PROTOXIDE OF LEAD. 587 



perature, but they favour its union with oxygen from the air. 

 Its best solvent is nitric acid. Besides a protoxide, PbO, 

 which is a powerful base, lead forms a suboxide Pb 2 O, and a 

 peroxide Pb O 2 , which do not combine with acids. 



Suboxide of lead, Pb 2 O, was discovered by Dulong, and is 

 best obtained by heating the oxalate of lead to low redness in 

 a small retort. It is dark grey, almost black, and pulverulent, 

 and is not affected by metallic mercury. By the analysis of 

 Boussingault, it contains 1 eq. of oxygen to 2 eq. of lead. The 

 grey pellicle which forms upon lead exposed to the air is, ac- 

 cording to Berzelius, the same suboxide. 



Protoxide of lead, Pb O, 1394.5 or 111.73. When a stream 

 of air is thrown upon the surface of melted lead, the metal is 

 rapidly converted into the protoxide, of a sulphur-yellow colour. 

 -The oxidated- skimmings of the metal are, in this condition, 

 termed massicot, and were at one time used as a yellow pigment. 

 This preparation is fused at a bright red heat, and the oxide is 

 thus separated from some metallic lead, with which it is inter- 

 mixt in massicot. The fused oxide forms a brick red mass, 

 on solidifying, which divides easily into crystalline scales, which 

 are tough and not easily pulverised; they form litharge. The 

 protoxide of lead can be obtained distinctly crystallized by 

 various processes, but always presents itself in the same form, 

 an octohedron with a rhombic base (Mitscherlich). By ig- 

 niting the subnitrate of lead, the protoxide is obtained very 

 pure, and of a rich lemon yellow colour. Its density when 

 fused is 9.4214. 



When the acetate, or any other salt of lead, is precipitated by 

 potash, the protoxide falls as a white hydrate, which may be 

 dried at 212 without decomposition. It contains 3f per cent 

 water, and is, therefore, the hydrate 2PbO + HO, (Mitscherlich). 

 Oxide of lead likewise crystallizes anhydrous, from solution, at 

 the usual temperature, when water to combine with is denied 

 to it, in the circumstances of its precipitation. This oxide dis- 

 solves in above 12,000 times its weight of distilled water, which 

 acquires thereby an alkaline reaction ; but not in water contain- 

 ing any saline matter. It is soluble in potash or soda ; and the 

 solutions, when evaporated, afford small crystals of an alkaline 

 compound. A compound of lime and oxide of lead is obtained in 

 needles, when hydrate of lime and that oxide are heated together, 

 and the solution allowed to evaporate with exclusion of air. This 



