This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Carbonate of magnesia is prepared by precipitating a solution of sulphate of magesia by another of carbonate of soda. Mutual decomposition takes place, with the formation of sulphate of soda, which remains in solution, and carbonate of magnesia, which subsides. in order to produce complete decomposition, the mixed solutions are heated for a short time to the boiling point. The precipitate, having been well washed and dried, is ready for use. The carbonate is heavy or light, according as the solutions are very strong or feeble.
The British Pharmacopoeia directs two varieties of magnesia, one the light (Magnesia Carbonas Levis, Br.), the other the heavy (Magnesia Carbonas, Br.), for the distinct methods of preparing which, the reader is referred to the U. S. Dispensatory (12th ed., p. 523). The two varieties seem to have been introduced with a view to the preparation of the light and heavy magnesia. it is the light variety which is chiefly used internally, and the one contemplated in the U. S. Pharmacopoeia.
Sensible and Chemical Properties. The salt is in the form either of a powder, or of cubical blocks, which are very light, perfectly white, smooth to the touch, without smell, and with very little taste. it may be considered, for practical purposes, as insoluble, requiring 9000 parts of hot, and 2500 of cold water for solution. it is dissolved, however, by carbonic acid water, probably in consequence of being converted into the bicarbonate.
It is supposed to be a compound of the neutral carbonate, containing one eq. of acid and one of base, with hydrate of magnesia.
In compatibles. it is decomposed by the acids and acidulous salts, by potassa, soda, lime, and baryta, and by the soluble salts of iron, copper, zinc, antimony, lead, and mercury, most of which form insoluble carbonates with its carbonic acid. At a high heat, its carbonic acid and water are driven off, leaving the earth uncombined.
Carbonate of magnesia is gently laxative, and highly antacid. in the latter capacity, it will be treated of hereafter. its cathartic effect is attributable to the formation of soluble salts with the acids it encounters in the stomach and bowels; and, without the presence of acid in the primae viae, it would probably not act. Hence it is liable to accumulate in the bowels, if taken too often and too largely. Should it not operate, after having been taken a sufficient length of time, it should be followed by acidulous drinks, such as lemonade; arid, in case of accumulation, these drinks should be accompanied with efficient cathartics, as sulphate of magnesia, infusion of senna, etc. its antacid properties particularly adapt it to cases in which there is an excess of acid in the stomach and bowels, at the same time that a laxative effect is required. The particular affections in which it may be given are precisely those to which pure magnesia is adapted; and, in order that unnecessary repetition may be avoided, will be mentioned in connection with that medicine. it has the advantages over magnesia, that it has less taste, and, in consequence of the carbonic acid given out when it is decomposed in the stomach, has a tendency to relieve nausea and vomiting. it should, therefore, be preferably administered, when it is desired to correct acidity of stomach, associated with an irritable state of that organ. Even when there is no absolute existing gastric irritation, the carbonate, probably from the same cause, often sits better on the stomach, and is less disposed to nauseate than magnesia; and for this reason, as well as from its want of taste, it is preferred by many who habitually stand in need of an antacid laxative. By such persons, the lumps of the carbonate are sometimes carried about the person, and a small piece occasionally bitten off, as required. But the greater bulk of the carbonate, and the circumstance that the carbonic acid is sometimes evolved in the bowels, giving rise to flatulent pains, outweigh its advantages as a general rule, and magnesia is much more frequently used.
The dose of carbonate of magnesia is one or two drachms, which, for a child a year or two old, may be reduced to from five to twenty grains. it may be given suspended in water or milk. The neatest mode of preparing it for use is first to rub it up with a little syrup or ginger syrup, and then suspend it in water, mint water, fennel-seed tea, or milk.
It not unfrequently happens, in obstinate constipation or obstructive affections of the bowels, from various causes, that, when active cathartics have been employed unsuccessfully, and, if persevered with, only serve to aggravate the vomiting, the use of gentle laxatives, given in small and repeated doses, succeeds very happily; the bowels yielding to persuasive measures what they have obstinately refused to violence. Under these circumstances, the following combination, which I first learned from my friend, Dr. C. D. Meigs, often answers an excellent purpose. Two drachms of carbonate of magnesia and an ounce of flake manna are thoroughly mixed with eight fluidounces of hot fennel-seed tea, previously prepared with a drachm of the seeds; and of the mixture, when cold, from one to two fluidounces may be given every hour or two until the desired effect is produced.
A solution of carbonate of magnesia in carbonic acid water may be prepared by forcing carbonic acid into a reservoir, containing water mixed with carbonate of magnesia. A preparation of this kind has been employed, under the name of Dinneford's fluid magnesia. So long as the preparation is kept in air-tight bottles, it will remain clear; but, if the carbonic acid be permitted to escape by exposure, the neutral carbonate of magnesia is deposited in crystals. The solution, however, has a disagreeable taste, and is in no respect superior, either in acceptability to the stomach, or effect on the bowels, to the carbonate given as above directed.