gently for some time in the tube. An amalgam was obtained, which by distillation, out of the contact of the atmosphere, afforded a darkgray metallic film, infusible at the point at which plate-glass softened, and which in the process of the distillation of the mercury rendered the glass black at its point of contact with it. This film burned with a red light when heated strongly, and became converted into a white powder, which had the character of magnesia. When a portion of magnesium was thrown into water it sunk to the bottom, and effervesced slowly, becoming covered with a white powder. By adding a little muriatic acid to the water the effervescence was violent. The metal rapidly disappeared, and the solution was found to contain magnesia.

When magnesia is strongly heated, in contact with two volumes of chlorine, this gas is absorbed, and one volume of oxygen is disengaged. Hence it is evident that there exists a combination of magnesium and chlorine, or a true chloride. The salt called muriate of magnesia is a compound of the chloride and water. When it is acted on by a strong heat, by far the greatest part of the chlorine unites to the hydrogen of the water, and rises in the form of muriatic acid gas; while the oxygen of the decomposed water combines with the magnesium to form magnesia.

Magnesia is often associated with lime in minerals, and their perfect separation becomes an interesting problem in analysis. Mr. Longchamp has published a valuable paper on the subject. in the twelfth volume of the Ann. de Chim. et de Phys.

Magnesia is chiefly used as an antacid, purgative, and lithontriptic, in medicine. When incautiously used for a long time it may produce very serious evils, of which a remarkable case is narrated by Mr. Brande, in the first volume of his Journal. A lady was recommended to take magnesia, in consequence of some very severe nephritic attacks, accompanied with the passage of gravel. She was desired to take a tea-spoonful every night; and Henry's calcined magnesia was preferred, as that always operated upon the bowels, and carried itself off,' which other magnesia did not, but on the contrary felt heavy and uneasy on the stomach. The dose was gradually increased to two tea-spoonfuls, in order to produce effect upon the bowels, which this quantity never failed to do The symptoms for which it was ordered were soon removed, but the plan was persevered in for two years and a half, with little intermission; so that as the average weight of a tea-spoonful is at least forty grains, and the average dose was a tea-spoonful and a half, it may be presumed that she took, during the above period, between nine and ten pounds troy. 'In the course of the last autumn she became sensible of a tenderness in the left side, just above the groin, connected with a deeply seated tumor, obscurely to be felt upon pressure, and subject to attacks of constipation, with painful spasmodic action of the bowels, tenesmus, and a highly irritable state of stomach. These attacks recurred every two or three weeks, varying in violence, but requiring the use of active remedies. Several irregular lumps, of a soft lightbrown substance, were voided, having the

appearance of a large mass broken down, and when dry extremely friable. A part of each was subjected to analysis, and found to consist entirely of subcarbonate of magnesia, concreted by the mucus of the bowels, in the proportion of about forty per cent. She was cured by the use of other purgatives.' Another case is mentioned, in which not only large quantities of a concretion of a similar description were voided, but upon examination after death, which took place perhaps six months after any magnesia had been taken, a collection, supposed to be from four to six pounds, was found imbedded in the head of the colon, which was of course much distended.

MAGNESIA, HYDRATE OF, in mineralogy. This mineral was discovered by Dr. Bruce of New York, in small veins in serpentine at Hoboken, in New Jersey. Color white. Massive. Lustre pearly. Fracture foliated or radiated. Semitransparent in the mass; transparent in single folia. Soft, and somewhat elastic. Adheres slightly to the tongue. Specific gravity 2.13. Soluble in acids. Its constituents are magnesia 70, water 30, which approaches to one prime equivalent of each.―Jameson.

MAGNESIA, in ancient geography, a maritime district of Thessaly, lying between the south part of the Sinus Thermaicus and the Pegasus on the south, and to the east of the Pelasgiotis. Magnesius and Magnessus, the epithet. Horace.

MAGNESIA, a town of Asia Minor, on the Meander, about fifteen miles from Ephesus. Themistocles died there: it was one of the three towns given him by Artaxerxes, 'to furnish his table with bread.' It is also celebrated for a battle fought there, 190 years before the Christian era, between the Romans and Antiochus king of Syria. The forces of Antiochus amounted to 70,000 men according to Appian, or 70,000 foot and 12,000 horse according to Livy, which has been exaggerated by Florus to 300,000 men; the Roman army consisted of about 28,000 or 30,000 men, 2000 of whom were employed in guarding the camp. The Syrians lost 50,000 foot and 4000 horse; and the Romans only 300 killed, with twenty-five horse. It was founded by a colony from Magnesia in Thessaly.

MAGNESIA AD SIPYLUM, a town of Tantalis, the residence of Tantalus, and capital of Mæonia, where now stands the lake Sale; seated at the foot of mount Sipylus, east of the Hermus. It was adjudged free under the Romans, but was destroyed by an earthquake in the reign of Tiberius.

MAGNESITE. Color yellowish-gray, or yellowish-white, and marked with spets. It occurs massive, tuberose, reniform, and vesicular. Surface rough. Dull. Fracture conchoidal. Fragments rather sharp-edged. Opaque. Scratched by fluor spar, but it scratches calcareous spar. It adheres pretty strongly to the tongue. It feels rather meagre. Streak dull. Rather easily Frangible. Specific gravity 2.881. Infusible; but before the blow-pipe it becomes so hard as to scratch glass. Its constituents are, 46 magnesia, 51 carbonic acid, 1 alumina, 0.25 ferruginous manganese, 0.16 lime, 1 water.Bucholz. It is found at Hrubschitz in Moravia, in serpentine rocks.

MAGNET, n. s. MAGNETIC, adj. MAGNETICAL,

Lat. magnes; Gr. μayνητας. Mayvηsia, the country where magnets MAGNETISM, N. S.. were first, as it is said, discovered. The loadstone; the stone that attracts iron; any ore or metal thus attractive: magnetic and magnetical, having the power of a magnet; attraction in any way: magnetism, the power of the loadstone; attraction generally; the science that teaches the nature of magnetic powers or operations.

The moon is magnetical of heat, as the sun is of Bacon's Natural History.

cold and moisture.

She should all parts to reunion bow; She that had all magnetic force alone. To draw and fasten hundred parts in one.

MAGNETISM. This science may justly be considered as yet but in its infancy, although the facts elicited since the commencement of the nineteenth century, by Barlow, Morichini, and Davy, bid fair to throw considerable light on some of its more recondite principles.

The theory of magnetism bears a very strong resemblance to that of electricity, and this analogy has been pretty fully examined under the article devoted expressly to ELECTRO-MAGNETISM. We have seen the electric fluid not only exerting attractions and repulsions, and causing a peculiar distribution of neighbouring portions of a fluid similar to itself, but also excited in one body, and transferred to another, in such a manner as to be perceptible to the senses, or at least to cause sensible effects, in its passage. The attraction and repulsion, and the peculiar distribution of the neighbouring fluid, are found in the phenomena of magnetism; but we do not perceive any actual excitation, or perceptible transfer of the magnetic fluid from one body to another; and it has also this striking peculiarity, that metallic iron is very nearly the only substance capable of exhibiting any strong indications of its presence.

A magnet, whether natural or artificial, is always possessed of the following characteristic properties, which are inseparable from its nature; so that a body cannot be called a magnet, unless it be possessed of all those properties at the same time. 1. It attracts iron and other ferruginous bodies. 2. When a magnet is placed so as to be at liberty to move itself with sufficient freedom, it turns one, and constantly the same, part of its surface towards the north pole of the earth, or towards a point not much distant from it; and, of course, it turns the opposite part of its surface towards the south pole of the earth, or towards a point not much distant from it. Those parts on the surface of the magnet are therefore called its poles; the former being denominated its north pole, and the latter its south pole. This property itself is called the 'magnet's directive power,' or the magnetic polarity;' and when a magnetic body places itself in that direction, it is said to traverse. A plain perpendicula: to the horizon, and passing through the poles of a magnet when standing in their natural direction, is called the magnetic meridian; and the angle which the magnetic meridian makes with the meridian of the place where the magnet stands, is called the declination of the magnet, or more commonly of the magnetic needle, at that place; beserving this property, are generally made of a cause the artificial magnets, mostly used for obslender shape, and sometimes real sewing-needles, rendered magnetic, are used for this purpose. 3. When two magnets are placed so that the north pole of one of them is opposite to the south pole of the other, then they attract each other; but if the south pole of one magnet be placed opposite to the south pole of the other, or if the north pole of the one be brought near to the north pole of the other; in either case, a repulsion takes place. In short, magnetic poles of the same name repel each other; but those of different names attract each other. 4. When a magnet is situated so as to be at liberty to move itself with sufficient freedom, it generally inclines one of its poles towards the horizon, and of course it elevates the other pole above it. This is called the inclination, or dipping of the magnet, or of the magnetic needle. 5. Any magnet may, by proper methods, be made to impart those properties to iron, or to steel, or, in short, to most ferruginous bodies.

After

The earliest theories of magnetism partook of the systematic ideas that prevailed among the philosophers of the day. The vortices of Descartes captivated the mind to such a degree, that attempts were made to introduce them every where. They were given to electric bodies, and the magnet must also have its share. wards the idea suggested itself of simple effluvia of magnetic matter, the molecule of which advanced towards each other, or took a retrograde motion, according to the manner in which the respective effluvia of two magnets met. There were supposed to be in the iron a kind of small hairs that performed the office of valves, to aid

he passage of the fluid in one way, and to oppose its passage when it presented itself in a contrary direction. Such was, among others, the opinion of Dufay; and this philosopher, who had seen so clearly the principle of electric motion, when he came to apply it to maguetism, presented a machine of his own invention, instead of the mechanism of nature.

Epinus was the first, who, to explain the phenomena of magnetism, made use of simple powers subjected to calculation. The idea which served as the basis of his theory was suggested to him while holding a tourmalin in his hand. He had discovered that the effects of this stone were the result of electricity, and had remarked that it repelled on one side, and attracted on the other, a small electrised body. To these two sides he gave the name of poles, and this appellation, which might have passed for a convenient mode of expression only, became the word really expressive of the thing. He saw in the tourmalin a kind of small electrical magnet, and, comparing the phenomena of real magnet, with those of idio-electric bodies, he found that the action of the two fluids might be reduced to the same laws; and thus added to the merit of having improved the theory of electricity, and created, as it were, the theory of magnetism, that of combining in the same link these two grand portions of the chain of human science.

We may thus more simply illustrate the theory of Epinus. He imagined that there must exist a fluid capable of producing all the phenomena of attraction and repulsion, and with a subtilty so great, as to penetrate the pores of all bodies; and also of an elastic nature, its particles being repulsive of each other. At the same time he imagined a mutual attraction between the magnetic fluid and iron, or other ferruginous bodies. According to this hypothesis, iron and all ferruginous substances contain a quantity of magnetic fluid, which is equally dispersed through their substance, when those bodies are not magnetic; in which state they show no attraction or repulsion towards each other, because the repulsion between the particles of the magnetic fluid is balanced by the attraction between the matter of those bodies and the same fluid, in which case those bodies are said to be in a natural state; but when, in a ferruginous body, the quantity of magnetic fluid belonging to it is driven to one end, then the body becomes magnetic, one extremity of it being now overcharged with magnetism, and the other extremity undercharged. Bodies thus modified, or rendered magnetic, exert a repulsion between their overcharged extremities, in virtue of the repulsion between the particles of that excess of magnetic fluid which is more than overbalanced by the attraction of their matter. There is an attraction exerted between the overcharged extremity of one magnetic body and the undercharged extremity of the other, on account of the attraction between that fluid and the matter of the body; but, to explain the repulsion which takes place between their undercharged extremities, we must either imagine that the matter of ferruginous bodies, deprived of its magnetic fluid, must be repulsive of its own particles, or that the under

charged extremities appear to repel each other, only because either of them attracts the opposite overcharged extremities; both which suppositions are embarrassed with difficulties. A ferruginous body, therefore, is rendered magnetic by having the equable diffusion of magnetic fluid throughout its substance disturbed, so as to have an overplus of it in one or more parts, and a deficiency of it in the remainder, and it remains magnetic as long as its impermeability prevents the restoration of the balance between the overcharged and undercharged parts.

Such was the state of our knowledge as to this subject, when on Coulomb the task seems to have devolved of estimating those very small forces discoverable only by the most delicate attention. This philosopher took two magnetic bars which he so disposed on a right line that their opposite poles were about twentyfive millimetres from each other. In the intermediate space he placed, successively, a number of small cylinders made of different materials, and from seven to eight millimetres in length. Each cylinder was suspended freely to a silk thread, such as it comes from the silk-worm. Coulomb observed that this cylinder, of whatever material it was composed, always arranged itself exactly according to the direction of the bars, and, if it were moved out of that direction, it invariably returned to it again after a certain number of oscillations. Gold, silver, copper, lead, glass, chalk, the bones of animals, and different kinds of wood, were tried, and all these bodies felt the action of the magnetic bars.

Two ways suggested themselves of explaining these phenomena: the first, by supposing that all the elements which enter into the composition of our globe were, by their nature, susceptible of the magnetic virtue, but that in most bodies it is so trifling as to be nearly nugatory; and has scarcely been observed except in iron, which possesses it in an eminent degree. The other explanation supposed that the magnetic action exerted by the bars, in the experiments we have cited, was owing to molecule of iron, pervading in an imperceptible manner the different natural substances, and eluding the strictest investigation of chemical analysis. Coulomb, who was favorable at first to the former explanation, appears to have wavered since between the two, and has devised a set of experiments, which he has in part executed, the object of which is to measure the action of the bars on the different bodies, and ascertain, relative to the mass of each, what quantity of iron must be disseminated in its interior, to produce the number of oscillations made in a given time.

However that may be, the fact we have stated is the more interesting, as it leads us to consider the terrestrial globe, taken in its whole extent, as an entire magnet, the force of which is the aggregated amount of all those exerted by the moleculæ that enter into its composition. This fact being once fully established, with regard to all terrestrial bodies, would advantageously compensate for the hypothesis of an individual magnetic nucleus, which has the appearance of having been invented by naturalists, rather to support their own theories, than to give a fair representation of nature

We shall remark here, that M. Prevost had previously asserted that, to explain natural magnetism, it was not necessary to have recourse to the supposition of a particular nucleus. It is sufficient, according to this naturalist, that the decomposition of the fluid, which is only effected in the interior of the iron, by means in our own power, may have place even out of that metal from natural causes more powerful than the agents of art, and whose permanent influence would keep the two poles of the globe in two states of opposite magnetism.

The magnetising power of the more refrangible rays of light has long been a subject of deep interest to the scientific world, but it has unfortunately been involved in a degree of uncertainty which is seldom attached to a point of experimental enquiry. Dr. Morichini, a respectable physician at Rome, discovered this remarkable property of the violet ray. His experiments were successfully repeated by Dr. Carpi of Rome, and the marquis Cosimo Ridolfi, at Florence; but as M. Dhombre Firmas, who resides at Alais, and professor Configliachi of Pavia, had both failed in obtaining any magnetic effect from violet light; and as M. Berard, a most skilful experimenter, had observed only casual indications of magnetism, the discovery of Morichini was brought into considerable discredit both in France and England.

Fortunately, however, for the reputation of the Italian physician, his experiments were performed both before Sir Humphry Davy, and professor Playfair; before the former in 1814, and the latter in 1817. Sir Humphry Davy stated, that he had paid the most diligent attention to one of Morichini's experiments, and that e saw an unmagnetised needle rendered distinctly magnetic by violet light.

When professor Playfair was at Rome, he saw the experiment performed by Dr. Carpi, in the absence of Morichini, before a party of English and Italian gentlemen. The following account was drawn up by an observer from a conversation which he had with that philosopher:

The violet light was obtained in the usual manner, by means of a common prism, and was collected into a focus by a lens of a sufficient size. The needle was made of soft wire, and was found, upon trial, to possess neither polarity nor any power of attracting iron filings. It was fixed horizontally upon a support, by means of wax, and in such a direction as to cut the magnetic meridian at right angles. The focus of violet rays was carried slowly along the needle, proceeding from the centre towards one of the extremities, care being taken never to go back in

the same direction, and never to touch the other half of the needle. At the end of half an hour after the needle was exposed to the action of the violet rays, it was carefully examined, and it had acquired neither polarity nor any force of attraction; but after continuing the operation twentyfive minutes longer, when it was taken off and placed on its point, it traversed with great alacrity, and settled in the direction of the magnetical meridian, with the end over which the rays had passed turned towards the north. It also attracted and suspended a fringe of iron

filings. The extremity of the needle that was exposed to the action of the violet rays repelled the north pole of a compass-needle. This effect was so distinctly marked as to leave no doubt, in the minds of any who were present, that the needle had received its magnetism from the action of the violet rays.'

The

Such was the state of this subject when Mrs. Somerville directed her attention to it; and it is no slight praise to say, that she has set to rest a question on which the scientific world was divided, and that, by the sagacity and ingenuity with which this lady has conducted her experiments, she has rendered visible, even in the northern climate of Scotland, one of the most delicate of the magnetic influences, which, it was agreed, on all hands, required for its developement the serene sky of an Italian climate. following is a general outline of these interesting experiments :-Having obtained the prismatic spectrum, by means of an equiangular prism of flint-glass placed in a hole in the window-shutter, Mrs. Somerville took a sewing-needle, about an inch long, and entirely devoid of magnetisın. This was ascertained by its attracting indifferently either pole of a sewing-needle magnetised in the usual way. This magnetised needle was pushed through a piece of cork, in which was inserted a glass cap, and it was in that state made to revolve freely on the point of another sewing-needle. Conceiving that no polarity would be superinduced if the whole needle was exposed to its action, she covered one half of it with paper, and exposed the other half to the violet rays of the spectrum, cast upon a pannel at the distance of five feet. In about two hours the needle had acquired magnetism, the exposed end exhibiting north polarity. This experiment was often repeated, and always with the same result. By a similar process, Mrs. Somerville ascertained that the indigo rays had nearly as great an effect as the violet, and that the blue and green rays likewise produced the same effect, though in a less degree.

Mrs. Somerville next tried the yellow, orange, and red rays; but neither in them nor the calorific rays was the slightest effect produced, even when the experiments were continued for three successive days. Mrs. S. now applied the same method to pieces of clock and watch springs, about one inch and a half long, and from oneeighth to one-fourth of an inch broad, and they were found to receive a stronger degree of magnetism from the violet rays; an effect which was attributed to their blue color, and their greater extent of surface. Bodkins were not affected. When the violet ray was concentrated by a lens, the magnetic influence was imparted to the needles in a shorter time.

In order to give additional confirmation to these results, Mrs. Somerville exposed magnetised needles, half covered as formerly, to the sun's rays transmitted through glass colored blue by cobalt, and they were distinctly magnetised as before. Needles exposed under green glass received the same property. Mrs. Somerville now enclosed unmagnetised needles in pieces of blue and green riband, one-half of each being covered with paper, and after they had hung a day

in the sun's rays behind a pane of glass, they had acquired magnetic polarity, the exposed ends being north poles, as in the former experiments. When red, orange, or yellow riband was used, no magnetic influence was imparted. In performing these experiments, Mrs. Somerville found that the most favorable time of the day was from ten to one o'clock; and that, as the season advanced, the magnetism acquired was less permanent, as the needle required a longer exposure to acquire the same degree of magnetic

virtue.

Mr. Cavallo made several experiments, with a view of ascertaining the magnetism of brass, and investigating the cause of it. The result he gives is as follows:-It appears, he says, 1st. That most brass becomes magnetic by hammering, and loses the magnetism by annealing or softening in the fire, or at least its magnetism is so far weakened by it as afterwards to be only discoverable when set afloat on mercury. 2dly. That the acquired magnetism is not owing to particles of iron or steel imparted to the brass by the tools employed, or naturally mixed with the brass. 3dly. Those pieces of brass which have that property, retain it without any diminution after a great number of repeated trials, viz. after having been repeatedly hardened and softened. But I have not found any means of giving that property to such brass as had it not naturally. 4thly. A large piece of brass has generally a magnetic power somewhat stronger than a small piece; and the flat surface of the piece draws the needle more forcibly than the edge or corner of it. 5thly. If only one end of a large piece of brass be hammered, then that end alone will disturb the magnetic needle, and not the rest. 6thly. The magnetic power which brass acquires by hammering has a certain limit, beyond which it cannot be increased by farther hammering. This limit is various in pieces of brass of different thickness, and likewise of different quality. 7thly. Though there are some pieces of brass which have not the property of being rendered magnetic by hammering, yet all the pieces of magnetic brass that I have tried lose their magnetism so as no longer to affect the needle, by being made red-hot; excepting indeed when some pieces of iron are concealed in them, which sometimes occurs; but, in this case, the piece of brass, after having been made red-hot and cooled, will attract the needle more forcibly with one part of its surface than with the rest of it; and hence, by turning the piece of brass about, and presenting every part of it successively to the suspended magnetic needle, one may easily discover in what part of it the iron is lodged. 8thly. In the course of my experiments on the magnetism of brass, I have twice observed the following remarkable circumstance:- A piece of brass which had the property of becoming magnetic by hammering, and of losing the magnetism by softening, having been left in the fire till it was partially melted, I found, upon trial, that it had lost the property of becoming magnetic by hammering; but, having been afterwards fairly fused in a crucible, it thereby acquired the property it had originally, viz. that of becoming

magnetic by hammering. 9thly. I have likewise often observed, that a long continuance in a fire, so strong as to be little short of melting-hot, generally diminishes, and sometimes quite destroys, the property of becoming magnetic in brass. At the same time the texture of the metal is considerably altered, becoming what some workmen call rotten. From this it appears that the property of becoming magnetic in brass, by hammering, is rather owing to some particular configuration of its parts, than to the admixture of any iron; which is confirmed still farther by observing, that Dutch plate brass, which is made, not by melting the copper, but by keeping it in a strong degree of heat whilst surrounded by lapis calaminaris, also possesses that property; at least all the pieces of it, which I have tried have that property. From these observations it follows, that when brass is to be used for the construction of instruments wherein a magnetic needle is concerned, as dipping needles, variation compasses, &c., the brass should be either left quite soft, or it should be chosen of such a sort as will not be made magnetic by hammering, which sort, however, does not occur very frequently.'

From what we have already stated it will be perfectly evident that magnetic effects are produced by quantities of iron incapable of being detected either by their weight or by any chemical tests. Mr. Cavallo found that a few particles of steel adhering to a hone, on which the point of a needle was slightly rubbed, imparted to it magnetic properties; and Mr. Coulomb has observed that there are scarcely any bodies in nature which do not exhibit some marks of being subjected to the influence of magnetism, although its force is always proportional to the quantity of iron which they contain, as far as that quantity can be ascertained; a single grain being sufficient to make twenty pounds of another metal sensibly magnetic. A combination with a large proportion of oxygen deprives iron of the whole or the greater part of its magnetic properties; finery cinder is still considerably magnetic, but the more perfect oxides and the salts of iron only in a slight degree; it is also said that antimony renders iron incapable of being attracted by the magnet. Nickel, when freed from arsenic and cobalt, is decidedly magnetic, and the more so as it contains less iron. Some of the older chemists supposed nickel to be a compound metal containing iron, and we may still venture to assume this opinion as a magnetical hypothesis. There is, indeed, no way of demonstrating that it is impossible for two substances to be so united as to be incapable of separation by the art of the chemist; had nickel been as dense as platina, or as light as cork, we could not have supposed that it contained an considerable quantity of iron, but in fact the specific gravity of these metals is very nearly the same, and nickel is never found in nature but in the neighbourhood of iron; we may therefore suspect, with some reason, that the hypothesis of the existence of iron in nickel may be even chemically true. The aurora borealis is certainly in some measure a magnetical phenomenon, and, if iron were the only substance capable of ex