MAGNESIUM (symb. Mg. eq. 12—new system, 24 sp. gr. 1.74) is generally ranked with those metals whose oxides form the alkaline earths (baryta, strontia, lime), but in many respects it more closely resembles zinc. It is a malleable, ductile metal, of the color and brilliancy of silver. It fuses at about the melting-point of tin (about 442°), and at an extreme heat it tnay be distilled like zinc. When ignited in dry air or in oxygen gas, it Mims with extraordinary brilliancy, and is oxidized into magnesia. In dry air it undergoes little change, and is much less oxidizable than the other uietals of the same group. It does not decompose cold water; but if the water be heated to about 90°, there is a slight evolution of hydrogen ; and if the temperature is raised to 212°, hydrogen is given off rapidly and abundantly. When thrown into strong hydrochloric acid, it inflames and becomes converted into chloride of magnesium, while hydrogen is given off.
It is obtained from its chloride either by the action of sodium or potassium, or by simple electrolytic decomposition; but the ordinary processes are difficult, and yield the metal only in-minute quantities. A patent has, however, been taken out by Mr. Sonstadt for improvements in its manufacture, by which it can be produced by the pound.
_Magnesia (Mg0) is the only oxide of maomesium. It is a white bulky powder, devoid of taste or smell, and having a sp gr. of 3765; it is infusible, and almost insoluble in water; and when placed on nioistened test paper, is seen to have an alkaline reaction. When mixed with water; it gradullls- forms a hydrate (MgO,H0), without, as in the case of lime, any sensible elevation of heat, and this hydrate slowly absorbs carbonic acid front the atmosphere. Magnesia (loes not occur native, and is usually obtained by the prolonged application of heat to the carbonate. Hydrate of magnesia occurs natu rally in a crystalline form in the mineral brucite.
Magnesia alba, the common white magnesia of commerce, is a mixture of the hydrate of magnesia and of hydrated carbonate. It is zibtaincd by the precipitation of a hot solution of sulphate of magnesia by a hot solution of carbonate of potash or soda, and by then collecting and drying the deposit.
Of the magnesian salts, some are soluble and some insolulile in water. The soluble salts have a peculiar and very bitter taste, and hence the German name, bittererde (bitter. earth), for magnesia. All the salts which are iusoluble in water, except the silicate, dis
solve in hydrochloric and nitric acids.
Carbonate of magnesia occurs native in the mineral magnesite, and in association with carbonate of lime in dolomite, from which it may be manufactured in a very pule state by 3Ir. Pattinson's process, which consists essentially in the followim, steps: Finely ground dolomite is exposed for some time to a red heat, by which the carbonate of mag nesia is decomposed; the powder is then introduced into a, very strong vessel, where it is mixed with water, and carbonic acid gas forced in under heavy pressure till it ceases to be absorbed; the carbonate of magnesia becomes dissolved as bicarbonate, while the carbonate of lime remains unchanged; on boiling the clear liquid, carbonate of magnesia is deposited, and carbonic acid expelled.
Sulphate of magnesia, or _Epsom salts (MgO,S03-1-7Aq), is the most important of the magnesian salts. It is obtained from sea-water, or from magnesian limestone (dolomite), or from the mother-liquor of alum-works, by processes into which we have not space to enter, and is a common ingredient in mineral waters (see EPSOM SALT). It is soluble in three times its weight of water at 60°, and in less water at a higher temperature, the solution having a bitter, disagreeable taste.
_Nitrate of magne,sia (MgO,N0.+6Aq) occurs in certain mineral waters, but is of no special importance.
A phosphate of magnesia, having the formula H0,23IgO,P00-14Aq, is obtained by the mixture of solutions of sulphate of magnesia and of ordinary phosphate of soda. It occurs either in an amorphous state or in six-sided prisms, according as the solutions are more or less concentrated. This salt is a constituent of the seeds of wheat and the other cereals, of bones, and of various morbid concretions. The phosphate of ammonia and magnesia, known also as phosphate and as triple phosphate (NH40,23IgO,P0,1-12Aq), is a more important salt than the preceding. It occurs either in minute crystalline grains or in beautiful transparent four-sided prisms of consider able size. and with a very characteristic appearance. The formation of the salt, which is only slightly soluble in pure water, and is quite insoluble in water containing free ammonia. or its hydrochlorate, not only furnishes a very delicate test for the presence of magnesia, but enables us to determine its quantity.