378 DR T. THOMSON on Asbestus, Chlorite, and Talc. 

 Thus the composition of Swedish potstone is, 



2 atoms talc + 1 atom magnesia, 



-j- 1 atom protoxide of iron, 

 + 1 atom alumina, 

 + li atom water. 



It is probable that the black crystalline grains were composed 

 of, 



1 atom magnesia, 



1 atom alumina, 



1 atom protoxide of iron. 



They constitute, therefore, a new mineral species. But I found 

 it impossible to separate them from the white portion of the mi- 

 neral apparently constituting the talc, or to subject them to a 

 separate analysis. 



Swedish potstone would seem, from the above analysis, to be 

 a mixture of pure talc and of a black mineral hitherto unex- 

 amined. 



4. Hexagonal Talc. 



The mineral which I subjected to analysis was from Ala in 

 Piedmont. It constituted large six-sided plates in a granular 

 stone, usually distinguished by the name of granular talc. 



The colour of the crystals was dark green. The texture fo- 

 liated. The crystals had the appearance of very short regular 

 six-sided prisms, about an inch in diameter, but the edges were 

 not smooth nor well denned ; which prevented the possibility 

 of measuring the angles. 



Streaks white. Soft enough to be scratched by the nail. 

 Specific gravity 2.772. 



