176 AC1DIFIABLE COMBUSTIBLES. 



Let us calculate the composition of this min- 

 eral, on the supposition that the silica is com- 

 bined with three bases, atom to atom ; and that 

 an atom of silica weighs 2, an atom of alumina 

 2-25, an atom of protoxide of iron 4-5, and an 

 atom of potash 6. 



1 . 2'25 (an atom of alumina): 2 (an atom of silica) : : 



50 (alumina in the mineral) : 44-44 



= silica united to the alumina. 



2. 4-5 (atom of protoxide of iron) : 2 : : 2-5 (oxide in 



the mineral) : I'll 



silica united to the iron 



3. 6 (atom of potash) : 2 : : 1-5 (potash in the min- 



eral) : 0-50 



= silica united to the potash 



Total silica in the mineral = 46'05 



This exceeds the quantity found by Brandes 

 by only 0-05 grain a quantity so small, that it 

 might very easily be overlooked in an analysis. 

 Let us, however, suppose, that the analysis is 

 perfectly accurate, and let us deduce from it 

 the atomic weight of the silica. About zVth of 

 the silica is united to the oxide of iron and the 

 potash. Now, the 28th part of O05, the excess 

 of silica, is nearly 0-002, which being deducted, 

 leaves about 0-053. If we subtract this excess 

 from the 44-444 of silica, supposed above to be 

 united to the alumina, there will remain 44-391 

 grains. Hence, by Brandes' analysis of this 

 mineral, silicate of alumina is composed of 



