The loadstone. Calamita, lapis Lydius, antiphyson, lapis Heraclius, from Heraclea, a town in Lydia. The term, however, is singular in many respects. Its origin is uncertain, but its application and influence have been peculiarly extensive: various cities have been styled Magnesia, and the Magnetes constituted no inconsiderable nation in Asia. Many of these cities have been mentioned as the origin of the name; but it is not our object to determine the question. The stone itself was long known before it was employed to direct the course of the navigator; and is usually of a dirty-black colour, though in this respect it varies, and is sometimes whitish, from the mixture of silicious particles in such a proportion as to render it fusible. Whether from its colour, its weaker powers, or any other cause, this was styled the female magnet, and magnesia When the white earth, precipitated from salts, similar to the Epsom, was observed, this was supposed to resemble the female magnet, and, of course, called magnesia, with the distinction from its greater whiteness, of alba. W hen another dark metal, similar to the magnet, was discovered, it had the same appellation; but as it did not attract iron, some distinction was necessary, and it was called mangnet, manganet, and manganese. The magnet, our present object, is the amorphous,o\id\i\a.icd iron of Hauy, iv. 13, often found in Europe, in a matrix of magnesian earth, though sometimes in a ferruginous sand, or a sulphurated lime. Its obvious quality of attracting or repelling iron is well known, and in this experiment the north and south poles are attractive, and each repulsive to its own points in other iron or other magnets. Every, the smallest, portion of a magnet has its two poles, not verging to the real poles of the world, but to those of the magnetic meridian, which varies sometimes a. little to the east, sometimes to the west. The smaller magnets are more active in proportion than the larger. Magnets, like all iron ores, arc astringent, but not used in medicine. They have been recommended by ignorant quacks in ruptures, to attract the intestine upwards, and to destroy the lentor of the blood, by separating the particles of iron in it. Such are the absurdities that deceit will feign and credulity believe. See Magnetism.

Magnes arsenicalis, a preparation of arsenic, which we omitted under that article, but which we noticed under Cancer, vol. i. p. 332, col. 2, q. v. Its name was derived from its supposed power of attracting the morbid poison. Geoffroy adds, that it opens, cleanses, and heals scrofulous ulcers, without the assistance of an ointment of any kind. See Cancer.

MagNes epilepsiae. See Cinnabaris.

M Agnesia,(from magnesia, the female loadstone, magnes). (See Ethel.) Among the alchymists it means the matter of the philosopher's stone, or sulphur; it sometimes signifies melted tin, with which mercury is incorporated, forming into a brittle white mass; sometimes a mixture of silver and mercury, and a very fusile metal, called magnesia philosophorum. But enough of such nonsense.

MagNesia alba, was a general term which chemists formerly gave to all substances which had the power of attracting any principle from the air. Thus an earth which, exposed to the air, yielded vitriol, was called magnesia vitriolata. More modern chemists, supposing that it had attracted the nitrous acid, in it.; preparation, called it magnesia nitri: but, from its colour, it soon obtained its present name, the white magnesia, albus Romanus pulvis, Comitisstae Palme pulvis. It was introduced as a medicine in the beginning of the eighteenth century, by count di Palma, at Rome, and continued a very lucrative secret. It is a very white, subtile powder, and now known to be a peculiar earth, and the basis of the Epsom salt.

Magnesia was, as we have said, for a long time a lucrative secret, and it scarcely emerged from the language and appearance of a quack medicine, at the time it was first prepared by Mr. Glass of Oxford. Mr. Glass took the form of preparing the medicine from Hoffman, and was not aware of its nature, or the effect of the addition of the alkali. To Dr. Black we are indebted for the discovery of its being a distinct earth. The Oxford preparation was light and elegant, though unequal; but the imputation of its being adulterated with calcareous earth brought on a dispute between Dr. Glass, the brother of the proprietor, and Mr. Henry. It is now of little consequence where the truth lay; for it is everywhere prepared with sufficient fidelity. Mr. Henry's process we shall add.

"Dissolve any quantity of sal catharticum amarum in its own weight of water; filter, and add to it, by degrees, a filtered solution of pearl ashes, in an equal quantity of water, stirring them gently, until the mixed liquors have acquired the appearance of a complete co-aguluni; then cease adding any more of the alkaline lixivium, and immediately throw the mixture into a large vessel of boiling water; keep it boiling for a quarter of an hour, take it out, and put it into a glazed earthen vessel: as soon as the powder hath subsided, and bo-fore the water is quite cold, pour it off', and add a fresh quantity of boiling water, till the liquor hath entirely lost its saline taste; next let it be so agitated as to suspend the liner 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, until a considerable part of the humidity is absorbed; then wrap it up in sheets of white paper, and dry it before the fire. Pour hot water upon the remaining powder, stir and decant it in its turbid state, and separate the magnesia from the water as before; thus the whole, or the most of it, will be reduced to an equal degree of fineness.

"The larger the quantity of water into which the precipitated powder is cast, the more speedily and perfectly will the vitriolated tartar, which is formed by the alkali uniting with the acid of the sal catharticum, be washed off. The neutral salt should be washed off as quick as possible, otherwise, 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 re-dissolved by any washing, however long continued. Dr. Black orders four times the quantity of water to that of the solution to throw the coagulum into, but that is far too little. The water should be pure; distilled is the best; but it should be kept until its empyreuma is gone off. Hard or impure water makes magnesia coarse and disagreeable. The chalk stones on which the magnesia is dried should be exposed to a moderate heat, that the moisture may evaporate quickly. Cleanliness should be particularly attended to through the whole."

Magnesia, when pure, is white, loose, and light, of the specific gravity of 2.330 nearly. It is perfectly infusible in the focus of the most powerful mirror, except when it contains particles of flint, which, if the alkali is impure, sometimes happens. When the volatile alkali is employed in the process, no flinty particles are found in it. Magnesia melts, however, with borax, and with some of the earths, though more certainly when the earths and alkalis are united. A new manufactory of china, resembling the seve, is established at Berlin, in which, instead of the kaolin, a magnesian earth, containing flint and an alkali, is the chief ingredient. Magnesia is nearly insoluble in water, but retains a small portion of this fluid within the interstices of its particles with some obstinacy. When, however, the carbonic acid gas is previously united with the water, the magnesia dissolves readily.

Magnesia contributes to the diffusion and suspension of many resinous substances, and, triturated with camphor, renders this medicine more miscible with water. It is supposed also to increase the solubility of bark in water, if triturated with it previous to infusion or decoction: but it seems to produce some chemical change in the constituent principles of the medicine, as the colour is not only deeper but more red. Whether it is more active as a medicine than the common decoction has not, we believe, been ascertained.

As magnesia contains about seven-twelfth parts of fixed air, it should be calcined before it is administered, at least when flatulence abounds. The air, however, which is expelled by heat, is greedily recovered by exposure to the atmosphere, so that it should be kept in a phial carefully closed. The magnesia contracts no acrimony by calcination.

Like all absorbents, it corrects acidities in the stomach, relieves the heart burn and pain in the stomach, colics and convulsions in children, with every other complaint arising from acidity. It is preferred to other absorbents, on account of its laxative quality, when united with an acid. If mixed with rhubarb, it is said to prevent the rhubarb from leaving a costive habit. If the magnesia does not meet an acid, it is inert, and is sometimes supposed to load the stomach as a heavy cold mass. It has been doubted whether it is proper in bilious or putrid fevers, and much idle disquisition has been employed on this subject; for a prudent practitioner will be led, in such cases, to employ medicines of very different qualities. Magnesia can do no good in either disease. See Hoffman's Observationes Physicae Chemicae, lib. iv. Obs. ii.; Black's Observations on the Magnesia Alba, in the Essays Philosophical and Literary of Edinburgh, vol. ii; London Medical Transactions, vol. ii.

Magnesia opalina; magnesia rubicunda antimonii. Opaline or ruby coloured magnesia of antimony. In making the hepar antimonii, decrepitated sal ammoniac is sometimes added to the antimony and nitre, and the result is, the opaline magnesia. It is a weaker emetic than the liver of antimony, and may be given in considerable doses to horses, to produce sweating. Lemery directs it to be made of equal parts of antimony, nitre, and decrepitated sea salt.

Magnesia vitriolata. See Catharticus Sal.