every one. It is easily known by the dull bluish-gray colour it assumes when exposed some time to the air, by the brilliant bluish-gray colour of a fresh surface, and by the facility with which it may be cut. The compounds which require particular notice are four in number, litharge, red lead, white lead, sugar of lead, and Goulard's extract. The first three are very much used by house-painters and glaziers, the last two are extensively employed in surgery, and the sugar of lead is also used in many of the arts.
2. Of Litharge and Red Lead.
Litharge is the protoxide of lead in a state of semivitrification. Red lead is a compound of two equivalents of protoxide and one of deutoxide. The former is generally in the form of a grayish-red heavy powder, sometimes partly crystalline; the latter in the form of a bright red powder approaching in colour to vermilion. They may be known by their colour;—by their becoming black when suspended in water and treated with a stream of sulphuretted-hydrogen gas;—and by litharge being entirely, and red lead partly, soluble in nitric acid, and forming a solution which possesses the properties to be mentioned presently for solutions of the acetate. The chemical actions concerned in these changes are obvious, except in the instance of nitric acid on red lead. Here the acid dissolves the protoxide only, and the deutoxide, which seems to act the part of an acid in the pigment, is separated in the form of a brown powder. 2. Of White Lead.
White lead, which is the carbonate of the metal, is in the form of a heavy snow-white powder, or in white chalk-like masses. It consists of variable proportions of the hydrated oxide and neutral carbonate; those specimens are the whitest which contain most carbonate; and the best English white lead I find to contain four equivalents of carbonate and one of hydrated protoxide. The grayer variety, formed by the action of distilled water on metallic lead, consists of only two of the former to one of the latter.[1] It may be known by its being blackened like the two former compounds by sulphuretted-hydrogen,—by being soluble with effervescence in nitric acid,—and by becoming permanently yellow when heated to redness, in consequence of the expulsion of its carbonic acid, and its conversion into protoxide. These tests, however, apply with exactness only to the pure carbonate, in which state white lead is not often met with in the shops. It is generally adulterated with sulphates, in consequence of which it is only partially acted on by nitric acid, and does not become distinctly yellow under a strong red heat. Dutch white-lead contains no less than between 78·5 and 25 per cent. of impurities insoluble in nitric acid, Venetian white-lead from 11 to 14·5 per cent., Munich white-lead between 1 and 7·5 per cent.[2] I have met, however, with perfectly pure specimens in the shops of this city.
