Magnesium (sym. Mg, equiv. 24) is a metal which is very widely distributed over the globe. It is present in many minerals—e.g. dolomite—a carbonate of lime and magnesia; asbestos—a silicate of lime and magnesia; and meerschaum—a silicate of magnesia. It exists in mineral waters and in the sea as sulphate, and as chloride, the former being known as Epsom salts. It was from the Epsom spring, in 1695, that Drew extracted this well-known salt, and in the beginning of the 18th century Magnesia alba, so called to distinguish it from what was already known as Magnesia nigra (black oxide of manganese—so called from its resemblance in colour, weight, &c. to the magnet) was discovered. The metal was first prepared by Davy, and for long its manufacture was limited to a small scale. Now, however, it is made in quantity by fusing together the chlorides of potassium and magnesium and fluor spar, and adding metallic sodium with great care. The crude metal is finally distilled and pressed in a semi-fluid state into ribbon or wire.
Magnesium has a silver-white colour, which is tarnished by moist air. It is a very light metal, its specific gravity being only 1.75. It is readily volatile, and, when lighted, burns in air with an intensely brilliant light rich in chemical rays. On this account it was, till superseded by the electric light, much used in photography, while in signalling and pyrotechny it plays an important part.
When magnesium burns in air it forms a white ash consisting of the oxide, magnesia, MgO (which may be also prepared by heating the carbonate). This is a very infusible substance, and is much used in medicine under the name of calcined magnesia. The carbonate, MgCO3, is found in nature, but for medical purposes it is prepared by precipitating a soluble magnesium salt with carbonate of soda. According as it is prepared in the hot or cold, the resulting carbonate forms the ponderous and dense or the light variety. Although insoluble in water, this substance readily dissolves in water containing carbonic acid, and this solution is known as fluid magnesia. The sulphate, MgSO4·7H2O, or Epsom salts, occurs in nature, and is well known as a domestic remedy. It is much employed in febrile affections, but it may be used in any case in which a mild but efficient laxative is required. Its dose varies from to 1 ounce, but in order to promote its full efficacy it should be taken along with copious draughts of water. In combination with infusion of senna it forms the ordinary black draught. Magnesia and the carbonate are employed in small doses as an antacid, but in larger quantity they have a distinct purgative action. Fluid magnesia (see above) is a valuable aperient for women and children. Citrate of magnesia is the popular name for a granular, effervescing aperient, now much in use. It consists of a mixture of bicarbonate of soda, tartaric and citric acids, sugar, and a small trace (1 to 5 per cent.) of Epsom salts.