An account of an improved method of preparing MAGNESIA ALBA.

ALTHOUGH Magnesia Alba is a medicine which has been in general use for many years, yet the proper mode of making it is very little known. Our pharmacopÆia affords us no information about it; and the formula which is given by the Edinburgh College, as well as that with which the ingenious Doctor Black[b] has favoured us, is deficient in several circumstances. Hence the preparation of pure Magnesia has been confined to very few persons, who have preferred the emolument they have received by keeping their method secret, to the more diffusive utility of which a publication of it would have been productive.[c] I therefore beg leave to lay before the public a process for the preparation of Magnesia, by which it will be in the power of every Apothecary to make it himself, in all respects equal to that which is sold by those who conceal their method.

I am the more strongly induced to make this communication, because the Magnesia which is generally to be found in the shops, is either extremely coarse and ill prepared, or, which is still worse, sophisticated with calcareous substances, differing greatly from true Magnesia. I have been assured by some Physicians, that they have met with it mixed with chalk, and even with lime, and I have sometimes seen it so adulterated: a fraud of very dangerous tendency, as this powder is so frequently administered to very young infants, and to adults of tender bowels and costive habits.

This medicine was originally prepared abroad, from the liquor called the mother of nitre, which is composed of a light earth united with an acid; and these being separated, either by a strong fire, or by the addition of an alkali, the powder was washed in water, and obtained the name of Magnesia Alba. Hoffman afterwards prepared it from the bittern remaining after the crystallization of sea salt, which he found to be similar to the mother of nitre. And the factitious Epsom salt being prepared from this bittern, and evidently composed of Magnesia and the vitriolic acid, Dr. Black, who has favoured the world with a number of very valuable experiments on this subject, made use of this salt with success for the same purpose.

Happening some years ago to live in the neighbourhood of a gentleman who has long been celebrated as the preparer of the most genuine Magnesia, and never having been able myself to make Magnesia comparable to his, by the commonly known methods, I was desirous of gaining some intelligence as to his process, and was at last so fortunate as to obtain some useful hints.

I availed myself of these, and after repeated trials, produced Magnesia equally pure, white, tasteless, light and impalpable with that of Mr. Glass; nay sometimes that of my own preparing has been superiour to his, and in one respect has generally the advantage of it, namely, that mine is not so stiff when dried, and may be reduced to the finest powder by simple pressure; whereas his requires some degree of trituration to break the lumps effectually; which I imagine may be owing to his pursuing Dr. Black's method of drying it, by straining and pressing out the water through a cloth.

The following is the manner of preparing it, which I have found successful.

Dissolve any quantity of sal catharticus amarus, commonly called Epsom salts, in its own weight of water; filter the liquor, and add to it by degrees a filtrated solution of pearl ashes in an equal quantity of water, stirring them gently until the mixed liquors have acquired the appearance of a complete coagulum: then cease adding any more of the alkaline lixivium; and, having diluted the precipitate, and mixed it intimately with a small quantity of hot water, immediately throw the mixture into a large vessel of boiling water. Keep it boiling for a quarter of an hour, then take it out, and put it into glazed earthen vessels. As soon as the powder has subsided, and before the water be quite cold, pour it off, and add a fresh quantity of boiling water: repeat these ablutions with several parcels of hot water, till the liquor has entirely lost its saline taste. Then let it be so agitated as to suspend the finer parts of the powder; in which state decant it into other vessels, and having separated the water from the Magnesia by inclination, put it on large chalk stones, till a considerable part of the humidity be absorbed. Then wrap it up in sheets of white paper, and dry it before the fire. Pour hot water on the remaining powder, stir it, decant it in its turbid state, and separate the Magnesia from the water as before. By these means, the whole, or most of it, will be reduced to an equal degree of fineness.

The separation of the Magnesia will be promoted by heating the saline lixivia before they are mixed; and the larger the quantity or water into which the precipitated powder is cast, the more speedily and perfectly will the vitriolated tartar, which is formed by the alkali of the sal catharticus, be washed off. Dr. Black directs that three or four times the quantity of water, to that of the solutions, should be added; but this I have found greatly insufficient. The neutral salt should be washed off as quickly as possible; otherwise, as he justly observes, by allowing the mixture to stand for some time, the powder concretes into minute grains, which when viewed with a microscope, appear to be assemblages of needles diverging from a point. These concretions cannot be redissolved by any washing, however long continued. His intention, in boiling the mixture, is much better answered, by adding it to the water when in a state of ebullition; and once boiling in this manner is more effectual than a dozen washings in hot water.

Much depends on the purity of the water used in the process. If it be hard pump water, the selenites with which it is impregnated will be decompounded, and the calcareous earth be deposited, after boiling; which mixing with the Magnesia will render it impure, gritty and discoloured. Rain water collected free from impurities, or clear river water, are most eligible; but if the situation of the operator does not permit him to procure these in a proper state, he should either use distilled water, which has been kept till the empyreuma is gone off, or at least such pump water as is free from any calcareous or saline impregnation. When poured on the Magnesia, it should be strained through a thick linen cloth, so as to intercept any accidental impurities which it may acquire in heating.

The drying should be performed with expedition. To this end, the chalk stones should be exposed to a moderate degree of heat; and when they have been employed two or three times, should be dried before a fresh quantity of the Magnesia is put on them. Cleanliness should be particularly attended to through the whole process; and the vessels ought to be carefully covered, that no dust may enter.

We may safely make use of a large copper brewing-pan, to boil the Magnesia in; for as the acid is perfectly neutralized, there can be no danger of its quitting the alkali, to which it has a greater affinity than to the metal; and copper does not readily dissolve, even in acids, when boiling hot; nor have I ever observed the least corrosion, though I have frequently used such vessels for this purpose.