Lead

Production.--At

the beginning of the 19th century the bulk of the world's supply of lead was obtained from England and Spain, the former contributing about 17,00o tons and the latter 0,000 tons annually. Germany, Austria, Hungary, France, Rus sia and the United States began to rank as producers during the second and third decades; Belgium entered in about 1840; Italy in the 'sixties; Mexico, Canada, Japan and Greece in the 'eighties; while Australia assumed importance in 1888 with a production of about 18,000 tons, although it had contributed small and varying amounts for many preceding decades. The present state and gen eral trend of the lead industry may be seen from the following table, which gives the production of smelter (in metric tons) for the countries having the largest output.

Minerals Yearbook 1938, U. S. Bureau of Mines. *Lead refined from foreign base bullion not included.

Properties of Lead.

Pure lead is a feebly lustrous bluish white metal, endowed with a characteristically high degree of softness and plasticity, and almost entirely devoid of elasticity.

Its breaking strain is very small: a wire in. thick is tured by a charge of about 3o lb. The specific gravity is 11.352 for ingot, and from 11.354 to 11.365 for sheet lead (water at C=i). The expansion of unit-length from o° C to ioo° C is 0.002948 (Fizeau). The conductivity for heat (Wiedemann and Franz) or electricity is 8.5, that of silver being taken as ioo. It melts at 327.7° C (H. L. Callendar) ; at a bright-red heat it per ceptibly vapourizes, and boils at a temperature between 1,450° and 1,600°. Its vapour at 1,870° is monatomic. The specific heat is 0•0314 (Regnault). Lead exposed to ordinary air is rapidly tarnished, but the thin dark film formed is very slow in increasing. When kept fused in the presence of air lead readily takes up oxygen, with the formation at first of a dark-coloured scum, and then of monoxide PbO, the rate of oxidation increasing with the temperature.

Water when absolutely pure has no action on lead, but in the presence of air the lead is quickly attacked, with formation first of the hydrate and finally of which is ap preciably soluble in water to give an alkaline liquid. When car bonic acid is present the dissolved oxide is soon precipitated as basic carbonate, so that the corrosion of the lead becomes con tinuous. Since all soluble lead compounds are strong cumulative poisons, danger is involved in using lead cisterns or pipes in the distribution of pure waters. The word "pure" is emphasized be

cause experience shows that the presence in a water of even small proportions of calcium bicarbonate or sulphate prevents its action on lead. All impurities do not act in a similar way. Ammonium nitrate and nitrite, for instance, intensify the action of a water on lead. Even pure waters, however, such as that of Loch Katrine (which forms the Glasgow supply), act so slowly, at least on such lead pipes as have already been in use for some time, that there is no danger in using short lead service pipes even for them, if the taps are being constantly used. Lead cisterns must be unhesitatingly condemned.

Aqueous non-oxidizing acids generally have little or no action on lead in the absence of air. Dilute sulphuric acid (say an acid of 20% or less) has no action on lead even when air is present, nor on boiling. Strong acid does act, the more so the greater its concentration and the higher its temperature. Pure lead is far more readily corroded than a metal contaminated with 1% or even less of antimony or copper. Boiling concentrated sulphuric acid converts lead into sulphate, with evolution of sul phur dioxide. Dilute nitric acid readily dissolves the metal, with formation of nitrate Pb(NO3)2. Lead has the peculiar property of making a lustrous black streak on paper.

Lead Alloys.

Lead unites readily with almost all other met als ; hence, and on account of its being used for the extraction of (for instance) silver, its alchemistic name of saturnus. Alloys with the following metals may be named : Antimony.—An alloy of 83 parts of lead and 17 of antimony is used as type metal ; other proportions are used, however, and other metals added besides antimony (e.g., tin, bismuth) to give the alloy certain properties.

Arsenic renders lead harder. An alloy made by addition of about of arsenic has been used for making shot.

Bismuth and Antimony.—An alloy consisting of 9 parts of lead, 2 of antimony and 2 of bismuth is used for stereotype plates. Bismuth and Tin.—These triple alloys are noted for their low fusing points. An alloy of 5 of lead, 8 of bismuth and 3 of tin fuses at 94.4° C, i.e., below the boiling-point of water (Rose's metal). An alloy of 15 parts of bismuth, 8 of lead, 4 of tin and 3 of cadmium (Wood's alloy) melts below 7o° C.