344 Mr. A. B. Northcote on the Constitution of AUophanc. 



If the formulae obtained by these analyses may be considered 

 as expressing the real constitution of this mineral, it would appear 

 that it consists of a mixture of neutral silicate of alumina with a 

 variable amount of neutral hydrate ; the former mineral is not, 

 I believe, known in an isolated state, but the latter as Gibbsite 

 frequently occurs : were we acquainted with the former also, we 

 might imagine allophane to be the transition, or a scries of trans- 

 ition stages between it and Gibbsite ; this is well shown by the 

 collected formulse : — 



?{AP03.3Si03. 



T / AP03.3Si03 

 ^- \3(AP03.3HO). 



.. r AP03.3Si03 



^^- \4(AP03.3HO). 



.XX / AF03.3Si03 



^^^* \5(A12 03.3HO). 



f AP03.3Si03 



jy I 4(AF03.3H0) 



■ \ |^r.g}3H0. 



Gibbsite. {AP03.3HO. 



What relation the specimens formerly examined by various 

 analysts bear to those now described, cannot, unfortunately, be 

 ascertained, since in one case only can I find a sample which 

 had been dried at 100° C, and that still contains an amount of 

 water quite incompatible with any formula analogous to those 

 preceding. 



I will subjoin the analyses containing the maximum and mini- 

 mum amounts of water which have been hitherto obtained*, for 

 the sake of comparison with my per-centage numbers : — 



Collyrite from Allophane from Allophane from 



Fermi by Beauvais by 



Guillemin. Berthier. 



39-68 29-2 



23-76 21-9 



35-74 44-2 



.g5|Sulphuric ^.y^j^y^ 



99-8 99-83 100-0 



This is the dried specimen 

 above alluded to. 



* Gmelin, Cav. See. vol. iii. p. 414. 



