418 Dr. Thomas Thomson on the 



Dividing these numbers by the atomic weig-lits of the 

 bodies, we obtain, 



Atoms. 



Columbic acid, .... 3-33 or 1* 

 Protoxide of iron, . . . 2*88^ ^p o-97 

 Protoxide of manganese, . 0*36 S 

 obviously, 1 atom columbic acid 



1 atom protoxide of iron and manganese. 

 The difference between it and common tantalite is, that 

 instead of 1 atom protoxide of iron and 1 atom protoxide 

 of manganese, united with two atoms of columbic acid, it 

 consists of 2 atoms of columbic acid, combined with 1-777 

 atoms protoxide of iron, and 0*223 atonjs of protoxide of 

 manganese. Or, which comes to the same thing, we may 

 consider it as composed of 



9 atoms columbic acid 

 8 atoms protoxide of iron 

 1 atom protoxide of manganese 

 So that the formula indicating its constitution is, 8/ CI + 

 mn CI. 



Such are the characters, and such the constitution of the 

 four species of minerals composed of columbic acid, united 

 to protoxide of iron and protoxide of manganese. The 

 formulas indicating their constitution are as follow : 



1. Torrelite, . . . 2p CI + mn^ CI. 



2. Columbite, . . . /^ CI + mn CI. 



3. Tantalite, . . . fC\ + mnC\. 



4. Ferrotantalite, . 8/Cl + mnC\. 



Let us now compare their specific gravities with each other. 



1. Torrelite, . . . 4-8038 



2. Columbite, . . . 6-0380 



3. Tantalite, . . . 7-3000 



4. Ferrotantalite, . 7-6550 



Thus, as the columbic acid increases, the specific gravity 

 increases, shewing the high specific gravity which must 

 belong to columbic acid. 



The crystalline shape of torrelite and columbite is dif- 

 ferent. It is probable, that tantalite and ferrotantalite have 

 each a peculiar crystalline shape, although, from the variety 

 of their minerals, and the imperfect state of the crystals, 

 that point has not been ascertained. 



