166 Mr. Brooke on Isomorphism. 



in which these may be distributed in forming binary com- 

 pounds ; and hence what might appear as isomorphous sub- 

 stitution under one formula, might become foreign matter 

 under another, which equally well represents the original 

 analysis. 



Biit even if the varying results of analysis had supported 

 the new theory, it might fairly have been inquired, whether 

 the known disagreements in chemical composition could not 

 be accounted for upon any other probable supposition. 



Now the obvious one of accidental mixture, which is so 

 apparent in the Fontainbleau sandstone, might probably have 

 rendered any new theory unnecessary, — for the observation 

 and the statements of Haiiy already given might generally 

 have furnished an explanation of the disagreements in ques- 

 tion ; yet even if those disagreements could not in all cases 

 have been explained by reference to the matrix, but that the 

 mineral should be found to contain matter which is also foreign 

 to that matrix, such matter might still have been present at 

 the time the mineral was formed, and hence might have be- 

 come enveloped in it. 



But the supporters of isomorphous substitution will pro- 

 bably say, that all which might be considered foreign matter, 

 ought to appear in the mineral in indefinite quantities, and not 

 in those near approaches to definite proportions which have 

 given rise to the new theory. This argument is not however 

 really opposed to the supposition of the disagreements being 

 occasioned by mixture of foreign matter. 



I shall suppose, for the purpose of illustration only, that 

 amphibole consists essentially of a single atom of tri-silicate 

 of lime, and that all else which might be discovered by 

 analysis is accidental mixture. But it may be said, that the 

 magnesia and iron and manganese which are present in am- 

 phibole are frequently combined nearly in definite pi'oportions 

 with the silica, and perhaps as tri-silicates, affording a pre- 

 sumptive agreement in favour oi substitution. 



But if we suppose the proportion of silica present when the 

 mineral was formed to have exceeded that which was neces- 

 sary to combine with the lime in the production of our sup- 

 posed amphibole, there does not appear any ground for con- 

 cluding that the remainder of the silex might not combine in 

 definite proportions with the other matters tliat were present, 

 although foreign to the constitution of amphibole ; that the 

 same circumstances which determined tlie composition of a 

 /;7-silicate in the amphibole, might not occasion the produc- 

 tion of fre-silicates of the foreign matter ; and that the new 



and 



