"Mr. Bvooke o?i Isomorphism. 167 



and extraneous silicates thus produced might not be cemented 

 together and cased up, as it were, by the true amphibole, in 

 the same manner as the sand in the Fontainbleau mineral : 

 and if this were the case, the result of analysis would be such 

 as it now appears, approaching more or less nearly to definite 

 composition, as the different foreign ingredients happened to 

 be present in quantities more or less nearly corresponding to 

 definite proportions. 



But the theory of isomorphous substitution appears to 

 involve the much more general chemical law, that any indefi- 

 nite number of binary compounds may combine together in all 

 possible proportions ; that 1 atom of silicate of lime may, not 

 merely mix, but combine with 100 or 1000 atoms of silicate of 

 soda, together with as many of silicate of potash, and a dozen 

 other substances. For the doctrine of substitution implies, 

 that if 1000 atoms of silex should exist in circumstances fa- 

 vourable to the production of a compound mineral, and that 

 in the formation of such mineral there should be required 

 1000 atoms of lime to be combined with the silex, and only 

 999 atoms should be present, these would, according to the 

 existing doctrines of chemistry, combine with 999 parts of the 

 silex, and the remaining part would combine with a particle 

 of magnesia or iron, or some other matter supplied from the 

 other elements surrounding it. But this separate binary atom 

 must, according to the new theory, become at the same time 

 chemically associated with the 999 parts of silicate of lime ; for 

 it would otherwise be simply mixed, as any other foreign 

 matter accidentally present might be, and would not then be 

 a constituent part of the mineral, as the theory of substitu- 

 tion requires. 



But an inquiry here presents itself, whether, if the differing 

 atom be only plesiomorphous, it may still act as the substitute 

 for the atom of lime; and if it may, would the crystal so formed 

 be isomoiphous, or only plesiomorjjhous. 



M. Beudant appears to adopt the theory of isomorphism, in 

 its s/77c^ sense, in those compounds in which silica and alumina 

 act the part of acids; but in relation to certain carbonates, he 

 assumes the theory of plesiomorphism, and ingrafts upon it 

 a new theory of his own, relative to the dependence of the 

 angle of the plesiomorphous crystal upon the number of its 

 substituted or replaced atoms. He says, in page GO of his 

 treatise on Mineralogy, that where the carbonates of lime, 

 iron, or magnesia exist togetlier in a mineral in any propor- 

 tions, the angle of its primary form is an arithmetical mean 

 of the angles of the primary forms of its con)ponent parts. 



Thus 



