168 Mr. Brooke on Isomorphism. 



Thus if the primary rhomboid of carbonate of lime and mag- 

 nesia, consisting of 1 atom of each, measure 106° 15' 



And if carbonate of lime measure 105° 5' 



Carbonate of magnesia must measure 107 25 



together 212 30 



the half, or 106° 15', being the angle of the compound. 



Again, if a mineral contain 10 atoms of carbonate of lime, 

 and 1 of carbonate of magnesia, its angle may, according to 

 this theory, be found as follows : 



10 times 105° 5', the angle of carb. of lime, is 1050° 50' 



once ... 107 25, the assumed angle of car-^ . ^ ^- 



bonate of magnesia J 



together 1158 15 



of which 1-1 1th, or 105° 17' 4-3", should be the angle of the 

 compound mineral. 



But this theory is at variance with accurately measured cry- 

 stals, and has doubtless arisen from some error in M. Beudant's 

 experiments which has escaped his notice. 



A mineral from Zillerthal named Breiinnerite may be ad- 

 duced as an instance of disagreement with M. Beudant's 

 theory. The mineral was analysed by myself, and afterwards 

 by Stromeyer, and found to consist of 



Carbonate of magnesia 86 or 9 atoms 1 , 



Carbonate of iron 14 or 1 atom j ' ^* 



If the angle of carbonate of magnesia be, as before given, 

 107° 25', that of carbonate of iron being known to be 107°, 

 the angle of the compound must obviously, according to M. 

 Beudant's theory, be less than 107° 25', whereas it has been 

 found by repeated measurements to be 107° 30'.* 



But the theory of isomorphous substitution appears also to 

 be at variance with itself by affording contradictory results. 



Thus it is said by Berzelius that paranthine may be com- 

 posed of bi-siiicate of lime and silicate of alumina, or bi-sili- 

 cate of soda and silicate of alumina. Hence these two com- 

 pounds are regarded as isomorphous, and may produce 

 square p}'is?ns of paranthine. But socialite, whose crystals 

 are regular dodecahedrons, are composed as the soda pa- 

 ranthine it is said may be; and hence the two paranthine com- 

 pounds may give dissimilar forms, and the elements lime and 

 soda are and are not isomorphous. 



Again, according to the formula given by Berzelius to ex- 

 press the composition of eudyalite, zirconia being isomorphous 



• See Phil. Mag. and Annals, N.S. vol. i. p. 397.-Edit, 



wi 



ith 



