Experiments and Observations on the Following Subjects / 1. On the preparation, calcination, and medicinal uses of Magnesia Alba. 2. On the solvent qualities of calcined Magnesia. 3. On the variety in the solvent powers of quick-lime, when used in different quantities. 4. On various absorbents, as promoting or retarding putrefaction. 5. On the comparative antiseptic powers of vegetable infusions prepared with lime, &c. 6. On the sweetening properties of fixed air.

by Thomas Henry

CHAP. II.

Miscellaneous Observations.

SINCE the drawing up of the paper which has been the subject of the last chapter, some observations have occurred, which are either so immediately connected with, or at least deduced from it, that it may not perhaps seem inexpedient to introduce them in this place.

I was very much surprised to observe, in the Lectures lately published, as delivered by the very learned and ingenious Dr. Cullen, that Magnesia is there said to be no more purgative than any other absorbent earth. The sentence is this; (speaking of other absorbents) "Magnesia alba should have been added to this set. It has had a considerable reputation as an absorbent, and when neutralized, as a purgative; but I find it is not more absorbent than any of the rest, nor more purgative in less quantity, as chalk or crab's eyes given in the same quantity, viz. dramij, will have the same effect. Therefore it may be neglected."

Surely this must have been an error of the person who wrote down the lectures, and have escaped the notice of the ingenious editors; who, from their skill in chemistry, must know that pure Magnesia differs from every calcareous or testaceous earth with which we are hitherto acquainted. These earths are nearly insoluble in the vitriolic acid, and what part does unite with it, forms a selenitical salt, the most difficult of solution of all others, and of an astringent nature: whereas the Magnesia[d] united with the same acid, produces what is commonly called Epsom salt, easy of solution, and purgative to the bowels. The former, with the nitrous acid constitutes a calcareous nitre, incapable of crystallization; with the marine acid a calcareous muriatic salt; and when dissolved in vinegar, the mixture spontaneously dries up into a friable sub-astringent salt: whereas Magnesia, with all these acids, forms purging salts; that with the nitrous acid, yellow, capable of being reduced into crystals retaining their form in a dry air, but melting in a moist one: with the muriatic acid, a salt is produced which does not crystallize, and easily melts when exposed to the air: with distilled vinegar, a saline uncrystallizable mass is formed, resembling glue both in colour and consistence while warm, but becoming brittle when cold. Dr. Black says, that two drachms of this salt purged a middle aged man four times; and half an ounce of the same gave a woman of a strong constitution no less than ten stools.[e]

Besides, where an acid prevails, much smaller doses than two drachms of Magnesia prove purgative; and it seldom happens that even that dose of the other absorbents[f] will produce the same effect. Nor am I singular in my opinion, when I declare my doubt whether Magnesia be not of itself in some degree purgative, independent of its junction with any acid whatsoever. It appears to be an earth sui generis. That of alum resembles it in some respects, yet differs from it essentially, when combined with the vitriolic acid: the alum is strongly astringent and antiseptic, the Epsom salt purgative and septic.

I have very lately seen a paper signed by Doctor Cadogan and dated in the year 1767, in which he complains grievously of the advertisers of Magnesia, making use of his name without his consent, and has published the process for making his Magnesia. The doctor's intent in this was doubtless benevolent, but his manner of preparing this powder is unnecessarily expensive and wasteful. He directs only one pound of lixivium tartari to five pounds of sal catharticus amarus, which is greatly insufficient to precipitate all the Magnesia. And he insists strongly on the superiority of the lixivium prepared from salt of tartar, to that made of potashes, as if the chemical effects of one, were different from the other. But, says the Doctor, potashes render the Magnesia bitter. Surely the vitriolated tartar produced by a union of one vegetable fixed alkali with the vitriolic acid, is equally soluble in water with that prepared with any other, and if so, will be as easily washed off from the Magnesia.

But behold a champion steps forth, and at one blow levels to the ground the whole tribe of Magnesia makers, who have procured it from the factitious Epsom salts. I confess I have not had the happiness to peruse this ingenious gentleman's pamphlet on the subject, but I have formed a very extraordinary opinion of his candour, modesty, and knowledge, from the very curious paper which he distributes with his Magnesia. Notwithstanding Doctor Black, and since him Mr. Glass and several others, have procured pure Magnesia from the factitious Epsom salts, Mr. Dale Ingram, assures us, that he has made an improvement, "which is by the learned esteemed one of the greatest acquisitions to the materia medica." And wherein does this mighty discovery consist? even that Magnesia prepared from the waters of Epsom, is superiour to that prepared from the bitter purging salt; and he assures us that the Magnesia sold by him is so prepared.

To the first assertion I shall only reply, that every person at all conversant in chemistry knows that Magnesia earth is the same, from whatever substance it can be separated in a pure state; that the factitious Epsom salt yields it in as great a degree of purity as the salt of the Epsom water, and that Dr. Alston assures us, the artificial salt "by various and repeated experiments, made in France as well as in Britain, is demonstrated to be every way as good as, yea to be the very same with, the genuine made of the Epsom waters."[g]