SODIUM (from °soda," which word was used in the Middle Ages to designate alkaline substances in general), a metallic element, first prepared by Davy in 1807, by the electrolysis of molten caustic soda, or sodium hydroxide. In 1808 Gay-Lussac and Thenard showed that the metal can be prepared by reducing the hydroxide with finely divided metallic iron. Brunner, about 1823, obtained metallic sodium by reducing sodi um carbonate with carbon; and Deville, in 1855, improved Brunner's process so as to make it commercially practicable for the manufacture of sodium on the large scale. Deville's method, as practised in recent times, consists in distilling a mixture of 30 parts of calcined carbonate of sodium, 13 parts of coal and 7 parts of lime; the reduction taking place at a temperature of about 2.500° F., the metal being given off in the form of a vapor, which is then passed into a condenser and allowed to liquefy and solidify. As thus obtained, sodium contains numerous impurities, most of which may be removed by straining the melted metal through linen under rock oil, at a temperature of about 212° F. Deville's process of manufacture was used al most exclusively until about 1886, when the well known Castner process came into use. Castner reduced' sodium hydroxide by a combination of carbon and iron, prepared by coking a mixture of pitch and finely divided iron; the coked mass having approximately the composition of Fe.C. and being commonly known as "carbide of iron," although it is doubtful if this name is chemically justifiable. The Castner process possesses many practical advantages over that of Deville, and hence it rapidly came into almost universal favor. A considerable quan tity of metallic sodium is now prepared by the direct electrolysis of molten sodium hydroxide, or sodium chloride (common salt). Sodium possesses powerful reducing properties, and the metal is used in the preparation of silicon, boron, aluminum and other elements whose re duction by means of carbon is either difficult or impossible. Deville, in fact, developed his method of preparation with the object of using the sodium that it yielded in the subsequent preparation of metallic aluminum (q.v.) ; and for many years the commercial supply of aluminum was practically all prepared by re ducing aluminum compounds with metallic sodium. Since the development of the Hall process for the manufacture of aluminum by electrolysis, metallic sodium has lost a con siderable part of its former great commercial importance.
Sodium is a silvery-white metal, hard at 0° F., ductile at 32°, wax-like at ordinary tem perature and pasty at 120°. It melts at 207° F., and boils at about 1,700°. It oxidizes with great facility, and must be kept under benzene, or petroleum, or some other fluid that is devoid of oxygen, or in a neutral atmosphere of hydrogen, coal gas or some other substance for which it has no affinity. It may be pre pared in the form of lustrous, octahedral crystals, white in color but with a rosy sheen. Like potassium, it dissolves in anhydrous liquid ammonia (NH,), forming a blue solution from which the metal may be again obtained by the evaporation of the solvent. Sodium has a specific gravity of about 0.98, a specific heat (in the solid form) of 0.273, and a coefficient of linear expansion (Fahrenheit scale) of 0.0000395. At the freezing point of water, the specific electrical resistance of sodium is about one-eighteenth of'that of mercury.
Chemically, sodium is a monad. It has the chemical symbol of Na (from "natron," the Spanish name for native carbonate of sodium), and its atomic weight is 23.00 if 0 =16, or 22.82 if H =I. It forms a multitude of com pounds, many of which are of great importance in the arts. The chloride of the metal occurs in great abundance in nature, as common salt. (See SALT). Its formula is NaCI, and it is extensively used as a source of sodium in the preparation of other compounds of the metal. Sodium nitrate, NaNO,, commonly known as "Chile saltpetre," occurs native in large quan tities in Chile, and as it is much cheaper than the native nitrate of potassium, it is used largely in the manufacture of ordinary nitre; chloride of potassium and nitrate of sodium re acting together to form chloride of sodium and nitrate of potassium. (See NITRE). Sodium bromide, NaBr, which is extensively used in medicine as a sedative, is prepared by adding bromine to a solution of pure sodium hydrate, NaOH, till the liquid becomes slightly yellow. It is then evaporated to dryness, and strongly heated to decompose the bromate, NaBrOs, which is formed simultaneously with the bromide; after which the residue is redissolved and crystallized by evaporation.