Mr. A. B. Northcote on the Constitution of Allophane. 343 



hope of finding that it was an iron substitution compound : it 

 was selected with considerable care, and freed as perfectly as 

 possible from the ordinary allophane with which it was iuterstra- 

 tilied. Its analysis yielded the following results i^ — 



Specimeji IV. 



I. II. 



Alumina .... 32-88 

 Sesquioxide of iron . 6'51 6'67 



Silica 16-90 17-21 



Water 40-34 40-29 



Lime ]-34 



Carbonic acid . . . 1-85 1-79 



Loss per cent.atlOO°C. < Carbonic acid exist-") 



99-99 

 f Water .... 21-97 



•75 



{_ ing as bicarbonate J 



Treating these constituents as in the former cases, we obtain 

 the following approximate arrangement :■ — 



Alumina 32-88 



Sesquioxide of iron . . 



Silica 



Water fixed at 100° C. . . 

 Water expelled at 100° C. 

 Carbonate of lime . . . 

 Carbonic acid existing as bicar 

 bonate 



} 



6-59 

 17-05 

 18-35 

 21-97 



2-41 



•75 



100-00 



These result8,if the former method of exclusion is again adopted, 

 give the following as the true constituents of the mineral : — 



Alumina .... 32-88 43-92 



Sesquioxide of iron. . 6*59 8-80 



Silica 17-05 22-77 



Water fixed at 100° C. 1835 2451 



74-87 100-00 



The formula given by these numbers places this specimen in 

 close relationship with the last mentioned, the sesquioxide of 

 iron replacing a small portion of the alumina, as will be seen by 

 the following expression, which is that which the above compo- 

 sition yields : — 



AF0^3Sio^4|^}:o:^•fHO)^HO 



^-'--3|3HO 



