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ras. Magnesia had, before his time, been ret of magnesia decomposes rapidly when frequently confounded with lime; he, how. exposed to the air. ever. by the most accurate experiments Sulphate of magnesia is a compound of shewed that it possessed properties differ- sulphuric acid and magnesia, and is found ent from all the other earths. Although in sea water, and in many mineral springs. magnesia exists in great abundance in com- Those at Epsom once afforded a large part bination with other substances, it has ne of what was used in commerce, hence the ver been found perfectly pure in nature. name of Epsom salt. Now indeed it is It is an ingredient in many fossils; and commonly obtained from sea-water. The several of the salts, which it forms by com- bittern water, or, as it

bittern water, or, as it is usually called the bination with the acids, are found in mine- mother water of common salt, that is, the ral springs, and in the water of the ocean water which remains after the crystallizaFrom these combinations magnesia is ob- tion, consists chiefly of sulphate of magnetained by different artificial processes. Mr. sia. The constituent parts are, according Murray mentions the sulphate of magnesia, to Bergman, or Epsom salt, as well adapted to this pur.

Sulphuric acid .............. 33 pose. One part of this salt is to be dis

Magnesia .................... 19 solved in twenty of water, and the solution:

Water ........................ 48 filtered; to this is added, while bot, a solution of pure potash or soda, as long as precipitation is produced. The alkali combines with the sulphuric acid, and the mag. But Mr. Kirwan gives a different result. nesia is separated: being insoluble in water,

In crystals. Dry. it falls down in white powder: it is then

Sulphuric acid ...... 29.35 63.32 washed in water till the fluid comes off Magnesia ............. 17.00 36.68 tasteless. This carth exists under the Water ................ 53.65 form of a white spongy powder, soft to the

100 100 touch, without smell, and having a slightly bitter taste. Its specific gravity is 2.3. It

Sulphate of magnesia is formed by passslightly changes vegetable colours to a green. ing sulphurous acid through water, in which Magnesia, when quite pure, is infusible, ma co

e, magnesia is diffused. At first it is in a though exposed to the most intense heat:

state of powder, which is gradually diseven in the focus of the very powerful

solved, and by exposure to the air, it deburning mirror, or in the heat excited by

posits crystals, and passes into sulphate of oxygen gas, it cannot be melted. When

en magnesia. It consists of made into a paste with water it contracts like alumina, if exposed to a sudden heat.

Sulphurous acid ............. 39

... 16 It is almost insoluble in water.

Magnesia ................
There is

45 no action between magnesia and hydrogen,

Water .................. or carbon, and very little between it and

100 phosphorus. It combines readily with the acids, and with them forms neutral salts. Carbonate of magnesia, or the magnesia Of these the greater number are soluble or alba, of the physicians, is a very important crystallizible, and have a bitter taste. It compound. The manufacture of this op does not enter into combination with the the large scale is thus conducted. Instead fixed alkalies, but in combination with of the pure sulphate of magnesia, the bit

some of the other earths, it is fusible by tern, or liquor remaining after the crystalli- means of a very strong heat. With lime, zation of sea salt is used, and the magnesia in certain proportions, it forms a greenish is precipitated by carbonate of potasb. yellow glass. It is much used in medicine When properly prepared it is perfectly as a gentle laxative, and as an absorbent to white, nearly or wholly tasteless, and very destroy acidity in the stomach. It is also sparingly soluble in water. The magnesia, employed to aid the solution of resinous of commerce is composed of and gummy substances, as camphor and

Fourcroy. Kirwan. opium in water. We shall notice only a

Carbonic acid ............ 48 - 34 few of its combinations.

Magnesia ................. 40

45 Magnesia combines with suphar either

Water ...................... 12 in the dry or humid way, forming thereby a sulphuret of magnesia. The solid sulphu.

21

100

Wben commou carbonate of magnesia is impulse which agitated these bars that had exposed to a moderate heat, it is decom- been magnetised by friction, existed only in posed: its carbonic acid disengaged. It the vicinity of Naples, or was general. loses about half its weight, and the magne. The result of his researches appears to be, sia remains nearly pure.

that the influence was general, but that the Under the magnesian genuis of fossils are magnets were rendered extremely variable, comprehended, not only those in which and fluctuated much, when near large magnesia is the ingredient which is present masses of iron. The experiments of Gioja in largest proportion, but those also in which, gave birth to many others, and at length to though in a smaller proportion, there exista trial of the magnetic influence on the surthe characters in some measure peculiar to face of the water. To establish this, a vesthis genus. These are softness, unctuosity, sel was moored out at sea, in a direction and being in general destitute of hardness, corresponding with that of the magiet; and lustre, and transparency, which are conspia boat, having a magnet equipoised on a cuous in many of those which belong to the pivot at its centre, was sent out at night in silicious and argillaceous genera. Magne. the exact line indicated thereby; which, sian fossils have usually a green colour more being daly followed, carried them close to or less deep.

the vessel that was at anchor. Thus the MAGNET. See MAGNETISM.

active power of attraction appeared to be MAGNETISM is supposed to have been established on both elements, and in the first rendered useful about the end of the course of time the magnet was fixed to twelfth, or at least very early in the thira card, marked with thirty-two points, teenth century, by John de Gioja, a handi. whereby the mariner's compass was precraft of Naples, who noticed the peculiar sented to us. The points to which the attraction of metals, iron in particular, magnet always turned itself, being genetowards certain masses of rude ore; the rally in correspondence with the meridiar touch of which communicated to other of the place where it acted, occasioned the substances of a ferruginous nature, especi. extremities of the bars to be called poles. ally iron or steel bars, the same property Succeeding experiments proved, that the of attraction: these touched bars he ob- magnetic bar never retained an exactly served to have a peculiar and similar ten- horizontal position; but that one of its poles dency towards one particular point ; that invariably formed an angle with any perfect when suspended in equilibrio, by means of level, over which it was placed: this was threads around their centres, they inva- not so very measurable in a short bar, but riably indicated the same point; and that, in one of a yard in length was found to give when placed in a row, however adversely several degrees of inclination. This, which directed; they soon disposed themselves in is called “ The Dip of the Needle,” (or perfectly parallel order. In this instance magnet) seems to indicate that the attracthe improved upon the property long kpown , ing power is placed within the earth. What to, but not comprehended or applied to that attracting power is we cannot deter. use by the ancients, who considered the mine; some consider it to be a fluid, while load-stone simply as a rude species of iron others conjecture it to be an immense mass ore, and curions only so far as it might of load-stone situated somewhere about the serve to amuse. Gioja being possessed of north pole. The difficulty is, however. a quick understanding, and of a strong considerably increased by the known fact of mind, was not long in further ascertaining the needles of compasses not always pointthe more sensible purposes to which the ing due north ; but in many places varying magnet might be appropriated. He ac- greatly from the meridional lipes respeccordingly fixed various magnets apon pivots, tively; and from each other at different supporting their centres in such rianner as times and places. The facility with which allowed the bars to traverse freely. Find- a meridional line may be drawn by solar ing that, however situated within the reach observation, and especially by takiug an of observation and comparison, they all azimuth, fortonately enables navigators to had the same tendency, he naturally con- establish the variation between the true cluded them to be governed by some at- northern direction, and that indicated by traction which might be ultimately ascer- the magnet attached to the card of the tained and acted upon. He therefore re- compass. Nevertheless, we have great moved into various parts of Italy, to satisfy reason to believe, that, for want either of limself whether or not the extraordinary accurate knowledge of the prevalent varia

VOL. IV.

tions, or from inattention thereto, many they will, instead of cohering, be obviously vessels, of which no tidings were ever heard, repelled to à distance corresponding with have been cast away; it being obvious that their respective powers of attraction, when a false indication of the northern point, in applied individually to unmagnetised neemany places amounting to nearly the ex- dles. The south poles will, in like manner, tent of twenty-five degrees, must produce repel each other; but the north pole of one, so important an error in a vessel's course, and the south pole of the other, will, when as to subject her to destruction on those approximated, be evidently attracted, and very shoals, rocks, &c. which the navigator will cohere so as to sustain considerable unhappily thinks he steers wide of. To ob- weights. Iron is the only metal, hitherto viate such danger, as far as possible, all known, which is capable of receiving and modern sea-charts have the variations of communicating the magnetic power : but the coopass in their several parts duly quiet, and the absence of contact, in some noted down; and in reckoning upon the respects, are indispensably necessary tocourse steered by compass, an allowance is wards its perfect retention. Thus, when a usually made for the difference between bar has been impregnated, however abunthe apparent course, by the compass, and dantly, with the magnetic principle, if it be the real course, as ascertained by celestial heated or hammered, the power of attracobservation. Under circumstances so com- tion will be dissipated; or if a tube filled pletely contradictory, the principle of mag. with iron filings have their surface magne. netism must remain unknown: we know tised, by shaking the tube the magnetic not of any hypothesis which strikes convic- influence will likewise be lost. In some tion on our minds, or which seems to con- respects the magnetic influence resembles vey any adequate idea of the origin, or mo. caloric; for it very rapidly communicates dus operandi, of this wondrous influence. to iron, devoid of magnetism, a certain porAll we can treat of is the effect; also of the tion of its own powers; which, however, appearances which guide our practice, and appear to be reproduced instantaneously. of the manner in which the attractive power As various small fires under one large vessel may be generated and increased.

will thereby beat it, and cause the water it in regard to the latter point, namely, the contains to boil, though either of them generation and increase of the magnetic individually would not produce that effect : attraction, we shall endeavour to give a so many weak magnets may, by causing brief but distinct view of what relates there each to communicate a power equal to its to : observing that where volcanic eruptions own, be made to create an accnmulated are frequent, and in those latitudes where power, larger than that contained by either the aurora borealis is distinctly seen, the of them individually: there is, however, a needle or magnet is sensibly affected.' Pre- seeming contradiction to be found in some viously to earthquakes, as well as during their authors, who recommend that the weakest action, and while the northern lights are in magnets should be first applied, and those fall display, no reliance can be placed on more forcible in succession according to the the compass ; of which the card will appear power they may possess; the reason assign. much agitated. This has given rise to the ed being, that the weaker magnets would opinion held by some, that the power is a else, in all probability, draw off some of fuid: to this, however, there appear so the accumulated power from the new mag. many objections, that we are more disposed net. Of this there appears no danger, since to reject than to favour it, although under experience proves that magnets rather gain the necessity of confessing that we are not than lose efficiency by contact, not only able to offer one that may account satisfac- with each other, but even with common torily for the various phenomena attendant iron. In fact, the magnetic power may at upon magnetism.

any time be created by various means: the We have already stated, that every mag. friction of two pieces of flat and polished net has two poles; that is, one end is called bars of iron will cause them for a short the north, the other the south, pole: the while to attract, and to suspend, light former being considered as capable of at weights. Soft iron is more easily influ. · traction; the other, as we shall infer from enced, hut steel will retain the influence

the subjoined explanations, being far more longer. Lightning, electricity, and galva. inert, if at all possessed of an attractive nism, being all of the same nature, equally power. When two magnets are brought render iron magnetic. It is also peculiar, together with their north poles in contact, that when two or more magnets are left for

any time with their several north poles in and within the influence of its pole, that contact, the whole will be thereby weakneedle also becomes magnetic; or, rather, ened; whereas, by leaving a piece of com- a conductor, possessing a certain portion of mon iron attached to a magnet, the latter attractive power : and it is no less extraorwill acquire strength. It is also well known dinary, that the magnet retains its power that some pieces of steel qnickly receive even in the exhausted receiver of an airthe magnetic infinence, while others require pump: this seems to be a formidable ob. considerable labour, and after all are jection to its being influenced by any fluid. scarcely impregnated. The oxide of iron Perhaps the opinion entertained by many cannot be impregnated, and those bars of our most popular lecturers on this subthat have been so, when they become par-ject, riz. that the earth itself is the great tially oxydized, lose their power. Hence attractor, may be nearest the truth. We we see the necessity of preserving the nee. are the more supposed to incline towards dles of compasses from rust.

such an hypothesis, knowing that, at the Magnets have the power to act notwith- true magnetic equator, the needle does standing the intervention of substances in not dip; and from the well ascertained any degree porous between them, and the fact, that bars of iron, placed for a length body to be acted npon: thus, if a needle of time exactly perpendicular, receive a be put on a sheet of paper, and a magnet strong magnetic power, their lower ends be drawn under it, the needle will follow repelling the south, but attracting the north the course of the magnet. The peculiar poles of magnets applied to them reaffinity of the load-stone for iron is em- spectively. The direction of the dipping ployed, with great success, by those who needle was ascertained by one Robert Nor. work in precious metals, for the separation man, about 250 years ago. He suspended of filings, &c. of iron from the smaller parti. a small magnetic needle, by means of a fine cles of gold, &c. A magnet beinig dipped thread around its centre, so as to balance into the vessel, in which the whole are perfectly, over a large magnet : the south blended, will attract all ferruginous parti- pole of the former was instantly attracted cles.

by the north pole of the latter. He found To communicate the magnetic power to that so long as the needle was held exactly a needle, let it be placed horizontally, and centrical, at about two inches above the with a magnet in each hand, let the north magnet, it remained horizontal ; but so soon pole of one, and the south pole of the other as withdrawn a little more towards one end be brought, obliquely, in contact over the than the other of the magnet, the equilicentre of the needle ; draw them asunder, brium was destroyed, and that pole of the taking care to press tirmly, and preserving needle which was nearest to either pole of the same angle or inclination to the very the magnet was instantly attracted, and ends of the needles, which shonld he sup- pointed downwards thereto. By the magported by two magnets, whose ends ought petic equator, we mean a circle passing to correspond in polarity with those of the round the earth at right angles with the

needle. Observe to carry the magnets you magnetic poles, which do not correspond . press with clear away from the ends of the with the geographical poles, as may be needle, at least a foot therefrom; repeat fully understood by the indications of all the friction in the same manner several compasses to points differing from the lat. times, perhaps six, eight, or ten times, and ter; and as the indications of compasses the needle will be permanently magnetized. vary so much both at different times and As we have already stated, by using other places, we may reasonably conclude, that magnets in succession, the powers of the the magnetic poles are not fixed. The varianeedle will be proportionably increased. tion of the dipping needle has not, in our But no effect will result from the friction latitude at least, varied more than half a if the bars are rusty, or, indeed, not highly degree since its depressive tendency was polished; their angles must be perfect, and first discovered by Norman their several sides and ends completely T he suspension of Mahomet's body, in flat.

the temple where it was deposited, is snpIt is, perhaps, one of the most curious of posed to have resulted entirely from mag. the phenoniena attendant upou this occult netism, with which the Arabians were com. property, that the centre of every magnet pletely unacquainted. is devoid of attraction ; yet, that when a MAGNETISM, animal. About 30 years needle is placed in a line with a magnet, ago, Father Hehl of Vienna, imposed on his countrymen, and indeed on the greater under the same or equal angles, appear part of the civilized world, a pretended equal; and, vice versa. Mr. Maclaurin mode of curing all kinds of disease by observes, that geometrical magnitudes may means of a sympathetic affection between be usefully considered as generated or prothe sick person and the operator. The duced by motion. Thus, lines may be conremedy was supposed to depend upon the ceived as generated by the motion of points; motions of the fingers, and the features of surfaces, by the motion of lines; solids, by the latter ; he placing himself immediately the motion of surfaces; angles may be supbefore the invalid, whose eyes were to be posed to be generated by the rotation of fixed on his, and performing a number of their sides. Geometrical magnitude is alantic and unineaning changes, accompanied ways understood to consist of parts; and to by various grimaces, or infections of the have no parts, or to have no magnitude, are principal muscles of the visage. This rarely considered as equivalent in this science. failed to excite a certain degree of appre. There is, however, no necessity for consihension in the mind of the sick; which, bydering magnitude as made up of an infinite creating a new action of the system, often number of small parts; it is sufficient that frightened them into convalescence. That no quantity can be supposed to be so small, such effects may have been produced among but it may be conceived to be farther di. the credulous and timid, we shall not con- minished: and it is obvious, that we are trovert; but, on the other hand, it is assert- not to estimate the number of parts that ed that numbers have been so far overcome may be conceived in a given magnitude, by with terror and fatigue, (for, like Dr. San- those which, in particular determinate cirgrado, the operator was never satisfied cumstances, may be actually perceived in it while any strength to undergo the process by sense, since a greater number of parts be. remained) that consequences highly danger- come sensible, by varying the circumstances ous, and in some instances fatal, were in- in which it is perceived. duced. Notwithstanding the obvious folly MAGNOLIA, in botany, so named in of the pursuit, there were found many gen- honour of Pierre Magnol, professor of metlemen of great respectability and talents dicine, and prefect of the botanic garden at among its followers; hence a certain degree Montpelier, a genus of the Polyandria Po. of credit was established, and there were lygynia class and order. Natural order of not wanting persons foolish enough to cer- Coadunatæ. Magnoliæ, Jussieu. Essentify many cases, and to give a celebrity tial character: calyx three-leaved; petala which was in a very short time found to be nine : capsule one-celled, two-valved; seeds misapplied. It is a lamentable case, that, berried, pendulous. There are seven spe. throughout the world, impositions of this cies; of which M. grandiflora, great laurelnature are always tolerated long enough to leaved magnolia, or tulip tree, in the southanswer the purposes of the fabricator, and erp provinces of North America, grows to to encourage others in similar deceptions. the height of eighty feet: the trunk is more Our readers may recollect many instances than two feet in diameter; the leaves are of notorious character, among which the nine or ten inches long, and three broad in metallic tractors, which were at one time the middle, of a thick consistence, resemb. asserted to be allied to metallic-magnetism, ling those of the common laurel, but much may, perbaps, serve as a proper illustration larger; of a lucid green, sessile, and placed and proof.

without order on every side of the branches ; MAGNIFYING, in philosophy, the mak- continuing green all the year, falling off ing of objects appear larger than they would only as the branches extend, and the new otherwise do ; whence convex lenses, which leaves are produced. The flowers come have the power of doing this, are called out at the ends of the branches: they are magnifying glasses; and of such glasses are large, and composed of eight or ten petals, microscopes constructed.

which are narrow at the base, broad, roundMAGNITUDE, whatever is made up of ed, and a little waved at their extremities; parts locally extended, or that hath several they are of a pure white colour, possessing dimensions; as a line, surface, solid. The an agreeable scent. The summers in Eng. apparent magnitude of a body is that mealand are not warm enough to briug the fruit sured by the visual angle, formed by rays to perfection. This fine tree is a native of drawn froin its extremes to the centre of Florida and Carolina, and, in common with He eve; so that whatever things are seen many of the trees and plants of that coun

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