MAG

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M A G

^er, soft to the touch, without smell, «nd having a slightly bittertaste. Its specific gravity is 2.3. It slightly changes vegetablecolours to a green. Magnesia, when quite pure, is infusible, thoughExposed to the most intense heat: even in the focus of a verypowerful burning mirror, or in the heat excited by oxygen gas, itcannot Ire melted. When made into a paste with water it con-h'aets like alumina, if exposed to a sudden heat. It is almost in-soluble in water. There is no action between magnesia and hy-drogen, or carbon, and very little between it and phosphorus. Itcombines readily with the acids, and with them forms neutral salts,ell these the greater number are soluble or crystallizable, and havea bitter taste. Although magnesia exists in great abundance incombination with other substances, it is not found native in quan-*-"ies sufficiently large and pure for general use. It nray, how-ev er, be prepared by dissolving separately equal portions of F.p-So "\ salt and pearl-ashes, in double their quantity of warm water,' v hen the sediment is to be strained : the two liquids are now toft# mixed ; adding eight times their proportion of warm water,ijie whole is suffered to boil a few minutes, being carefully stirreddh a wooden spatula, to prevent the powder from adhering to:!' e bottom of the vessel. Now the liquid is to stand at rest, till•be heat be somewhat diminished, when it should be filteredbfoiigh a cloth, on which the magnesia will remain : lastly, it is0 be washed till it be totally divested of its saline taste. White’“'agnesia is of considerable service for preventing or removingb'nny disorders of children, especially of such as are troubled with.redundance of acid in the first passages; for which purpose it^ preferable to the calcareous absorbents. But even magnesia islft queutly misapplied, and ought never to be given to infants dis-

a. ; .— A—

|i posed to flatulency, or where no symptoms of acidity can be dis-

I covered ; as it is otherwise apt to lodge in the bov,els, and pro-duce obstinate costiveness; being in itself an inactive earth, unlesscombined with acids. Ilence it is often, and very properly, con-joined with rhubarb ; so that children above one year old maytake from five'to ten grains of the former, and from one to twograins of the latter ; while adults generally require one or twoscruples of magnesia, and from five to ten grains of rhubarb for amoderate 'dose.

MAGNET, or LOADSTONE, a sort of ferruginous stone;in weight and colour resembling iron-ore, though somewhat hard-er and more heavy; endowed with various extraordinary proper-ties, attractive, directive, inclinatory, Sic. See Magnetism.It is also called Lapis Heracbuis, from Ileraclea, a city of Mag-nesia, a district of ancient Lvdia, where it is said to have beenfirst found, and from which it is usually supposed to have takenits name. Others derive the word from a shepherd namedMagnes, who first discovered it with the iron of bis crook onmount Ida. It is also called Lapis Nauticus, from its use in navi-gation ; and siderites, from its attracting iron, which the Greekscall The magnet is usually found in iron-mines, and

sometimes in very large pieces half magnet half iron. Us colouris different according to the different countries it is brought from.Norman observes, that the best are those brought from China andBengal, which are of an irony or sanguine colour; those of Arabiaare reddish ; those of Macedonia, blackish ; and those of Hun-gary, Germany , England, &c. the colour of unwrought iron.Neither its figure nor bulji is. determined ; being found of differentforms aud sizes,

MAGNETISM.

^AGNETISM, that power by which the loadstone is inllu-’ ce ‘l, manifesting itself by certain attractive and directive virtues,!| eh constitute the properties of the magnet. The attractiveefl" Cr the magnet was known to the ancients; and is lnention-^ even by Plato and Euripides , who call it the llercuiean-stone,th' c ? llse it commands iron which subdues every thing else. Blitkrmvvledge of its directive power, whereby it disposes its poles0 j. , n S the meridian of everyplace, and occasions needles, pieces"on, & C- touched with it, to point nearly north and south, is ofti^lfh later date ; though the exact time of its discovery, andtec , Cover er himself, are not ascertained. The earliest accountt 0 L >ri * t ‘d of these is in 1260, when Mark Polo the Venetian is saidv e ave introduced the mariner’s compass; though not as an in-to,‘^n of bis own, but as derived from the Chinese , who are saidlljg 1 ' 1 '', 1 -'had the use of it long before; though some imagine thatll e J“kmese rather borrowed it from the Europeans. FlavioU S ’ll°. or Gioia, a Neapolitan, who lived in the 13th century, is&i f V-,y supposed to have the best title to the discovery ; thoughiiiiicl "feeler mentions, that he. had seen a book of astronomy‘'■Plfli' w inch supposed the use of the needle ; though not as

dep. 1° the uses of navigation, but of astronomy. And Guyot‘‘ !i p' 0Vll!S ’ an ancient French poet, who wrote about 1180, makes iliii,( s , s * m ention of the loadstone and the compass; and obliquely jits d p a {- t ' lc .' r Ulie in navigation. The variation of the magnet, ora V e l. C J! 1 ' 1 *' 0 . 11 from the pole, was first discovered by Seb. Cabot, !G elu , ni a», in 1500; and the variation of that variation, by Mr,dip oi- r - an c’ ai } Englishman, about the year 1625. Lastly, theto a I'm''' nation of the needle, when at liberty to play vertically,otir Co.' 4 “ en eath the horizon, was first discovered by another ofMrymen, Mr. 11. Norman, about 1576.

j Principal Phenomena of the Magnet.

Sl fristan c mas '? f ? t ’ "'fr et ' iel ’ natural or artificial, attracts iron, and allCa flefl nfki ' t * 1 cmita ' n >t in its metallic state. The semimeta!het, thousl c am ' P er ' ln P s some others, are attracted by the mag-2. jf ° * * ree d from iron as much as possible.

Pivot, 0r r J la 6 ne tbe suspended by a thread, nicely placed on a•‘tantiy tho , to ^°?t a bason of water, it will turn one and con-same side nearly towards the north pole of the earth,

the other of course turning towards the south. lienee these partsof the magnet have been called its poles, taking the designationsof north aud south from those parts of the world towards whichthey turn. This property is called the polarity of the magnet;and when it is in the act of turning itself into tins position,'it issaid to traverse. A plane drawn perpendicular to tin: horizonthrough both poles of a magnet, alter it has turned itself, is calledthe magnetic meridian ; aud the angle it makes with the meridianof the place is called the declination of the magnet or .of the mag-netic needle.

3. When either the north or the south poles of two magnets areplaced near to each other, they repel; hut a north and a southpole attract each other.

4. A magnet placed in such a manner as to be entirely at liber-ty, inclines one of its poles to the horizon, and of course elevatesthe other above it. This property is called the inclination or dip-ping of the magnet; and is most conspicuous in artificial magnetsor needles, which may be accurately balanced before the magneticvirtue is imparted to them.

5. By proper management any magnet may be made to com-municate its virtue to a piece of steel or iron, which virtue it willretain for a longer or shorter time according to circumstances.

6. Both the declination and the dip are different in different partsof the earth, and neither of them remains the same at the sameplace in different periods. Concerning this variation in the dipor inclination..of the magnetic needle, there have been many con-jectures and opinions offered by writers on this subject; and ther-orists have framed theories, some of which are not a little curiousbut none of them in any degree satisfactory, so that the know-

i ledge of the cause of this surprising and important phenomenon; still remains in darkness, and among the desiderata ot the philoso-pher. The variations are indeed so irregular and so different indifferent places, that the observations on them afford but few datafor investigating the cause : it is evident that more than one causeoperate in this instance : for besides the general change of decima-tion arising from some unknown cause; there is a difference of (hedeclination arising from heat, so that according to the tempi re-ture, and hence according to the time of the day, or season of the'year, the declination is found to,increase or decrease-

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