248 HISTORY OF MINERALOGY. 



This discovery of isomorphism was of great 

 importance, and excited much attention among the 

 chemists of Europe. The history of its reception, 

 however, belongs, in part, to the classification of 

 minerals; for its effect was immediately to meta- 

 morphose the existing ^chemical systems of arrange- 

 ment. But even those crystallographers and che- 

 mists who cared little for general systems of clas- 

 sification, received a powerful impulse by the 

 expectation, which was now excited, of discovering 

 definite laws connecting chemical constitution with 

 crystalline form. Such investigations were soon 

 carried on with great activity. Thus at a recent 

 period, Abich analyzed a number of tessular mi- 

 nerals, spinelle, pleonaste, gahnite, franklinite, and 

 chromic iron oxide ; and seems to have had some 

 success in giving a common type to their chemical 

 formulae, as there is a common type in their crystal- 

 lization (o). 



Dimorphism. My business is, to point out the 

 connected truths which have been obtained by phi- 

 losophers, rather than insulated difficulties which 

 still stand out to perplex them. I need not, there- 

 fore, dwell on the curious cases of dimorphism; 

 cases in which the same definite chemical com- 

 pound of the same elements appears to have two 

 different forms ; thus the carbonate of lime has two 

 forms, calc-spar and' arragonite, which belong to 

 different systems of crystallization. Such facts may 



