245 



CHAPTER VI. 



CORRECTION OF THE LAW OF THE SAME ANGLE FOR 

 THE SAME SUBSTANCE. 



ISCO VER Y of Isomorphism. Mitscherlich. 

 The discovery of which we now have to 

 speak may appear at first sight too large to be 

 included in the history of crystallography, and may 

 seem to belong rather to chemistry. But it is to be 

 recollected that crystallography, from the time of its 

 first assuming importance in the hands of Haiiy, 

 founded its claim to notice entirely upon its con- 

 nexion with chemistry ; crystalline forms were pro- 

 perties of something ; but what that something was, 

 and how it might be modified without becoming 

 something else, no crystallographer could venture to 

 decide, without the aid of chemical analysis. Haiiy 

 had assumed, as the general result of his researches, 

 that the same chemical elements, combined in the 

 same proportions, would always exhibit the same 

 crystalline form; and reciprocally, that the same 

 form and angles (except in the obvious case of the 

 tessular system, in which the angles are determined 

 by its being the tessular system,) implied the same 

 chemical constitution. But this dogma could only 

 be considered as an approximate conjecture; for 

 there were many glaring and unexplained excep- 



