LEAD, one of the most useful metallic elements, well known in chemistry and in the arts. From early times references to it are found in literature. It is mentioned in Exodus xv, 10• Numbers xxxi, 22; Job xix, 24; and Ezekiel xxvii, 12, and is supposed to have been imported into Palestine from Tyre, although it should be remembered that there are lead mines in Sinai and the Lebanon, as well as in Egypt. Articles made of lead by the ancient Romans, such as water-pipes, water-tanks, weights. rings and small ornamental cylinders, are still preserved. Examples in the grounds of old churches show that the Roman method of mak ing pipes from sheet-lead continued in use till late in the Middle Ages. Small lead-weights of curious forms have been found among Vi king remains dating as early as the 10th cen tury. Many of the European countries are known to have produced lead as early as from the 10th to the 14th century.
Physical Properties.— Lead (chemical sym bol Pb. from the Latin name of the element, ni plubism, atomic weight about 207), is a soft metal of a bluish-white color, tending to gray, with a bright metallic lustre when newly cut or melted. It is tasteless but has a distinctive odor when rubbed. It soon tarnishes when ex posed to the air, taking on a thin film sup posed to be suboxide. But lead suffers less than most metals either from atmospheric agencies, or damp soils. It can be scratched with the nail, and easily cut and makes a mark upon paper. Its specific gravity varies from 11.352 in the ingot to 11.365 when rolled into sheets. It begins to melt at 619° F., and is completely liquid at 638°. Its boiling point is about 2,700'. In cooling from a molten condition lead crystallizes into regular octa hedrons from which the still liquid lead may be poured away. It may also be obtained in a crystalline tree-like formation known as arborescent lead by decomposing a solution of lead acetate by the electric current. Its hard ness is increased by repeated melting-s. It is highly malleable and in a less degree ductile, but its tenacity is small—a wire 1-12th of an inch being unable to carry a load of 20 pounds. Lead is not a good conductor of beat or electricity. It can be welded at ordi nary temperatures by pressing two freshly cut surfaces together; and when gently heated it can be forted by pressure through perforations and pipes. Water containing carbonic acid has
a slight action on lead, carbonate being formed and dissolved, but this action is apparently hindered by the presence of some salts and accelerated by others. When lead is heated to redness in air it is oxidized, litharge (PbO) being formed. Lead is attacked by nitric acid and hot strong sulphuric acid, but dilute sulphuric or hydrochloric acids have but little action. It is, therefore,. largely used for the construction of sulphuric acid chambers and for chemical works plants. It alloys readily with many metals.
Lead Ores.— Native lead occurs sparsely in some localities in Mexico, India and Wales. Its ores are found extensively, more or less pure or associated with other ores, in various parts of the United States, in Great Britain, Germany, Spain and other European countries. The most common form is the sulphite (PbS), known as galena. It occurs usually in veins, and is often associated with zinc and fre quently with silver. Ores of lead are found most, plentifully in rocks belonging to the Silurian Age, chiefly in the mountain lime stones. Some occurrences are true veins, others beds or pockets. In the United States the most common occurrence is as a filling a vertical crevice, and is in such cases generally a very pure galena. The lead-silver ores of the Cur d'Alene district in Idaho are in the form of veins, while those of the Leadville district in Colorado occur largely as irregular replacement deposits of great size in lime stones. In both the above districts, the ores are believed by some authorities to have been introduced by heated waters connected igneous activity. Others argue that the sence of other hot water sedimentary sub stances make this theory untenable. In the Flat River district of southeastern Missouri, the ores carry little silver, and occur as irregular disseminations in Cambrian dolomites. They are believed to have been introduced by cold waters which leached the ores from adjacent rocks where they occurred in minute quan tities. In the Joplin district of Missouri and in southwestern Wisconsin, considerable lead is associated with the zinc ores (q.v.). Cerus site, the lead carbonate, is common in the upper parts of certain lead deposits, where it is supposed to have resulted from the weather ing of the galena of the original ore.