SULPHATE OF MAGNESIA.

365

Magnesia Sulphas. — Sul'phate of Magne'sia.

History. —This salt was originally procured from the Epsom watersby Dr. Grew in 1675. It has had a variety of names, such as Epsom orthe bitter purging salt, sal Anglicum, sal Seidlitzense, sal catharticum,vitriolated magnesia, &c.

Natural History. —It is a constituent of sea and many mineralwaters: it occurs as an efflorescence on other minerals, forming the hairsalt of mineralogists ; and with sulphate of soda and a little chloride ofmagnesium, constitutes Reussite.

Preparation. —The two great sources of the sulphate of magnesia ofEnglish commerce are dolomite and bittern. Dolomite or magnesianlimestone is a compound of carbonate of lime and carbonate of magnesia.It occurs in enormous quantities in various counties of England (as thoseof Somerset, York, and Nottingham ), and is employed for building.York Minster and Westminster Hall are built of it. Bittern is the resi-dual liquor of sea-water, from which common salt (chloride of sodium)has been separated. It contains chloride of magnesium and sulphate ofmagnesia.

Sulphate of magnesia may be extracted from bittern by evaporation,a process practised at Lymington, in Hampshire (Dr. Henry, Phil. Trans.1810, p. 94). If sulphuric acid be added to bittern, a further quantity ofI sulphate may be obtained by the decomposition of chloride of mag-nesium. Sulphate thus procured is preferred at Apothecaries’ Hall formaking magnesia, as it yields a whiter product than sulphate made fromdolomite.

Various methods of manufacturing sulphate of magnesia from dolomitehave been described. The usual method is to treat this mineral bydilute sulphuric acid: carbonic acid escapes, and a residue, composed ofsulphate of magnesia and sulphate of lime, is obtained. These two saltsare separated from each other by crystallization.

In 1816, Dr. William Henry, of Manchester (Repert. of Arts,v ol. xxx.p. 142, 2nd Ser.) took out a patent for the following process:—Calcinemagnesian limestone, so as to expel the carbonic acid; then convert thecaustic lime and magnesia into hydrates by moistening them with water.Afterwards add a sufficient quantity of hydrochloric (or nitric or acetic)acid (or chlorine), to dissolve the lime, but not the magnesia, which,after being washed, is converted into sulphate by sulphuric acid (or,where the cost of this is objectionable, by sulphate of iron, which iseasily decomposed by magnesia). Or the mixed hydrates of lime andmagnesia are to be added to bittern: chloride of calcium is formed insolution, while two portions of magnesia (one from the bittern, the otherfrom the magnesian lime) are left unacted on. Or hydrochlorate of ammo-nia may be used instead of bittern : by the reaction of this on thehydrated magnesian lime, chloride of calcium and caustic ammoniaremain in solution, while magnesia is left undissolved: the ammonia isseparated from the decanted liquor by distillation.

Carbonate of ammonia has also been employed to separate lime frommagnesia: carbonate of lime is precipitated, and the magnesia remainsin solution, from which it may be easily separated by ebullition ( Journ.of Science, iii. 217; vi. 313; ix. 177). At Monte della Guardia, near