1825.] Scientific Noticei-^Mineraldgt^. 233 



Magnesia 66-67 



Oxide of manganese 1*57 



Protoxide of iron 1*18 



Lime G-19 



Water 30-39 



100-00 

 4. Magnesite. 



100 parts of magnesite, from Salem, in India, gave the same 

 accurate analyst : 



Carbonic acid 51-83 



Magnesia 47-88 



Lime 0-28 



Oxide of iron Trace 



99-99 

 5. Seleniuret of Lead. 



MM. Stromeyer and Hansmann have examined an ore from 

 the Laurence Mine, at Clausthal, which proves to be a seleniuret 

 of lead. The ore is combined with brown spar, and from its 

 imparting a smalt blue colour to glass, was supposed to contain 

 cobalt, and had been described under the name of cobalt hley- 

 erz, by M. Hausmann. 



Externally, seleniuret of lead has a greater resemblance to 

 some varieties of galena than to any other substance, but its 

 colour is peculiar, and partakes more of a blue tint than even the 

 wasserblei of the German mineralogists (sulplmret of moluhdena). 

 It has a tendency to crystallization, but its crystalline form has 

 not yet been ascertained. The fracture is fine grained ; lustre 

 metallic, not very brilliant. It is inferior in hardness to galena, 

 is not harsh to the touch, gives a dark streak, and retains its 

 metallic lustre after being rubbed ; its specific gravity = 7-697. 

 It becomes negatively electrical by friction. 



Seleniuret of lead t'usejs readily before the blowpipe, gives out 

 a powerful smell resembling that of rotten turnips, and becomes 

 covered with a brownish red crust, which is succeeded by a 

 coating of yellow oxide of lead. When the flame is directed on 

 the ore, a bright blue tint is developed ; fused with borax it 

 gives a pale smalt coloured glass. 



Heated in a glass tube by a spirit lamp, selenium sublimes, 

 exhaling its peculiar offensive odour, and the surface of the 

 glass becomes covered with a light sublimate of a brownish red. 

 colour. By heating the tube to redness, the ore fuses, but suf- 

 fers no other apparent change ; but on continuing the heat, the 

 brownish red sublimate gradually disappears, and is succeeded 

 by a white, acicular crystalline substance, which attracts 

 moisture by exposure, and deliquesces. It reddens htmus paper 



