So'dium, a metallic element discovered by Sir Humphry Davy in the year 1807. It is silver-white, and has a very high luster. It melts at 204 degree F. and oxidizes rapidly in the air, though not so rapidly as potassium. It decomposes water instantly, but does not spontaneously take fire when thrown on water, unless the water be somewhat warm or unless the progress of the globule of sodium upon the surface of the water be impeded. When heated in air or oxygen it takes fire and burns with a very pure and intense yellow flame. It is perhaps more abundant in our globe than any other metal, for it constitutes two-fifths of all the sea salt existing in sea water and is present in the water of springs, rivers and lakes, in almost all soils and in the deposits of rock salt. Sodium is also contained in sea plants and in land plants growing near the sea. It occurs, also, in most animal fluids. It is used as an agent in the manufacture of aluminium and magnesium, and as a reagent in chemical operations. Common salt is a compound of chlorine with sodium (See SALT). Sodium also occurs as oxide of sodium in a good many minerals; but it is most common in the form of carbonate, nitrate and borate of soda. Of the numerous salts of sodium, many are important medicinal agents, and others are valuable in other ways. Sodium arsenate is used as a substitute for arsenic and is an important element in fly poisons; sodium bromide and sodium iodide are prescribed for quieting the nerves; sodium bicarbonate is one of the important ingredients of baking powder and is used in the manufacture of mineral waters; sodium hypophosphite is another remedy which is prescribed for restoring health to the nervous system. See SALTPETRE; WATER GLASS.