SODIUM [symbol Na; atomic number atomic weight 23.00 (0= i6)], a chemical element belonging to the group of alkali metals. It is abundantly and widely diffused in nature, but always in combination. Sodium chloride, or common salt (q.v.), is exceedingly common, being the chief salt present in sea-water, besides occurring in extensive stratified deposits. So dium carbonates are also widely dispersed in nature, forming constituents of many mineral waters, and occurring as principal saline components in natron or trona lakes, as efflorescences in Lower Egypt, Persia and China, and as urao in Mexico, Colom bia and Venezuela. The solid crusts found at the bottom of the salt lakes of the Araxes plain in Armenia contain about 16% of carbonate and 8o% of sulphate. In Colombia there occurs a double salt, known as gay-lussite. In Wy oming, California and Nevada enormous deposits of carbonates, mixed in some cases with sulphate and with chloride, occur. Vast areas of the steppes in Hungary contain sodium carbonate in the soil. Natural sulphate occurs in an anhydrous condition as thenardite, at Tarapaca, Chile, and in the rock-salt deposits at Espartinas near Aranjuez, Spain. Hydrated sulphates occur at several localities in certain provinces of Spain, and at Miihlingen in Aargau, and copious deposits of glau'berite, the double sulphate of sodium and calcium, are met with in the salt mines of Villarrubia in Spain, at Stassfurt, and in the province of Tarapaca, Chile, etc. A native nitrate of soda is obtained in great abundance in the district of Atacama and in Tarapaca, and was formerly imported into Europe in enormous quantities as cubic nitre for the preparation of saltpetre. Cryolite, a fluoride of aluminium and sodium, is extensively mined in Greenland and elsewhere for industrial purposes. These form the principal natural sources of sodium compounds—the chloride as rock salt and in sea-water being of such predominating importance as quite to outweigh all the others. But it is questionable whether, taken altogether, the mass of sodium they represent is as much as that disseminated throughout the rocky crust in the form of soda fel spar (i.e., as silicate of soda) and in other soda-containing rocks. From this source all soils derive small proportions of sodium in soluble forms, hence the ashes of plants, although they preferably imbibe potassium salts, contain traces and sometimes notable quantities of sodium salts, which also form essential ingredients in all animal juices.
Although many sodium compounds have been known from very remote times, the element was not isolated until 1807, when Sir H. Davy obtained it by electrolysing caustic soda. This method was followed by that proposed by Gay-Lussac and Thenard, who decomposed molten caustic soda with red-hot iron; and this in turn was succeeded by Brunner's process of igniting sodium carbonate with charcoal. In spite of many attempts, however, the metal could not be cheaply produced until electrolytic meth ods were perfected, and that patented by Castner in 1890 formed the basis of subsequent successful methods. If sodium chloride is used, there are several disadvantages : the fused salt and both the products of its electrolysis exert a destructive action upon containing vessels, and, further, the boiling point of the metal (877°) is inconveniently near the melting point (775°) of the salt. Borchers endeavoured to contend against the first diffi culty by employitig an iron cathode vessel and a chamotte (fire clay) anode chamber united by a specially constructed water cooled joint. The other difficulty is to some extent met by using a mixture of sodium, strontium and potassium chlorides, which melts at a lower temperature than the pure chloride. In Castner's process (as employed at Oldbury and Niagara Falls and in Ger many) fused caustic soda is electrolysed. The apparatus consists of an iron cylinder heated by gas rings below, with a narrower cylinder beneath, through which passes upwards a stout iron cathode rod cemented in place by caustic soda solidified in the narrower vessel. Iron anodes are suspended around the cathode, and between the two is a cylinder of iron gauze at the bottom with a sheet-iron continuation above, the latter being provided with a movable cover. During electrolysis, oxygen is evolved at the anode and escapes from the outer vessel, while the sodium deposited in globules on the cathode floats upwards into the iron cylinder, within which it accumulates, and from which it may be removed at intervals by means of a perforated iron ladle, the fused salt, but not the metal, being able to pass freely through the perforations. The sodium is then cast into moulds. Sodium hydroxide has certain advantages compared with chloride, al though it is more costly; its fusing-point is only 32o° C, and no anode chlorine is produced, so that both containing vessel and anode may be of iron, and no porous partition is necessary.