1815.) in the Physical Sciences, 47 



Boracic acid 6 



Magnesia 3A 



Oxide of iron Jg. 



Silica ^ 



Loss 1-5^ 



11 



Thus it is free from lime, and therefore verifies the previously 

 received opinion that boracite is essentially a borate of magnesia. 



11. Vitreous Black Oxide of Iron. — This is a mineral mentioned 

 by Haiiy in his Tableau Comparatif, p. 98 and 274, as a black 

 vitreous substance found in the department of the Lower Rhine. 

 It scratches glass, and has a specific gravity of 3-2. When heated 

 to redness, it becomes magnetic. Vauquelin found it composed of 

 oxide of iron 80-25, water 15, silica 3-75. Bucholz has lately 

 analysed a small quantity of this mineral. He found its composition 

 as follows : — 



Oxide of iron 68*5 



Silica 10-0 



Oxide of manganese 5'5 



Loss 16-0 



100-0 

 If we consider this loss as water, which seems from the analysis 

 of Vauquelin to be the case, the mineral will be a hydrate of iron 

 and manganese united with silica. 



12. Si/Ipkuret of Cobalt. — This mineral occurs in Sweden, at 

 Nya Bastnas, near Riddarhyttan. Hisinger found its constituents 

 as follows : — • 



Cobalt 4I5-20 



Copper 14, -4.0 



Iron 3-53 



Sulphur 38-50 



Earthy matter 0*33 



99-96 



13. Kutine Yellow Oxide of Ltad.—'Vhh rare mineral has been 

 lat«ly examined by Dr. .John. Its characters are the following :— 



External colour, between sulphur and lemon-yellow ; internal, 

 between honey-yellow and Aurora red. 

 Amorphous. 



Fracture, earthy. In some places there is a tendency to tiic 

 foliated fracture. 



External lustre, dull ; internal, semi-metallic. 



Opake. 



Semihard. 



Hriltle. Mfxltrately easily frangible. 



Sj)eei(i(: gravity 8000. 



