above-mentioned compounds will present themselves in the following form:--Lead oxide, Pb_{12}O_{12}, its crystalline hydrate, Pb_{12}O_{8}(OH)_{8}, lead chloride, Pb_{12}Cl_{24}, lead oxychloride, Pb_{12}Cl_{12}O_{6}, the other oxychloride, Pb_{12}(OH)_{8}Cl_{6}O_{6}, mendipite (_see_ Note 51), Pb_{12}Cl_{8}O_{8}, normal lead carbonate, Pb_{12}(CO_{3})_{12}, crystalline basic salt, Pb_{12}(OH)_{6}(CO_{3})_{6}, white lead, Pb_{12}(CO_{3})_{8}(HO)_{8}, corneous lead, Pb_{12}Cl_{12}(CO_{3})_{6}, lanarkite, Pb_{12}(CO_{3})_{6}(SO_{4})_{6}, leadhillite, Pb_{12}(CO_{3})_{9}(SO_{4})_{3}, &c. The number 12 is only taken to avoid fractional quantities. Possibly the polymerisation is much higher than this. The theory of the polymerisation of oxides introduced by me in the first edition of this work (1869) is now beginning to be generally accepted. Amongst the soluble lead salts, that best known and most often applied in practical chemistry is _lead nitrate_, obtained directly by dissolving lead or its oxide in nitric acid. The normal salt, Pb(NO_{3})_{2}, crystallises in octahedra, dissolves in water, and has a specific gravity of 4·5. When a solution of this salt acts on white lead or is boiled with litharge, the basic salt, having a composition Pb(OH)(NO_{3}), is formed in crystalline needles, sparingly soluble in cold water but easily dissolved in hot water, and therefore in many respects resembling lead chloride. When the nitrate is heated, either lead oxide is obtained or else the oxide in combination with peroxide. _Lead chloride_, PbCl_{2}, is precipitated from the soluble salts of lead when a strong solution is treated with hydrochloric acid or a metallic chloride. It is soluble in considerable quantities in hot water, and therefore if the solutions be dilute or hot, the precipitation of lead chloride does not occur, and if a hot solution be cooled, the salt separates in brilliant prismatic crystals. It fuses when heated (like silver chloride), but is insoluble in ammonia. This salt is sometimes met with in nature, and when heated in air is capable of exchanging half its chlorine for oxygen, forming the basic salt or lead oxychloride, PbCl_{2}PbO, which may also be obtained by fusing PbCl_{2} and PbO together. The reaction of lead chloride with water vapour leads to the same conclusion, showing the feeble basic character of lead 2PbCl_{2} + H_{2}O = PbCl_{2},PbO + 2HCl. When ammonia is added to an aqueous solution of lead chloride a white precipitate is formed, which parts with water on being heated, and has the composition Pb(OH)Cl,PbO. This compound is also formed by the action of metallic chlorides on other soluble basic salts of lead.[51]