342 Mr. A. B. Northcote on the Constitution of Allophane. 



disturbing causes which I have before mentioned, we obtain the 

 formula 



AP 03 . 3Si03, 4(A12 0=^ . 3II0) 



as the sinipk'st expression in whole numbers of the constitution 

 of this specimen. 



Specimen III. 



I. II. 



Losspercent. atlOO'^'C'l Carbonic acid existO .on 

 1^ ing as bicarbonatej 



These constituents may be thus approximately arranged : — 



Alumina 39-09 



Silica " 17-00 



Water fixed at 100° C. . . 20-82 

 Water expelled at 100° C. . 20-10 

 Carbonate of lime .... 2*68 

 Carbonate of protoxide of iron, trace 

 Carbonic acid existing as bicar-l ,n-, 



bonate \_ 



100-00 



If we again assume the water expelled at 100° C, the carbon- 

 ate of lime, and the excess of carbonic acid, to be non-essential 

 to the constitution of the mineral, the following numbers will 

 represent its real composition : — 



Alumina 39-09 50-82 



Silica 17-00 22-11 



Water fixed at 100° C. 20-82 27-07 



76-91 100-00 

 The precise formula given by these numbers is 

 A12 0».3Si03, 5(AP03.3HO)+3HO; 

 or if we exclude, as before, the surplus water, 



AP 03 . 3Si03, 5(AF 0^ . 3H0). 

 The fourth specimen, which from its appearance evidently con- 

 tained some quantity of sesquioxide of iron, I examined in the 



