. PAINTING. 259 



ores of lead, and probably resulting from their oxidation. In 

 some localities it accompanies cerusite or white-lead ore. 



" When prepared for analysis, or when the commercial 

 article is freed from the protoxide by digestion with a solu- 

 tion of acetate of lead, it contains 90.63^ of lead and 9.37$ of 

 oxygen, numbers agreeing exactly with the formula Pb 3 O 4 . 



* ' It may be regarded either as a compound of the protoxide 

 and peroxide of lead PbO.PbO,, or perhaps of the protoxide 

 and sesquioxide, PbO.Pb^Oj, analogous to the magnetic 

 oxide of iron. Its specific gravity ranges from 8.6 to 8.94. 



'The commercial red oxide of lead is formed when the pro- 

 toxide is kept at a low red heat for a considerable time in 

 contact with air; also, after the previous formation of hy- 

 drated protoxide and basic carbonate of lead, when lead shav- 

 ings are strewn upon the water, the vessel being loosely cov- 

 ered and set aside for some months, the formation of red 

 lead taking place upon the surfaces of the lead exposed to the 

 air. . . . Commercial red lead contains all of the foreign 

 metallic oxides such as the oxides of silver, copper, and iron 

 with which the massicot or litharge used in preparing it is 

 contaminated. It is also adulterated with red oxides of iron, 

 boles, or brick-dust ; these substances remain undissolved 

 when the red lead is digested in warm dilute nitric acid ; 

 boiling hydrochloric acid extracts the sesquioxide of iron from 

 the residue. . . . The use of red lead as a pigment is pos- 

 sibly of earlier origin than any of the oxides of iron, ochres, 

 and other substances, natural or artificial, of which we have 

 any record, unless it be asphaltum or lampblack. The many 

 miscellaneous pigments which have come forward, been tried, 

 and found wanting in some one or other of the qualities which 

 constitute a good paint are almost numberless. There is no 

 other color-pigment whose use as a protective covering to 

 wood, brick, stone, or metal has been so uniformly satisfactory 

 and successful as red lead, and any failure to fulfil its mission 



