188 ACID1FIABLE COMBUSTIBLES. 



Let us calculate the composition of this min- 

 eral on the supposition that it is a silicate of iron 

 and manganese ; and that the atomic weight of 

 silica is 2 9 of protoxides of iron and manganese, 

 each 4\5. 



(1.) 4-5 (protoxide of iron atom) : 2 (atom of silica) 



: : 32 : 14-22 = silica united to oxide of iron 14-22 



(2.) 4-5 (atom of protoxide of manganese) : 2 : : 35 : 



15-55 = silica united to the oxide of manganese 15-55 



Total silica united with the bases = 29-77 



The silica found in the mineral by Dobereiner 

 exceeds this quantity by 2*72 grains. This ex- 

 cess might be ascribed to the presence of a cer- 

 tain quantity of trisilicate of manganese. Pro- 

 toxides of iron and manganese having the same 

 atomic weight, the silicates of these two bases 

 should have the same weight. Hence, it is not 

 unreasonable to suppose, that only 32 grains of 

 the protoxide of manganese were in the state of 

 silicate the other 3 grains may have existed in 

 the state of trisilicate. Now, 4%5 (atom of pro- 

 toxide of manganese) : 6 (3 atoms silica) : : 3 : 

 4 = silica united to the 3 manganese. The con- 

 stituents of the mineral may be thus stated : 



Silicate of iron . 46-22 

 Silicate of manganese 46-22 

 Trisilicate of manganese 7'00 



99'44 



