Mr. Brooke's Additional Remarks on Isomorphism. 4<25 



they belong, the last which ought to be adduced as evidence 

 either of the truth or fallacy of isomorphism. 



The second paragraph to which I have alluded is as fol- 

 lows: — 



" It may be remarked, that in the case of Stilbite iBhich 

 Mr, Brooke has brought for-ward as an exception, he has over- 

 looked the presence in it of 6 atoms of water, — a circumstance 

 which constitutes a chemical ditference between that mineral 

 and paranthine." 



This however involves an entire misrepresentation of what 

 I have stated ; — unintentional I have no doubt, and occasioned 

 by hasty perusal of what I wrote. I did not, as Dr. Daubeny 

 will perceive on looking again at my paper, raise any question 

 about the form of Stilbite. I referred to Eudi/alite as an ex- 

 ample in which soda and lime are said to be isomorphous in 

 respect of 1 atom of silica ; to Paranthine, in which they are 

 said to be isomorphous in respect of 2 atoms of silica ,• and 

 to Stilbite, in which they stand in the same relation to 3 atoms 

 o{ silica. And 1 did this merely to adduce the inference that 

 all other elements which might be supposed isomorphous in 

 respect of 1 atom of silica, ought to be equally so when com- 

 bined with 2 atoms, or with 3 atoms ; and hence it was unne- 

 cessary even to consider the water or any of the other com- 

 ponent elements of either of these minerals. 



With regard to plesiomorphism, it does not appear that any 

 limit has been assigned to the difference in the plesiomorphous 

 angles. Silica and alumina are said to be isomorphous isohen 

 alumina acts the part of an acid. But as the question raised 

 is, whether the atoms o^ silica and alumina are isomorphous, — 

 unless those oi alumina differ according as this substance acts 

 the part of acid or of base, and in this case it is not easy to 

 conceive what an atom means, — silica and alumina should be 

 isomorphous generally. Now the primary form of silica, or 

 quartz, is a rhomboid of 94° 15', and that of alumina, or co- 

 rundum, also a rhomboid, but measuring 86° 5'. Hence these 

 bodies are, when they occur singly, only plesiomorphous, with 

 a difference in their angles of 8° 10', notwithstanding which 

 they are lequired by the theory to pass for strictly isomorphous 

 elements when alumina becomes a substitute for silica. When 

 however they enter into mutual combination in cijanilc, which 

 is regarded as a pure silicate of alumina, both isomorphism and 

 plesiomorphism at once vanish, and a nc^a class even of form 

 is produced, a doubly oblique prism, as irregular and remote 

 a form from the rhomboid as can be found amongst crystals. 



How these facts are to be reconciled with the new theory, 

 I must leave to the better consideration of its supporters. 

 N.S. Vol. 10. No. GO. Dec. 1831. 3 I LVI. Notices 



