Land. Dub. Sulphas Mag-nesiae, Edin. Sulphate of Magnesia. Bitter purging Salt.

Syn. Sulphate de Maguesie (F.), Schwefelsaure Magnesia (G.), Englesk laxeersali (Dan.), Sale amaro, Ossisolfato di Magnesia (I.), Sal Amargo (S.), Sal Cathartico amargo (Portug.).

This salt is found native in a pure state1; but it is more commonly combined with gypsum2 and other salts, and in solution in sea-water, and several mineral springs. It was first artificially obtained in England in 1675, by Dr. Grew, from the evaporation of the water of the Epsom spring : whence it was named Epsom salt: and in 1700 it was made in considerable quantity from two springs at Shooter's hill in Kent3; but the discovery of it in bittern, or the residual brine after the crystallization of sea-salt, soon opened a more copious source from which it might be obtained at all times; and, for many years past, a great part of the sulphate of magnesia used in this country has been manufactured from bittern. This substance consists chiefly of hydrochlorate of magnesia, hydro-chlorate of lime, some common salt, and a small portion of sulphate of lime; and, therefore, it is probable, that the sulphate of magnesia is obtained by decomposing the hydro-chlorate by means of sulphate of iron, or sulphuric acid in some form, although some affirm that the bittern is only boiled down to a high point of concentration, when the sulphate of magnesia forms, and is purified by a second solution and crystallization.

Much of the sulphate of magnesia, however, now sold, is prepared from magnesian limestone, Dolomite4, by a process invented by Dr. Henry of Manchester : the Dolomite is first calcined, then treated with hydrochloric acid, which takes up the lime and leaves the magnesia, which is then con-

1 In the mercury mines of Idria it is found crystallized, and named by the Germans Haarsalz. According to Klaproth, it contains 1 per cent, of oxide of iron. Analyt. Ess. 80. It also abounds in the great caverns in the Allighany mountains, in the United States of America.

2 It is found in the gypsum quarries of Piedmont; and, as Proust relates, it abounds so much in Spain, that in Andalusia large tracts are covered with an efflorescence of it after floods. Journ. de Physique, xxxiii. 312.

3 It is also made in Bohemia from the mineral water of Seidlitz.

4 In the neighbourhood of Genoa it is prepared by roasting a schistose rock which contains magnesia and sulphuret of iron; and then exposing it to the air under a shade for six months, occasionally watering it, and then lixiviating. Near Baltimore it is made by pulverizing a siliceous hydrate of lime, which abounds there, and saturating it with sulphuric acid, verted into the sulphate. As it contains no hydrochlorate of magnesia, it attracts no moisture from the atmosphere, and does not deliquesce, as that prepared from bittern. This sulphate is sometimes adulterated with Glauber salt, which is made to resemble Epsom salt, by stirring it briskly when it is about to crystallize. It may be detected by precipitating the magnesia by pure ammonia, aided by heat; filtering, and evaporating the filtered fluid to dryness, by a heat sufficient to volatilize the sulphate of ammonia: if it contain Glauber salt, the soda will remain fixed.

Qualities.-Sulphate of magnesia is inodorous, and has a very bitter, nauseous, saline taste. It is usually in small needle-like quadrangular crystals, but the form of its larger crystals is a hexangular prism, acuminated by six planes. When pure, it effloresces; and is soluble in its own weight of water at 60°, increasing the volume of the fluid rather more than 4-tenths, or a solution of j. of sulphate of magnesia in f j. of water measures eleven fluid drachms and a quarter. Heat expels its water of crystallization; and the mass is melted, but not decomposed: it loses merely its water, and a minute portion of its acid. According to Dr. Henry, the composition of this sulphate is acid 38, magnesia 18, and water 44 parts, in 100 of the salt. Mr. Phillips (Translation of the Pharmacopoeia) states the components to be sulphuric acid 32.52, magnesia 16.26, water 51.22: or we may regard them as 1 equiv. of magnesia=20.7 +1 of acid 40.l+7 of water = 63, making the equivalent of the salt 123.8. Its specific gravity is 1.66. It is decomposed by the alkalies and their carbonates, by lime-water, the hydrochlorates of ammonia, of baryta, and lime, nitrate of silver, and the acetates of lead, which are therefore incompatible with it in prescriptions : but it is not precipitated by the sesquicarbonate of ammonia.

Medical properties and uses.-This salt is purgative and diuretic. It operates readily, without griping; and, notwithstanding its nauseous taste, is generally retained by the stomach when almost all other things are rejected, especially when it is administered in small, repeated doses, largely diluted, or united with acidulated, infusion of roses. In these forms it is a useful purgative in hypochondriasis, colica pic-tonum, ileus, puerperal fever, and in all acute diseases. In small doses, combined with compound spirit of ammonia, and dissolved in the infusion of quassia or gentian, it has been found very useful in dyspepsia accompanied with costiveness. It is also used as an adjunct to stimulating clysters. By moderate exercise in the open air while taking this salt, the purgative effect is diminished, and its diuretic property increased. The dose is from 3ss. to ij. dissolved in water, gruel, or

8 any other vehicle; and taken either at once, or in divided doses frequently repeated.

Officinal preparations.-Enema Catharticum, D. Enema foeti-dum, D. Magnesiae Carbonas, L. E. D.