HISTORY OF MINERALOGY 



CHAPTER VI. 



CORRECTION OF THE LAW OF THE SAME ANGLE FOR THE SAME SLB 



STANCE. 



T^ISCOVERY OF ISOMORPHISM. MITSCHERLICH. The discovery of 

 -L' which we now have to speak may appear at iirst 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 crystallo- 

 graphy, from the time of its first assuming importance in the hands 

 of Haiiy, founded its claim to notice entirely upon its connexion with 

 chemistry ; crystalline forms were properties 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. Hatty 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 exceptions to it. The 

 explanation of several of these was beautifully described by the dis- 

 covery that there are various elements which are isomorphous to each 

 other ; that is, such that one may take the place of another without 

 altering the crystalline form ; and thus the chemical composition may 

 be much changed, while the crystallographic character is undisturbed. 

 This truth had been caught sight of, probably as a guess only, by 

 Fuchs as early as 1815, In speaking of a mineral which had been 

 called Gehlenite, he says, "I hold the oxide of iron, not for an essen- 

 tial component part of this genus, but only as a vicarious element, 

 replacing so much lime. We shall find it necessary to consider the 

 results of several analyses of mineral bodies in this point of view, if 

 we wish, on the one hand, to bring them into agreement with the 

 doctrine of chemical proportions, and on the other, to avoid unneces 

 warily splitting up genera." In a lecture On the Mutual Influence of 



