Syn. Calcined Magnesia.

Preparation

Magnesia is prepared by exposing carbonate of magnesia to a red heat, which drives off the carbonic acid and water. "Various circumstances influence more or less the physical character of the resulting preparation; so that it may be harsh or smooth, and light or heavy, according to the method used. These circumstances need not be detailed here, as they belong rather to pharmacy than therapeutics. Two varieties of magnesia are kept in the shops, one light, made in the ordinary officinal method, the other heavy, and prepared by processes somewhat peculiar to the several manufacturers, under whose names it is sold, as Henry's Magnesia, Husband's Magnesia, Ellis's Magnesia, etc. in the British Pharmacopoeia, the former is designated as Magnesia Levis, and directed to be prepared from the light carbonate of magnesia; the latter, simply Magnesia, and ordered to be prepared from the heavy carbonate. Both kinds are often kept in wide-mouthed four-ounce glass bottles, so as to prevent the absorption of carbonic acid and water from the air. Magnesia not unfrequently contains a portion of carbonate from insufficient calcination, and, what is much worse, some caustic lime, which gives it an unpleasant taste, and may interfere with its remedial effects. The former impurity is readily detected by the effervescence caused by the addition of a dilute acid; the latter by the precipitate produced, in a neutral solution of the magnesia in a dilute acid, by a solution of oxalate of ammonia, or bicarbonate of potassa.

Sensible and Chemical Properties. Magnesia is a very light, perfectly white powder, without smell, and of a slight alkaline taste. it may be considered, for remedial purposes, as insoluble in water; more than 5000 parts of cold, and 36,000 of boiling water being required to dissolve it. in this greater solubility in cold than hot water, it resembles lime. it consists of one equivalent of a peculiar metal called magnesium, and one of oxygen. it forms soluble salts with most of the acids; and the solutions of these salts are precipitated by the alkaline carbonates, though not the bicarbonates, and by a mixture of ammonia and phosphate of soda; the ammoniaco-phosphate of magnesia being thrown down in the latter case. Magnesia decomposes most of the native salts of the vegetable alkaloids, causing precipitation of the insoluble base.

The heavy magnesia (Henry's, etc.) is much heavier than the ordinary kind, usually smoother, and more readily miscible with water, and on these accounts is often preferred.

Medical Effects and Uses

Magnesia is at once antacid and laxative. it is probably solely by combination with the acids of the primae viae that it is enabled to operate on the bowels; and, consequently, it is a soluble salt of magnesia that really acts. Hence, its essential nature as a cathartic must be the same as that of the saline cathartics generally (see Saline Cathartics); though it is much feebler, in the ordinary doses in which they are respectively given. From the limited amount of acid in the primae viae, no quantity of it, however large, can in general be made to act so powerfully as a full dose of one of the purgative salts, unless its administration is accompanied with that of an acid. it may, therefore, be considered as belonging properly to the laxatives. its great advantage, independently of its mildness, is its property of neutralizing acids in the alimentary canal, which renders it useful in a large number of diseases. it is applicable to all cases which offer joint indications for an antacid and very mild cathartic. it is of great service in dyspepsia, and will often afford relief to very unpleasant gastric symptoms in that complaint. Sick-headache, dependent on acidity of stomach, is relieved by it, and, if anticipated by the administration of magnesia, upon the occurrence of its first preliminary symptoms, may often be prevented. Diarrhoea not unfrequently, especially in children, either originates in, or is sustained by an excess of acid in the stomach and bowels, and may be advantageously treated with magnesia in its earlier stages, especially in connection with rhubarb. The same may be said of other affections of the intestinal canal in children, as vomiting, colic, and flatulence. in many cutaneous affections, it proves extremely useful by correcting the acidity of the primae viae, by which they are produced or aggravated. This is especially true of urticaria, strophulus, and lichen; and it is a good general rule, in the treatment of cutaneous diseases, to employ magnesia, alone or in combination, when a laxative effect is demanded. in gout, so often complicated with an excess of acid, magnesia is very useful; and it is often administered, both in this disease and rheumatism, mixed with sulphate of magnesia, or some other of the saline cathartics, and the wine of colchicum.

In the course of febrile diseases, and inflammatory affections of all kinds, it not unfrequently happens that the indications for this laxative are presented. To children it is peculiarly appropriate, by its comparative want of taste, and its mildness; and it is very often called for in their diseases, in consequence of their extreme proneness to an excess of acid in the stomach and bowels. Freshly precipitated magnesia may be used as an antidote to arsenic, in the absence of the hydrated sesquioxide of irom in administering a fluid mixture of magnesia, it must always be remembered that, in certain proportions of the ingredients, the liquid solidifies, so that it cannot be exhibited. A mixture of ten parts of distilled water and one of freshly calcined magnesia will become solid in twenty-four hours. Not less than fourteen or fifteen times its weight of water should always be employed. The conjecture has been advanced, that the medicine might sometimes do harm, if given with too little of the liquid vehicle, or in the state of powder, by solidifying the contents of the stomach. Like the carbonate, magnesia may sometimes accumulate injuriously in the bowels, if it meet with insufficient acid. The remedy is to give freely of acidulated drinks with cathartics.* a. Vegetable Purgatives.

Administration

The laxative dose of magnesia is for an adult half a drachm or a drachm; for a child one or two years old, from two or three to ten or fifteen grains; but the largest of these doses may often be exceeded with propriety, in infantile cases, when a purgative effect is indicated. it may be administered simply mixed with water or milk, or, as in the case of the carbonate, by being first well rubbed with syrup or ginger syrup, so as to enable it to make a smooth mixture. As often given, it is apt to leave minute lumps in the/mouth, between the teeth, which sustain a slightly unpleasant taste, and by association with the occasional nauseating effect of the medicine, give rise sometimes to an unconquerable aversion to it. This may be obviated by administering it as above recommended. One of the aromatic waters or infusions, or carbonic acid water, may often be used with advantage as the vehicle.