ALU-I1NA. 299 



Let us calculate the composition of this mine- 

 ral, on the supposition that each of the three 

 bases are combined with two atoms of silica, con- 

 stituting as many bisilicates : 



(1.) 2-25 : 4 : 23-1 : 41-066 = silica united to alumina. 

 (2.) 6:4: :2M5: 14-1= silica united to potash. 

 (3.) 4-5:4:: 0-95 : 0-844 = silica united to iron. 



Now, 41-066 + 14-1 + 0-844 = 56-01, which is 

 less than the silica actually found in the mineral 

 by 0-09 grain. 



The silica united to the potash and the iron 

 amounts to 3^Vth of the whole. The 3^43 th of 

 0-09 is 0-024, which, being subtracted, leaves 

 0-066 to be added to 41-066, in order to have the 

 silica actually united to the alumina in the mine- 

 ral, it will amount to 41-132 grains. And 



41-132 : 23-1 : : 4 : 2-2464 = atomic weight of alumina. Gives 



2-2464. 



9. THOMSON1TE. 



ite 



This mineral, which hitherto has been found Thomson, 

 only in the Kilpatrick hills, near Glasgow, and 

 which had been considered as a mesotype, has 

 been constituted into a new species by Mr. 

 Brooke, on account of the peculiar form of its 

 primary crystal. I made a very careful analysis 

 of this mineral,* and found its constituents as fol- 

 lows: 



* Annals of Philosophy, XVI. 



