MAGNESIA, or MAGNESIA ALBA, in mineralogy and chemistry, a kind of earth only discovered since the beginning of this century. It first began to be known at Rome by the name of the Count de Palma's powder, which a canon there offered as a general remedy for all disorders. It was by many considered as a calcareous earth; but F. Hoffman showed it to be essentially different. The same was afterwards done by
Magnesia. Dr Black of Edinburgh and M. Margraaf of Berlin, though unknown to each other at the time. When pure, it is extremely white, loose, and light; the specific gravity about 2.330. It is one of the most infusible substances in nature; neither melting, nor even hardening nor contracting, in the focus of the most powerful burning-glass. An experiment was made on some of this earth in the summer of 1782 by M. Magellan, with Mr Parker's burning-glass; the effects of which are more powerful than those of any other, though its diameter is only 32 inches. The event seemed at first to be unfavourable to the conclusion above mentioned; for a cubic inch of magnesia, a quarter of an inch each side, being put into its focus, was hardened, and reduced to less than a third part of its bulk each way, viz. from .25 of an inch to .08. On applying a similar cube of magnesia, however, from Mr Henry's manufacture at Manchester, it neither became harder nor sensibly diminished in size. Bergman informs us, that magnesia, unless precipitated by the volatile alkali, or that by the neat alkalised tartar, always contains some siliceous or calcareous earth. Almost the same thing happens when it is separated by calcination from the remaining lixiviations of the nitrous and marine acids; in which case, by such a violent fire, it adheres together, and even shows a tendency to vitrify.
Notwithstanding this extreme refractoriness of magnesia by itself, it melts easily with borax, though scarce affected by alkalies or the calces of lead; when mixed with other earths it produces hard masses of various kinds; when mixed with calcareous, argillaceous, or siliceous earths, it melts in the fire; and if four times its weight of green glass be added to it, the mass forms a kind of porcelain so hard as to strike fire with steel. But neither an equal part of the above earths, nor of ponderous earth, glass of lead, vegetable alkali, nor vitriolated tartar, added separately to magnesia, will melt in the fire: however, when mixed with common argillaceous earth, it melts into a hard mass. Magnesia differs from calcareous earth in having a much smaller attraction for fixed air. In this respect it is inferior even to fixed alkaline salts; so that it will not render any of these caustic, though it will do so to the volatile alkali. It also parts very readily with its own fixed air by mere heat; and it was by making experiments on this substance that Dr Black made his first discoveries concerning fixed air. In its calcined state, however, it does not show any of the causticity of lime, but may be safely taken internally; and is even preferred by some to that which contains fixed air. In this state it is much less soluble than when combined with fixed air, and does not effervesce with any acid. When mixed with water, a very small degree of heat is excited, and in about 7962 times its weight of water it totally dissolves. It dissolves also very readily in aerial acid; by which means it is frequently united with fresh water. For the same reason, when we mix a solution of perfectly mild alkali, either fixed or volatile, with a solution of magnesia, no precipitation follows; because the great quantity of fixed air extricated by the union of the acid and alkali, instantly dissolves the precipitate as fast as it is formed. But if we put this mixture over the fire, it will grow thick, and coagulate as soon as it is heated to a cer-
tain degree; because the magnesia is unable to retain, in any considerable heat, as much fixed air as is necessary for its solution.
On putting magnesia into water, and afterwards drying it, it is found to retain of its weight of aqueous fluid; but when fully saturated with aerial acid, it will absorb and retain of the same. When fully saturated with aerial acid, it is more soluble in cold than in hot water; because the heat of the latter dissipates part of the fixed air, as was observed concerning the alkaline salts.
Magnesia, when combined with different acids, forms salts exceedingly different from those produced by calcareous earth under similar circumstances; and of which an account is given under the article CHROMISTRY. It is usually prepared either from the bitter of sea-salt, or from the salt prepared from that liquid under the name of Epsom salt. The magnesia prepared directly from the bitter, however, is by no means equal in purity to that produced from the finer kinds of Epsom salt. Hence, in order to have pure magnesia, Bergman gives the following directions: "Let Epsom salt, in well-formed crystals, be dissolved in distilled water; and from this the magnesia is to be precipitated by mild volatile alkali. Some of this earth that remains suspended in the solution, by means of aerial acid, may be easily precipitated by a simple ebullition. An hundred pounds of this magnesia, when rightly prepared, contains near 25 parts of fixed air, 30 of water, and 45 of pure earth. Its specific gravity is then 2.155. This method of preparation may answer very well for having a very pure magnesia; but when it is required to have it very light and spongy, which, by those who use it, is looked upon to be the only criterion of its goodness, we must use the following method:
Take any quantity of Epsom salt, dissolve it in boiling water, and filter the solution. Dissolve also half the quantity of good pearl-ash, and filter this solution. Both of these solutions ought to be somewhat diluted; and it will be proper to use twice the quantity of water which would fairly dissolve the salts. Mix the two solutions when nearly cold, and stir them very well together. Let the mixture stand for some hours until the precipitate has fallen to the bottom in form of a coarse gritty powder. Put the whole then into a clean copper kettle, under which a moderate fire is made. Stir the matter incessantly with a large wooden spatula, to prevent the powder from sticking to the bottom. As the mixture heats, the powder begins to lose its sandy appearance, and to increase greatly in quantity; so that, though at first the mixture was quite thin, with only a small portion of sandy matter amongst it, before it has attained the boiling heat it will be so thick that it can scarcely be stirred. When the grittiness is quite gone, the matter must be put upon a filtering cloth, and warm water poured upon it till it runs insipid. The magnesia is then to be put upon chalk stones, which will absorb the greatest part of the moisture; and it may at last be fully dried in a stove.
Magnesia alba is a good absorbent; and undoubtedly to be preferred to crab's-eyes, on account of its purgative quality when united with an acid, which the other has not. It has been esteemed hurtful in bi-
Magnesia. lions habits where there is a disposition in the stomach contrary to acidity. This, however, according to Mr Henry, is doubtful: and where putrid bile is to be corrected, he thinks good purposes may be answered by taking magnesia with an acid in a state of effervescence; as the fixed air, thus extricated, will correct the putridity of the contents of the intestines, while they are at the same time evacuated downwards. He is also of opinion, that in cutaneous diseases it may enter the circulation in form of a neutral salt, and, by acting as a diaphoretic and diuretic, prove an excellent alternative.
For some medical purposes, magnesia is used in a calcined state; in which case it is deprived of its fixed air, and then it proves nearly as aperient as a double quantity of magnesia in its uncalcined state. Mr Henry is of opinion, that it may be useful in distensions of the bowels arising from flatus; that it may be successfully employed as a cathartic with patients labouring under the stone, who are using the lixivium saponaceum; and that, joined with warm aromatics, it may be of service in correcting the great flatulency which so much afflicts people of a gouty disposition. From several experiments made by the same author, it also appears that magnesia has a considerable antiseptic power. The like virtue he ascribes to all kinds of testaceous powders: whence he concludes, that medicines of this kind are by no means improper in fevers of a putrescent type; that where bile is suspected to be the cause of any putrid disease, those antiseptics should be prescribed which particularly impede its corruption; that, as calcined magnesia is a more powerful antiseptic than most other absorbents, it merits a preference to these; and that where an acid caecochymy prevails, magnesia or other absorbents, taken immediately before or after meal-time, may, by increasing the putrefactive fermentation of animal food, be of very great service. He hath also found, that magnesia hath a power of promoting the solution of resinous gums in water; and thus we have an elegant and easy method of preparing aqueous tinctures from these substances. Such tinctures, however, are calculated only for extemporaneous prescription, as most of them deposit a sediment when they have been kept a week or two.
Black Magnesia. See MANGANESE.