in Reply to Mr. Brooke. 411 



that in some specimens of arragonite a portion of strontia enters 

 into the composition of the mineral ; which is what we might 

 expect, carbonate of strontia having a plesiomorphous relation 

 to arragonite. 



In other cases, such as those adduced by Mr. Brooke, of 

 paranthine and sodalite ; of eudialyte, zircon, and olivine ; it is 

 very far from being clearly established, as yet, that the che- 

 mical formulse of these groups can be made respectively iden- 

 tical, even by the substitution of isomorphous elements ; and 

 it certainly has not yet been proved that iii these cases the 

 elements are isomorphous. If there be really dimorphous 

 minerals of this kind, the observations of the last paragraph 

 may be applied to them : at present we do not appear to pos- 

 sess a sufficient number of good analyses of these species, to be 

 able to determine how far they are examples of the theory. 

 But certainly the theory of accidental mixture cannot possibly 

 explain any difficulties of this kind, which the theory of isomor- 

 phism will not explain at least as well. 



I am therefore surprised that Mr. Brooke should have 

 brought forwards such cases as objections to the isomorphous 

 theory. But I am still more surprised that he should have 

 brought forwards as objections the differences which occur in 

 other cases, where the very essence of the theory requires that 

 there should be a difference, and when the theory could not 

 stand a moment if there were none. " Arsenic," he says, 

 " combines with sulphur in two different proportions, and 

 producing different primary forms. Hence the proportion of 

 a common element, and therefore an isomorphous one, occa- 

 sions a change, even in the system of crystalHzation." No 

 doubt the change of the proportion of an element occasions 

 a change in the crystallization : we should have little chance 

 of finding any connection between the elements and the cry- 

 stallization if it were not so : as + 2s (realgar) and as + 3s 

 (orpiment) have different chemical formulae, and accordingly 

 we find they have a different system of crystallization. How 

 Mr. Brooke conceives that the theory of isomorphism can 

 identify these two compounds, I cannot understand. If, when 

 he says, " a common element, and therefore an isomorphous 

 one," he means that two atoms of an element are isomorphous 

 with three, he certainly supposes what no advocate of isomor- 

 phism ever dreamt of. 



In like manner I cannot see how the isomorphous theory 

 offers any ground for the expectation, of which he speaks, 

 that the siilpliuret of silver, the sulphuret of copper, and the 

 sulphuret of bismuth sliould present similar forms. For it is 

 not asserted, so Itir as I am aware, either that the number of 

 •i Ci 2 atoms 



