364
ELEMENTS OF MATERIA MEDICA.
Composition.— The following is the composition of subcarbonate ofmagnesia of the shops :—
Kirwan. Bergman. Klaproth . Biichol z.
Light. Heavy.
Magnesia .... 45 . 45 40 33 42 ...
Carbonic Acid 34 . 25 33 32 35 ...
Water. 21 30 27 35 23 ...
Magnesia alba 100 . 100 100 100 100 100*00 — 100*0 . 100*0
Several reasons have led chemists to reject the idea of this compoundbeing an ordinary subsalt (vide Berzelius , Traite de CMm. iv. 101); butthey are not agreed as to the precise mode in which the constituents arecombined, as the following table shews:—
Phillips.
41*60 to 43*2'36*58 — 36*4 ,21*82 — 20 4
40*8
36*0
23*2
il Eq.
Eq.Eq.Wt. 1 Eq.Wt.
IIydr«i. Carb . Magn. 3 148*59 Carb. Magn. 1 42>IIydr te . Magnes... 1 29*72 ,Quadrihydr. 1 56$
Magnesia alba .... 1 178*3L1 1 98
Brande.
Eq.
Eq.Wt.
(Hydrd.Carb. Magn. 1 51OI ^Trihydrie.Magn. .. 1 47
Phillips.
l Eq.
! Eq.Wt.
Hydrd.Carb. Magn. 4 204iBihydrd.Magn. .. 1 38
I 9E
I 242
Purity.' —Subcarbonate of magnesia should be perfectly white andtasteless. The water in which it has been boiled should have no alka-line reaction on turmeric paper, nor throw down anything on the addi-tion of chloride of barium or nitrate of silver: by which the absence ofalkaline carbonates, sulphates, and chlorides, is proved. Dissolved indilute sulphuric acid the oxalates and bicarbonates should occasion noprecipitate, by which the non-existence of any calcareous salt is shewn.
Physiological Effects. —The effects of subcarbonate of magnesiaare nearly the same as those of pure magnesia. We can readily con-ceive that the local operation of the first is somewhat milder than that ofthe latter (as in the case of the alkalies and their carbonates), but thedifference is hardly perceptible in practice. As the subcarbonate effer-vesces with acids it is more apt to create flatulence when swallowed.
Uses. —The uses of the subcarbonate are the same as those of calcinedmagnesia ; except where the object is to neutralize acid in the alimentarycanal (as in cardialgia and in poisoning by the mineral acids), when thelatter preparation is to be preferred on account of its not effervescingwith acids, and thereby not causing flatulency.
In the Pharmacopoeia , subcarbonate of magnesia is directed to beemployed for the extemporaneous preparation of the so-called DistilledWaters (aquae distillates). A drachm of any distilled oil is to be carefullytriturated with a draclmi of subcarbonate of magnesia, and afterwardswith four pints of distilled water; the water is then to be strained. Forordinary purposes no objection exists to the use of waters thus prepared,but they are incompatible with bichloride of mercury, on account of thesmall quantity of magnesia which they hold in solution.
Administration. —The dose of subcarbonate of magnesia, as a pur-gative, is from ten grains to a drachm; as an antacid, from five grains toa scruple.
Aqua Magnesle Bicarbonatis. (Magnesia Water: Aerated MagnesiaWater .)—This solution is prepared by passing a stream of carbonic acidthrough water in which a given quantity of subcarbonate of magnesia issuspended. Or it may be made extemporaneously by adding subcarbo-nate of magnesia (or a mixture of sulphate of magnesia and carbonate ofsoda) to the ordinary bottle-soda water (carbonic acid water.) It is anagreeable mode of exhibiting magnesia in nephritic affections.