246 HISTORY OF MINERALOGY. 



tions to it. The explanation of several of these was 

 beautifully described by the discovery 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 speak- 

 ing 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 pro- 

 portions, and on the other, to avoid unnecessarily 

 splitting up genera." In a lecture On the Mutual 

 Influence of Chemistry and Mineralogy 1 , he again 

 draws attention to his term vicarious (vicarirende,} 

 which undoubtedly expresses the nature of the 

 general law afterwards established by Mitscherlich 

 in 1822 (N). 



But Fuchs's conjectural expression w r as only a 



prelude to Mitscherlich's experimental discovery of 



isomorphism. Till many careful analyses had given 



substance and signification to this conception of 



1 Munich, 1820. 



