PAKT II.
Materia Medica.
245
commonly combined with gypsum 1 ; and in solution in sea-water, and several mineral springs. It was first artificiallyobtained in England in 1675, from the evaporation of the waterof the Epsom spring : whence it was named Epsom salt: andin 1700 it was made in considerable quantity from two springsat Shooter's-hill, in Kent *; but the discovery of it in bittern,or the residual brine after the crystallization of sea-salt, soonopened a more copious source from which it might be obtainedat all times; and for many years past, all the sulphate of mag-nesia used in this country has been manufactured from bittern.This substance consists chiefly of muriate of magnesia, muriateof lime, some common salt, and a small portion of sulphate oflime; and therefore it is probable, that the sulphate of mag-nesia is obtained by decomposing the muriate by means of sul-phate of iron, or sulphuric acid in some form, although someaffirm that the bittern is only boiled down to a high point ofconcentration; when the sulphate of magnesia forms, and ispurified by a second solution and crystallization. The sul-phate found in the shops generally contains some muriate ofmagnesia, which renders it deliquescent; and consequently itrequires to be preserved in close covered jars. It is often adul-terated with Glauber salt, which is made to resemble Epsomsalt, by stirring it briskly, when it is about to crystalise. Itmay be detected, by precipitating the magnesia by pure am-monia, aiding by heat; filtering, and evaporating the filteredfluid to dryness, by a heat sufficient to volatilize the sulphateof ammonia. If it contained glauber salt, the soda will remainfixed. Muriate of lime is detected by the oxalic acid.
Qualities. —Sulphate of magnesia is inodorous, and has avery bitter nauseous saline taste. It is usually in small needle-like crystals, but the form of its regular crystal is a quadran-gular prism, acuminated by four planes. When pure it efflo-resces; 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 3j of sulphate of magnesia in 13J of water, mea-sures eleven fluid drachms and a quarter. Heat expels itswater of crystallization; and the mass is melted, but not de-composed ; it loses merely its water of crystallization, and aminute portion of its acid. According to Bergman, 100 partsconsist of 29*35 of sulphuric acid, 17 of magnesia, and 53'65 ofwater of crystallization. 3 Its specific gravity is 1 "66. It is de-
' In the gypsum quarries of Piedmont; and, as Proust relates, it abounds so much inSpain, that in Andalusia large tracts are covered with an efflorescence of it after iloods,Joitru. de Physique, xxxiii. 312.
1 It is also made in Bohemia from the mineral water of Sedlitz.
l According to Dr. Henry, the composition is, acid 38, magnesia 18, and water 44parts, in 100 of the salt.
R 3