SULPHATE OF MAGNESIA.

513

posed to the air they very slowly and slightly efHoresce. They dissolve in theirown weight of water at 60°, and in three-fourths of their weiglit of boiling water.They are insoluble in alcohol.

Cliaracleristics. —Sulphate of magnesia is known to contain sulphuric acid bythe tests for the sulphates already mentioned (p. 406.) The nature of its baseis shown by the tests for magnesia before described (p. 506.)

Composition. —The following is the composition of crystallized sulphate ofmagnesia:—

Magnesia......

Sulphuric Acid.Water..........

Atoms... 1 ... ] .

Eq. wt.

,. 20 ..,. 40 .... 03 ..

Per Cent. Gay-Lussac. Wenzel.

10-26 ...... 1004 ...... 16-88

32-5251-S2

32 53 ...... 3004

51-43 ...... 52 50

Crystald Sulphate of Magnesia ,

123

10000 ...... 10000

Purity. —The sulphate of magnesia raet with in the shops is usually suffi-ciently pure for all medicinal and pharmaceutical purposes. It should be colour-less, and its dilute Solution should undergo no change when mixed with ferroey-anides or hydrosulphurets. When obtained from bitteru it is sometimes conta-rninated with chloride of magnesium, which, by its affinity for water, keeps thesulphate in a damp State. By digestion in alcohol the chloride is dissolved; and,by evaporation, the spirituous Solution may be obtained in the solid State. It issaid, that occasionally small crystals of sulphate of soda are intermixed with thoseof sulphate of magnesia—a fraud I have never met with in English commerce,nor is it likely to oeeur at the present low price of the magnesian salt. Shouldsuch an adulteration be suspected, there are several melhods of detecting it: thesophisticated salt would efHoresce more rapidly than the pure salt, and wouldcommunicate a yellow tinge to the flame of alcohol. Boiled with caustic limeand water, all the magnesian sulphate would be decomposed, and the liquor beingfiltered (to separate the preeipitated magnesia and sulphate of lime) would yield,on evaporation, sulphate of soda. If shaken in the cold with carbonate of baryta,a Solution of carbonate of soda would be obtained, easily recognised by its alka-line properties.

Very rcadily dissolved by water. Sulphuric acid dropped into the Solution does not expelany hydrocliloric acid. 100 grs. dissolved in watcr, and mixed with a boiling Solution of car-bonate of soda, yield 34 grains of carbonate of magnesia when dried. Pli. Land.

The avolution of hydrochloric acid gas would be a proof of the presence of achloride. If less than 34 grs. of carbonate of magnesia bc obtained, the presenceof sulphate of soda may be suspected.

" Ten grains dissolved in a. fluid-ounce of waler, and treated with a Solution of carbonateof ammonia, are not cntirely preeipitated by 280 minims of Solution of phosphate of soda."Ph.Ed.

Physiological Effects. — Sulphate of magnesia is a mild and perfectly safeantiphlogistic purgative, which promotes the secretion as well as the peristalticmotion of the alimentary canal. It is very similar in its Operation to sulphate ofsoda, than which it is less likely to nauseate, or otherwise disorder the digestivefunetions, while it acts somewhat more speedily on the bowels. It does not oc-casion nausea and griping, like some of the vegetable purgatives, nor has it anytendency to create febrile disorder or inflammatory Symptoms; but, on the otherhand, has a refrigerant iiiflueiice: hence it is commonly termed a cooling purga-tive. In small doses, largely diluted with aqueous fluids, it slightly promotesthe action of other emunetories: thus, if the skin be kept cool, and moderate exer-cise be conjoined, it acts as a diuretic; whereas, if the skiri be warm, it operatesas a diaph'oietic. Dr. Christison [Trealise on Poisons, 3d edil, p. 603.) men-tions a case of supposed poisoning, in a boy of ten years old, by two ounces ofEpsom salts. The Symptoms were staggering, imperceptible pulse, slow anddifficult breathing, extreme debilitv, and death within ten minutes, withouf vo-

Vol. I.— 65

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