Lead

Tin unites with lead in any proportion with slight expansion, the alloy fusing at a lower temperature than either component. It is used largely for soldering.

"Pewter" (q.v.) may be said to be substantially an alloy of the same two metals, but small quantities of copper, antimony and zinc are frequently added.

Lead and Radioactivity.

Lead is the end product of sev eral series of radioactive decay (see RADIOACTIVITY), and has been proved to be identical with radium-D. The atomic weight of lead therefore depends upon the series from which it is de rived, and atomic weights as low as 206.0 have been found for lead derived from uranium, as in uraninite, and from several min erals in the Belgian Congo.

Compounds of Lead.

Lead generally functions as a bivalent element of distinctly metallic character, yielding a definite series of salts derived from the oxide PbO. At the same time, however, it forms a number of compounds in which it is most decidedly quadrivalent; and thus it shows relations to carbon, silicon, ger manium and tin.

Hydrides.—In 1919, F. Paneth found that traces of a hydride were produced by dissolving a lead-magnesium alloy in acids or by electrolytic reduction of solutions of lead salts. In 1925, E. J. Weeks obtained a hydride, as a grey deposit on aluminium foil placed in a solution of an alkali plumbite.

Oxides.—Lead combines with oxygen to form five oxides, viz. PbO, and The suboxide, is the first product of the oxidation of lead, and is also obtained as a black powder by heating lead oxalate to 3o0° out of contact with air. It ignites when heated in air with the formation of the monoxide ; dilute acids convert it into metallic lead and lead monoxide, the latter dissolving in the acid. The monoxide, PbO, occurs in nature as the mineral lead ochre. This oxide is produced by heating lead in contact with air and removing the film of ox ide as formed. It is manufactured in two forms, known as "massi cot" and "litharge." The former is produced at temperatures below, the latter at temperatures above the fusing-point of the oxide. The liquid litharge when allowed to cool solidifies into a hard stone-like mass, which, however, when left to itself, soon crumbles up into a heap of resplendent dark yellow scales known as "flake litharge." "Buff" or "levigated litharge" is prepared by grinding the larger pieces under water. Litharge is much used for the preparation of lead salts, for the manufacture of oil var nishes, of certain cements, and of lead plaster, and for other pur poses. Massicot is the raw material for the manufacture of

"red lead" or "minium." Lead monoxide is dimorphous, occurring as cubical dodecahe dra and as rhombic octahedra. Its specific gravity is about 9; it is sparingly soluble in water, but readily dissolves in acids and molten alkalis ; it is used in making flint glass and glazes. A yellow and a red modification have been described. The corre sponding hydrate, PbO,Pb(OH)2, is obtained as a white crystal line precipitate by adding ammonia to a solution of lead nitrate or acetate. It dissolves in an excess of alkali to form plum bites of the general formula Pb(0M)2. It absorbs carbon diox ide from the air when moist.

Lead dioxide, also known as puce oxide, occurs in nature as the mineral plattnerite, and may be most conveniently prepared by heating mixed solutions of lead acetate and bleaching powder until the original precipitate blackens. The solution is filtered, the precipitate well washed, and, generally, is put up in the form of a paste in well-closed vessels. It is also obtained by passing chlorine into a suspension of lead oxide or carbonate, or of magnesia and lead sulphate, in water ; or by treating the ses quioxide or red oxide with nitric acid. The formation of lead dioxide by the electrolysis of a lead solution, the anode being a lead plate coated with lead oxide or sulphate and the cathode a lead plate, is the fundamental principle of the storage cell (see Heating or exposure to sunlight reduces it to the red oxide; it fires when ground with sulphur, and oxidizes ammonia to nitric acid, with the simultaneous formation of am monium nitrate. It oxidizes a manganese salt (free from chlorine) in the presence of nitric acid to a permanganate; this is a very delicate test for manganese. It forms crystallizable salts with potassium and calcium hydrates, and functions as a weak acid forming salts named plumbates. The Kassner process formerly used for the manufacture of oxygen depended upon the formation of calcium plumbate, by heating a mixture of lime and litharge in a current of air, decomposing this substance into cal cium carbonate and lead dioxide by heating in a current of car bon dioxide, and then decomposing these compounds with the evolution of carbon dioxide and oxygen by raising the tempera ture.