Syn. Epsom Salt.

Preparation

Sulphate of magnesia exists in nature as an ingredient of sea water and the water of certain springs, and is occasionally found in caves, and efflorescing on the surface of soils, which either contain it, or the materials out of which it is formed by chemical reaction. For use it has been procured from various sources. Formerly most of the salt employed was obtained by evaporation and crystallization from bittern, which is the mother-water left after the crystallization of common salt from concentrated sea water. Thus procured, however, it contained a little chloride of magnesium, which, from its deliquescent property, disposed it to be always moist. in Great Britain, most of it is now prepared from magnesian limestone, consisting of the carbonates of magnesia and lime, which, being treated with dilute sulphuric acid, yields the sulphates of magnesia and lime, which are separable by their very different solubilities. Our own markets are chiefly supplied with a beautiful form of the salt, largely manufactured, in Philadelphia and Baltimore, from mag nesite or silicated hydrate of magnesia, by saturating the powdered mineral with sulphuric acid, calcining the resulting compound in order to separate iron, and then purifying the sulphate of magnesia by repeated solution and crystallization.

Properties

As usually kept in the shops, sulphate of magnesia is in fine transparent acicular crystals, produced by agitating the solution at the moment of crystallization. When carefully crystallized, it forms larger four-sided prisms, terminated by two or four-sided summits. it is slightly efflorescent, inodorous, of a saline, bitter, nauseous taste, soluble in its own weight of water at 60° F., and in three-quarters of its weight of boiling water, and insoluble in alcohol. When heated, it melts in its water of crystallization, which is driven off by a continuance of the heat. The crystals contain about 51 per cent. of water of crystallization; and the salt is consequently twice as strong in the anhydrous form as in the crystallized. in the latter state, it consists of one equivalent of acid, one of base, and seven of water.

Incompatibles. This salt is decomposed by the alkalies and their carbonates; by lime, baryta, and their soluble salts; by the soluble salts of lead, with which it forms insoluble sulphate of lead; by a solution of nitrate of silver containing fifteen grains or more to the fluidounce; and by solutions of the protosalts of mercury, but not the persalts or corrosive sublimate.

Medical Uses

Sulphate of magnesia was known as a purgative so long since as near the close of the seventeenth century, when it was procured from the waters of Epsom springs, in England, by Dr. Grew; but it was relatively little employed until after the beginning of the present century. it is now the saline cathartic most used, having for more than forty years superseded sulphate of soda, and completely usurped the common name of salts, formerly attached to that cathartic. it is an excellent saline purgative, possessing all the properties of this subdivision of cathartics in a high degree, and preferable to sulphate of soda, which alone equals it in efficiency, on account of its less disagreeable taste, and greater acceptability to the stomach. it may, therefore, be used for all the purposes to which the saline cathartics are applicable. in colica pictonum, it has the additional advantage of forming an insoluble sulphate of lead with any salt of that metal which may happen to be in the primae viae, and thus acting, in some measure, as an antidote.

The full medium dose of the salt is an ounce; but it will generally operate in half the quantity; and even a drachm or two, taken before breakfast or at bedtime, will often open the bowels once at least. The dose may be given dissolved in from two to four ounces of water. When the stomach is irritable, the best mode of administration is in solution in carbonic acid water, flavoured with lemon syrup. The unpleasant effect of its taste may be entirely counteracted by the following method of exhibition. The requisite dose being dissolved in the least quantity of water, let the patient draw a full breath, then swallow the solution quickly, and immediately afterwards, before allowing the breath to escape, a little lemonade, or some other agreeably sapid liquid. Persons who usually shudder at the thought of the medicine, can take it in this way without the least inconvenience.

This salt is often given with the infusion of senna, the griping effect of which it is thought to counteract; and frequently also with magnesia, when there is excess of acid in the primae viae. in gouty and rheumatic diseases it is often associated with magnesia and wine of colchicum root, as recommended by Scudamore. The formula which I generally employ is half an ounce of the sulphate, half a drachm of the magnesia, and twenty drops of the wine.

The British Pharmacopoeia has an Enema of Sulphate of Magnesia (Enema Magnesia Sulphatis, Br.; Enema Catharticum, Ed., Dub.; Cathartic Clyster), made by dissolving an ounce of that salt in fifteen fluidounces of mucilage of starch, and mixing a fluidounce of olive oil with the solution. The whole is injected at once when there is an indication for a cathartic enema.