Epsom Salt, the name given in pharmacy to the hydrated sulphate of magnesia, which was obtained as far back as the year 1675 by evaporating the waters of some mineral springs at Epsom, England. Sea water was afterward found to contain it, the brine remaining after the separation of the common salt consisting of the sulphate of magnesia and the chlorides of magnesium and calcium. It was readily obtained by collecting the first crystals which formed, and washing them with a strong solution of the same salt. An excellent quality is manufactured at Baltimore and Philadelphia,, from the mineral magnesite, a silicious hydrate of magnesia, which is found in the serpentine of that region. The mineral, reduced to powder, is dissolved in sulphuric acid. The product after drying is calcined in order to decompose the sulphate of iron, and convert it into the peroxide of iron. It is then dissolved in water, and any iron present is precipitated by sulphuret of lime. The crystals of sulphate of magnesia are separated and dissolved again to complete their purification.

This salt, and calcined magnesia also, have been prepared from the dolomite or magnesian carbonate of lime, by the process of Mr. William Henry of Manchester. The mineral was calcined, and the lime and magnesia were then converted into hydrates by sprinkling with water; the former was dissolved out by a minimum quantity of hydrochloric acid, and the latter was converted into a sulphate by sulphuric acid. - Epsom salt is also found as a mineral substance, incrusting the walls of caves, in the form of an efflorescence, and also in silky fibres. In the Mammoth cave, Kentucky, loose masses of it adhere to the roof like snow balls, and in many other caves of the western states it is found upon the walls or mixed with the earth upon the floor. It occurs in some of the gypsum quarries near Paris, and in other parts of France; and wherever water becomes charged with gypsum or sulphate of lime, and flows over rocks containing carbonate of magnesia, the sulphate of magnesia is likely to appear from the result of mutual decomposition of the two salts.

Hydrated sulphate of magnesia consists of 1 equivalent of magnesia, 20; 1 of sulphuric acid, 40; and 7 of water, 63=123; or magnesia 16.26 per cent., acid 32.52, and water 51.22. It crystallizes in four-sided prisms with reversed dihedral summits, or four-sided pyramids, of hardness 2.25, and specific gravity 1.75. The crystals effloresce slightly in the air, and if they contain any chloride of magnesium this is shown in their deliquescing. They dissolve in their own weight of water at 60°, and in three fourths their weight of boiling water. Sulphate of soda is sometimes fraudulently mixed with Epsom salt. Its presence may be detected by dissolving 100 grains in water, and precipitating with a boiling solution of carbonate of potash. Unless this precipitate of carbonate of magnesia amounts when dried to 34 grains, sulphate of soda is no doubt present. - The salt is much used as a cathartic, and, being mild and cooling, is particularly adapted to the treatment of fevers and inflammatory affections. The medium dose is an ounce, and this is said to be deprived of its bitter taste, and rendered quite palatable, by being dissolved in about a pint of water, and boiled a few minutes with 1 1/2 grain of tannic acid or 2 or 3 drams of roasted coffee, strained, and sweetened with sugar.

If given in small doses and frequently repeated, its action may be directed to the kidneys rather than to the bowels; and the same happens if for any cause, as for instance intestinal obstruction, it fails to be excreted in the usual manner. The value and activity of many of the celebrated mineral waters of this country and Europe depend upon the Epsom salts or Glauber salts which they contain. The laxative action of sulphate of magnesia is increased by freely diluting it with water. An ounce dissolved in a quart of water will act upon the bowels much more decidedly than the same quantity dissolved in half a pint. This fact serves to explain partly the action of certain natural laxative waters.