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.[[1221]] 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.[[1222]] I have met, however, with perfectly pure specimens in the shops of this city.
3. Of Sugar of Lead.
Sugar of lead is the acetate of this metal. It is sold in the form either of a white heavy powder, or of aggregated masses of long four-sided prismatic crystals. It has a sweetish astringent taste, and a slight acetous odour. It is very soluble.
When in the solid state, it may be known by its solubility in water, and by the effects of heat. It first undergoes the aqueous fusion, then abandons a part of its acid empyreumatized, as may be perceived by the smell, next becomes charred, and finally presents globules of lead reduced by the charcoal of the acid. The best way of effecting its reduction on the small scale is to char it, and then direct on the mass the point of a blowpipe-flame: in an instant globules are developed. It is not easily reduced in a tube; at least I have never been able to succeed in that way.
In the fluid state the acetate of lead, as well as all its soluble salts, may be detected by the following system of reagents,—hydrosulphuric acid, bichromate of potass, hydriodate of potass, and metallic zinc,—which are the best of the numerous reagents yet proposed.
1. Hydrosulphuric acid causes a black precipitate, the sulphuret of lead. This is a test of extreme delicacy; and it acts in whatever state of combination the lead exists, whether fluid or solid.
It is preferable to the hydrosulphate of ammonia as a medico-legal test; for, as Fourcroy observed, the hydrosulphate of ammonia acts on many sound wines as if they contained lead,[[1223]] while hydrosulphuric acid never causes with them a black precipitate, unless they contain either lead or some other metallic impregnation. It must be remembered that many other metallic solutions, such as those of mercury, copper, silver and bismuth, yield a black precipitate with this test.
2. Chromate of potass, both in the state of proto-chromate and bichromate, causes a fine gamboge-yellow precipitate, the chromate of lead. For the characteristic action of this reagent, it is desirable that the suspected liquid be neutral. It forms with solutions of the sulphate of copper a precipitate nearly of the same colour as the chromate of lead.