MAGNESIA

A white, tastele , earthy substance. usually obtained by exposing its hydrated carbonate to a red heat. Its specific gravity is 2-3. It is almost insoluble; but when moistened and put upon turmeric paper it reddens it: this sometimes depends upon a trace of lime. It is an oxide of a brilliant white metal, which has been called magnesium,and which may be obtained by heating chloride of magnesium with pota ium: they act intensely upon each other, chloride of pota ium is formed, and magnesium separates: it may be washed with water and dried. Heated to redne in the air, it burns with great brilliancy into magnesia, 12 parts of the metal combining with 8 of oxygen to form 20 of magnesia. In commerce, pure magnesia is generally distinguished by the term calcined magnesia; and the hydrated carbonate of magnesia, obtained by precipitating a solution of sulphate of magnesia by carbonate of soda, and washing and drying the precipitate, goes by the name of magnesia, or magnesia alba. The chief use of magnesia and its carbonate is in medicine. Sulphate of magnesia is obtained by evaporating the residue of sea-water after the common salt has been separated, or by adding sulphuric acid to bittern and evaporating, so as to obtain the resulting sulphate of magnesia. This salt is also obtained by the action of dilute sulphuric acid on magnesian limestone, and it is not uncommon in mineral waters: it was formerly procured from certain springs near Epsom, in Surrey, and was hence termed Epsom salt. It crystallizes in four-sided prisms with dihedral summits. Its crystals are soluble in their weight of water at 00°, and in three fourths their weight at 212°. They melt when heated, and gradually lose their water of crystallization. They consist of 20 magnesia, 40 sulphuric acid, and 03 water. This salt is a useful purgative in medicine, and is the chief source of the other forms ■ magnesia] s.lts fa terish flavour. Magnesia is found native in the Btate-of hydrate and carbonate; it exists as a component part of several minerals, and many of them are soft or soapy to iho touch. [s. 712]