_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 (1/10)th in. thick is ruptured by a charge of about 30 lb. The specific gravity is 11.352 for ingot, and from 11.354 to 11.365 for sheet lead (water of 4°C. = 1). The expansion of unit-length from 0°C. to to 100°C. is .002948 (Fizeau). The conductivity for heat (Wiedemann and Franz) or electricity is 8.5, that of silver being taken as 100. It melts at 327.7°C. (H. L. Callendar); at a bright-red heat it perceptibly vapourizes, and boils at a temperature between 1450° and 1600°. The specific heat is .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 of the hydrate, Pb(OH)2, which is appreciably soluble in water forming an alkaline liquid. When carbonic acid is present the dissolved oxide is soon precipitated as basic carbonate, so that the corrosion of the lead becomes continuous. 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 because 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.