THE 



PHILOSOPHICAL MAGAZINE 



AND 



ANNALS OF PHILOSOPHY. 



[NEW SERIES.] 



DECEMBER 1831. 



LI. On Isomorphism. In Reply to Mr. Brooke. By Professor 



Whewell, of Cambridge *. 

 TN the Phil. Mag. and Annals for September last, appeared 

 ■*■ some observations " on Isomorphism," by Mr. Brooke, in 

 which a very unfavourable opinion was expressed of the doc- 

 trine so designated, and of its probable effect on the pro- 

 gress of mineralogical chemistry. Everything on this subject, 

 which comes from a gentleman of such exact acquaintance 

 with minerals as Mr. Brooke possesses, is well deserving no- 

 tice ; and the high claims which have been set up in favour 

 of the theoretical and practical consequences of the doctrine 

 in question make it very desirable to ascertain distinctly how 

 far its pretensions are well founded. I am one of those who 

 look to it for the solution of many difficulties in mineralogy, 

 hitherto insurmountable, and who expect that it will lead us 

 to the true line of junction between chemistry and crystallo- 

 graphy. I shall be glad, therefore, to state my views of the 

 evidence on which the doctrine of isomorphism rests, and of 

 the validity of its proof as compared with that of the theory 

 which Mr. Brooke opposes to it. I will begin with this last. 



I. On the Tlieory of essential Ingredients and accidental Mixture. 

 The old theory of the constitution of minerals, to which 

 Mr. Brooke seems inclined to adhere in opposition to the 

 doctrine of isomorphism, is, that each mineralogical species 

 consists of certain parts which are essential ; and that the 

 varieties of composition which we find in actual specimens 

 arise from the mixture of extraneous substances with these 

 fundamental elements. Thus Mr. Brooke supposes, at least 



• Communicated by the Autlior. 

 N. S. Vol. 1 0. No. 60. Dec. 1831. 3 F as 



