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

 Specimen I. Sp. gr. 1"77. 



99-71 

 rWater • • • , • 27-11 

 Loss percent. at 100°C. ■< Carbonic acid exist- I^.q^ 

 \_ ing as bicarbonatej 



These constituents may be approximately arranged as follows: — 



Alumina . 31*34 



Silica 20-50 



Water fixed at 100° C. . . . 15-80 

 Water expelled at 100° C. . . 27-11 

 Carbonate of lime .... 3-43 

 Carbonate of protoxide of iron. -49 

 Carbonic acid existing as bi-\ \-r\A 

 carbonate . . . . . J 



99-71 



If now we remove the carbonates of lime and iron, the loosely- 

 combined water and carbonic acid, which may, I presume, be 

 safely regarded as extraneous to the mineral, we obtain the fol- 

 lowing numbers expressing its composition : — 



Alumina . . . . 31-34 46-34 



Sihca 20-50 30-31 



Water fixed at 100° C. 15-80 23-35 



67-64 100-00 

 The actual formula deducible from these numbers is 

 A12 03 . 3Si03, 3(A12 0^ . 3H0) -f2iH0 ; 

 but I need scarcely say that it is my belief that the real formula 

 of this specimen is 



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



the additional water having very probably crept in as an erro- 

 neous result. This might happen in many ways : the protocar- 

 bonate of iron, if really so existing in the mineral, would cer- 

 tainly by drying at 100° C. become converted into hydrated 

 sesquioxide, by which means more water would be retained than 

 the alumina alone required ; or a slight variation in the relative 



