LAW OF THE ANGLE, ETC. 247 



vicarious elements, it was of small value. Perhaps 

 no one was more capable than Berzelius of turn- 

 ing 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 properties com- 

 mon ;" 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 (calc-spar,) of magnesia, 

 of protoxide of iron, and of protoxide of manganese, 

 agree in many respects of form, while the homolo- 

 gous 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. 



2 Essay on the Theory of Chemical Proportions, p. 122. 



