Lead chloride, PbC12, occurs in nature as the mineral cotun nite, which crystallizes in the rhombic system, and is found in the neighbourhood of volcanic craters. It is artificially obtained by adding hydrochloric acid to a solution of lead salt, as a white precipitate, little soluble in cold water, less so in dilute hydro chloric acid, more so in the strong acid, and readily soluble in hot water, from which on cooling, the excess of dissolved salt separates out in silky rhombic needles. It melts at 485° and solidifies on cooling to a translucent, horn-like mass; an early name for it was plumbum corneum, horn lead. A basic chloride, Pb(OH)C1, was introduced in 1849 by Pattinson as a substitute for white lead. An anhydrous basic chloride approximating to but not a definite compound, is obtained by the fusion of the two components ; it is used as a pigment under the name of "Cassel yellow." "Turner's yellow" or "patent yellow" is another artificially prepared oxychloride, used as a pigment (see PAINTS, CHEMISTRY or). Mendipite and matlockite are mineral oxychlorides, and respectively.
Lead fluoride, is a white powder obtained by precipitating a lead salt with a soluble fluoride ; it is sparingly soluble in water but readily dissolves in hydrochloric and nitric acids. Lead bro mide, a white solid, and lead iodide, a yellow solid, are prepared by precipitating a lead salt with a soluble bromide or iodide ; they resemble the chloride in solubility.
Lead carbonate, occurs in nature as the mineral cerus site (q.v.). It is produced as an almost insoluble white precipitate by the addition of a solution of lead salt to an excess of am monium carbonate. Of greater practical importance is a basic carbonate, substantially largely used as a white pigment under the name of "white lead." This pigment is of great antiquity; Theophrastus called it ahui)Ocov, and prepared it by acting on lead with vinegar, and Pliny, who called it cerussa, obtained it by dissolving lead in vinegar and evaporating to dry ness. It thus appears that white lead and sugar of lead were un differentiated. Geber gave the preparation in a correct form, and T. 0. Bergman proved its composition. This pigment is manu factured by several methods. In the old Dutch method, pieces of sheet lead are suspended in stoneware pots so as to occupy the upper two-thirds of the vessels. A little vinegar is poured into each pot ; they are then covered with plates of sheet lead, buried in horse-dung or spent tanner's bark, and left to themselves for a considerable time. By the action of the acetic acid and atmos pheric oxygen, the lead is converted superficially into a basic acetate, which is at once decomposed by the carbon dioxide, with formation of white lead and acetic acid, which latter then acts de novo. After a month or so the plates are converted to a more
or less considerable depth into crusts of white lead. These are knocked off, ground up with water, freed from metal particles by elutriation, and the paste of white lead is allowed to set and dry in small conical forms. The German method differs from the Dutch inasmuch as the lead is suspended in a large chamber heated by ordinary means, and there exposed to the simultaneous action of vapour of aqueous acetic acid and of carbon dioxide. An other process depends upon the formation of lead chloride by grinding together litharge with salt and water, and then treating the alkaline fluid with carbon dioxide until it is neutral. White lead is an earthy, amorphous powder. The inferior varieties of commercial "white lead" are produced by mixing the genuine article with more or less of finely powdered heavy spar or occa sionally zinc white (Zn0). Venetian white, Hamburg white and Dutch white are mixtures of one part of white lead with one, two and three parts of barium sulphate respectively.
Although inexpensive, these methods are very slow, and are being replaced by electrolytic methods based on the oxidation of lead to hydroxide and conversion of this by means of carbon dioxide. The covering power of white lead increases with its density, and the material is more efficient the nearer its composi tion is to the pure normal carbonate being quite useless. Lead poisoning (plumbism) has been found to be greatly decreased when the wet rubbing down is used instead of the dry.
Lead sulphide, PbS, occurs in nature as the mineral galena (q.v.), and constitutes the most valuable ore of lead. It may be artificially prepared by leading sulphur vapour over lead, by fusing litharge with sulphur, or, as a black precipitate, by passing sulphuretted hydrogen into a solution of a lead salt. It dissolves in strong nitric acid with the formation of the nitrate and sulphate, and also in hot concentrated hydrochloric acid.
Lead sulphate, occurs in nature as the mineral anglesite (q.v.), and may be prepared by the addition of sulphuric acid to solutions of lead salts, as a white precipitate almost insoluble in water (I in 21,739), less soluble still in dilute sulphuric acid and insoluble in alcohol. Ammonium sulphide blackens it, and it is soluble in solution of ammonium acetate, which distinguishes it from barium sulphate. Strong sulphuric acid dissolves it, forming an acid salt, Pb(HSO4)2, which is hydrolysed by adding water, the normal sulphate being precipitated; hence the milkiness ex hibited by samples of oil of vitriol on dilution.