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Characteristics. —It is soluble in the dilute mineral acids withouteffervescence. The solution does not occasion any precipitate with thef’errocyanides, hydrosulphurets, oxalates, or bicarbonates. The neutralalkaline carbonates, when unmixed with any bicarbonate, throw down awhite precipitate. Ammonia with phosphate of soda causes a whiteprecipitate (ammoniacal-phosphate of magnesia.) Magnesia is insolublein alkaline solutions, and is thereby distinguished from alumina. Itssolution in sulphuric acid is remarkable for its great bitterness.
Composition. —Magnesia has the following composition:—
Eq. Eq.Wt. PerCent. Wollaston. Gay-Lussac. Berzelius .
Magnesium .... 1 .... 12 .... 60 ... . 59*3 .... 59*5 .... 61*29
Oxygen. 1 .... 8 .... 40 ... . 40*7 .... 40*5 .... 38*71
Magnesia .... 1 .... 20 .... 100 .. . 100*0 . . . 100*0 .... 100*00
Purity. —Its freedom from any carbonate is shewn by its dissolving in
dilute mineral acids without effervescence. Its hydrochloric solutionshould occasion no precipitate with the oxalates, bicarbonatcs, andbarytic salts,by which the absence of lime and sulphates maybe inferred.
Physiological Effects. —When taken into the stomach, magnesianeutralizes the free acids contained in the stomach and intestines, andforms therewith soluble magnesian salts. In full doses it acts as a laxa-tive ; but as it occasions very little serous discharge, Dr. Paris ( Pharma -colo/jia, vol. i. art. Cathartics) ranks it among purgatives “ which urgethe bowels to evacuate their contents by an imperceptible action uponthe muscular fibres.” Part of its laxative effect probably depends on theaction of the soluble magnesian salts (chloride of magnesium and acetateof magnesia) whi cli it forms by union with the acids of the alimentarycanal. Magnesia exercises an influence over the urine analogous to thatof the alkalies, namely, it diminishes the quantity of uric acid in theurine, and when continued for too long a period occasions the deposit ofthe earthy phosphates in the form of white sand. (W. T. Brande, Phil.Trans. 1810, p. 136 ; and 1813, p. 213.) On account of its greater inso-lubility, it requires a longer time to produce these effects than thealkalies. When taken in too large quantities and for a long period it has. -sometimes accumulated in the bowels to an enormous extent, and beingconcreted by the mucus of the bowels, created unpleasant effects. Alady took every night during two years and a half, from one to two tea-spoonfuls of Henry’s calcined magnesia (in all between 9 and 10 lbs. troy)for a nephritic attack, accompanied with the passage of gravel; subse-quently she became sensible of a tenderness in the left side just above thegroin, connected with a deep-seated tumor, obscurely to be felt on pres-sure, and subject to attacks of constipation, with painful spasmodic actionof the bowels, tenesmus, and a highly irritable state of stomach. Duringone of these attacks she evacuated two pints of “ sand and on anotheroccasion voided soft light brown lumps, which were found to consistentirely of subcarbonate of magnesia concreted by the mucus of thebowels, in the proportion of 40 per cent. In another case a mass of asimilar description, weighing from 4 to 6 lbs., was found imbedded in thehead of the colon, six months after the patient had ceased to employ anymagnesia. (E. Brande, Quart. Journ. of Science, i. 297.)
Uses. —As an antacid it is as efficacious as the alkalies, while it has an