is a general property of crystalline 

 matter as universal as condensation and freezing. It may be 

 stated generally, therefore, that matter is capable of existing 

 in the gaseous, the liquid, and in various polymorphic crystalline 

 states, and it is safe to predict that unstable forms of most 

 crystalline solids could be obtained if they were sufficiently 

 closely studied during crystallisation under different conditions. 



