470 DR T. THOMSON'S Analysis of some Minerals. 



But, probably, the silica is in chemical combination with the 

 magnesia. I am disposed to consider this curious mineral as 



composed of 



.rev/ 



5 atoms silicate of magnesia, 



12 atoms bihydrate of magnesia, 

 1 atom ferrate of magnesia. 



Its symbol, on that supposition, will be, 

 5MS+ 1 



X. Brownspar and Pearlspar. 



These names have been applied to a variety of mineral spe- 

 cies, which have a considerable resemblance to each other, and 

 the crystalline form of which approaches more or less to that of 

 calcareous spar. MOHS and HAIDINGER have done a good deal 

 to disentangle this chaos, and have constituted several well de- 

 nned species out of minerals hitherto confounded under the 

 common name of Brownspar. But it is doubtful whether the 

 external characters alone afford sufficiently distinctive marks in 

 all cases, at least, for arranging the different brownspars under 

 their respective species. At any rate, it would be proper to sub- 

 ject them, in the first place, to a careful chemical analysis, in or- 

 der to discover with accuracy the number of true species under 

 which they should be arranged. It is with a view of forwarding 

 this desirable object that I shall here state the composition of 

 such species of brownspar in my own collection as I have hither- 

 to subjected to chemical analysis. 



First Variety. 



It is composed of plates which break into rhomboids, similar 

 in appearance to calcareous spar. But the angle, as has been 



