286 LEAD. 



is lead-grey ; the lustre metallic, and varying from splendent to 

 glistening. It occurs in amorphous masses, disseminated and 

 crystalized, in cubes and octahedrons. Fig. a represents the cube, 

 which is the primitive form ; b, the same truncated on the angles, 

 forming the passage into the octahedron, fig. c. Fig. d, e, and f, 

 represent various modifications of the octahedron. The structure 

 is lamellar, granular, or compact. It is soft, sectile, and easily 

 frangible. The specific gravity is 7*5. Before the blow-pipe it 

 decrepitates and melts, emits a sulphureous odour, and globules of 

 metallic lead are formed on the charcoal. A specimen from 

 Durham, analysed by Dr. Thomson, contained, lead 85' 13, sulphur 

 13"2. Some" varieties, particularly the granular galena, or steel- 

 grained lead ore, often contain a very considerable portion of 

 silver. Sulphuret of lead occurs in beds and veins in primitive, 

 transition, and secondary rocks, in various parts of the globe. 

 The most productive lead veins in England intersect mountain - 

 limestone in Derbyshire, Northumberland, Durham, Cumber- 

 land, and Yorkshire. Nearly all. the lead of commerce is obtained 

 from galena. After being broken in pieces the ore is washed, and 

 then exposed to a strong heat in a reverberatory furnace, till the 

 sulphur is all separated ; it is then brought into a state of fusion 

 with lime, and the scoria is raked oft*, while the lead is run out into 

 moulds through an aperture near the bottom ; in this state it is 

 called pig lead. The silver is separated by converting the lead 

 into litharge, and separating the silver from what remains by 

 cupellation. 



Sp. 2. Native Minium, or Native Red Oxtde of Lead. 

 Pl. XXXIX. fig. 3. — Plombe oxyde rouge, Hauy. — This is a 

 scarce ore of lead ; its colour is scarlet ; it is massive, amorphous, 

 and pulverulent. It occurs incrusting sulphuret of lead, and its 

 properties are the same as the common red -lead of commerce. 



Sp. 3. Carbonate of Lead, or White Lead-spar. Pl. 

 XL. fig. 1. — Weiss Bleirz, Werner ; Plomb carbonate, Hauy. — 

 Its colours are white, yellow, brown, and grey; sometimes coloured 

 green or blue by copper. It occurs massive, disseminated, in 

 membranes, and crystalized. The primitive, a cuneiform oc- 

 tahedron, is represented fig. a, and some of the more common 



