Magnesia Alba Pharm. Lond. & Edinb. Magnesia: a fine white earth; soluble readily in all acids, the vitriolic as well as the others, into a bitter purgative liquor.

This earth has not hitherto been found naturally pure or in a separate state: it was for feveral years a celebrated secret in the. hands of some particular perfons abroad, till the preparation was made public by Lancifi in the year 1717(a), and afterwards by Hoffman in 1722(b). It was then extracted from the mother-lye, or the liquor which remains after the cryftallization, of rough nitre; either by precipitation with a solution of fixt alkaline salt; or by evaporating the liquor, and calcining the dry residuum, so as to dissipate the acids by which the earth had been made dissobluble.

The magnesia, in this mother-lye, appears to have proceeded from the vegetable allies, which are either made ingredients in the compositions from which nitre is obtained, or else added in the-elixation of the nitre: for the allies of different woods, burnt to perfect whiteness, and freed from their alkaline salt, were found to be, in part, of the same nature with the true magnesia (c). But as quicklime also, in most of the German, French, and other European nitre-works, is commonly employed in large quantity, the earth obtained from the mother-lyes of those works is rather a calcareous earth than magnesia. What is now brought from abroad, under the name of magnesia, gives plain proofs of its calcareous nature, by its burning into quicklime, and forming a felenites with the vitriolic acid.

(a) Annot. in Mercati metallothec. vatican. Arm. ii. cap. x. P. 50.

(b) Observationes Pbyjico-chymiae, lib. ii. obf. 2.

(c) Of vegetable ashes, moderately or strongly calcined, only a part was found to dissolve in acids, and this part appeared to be perfect magnesia. It is probable that the remainder might be reduced to the same state by repeating the calcination.

The true magnesia is obtained in great purity, from a filtered solution of fal catharticus amarus, by adding a filtered solution of any alkaline salt. * This, by its superiour affinity with the vitriolic acid of the fal catharticus, precipitates its earthy basis, which is the magnesia. The method of conducting the process is thus directed in the last Edinburgh dispen-fatory. Dissolve separately equal weights of fal catharticus, and any pure fixed alkaline salt, in double their weight of water. Strain, and then mix them, and immediately add eight times the quantity of hot water. Let the liquor boil a while over the fire, and at the same time agitate it. Then let it stand till the heat be abated; and strain it through a linen cloth, on which the magnesia will be left. This is to be washed by affufions of pure water till it become perfectly tasteless. The London pharmacopoeia directs two pounds of each of the salts to be dissolved separately in ten pints of water, and the liquors strained and mixed. The mixture is now to be boiled a little, and then drained through linen as in the former process.

*A method of preparing magnesia in the most perfect and convenient manner, was pub-lished by Mr. Henry in vol. ii. of the Medical Tranfactions. The same writer, likewise, in a publication of Experiments and Observations on various subjects, recommends the calcination of magnesia, as rendering it a fitter medicine in certain cases. Magnesia is found by experiment to contain above half its weight of fixed air. The evolution of this in the stomach may in-creafe flatulency, and cause uneasiness in weak bowels. By strong calcination the air is expelled from magnesia, while its purgative virtue remains unimpaired; nor does it acquire any of the acrimony or causticity of lime. The London and Edinburgh colleges have now received this preparation, under the title of mag-nesia ufla.

The magnesia is recommended by Hoffman as an useful antacid, a safe and inoffensive laxative in doses of a dram or two, and a diaphoretic and diuretic, when given in small doses, as fifteen or twenty grains. Since this time, it has had a considerable place in the practice of foreign physicians, and has of late come into some esteem among us, particularly in heart-burns, and for preventing or removing the many disorders which children are thrown into from a redundance of acid humours in the first paffages. It is preferred, on account of its laxative quality, to the testaceous and other absorbent earths, which, unless gentle purgatives are given occa-fionally to carry them off, are apt to lodge in the body, and occasion a coftiveness very detrimental to infants. It must be observed, however, that it is not the magnesia itself which proves laxative, but the saline compound result-ing from its coalition with acids: if there are no acid juices in the stomach to dissolve it, it has no sensible operation, and in such cases it may be rendered purgative by drinking any kind of acidulous liquors after it. All the other known soluble earths yield with acids, not purgative, but more or less astringent compounds.

It may be proper to observe, that the name magnesia has been principally applied to a sub-stance of a very different kind; a native mineral, sound in iron mines, and in the lead mines of Mendip hills, in Somersetshire, usually of a dark grey colour, sometimes bright and striated like antimony, sometimes dull, with only a few small striae; remarkable for communicating, to Magnesia, Manganese, vitrariorum &mininerolo* a large proportion of glass in fusion, a purplish or red tinge, which disappears on a continuance of the fire, at the same time destroying the effect of many other colouring matters, and rendering foul or coloured glass clear: supposed to be an ore of iron, and recommended medicinally, when calcined by a strong fire, as an astringent; but yielding no iron, or marks of iron, on any of the common trials by which that metal is distinguished in ores; and in its nature and composition as yet little known. Mr. Pott relates, that on being calcined with sulphur, and afterwards elixated with water, it yielded a large quantity of a white crystalline salt, of a bitterish astringent taste, followed by a kind of sweetness; and that the salt, after strong calcination, tasted like burnt alum, but more acid (a); from whence it may be presumed, that this mineral consists in great part of an earth analogous to that of alum, which, in combination with acids, makes one of the strongest styptics.