MAGNESIUM, a metallic element whose compounds are abundant and widely distributed, but which does not occur, in nature, in the metallic form. Magnesium resembles calcium in its chemical deportment, and the oxides of the two metals were long confused with each other. Metallic magnesium was first prepared by Davy, in 1808, both by electrolysis and by the reduction of white-hot magnesia in an atmosphere of potassium vapor. In 1830 Bussy obtained a larger and purer yield of the metal, by heating a mixture of potassium and anhy drous magnesium chloride to redness. It is best prepared by the electrolysis of the fused anhydrous chloride or of a mixture of mag nesium chloride with the chlorides of sodium, potassium and ammonium. Pure magnesium is silvery-white in color, lustrous and moderately hard. It may be hammered, rolled, filed and polished. Its specific gravity is about 1.75 and its specific heat about 0.245. Its melting point is variously given, the estimates ranging from 850° F. to nearly 1,500° F. When raised to a bright-red heat (out of contact with the air) it volatilizes, depositing upon cool surfaces again in the form of lustrous silvery crystals which belong to the hexagonal system, and are isomor phous with those of zinc. It expands by 0.000015 of its own length, per Fahrenheit degree of rise of temperature; and at 32° F. its electrical resistance is 0.0438 of that of mercury.
Chemically, magnesium is a dyad. It has the symbol Mg, and an atomic weight of 24.36 if or 24.18 if H Its most important compounds are the °icicle, MgO, the chloride. MgCla, the sulphate, MgS02, and the carbonate, Mg00a. Metallic magnesium is not altered upon exposure to dry air, but ordinary air oxidizes it superficially. It dissolves readily in dilute acids, with the formation of the corre sponding salts. Chlorine, bromine, iodine, fluorine, sulphur, phosphorus and arsenic com bine with it directly. Red-hot metallic mag nesium also slowly combines with free nitrogen to form a solid nitride. In the isolation of argon, helium and the other rare gases of the atmosphere advantage is taken of this fact for separating these gases from the nitrogen of the air. (See ARGON). When strongly heated in the air, metallic magnesium takes fire and burns with an exceedingly brilliant white light that is rich in chemical rays; the product of the com bustion being magnesia, MgO. Advantage is taken of this property in photography, most of the gash-light° powders that are used consist ing essentially of pulverized magnesium, either alone or mixed with a small quantity of some explosive or oxidizing agent. Magnesium will also burn when sufficiently heated in steam, carbon dioxide or sulphur dioxide.
Magnesium oxide, or "magnesia.? MgO, is usually prepared by heating the nitrate or car bonate of the metal; and on account of this method of preparation it is commonly known as °calcined magnesia.? Magnesia is a white sub stance, without taste or odor. It does not have a strongly alkaline reaction, but it acts as a powerful base, reacting with acids to form the magnesium salts. It is scarcely soluble in water, but it slowly absorbs moisture and carbon dioxide from the air, becoming converted into a mixture of the hydrate and carbonate. When made into a paste with water, magnesia sets to a hard, white mass, consisting partly or wholly of the hydrate, Mg(OH)2; but this action does not occur if the magnesia has been previously heated to whiteness. One of the most distinctive characteristics of magnesia is its infusibility. Even when heated in the flame of the oxyhydrogen bloWpipe it does not melt, but gives out a bright white light, somewhat similar to that emitted by lime. (See CALCIUM LIGHT). In the fiercer heat of the electric fur nace, magnesia has been melted. On account of its infusibility magnesia is used in the manu facture of crucibles and of firebrick. It occurs native as the mineral periclase, which crystal lizes in the isometric system.
Magnesium chloride, MgCl, is prepared by dissolving magnesia in hydrochloric acid, and evaporating the solution after the addition of a certain quantity of sal ammoniac. Upon sub sequent fusion the sal ammoniac volatilizes and the magnesium chloride is left behind. This substance is largely used as a preventive of mildew, in the sizing of cotton cloth. The min eral carnallite contains magnesium chloride, having the composition MgC12-1- KCl + 6H10. Magnesium sulphate occurs native (in combina tion with one molecule of water) as kieserite, and it may also he prepared artificially by dis solving magnesium oxide or carbonate in dilute sulphuric acid. When combined with seven molecules of water, magnesium sulphate consti tutes the familiar substance known as Epsom salts (q.v.), which is largely used in medicine. Magnesium carbonate, MgCO,, is a white sub stance, insoluble in water, but soluble in a solution of ammonium chloride, and also in water that contains carbon dioxide in solution. It occurs in nature as the mineral magnesite, which crystallizes in rhombohedral forms, iso morphous with calcite. Dolomite, which occurs in nature in enormous quantities, is a carbonate of magnesium and calcium.
Magnesium salts are used to a considerable extent in medicine. The name °magnesium* is derived from °magnesia,* which substance is said to have been obtained from the province of Magnesia in Thessaly.