Chemical Examination of American Minerals. 26 



The only difference between the native and the artificial 

 soda-alum is, that the former contains only 20 atoms of water, 

 while the latter contains 25 atoms. This no doubt occasions 

 the difference in the shape of the crystals. Artificial soda- 

 alum crystallizes in regular octahedrons. But the native 

 seems to form prismatic crystals ; at least if any inference 

 can be drawn from its fibrous structure. I made some attempts 

 to obtain it in regular crystals ; but owing to the heat of the 

 weather, they were not attended with success. 



2. BISILICATE OF MANGANESE. 



This mineral was first observed in Sweden. I have speci- 

 mens of it from Cornwall, and it has been observed also in 

 other places. About two years ago, a specimen was sent me 

 by Dr. Torrey, from Cummington in Massachusetts, where 

 it occurs in considerable abundance. It had the usual red 

 color of this mineral, but was darker, and on one side almost 

 black ; and another specimen was quite incrusted with black 

 matter. Its specific gravity was 3*83, which is lighter than 

 the common bisilicate of Europe. This has a specific gravity 

 of 3- 5384. On treating the American specimen with an acid, 

 an effervescence took place, showing the presence of a little 

 carbonic acid. A careful analysis gave its constituents as 

 follows : 



Silica, 40-58 



Protoxide of manganese, _ - . - 38*92 



Protoxide of iron, ----- 13*50 



Water, 8-00 



Carbonic acid ------ 3'23 



99 23 



I did not weigh the carbonic acid ; but it is easy to deduce 

 its quantity from the composition of the mineral. If it be a 

 Vol. III. 4 



