LAW OF THE ANGLE, ETC. 335 



Chemistry and Mineralogy, he again draws attention to his term 

 vicarious (vicarirende), which undoubtedly expresses the nature of the 

 general law afterwards established by Mitscherlich in 1822. 



But Fuchs's conjectural expression was only a prelude to Mitscher- 

 lich's experimental discovery of isomorphism. Till many careful 

 analyses had given substance and signification to this conception of 

 vicarious elements, it was of small value. Perhaps no one was more 

 capable than Berzelius of turning to the best advantage any ideas 

 which were current in the chemical world; yet we find him, 2 in 1820, 

 dwelling upon a certain vague view of these cases, that " oxides 

 which contain equal doses of oxygen must have their general proper- 

 ties common ;" without tracing it to any definite conclusions. But 

 his scholar, Mitscherlich, gave this proposition a real crystallographical- 

 import. Thus he found that the carbonates of lime (calcspar,) of 

 magnesia, of protoxide of iron, and of protoxide of manganese, agree 

 in many respects of form, while the homologous angles vary through 

 one or two degrees only ; so again the carbonates of baryta, strontia, 

 lead, and lime (arragonite), agree nearly ; the different kinds of felspar 

 vary only by the substitution of one alkali for another ; the phosphates 

 are almost identical with the arseniates of several bases. These, and 

 similar results, were expressed by saying that, in such cases, the 

 bases, lime, protoxide of iron, and the rest, are isomorphous ; or in 

 the latter instance, that the arsenic and phosphoric acids are isomor- 

 phous. 



Since, in some of these cases, the substitution of one element of the 

 isomorphous group for another does alter the angle, though slightly, it 

 has since been proposed to call such groups plesiomorphous. 



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 metamorphose the existing chemical 

 systems of arrangement. But even those crystallographers and chemist.- 

 who cared little for general systems of classification, received a pow- 

 erful impulse by the expectation, which was now excited, of discover- 

 ing definite laws connecting chemical constitution with crystalline form, 

 Such investigations were soon carried on with great activity. Thus, 

 at a recent period, Abich analysed a number of tessular minerals, 

 spinelle, pleonaste, gahnite, franklinite, and chromic iron oxide ; and 



Munich, 1820. 2 Essay on the Theory of Chenvcal Proportions, p. 122. 



