164 Mr. Brooke on Isumorphism. 



According to this classification 1 atom of silica may com- 

 bine in eudyalite with 1 atom of soda or lime or zirconia or 

 j)rotoxide qjf iron or protoxide of manganese, without any 

 change of crystalUne form ; or it may combine with either of 

 these alone, or, according to the law of replacement or sub- 

 stitution, with several of tliem, or even all together, in any in- 

 definite proportions. In Jetstein it may combine with 1 «/o?« 

 of soda or potash. In a particular kind o^ garnet, with 1 atom 

 of lime or magnesia or protoxide of iron or protoxide of man- 

 ganese. In pyrope, with 1 atom of lime or magnesia or prot- 

 oxide of iron or oxide of chrome. Hence soda, potash, lime, 

 magnesia, protoxide of iron, protoxide of manganese, zirco7iia 

 and oxide of chrome, may mutually replace each other in 

 their combinations with 1 atom of silex, without affecting the 

 form of the crystallized compound ; and hence according to 

 the theory they are strictly isomorphous atoms. 



Paranthine is said to consist in part of 2 atoms of silex com- 

 bined with 1 of soda or lime ; and stilbite appears composed 

 in part of 3 atoms of silex combined with 1 of soda or lime. 

 But if soda and lime are isomorphous in relation to 1 or 2 or 

 3 atoms of silex, there is not any obvious reason why all 

 the other elements that are deemed isomorphous in relation to 

 1 atom should not be equally so in relation to 2 and 3 atoms; 

 and as it appears that some of these are isomorphous in re- 

 spect to alumina also, when that substance acts the part of an 

 acid and replaces the silex, as it may do according to this 

 theory, it follows that the others must also be so ; and hence 

 there is introduced a play of composition into the chemical 

 constitution of minerals, which in the happiest manner puts 

 an end to the chemical distinction of species. 



Berzelius has given the chemical constitution of some va- 

 rieties of amphibole, as follows : 



Grammatite 1 atom of tri-silicateoflime, 



and 1 hi-silicate of magnesia. 



Actinolite 1 atom of tri-silicateoflime, 



and 1 bi-silicate of magnesia, 



or of protoxide qfiro7i. 



Hornblende 1 atom of tri-silicate of lime, 



and 1 bi-aluminate of magnesia, 



or of protoxide of iron. 

 But if the doctrine of isomorphous substitution is to be 

 regarded as a general principle, these can be only particular 

 cases of a more general formula ; for it is not, I suppose, pre- 

 tended by the theorists, that atoms which are isomorphous in 



respect 



