Magnetism and chlorine form Chloride of magnesium (Mg Cl). Magnesia dissolves in hydrochloric acid, forming chloride of magnesium, but thin solution cannot bo eva porated to dryness without undergoing decomposition, miles, chloride of ammonium is present, when a stable double salt is formed. This salt loses its chloride of ammonium when heated to redness, and the fused chloride of magnesium may then be poured out on to a slab. Thna obtained, it forms a white crystalline semi-translucent solid, very deliquescent, very soluble in water, and tolerably so in alcohol. It quickly decomposes in contact with air, and must therefore be kept in well closed bottles.
Carbossau of magnesia (MgO, CO,), occurs native in acicular crystals. [Ilawresixt, in NAT. HIST. DIV.] It is also deposited in small pris matic crystals, containing three equivalents of water of crystallisation, when a solution of magnesia in carbonic acid is allowed to evaporate spontaneously.
Magnesia alba, the common magnesia of the shops, is a mixture of carbonate and hydrate of magnesia (MgO, CO, + MgO, HO) in very variable proportions, the carbonate however nearly always preponder ating. it is made by mixing together solutions of sulphate of magnesia and carbonate of soda, the latter being in excess, collecting, well washing, and drying the precipitate. It is an exceedingly light powder, inodorous, and almost tasteless. Its density varies according to the strength of the solutions employed, being greatest when the two salts are mixed together, without the addition of water, fused in their water of crystallisation, evaporated to dryness, and the sulphate of soda sub sequently washed out. The heavy carbonate thus produced is, however, liable to be semi-crystalline, or gritty.
Bicarbonate of magnesia (MgO, 2 CO,). This salt exists only in solution, and is made by passing carbonic acid through water in which carbonate of magnesia is suspended.
Sulphate of magnesia (MgO, SO, + 7 110), This salt was originally obtained in 1697, from the Epsom waters, by Dr. Grow, and hence its common name of Epson, salts. It occurs in sea-water, and in many mineral waters, and is largely prepared from bittern, or the mother liquor from which common salt has crystallised after sea-water has been evaporated; the first crystallisation is called single epsoms, and when recrystallieed is denominated double,. The principal source, however, of sulphate of magnesia is dolomite, or magnesia's limestone, a double carbonate of limo and magnesia. [31soxestax times-rose, in NAT.
Div.) Chipping. of this stone may be readily procured, as it is in very common use for building purposes, and on acting upon it with sulphuric acid, the carbonic, acid is evolved, sulphates of lime and magnesia are funned, and the latter is separated from the former by simple solutions In water, sulphate of magnesia being very soluble, while sulphate of lime is almost insoluble. On evaporating the solu tion, the sulphate of magnesia crystallises out. When prepared in this
way, however, It frequently contains nulphate of Iron, but this may, if necomary, be got rid of by calcining the salt. The sulphate of iron is then decomposed, peroxide of Iron remaining while the sulphate of magnesia is unaltered, and may be dissolved away from the oxide of Iran by water.
tan evaporating and slowly cooling a solution of sulphate of magnesia, large four-sided rhombic prisms are obtained, but by rapid cooling and frequent agitation small acioular crystals are deposited, forming the Bpstes salts as met with In pharmacy.
Sulphate ofmagnesia is colourless, transparent, and inodorom, but has an extremely bitter taste. Heated, it faces, parts with its water of crystallisation, and finally becomes anhydrous. The crystals slowly efllorosce in very dry air. Five parts dissolve in four of water at 66' Fehr., and in still less of boiling water. Sulphate of magnesia is also exceedingly soluble in hydrochloric acid, but insoluble In alcohol.
Nitrate of magnesia (31g0, NO. + 6 110). Formed by dissolving carbonate of magnesia In nitric acid, and concentrating by evaporation. It then crystallises out in rhombic prisms and needles, very deliques cent, very soluble in water, and soluble also in nine times their weight of alcohol. They are almost insoluble in absolute alcohol.
Phosphate of magnesia and ammonia (2 MgO, N11,0, PO, + 12 110). Precipitated in a granular form, when phosphate of soda is added to a solution of magnesia in chloride of ammonium, containing also ammonia. By heat it is converted Into bibasie phosphate, or pyre phosphate, containing per cent. of magnesia.
Silicates of magnesia. Many of these occur native. See Sxturrri-, 31tenscnaust, and llonNuLEXUE, in NAT. HIST. DIV.] Citrate of magnesia (3 Mg0, Ci, 110). Prepared by saturating citric acid with carbonate of magnesia. It is leas hitter than most of the maguesian salts.
Effervescent citrate of magnesia, or granular citrate, as it is sometimes called, ie nothing more than a mixture of bicarbonate of soda and tartaric acid, with a small but variable quantity of sulphate of magnesia : sometimes it contains a little citrio acid. It is made by well mixing the ingredients, slightly moistening till the mass begins to swell from disengagement of carbonic acid, and then rapidly drying over a water bath with constant stirring.
Double salts of magnesia, especially with alkaline salts, are very numerous.
Tests for magnesia. Magnesia Is characterised by its not being pre cipitated from solution by hydrosulphate or carbonate of ammonia in the presence of chloride of ammonium, and by a granular precipitate with phosphate of soda and with emulate of ammonia, especially if free ammonia is present.
Estimation of magnesia. This is generally effected in the state of bibmic phosphate (21g0, P0,), in the manner already mentioned under phosphate of magnesia.