186 METALS AND THEIR COMBINATIONS. 



crystalline scales, known as litharge ; by heating still further in con- 

 tact 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 probably is a mixture (or combination) of oxide and 

 dioxide of lead, PbO 2 (PbO) 2 . 



Lead oxide is used in the manufacture of lead salts, lead plaster, 

 glass, paints, etc. 



Mtric acid when heated with red lead combines with the oxide, 

 while lead dioxide, PbO 2 , is left as a dark-brown powder, which, on 

 heating with hydrochloric acid, evolves chlorine (similar to man- 

 ganese dioxide). 



Lead nitrate, Plumbi nitras, Pb(NO 3 ) 2 = 330.4. Obtained by 

 dissolving the oxide in nitric acid : 



PbO + 2HNO 3 = H 2 O + Pb(N0 3 ) 2 . 



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, astrin- 

 gent, and afterward metallic taste. 



Lead carbonate, Plumbi carbonas, 2(PbCO 3 ).Pb(OH) 2 ^773.2 



( White-lead). This compound may be obtained by precipitation of 

 lead nitrate with sodium carbonate, but is manufactured on a large 

 scale directly from lead, by exposing it to the simultaneous action of 

 air, carbon dioxide, and vapors of acetic acid. The latter combines 

 with the lead, forming a basic acetate, which is converted into the 

 carbonate (almost as soon as produced) by the carbon dioxide present. 



The action of acetic acid on lead or lead oxide will be considered 

 in connection with acetic acid. 



Lead carbonate is a heavy, white, insoluble, tasteless powder ; the 

 white-lead of commerce frequently is found adulterated with barium 

 sulphate. 



Lead iodide, Plumbi iodidum, PbI 2 459.4 (Iodide of lead). 

 Made by adding solution of potassium iodide to lead nitrate (Plate 



III., 6)': 



Pb(N0 3 ) 2 + 2KI = 2KNO 3 + PbI 2 . 



It is a heavy, bright yellow, almost insoluble powder, which may 

 be distinguished from lead chromate by its solubility in ammonium 

 chloride solution on boiling, lead chromate being insoluble in this 

 solution. 



