164 METALS AND THEIR COMBINATIONS. 



In regard to valence, they show no uniformity whatever, 

 silver being univalent, copper, cadmium, and mercury bivalent, 

 bismuth trivalent, and lead either bivalent or quadrivalent. 



Lead, Pb u = 206.5 (Plumbum). This metal is obtained almost 

 exclusively from the native sulphide of lead, called galena, PbS. 



Lead owes its usefulness in the metallic state chiefly to its 

 softness, fusibility, and resistance to acids, which properties are 

 of advantage in using it for tubes or pipes, or in constructing 

 vessels to hold or manufacture sulphuric acid. Lead is also a 

 constituent of many alloys, as, for instance, of type-metal, 

 solder, britannia metal, shot, etc. 



Lead oxide, Plumbi oxidum, PbO = 222.5 (Oxide of lead, Lith- 

 arge). Obtained by exposing melted lead to a current of air, when 

 the metal is gradually oxidized with the formation of a yellow 

 powder, known as massicot ; at a higher temperature this fuses, 

 forming reddish-yellow, crystalline scales, and is then known as 

 litharge ; by heating still further in contact with air, a portion 

 of the oxide is converted into dioxide (or peroxide), PbO 2 , and 

 a red powder is formed, known as red lead (or minium), which is 

 probably a mixture (or combination) of oxide and peroxide of 

 lead, Pb0 2 2PbO. 



Oxide of lead is used in the manufacture of lead salts, lead 

 plaster, glass, paints, etc. 



Nitric acid when heated with red lead combines with the 

 oxide, while the dioxide is left as a dark brown powder, which 

 on heating with hydrochloric acid evolves chlorine (similar to 

 manganese dioxide). 



Lead nitrate, Plumbi nitras, Pb2N0 3 = 330.5 (Nitrate of lead). 

 Obtained by dissolving the oxide in nitric acid: 



PbO + 2HNO 3 = H 2 O + Pb2NO 3 . 



Lead nitrate is the only salt of lead (with a mineral acid) 

 which is easily soluble in water; it has a white color, and a 

 sweetish, astringent, and afterwards metallic taste. 



Lead carbonate, Plumbi carbonas, 2(PbC0 3 ).Pb2HO = 773.5 (Car- 

 bonate of lead, White-lead). This compound may be obtained by 



