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light, which often continue nearly a minute at rest. 3. Flashespointing upwards, or in the direction of the beams which theySucceed. These are only momentary, and have no lateral mo-tion. They appear broad and dili'usej and of a weaker light thanthe beams; they grow gradually fainter till they disappear, andcontinue for hours flashing at intervals. 4. Arcs nearly in theform of a rainbow: these, when complete, go quite across the hea-vens, from one point of the horizon to the opposite point. Theseappearances generally succeed each other in the following order:1 . the faint rainbow-like arcs; 2. the beams; and 3. the flashes.The-northern horizontal light appears to consist of an abundanceof flashes or beams blended together by the situation of the ob-server. The beams of the aurora appear at all places to be arcsof great circles of the sphere, with the eye in the centre ; andthese arcs, if prolonged upwards, would all meet in a point. Therainbow-like arcs cross the magnetic meridian at right angles.When two or more appear at once, they are concentric, and tendto the east and west: also the broad arc of the horizontal lighttends to the magnetic east and west, and is bisected by the mag-netic meridian; and when the aurora extends over any part ofthe hemisphere, the line separating the illuminated part of the he-misphere from the clear part is half the circumference of a greatcircle crossing the magnetic meridian at right angles, and termi-nating in the east and west. That point in the heavens to whichthe beams of the aurora appear to converge, at any place, is thesame as that to which the south pole of the dipping-needle pointsat that place. The beams appear to,rise above each other in suc-cession ; so that of any two beams, that which has the higher basehas also the higher summit. Every beam appears broadest at ornear the base, and to grow narrower as it ascends ; so that the con-tinuation of the bounding lines would meet in the common centreto which the beam tends. The height of the rainbow-like arcs ofthe aurora borealis is estimated by Mr. Dalton to be 150 milesabove the earth’s surface. The periods of the appearance of thesenorthern lights are very inconstant. In some years they- occur veryfrequently', and in others they are more rare; and it has been ob-served that they are more common about the time of the equi-noxes than at other seasons of the year. Dr. Halley (see Philos.Trans. No. 347, p. 406,) has collected together several observa-tions ; lie informs us, that the first account of similar plurnomena-recorded in the English annals, is that of lire appearance whichwas noticed January 30, 1560, and called “ burning spears,” bythe author of a book intitled “ A Description of the Meteors,”by W. F. D. D. reprinted at London in 1654. The next appear-ance of a like kind, recorded by Stow, occurred on October 7,1564. In 1574, as Camden and Stow informs us, an aurora bo-realis was seen for hvo successive nights, viz. the I4lh and 15th ofNovember, with appearances similar to those observed in 1716,and which are now commonly noticed. The same phenomenonwas twice seen in Brabant,-in 1575, viz. on the 13tli ot February,and the 28th of September; and the circumstances attending itwere described by Cornelius Gemma , who compares them tospears, fortified cities, and armies fighting in the air. .Dr. Halley-further observes, that the aurora borealis of 1716 was visible fromthe west of Ireland to the confines of Russia, and to the east of Po-land ; extending at least near 30° of longitude, and from about the50th degree of north latitude, 'over almost all the north of Europe ;and in all places at the same time, it exhibited appearances simi-lar to those which lie observed at London. Father Boscovich hasdetermined the height of an aurora borealis, observed on the 16th ofDecember 1737, by the Marquis of Poleni, to have been 825miles; and Mr. Bergman, from a mean of thirty computations,makes the average height of the aurora borealis to he 72 Swedish ,or (supposing a Swedish mile to be about 6£ English miles) 468English miles. Euler supposes the height to be several thousandsof miles; and Mai ran also assigns to these phamomena a very ele-vated region, the far greater number of them being, according tohim, about 200 leagues above the surface of the earth. Manyattempts have been made to determine the cause of this phamo-menon. Dr. Halley imagines that the vapours, or effluvia, ex-ceedingly rarefied by subterraneous fire, and tinged with sulphu-reous streams, which many naturalists have supposed to be thecause of earthquakes, may also be the cause of this appearance ;or that it is produced by a kind of subtile matter, freely pervad-' iag the pores of the earth, and which, entering into it nearer the

southern pole, passes out again with some force into the aither, atthe same distance from the northern. This subtile matter, bybecoming more dense, or having its velocity increased, may per-haps he capable of producing a small degree of light, after themanner of effluvia from electric bodies, which, by a strong andquick friction, emit light in the dark to which sort of light thisseems to have an affinity. On this subject see Philos. Tran*..No. 347 ; and also Mr. Cotes’s description of this pluenomenoivand his method of explaining it, by streams emitted from the he-terogeneous and fermenting vapours of the atmosphere, in Smith’sOptics , p. 69. Mr. Canton, soon after he had obtained electri-city from the clouds, ottered a conjecture, that the aurora is oc-casioned by the dashing of electric fire positive towards nega-tive clouds at a great distance, through the upper part of the at-mosphere, where the resistance is least: and lie -supposes that theaurora which happens at the time when the magnetic needle isdisturbed by the heat of the earth, is the electricity of the heat-ed air above it; and this appears chiefly in the northern regions,as the alteration in the heat of the air in those parts is the greatest.Dr. Franklin supposes, that the electric lire discharged into, thepolar regions, from many leagues of vaporised air raised fromtlie ocean between the tropics, accounts for the aurora borealis;.and that it appears first, where it is first in motion, namelv, inthe most northern part; and the appearance proceeds south-ward, though the tire really moves northward. Mr. Kirwan, in.the Transactions of the Royal Irish Academy , ann. 1788, lias also-some ingenious remarks on the aurora: boreales et australis. Hegives his reasons for supposing the rarefaction of the atmospherein the polar regions to proceed from them, and these from acombustion of inflammable air caused by electricity. M. Libesis of opinion, that the aurorae boreales are caused by nitric acid,,nitrous acid, and nitrous gas. He observes, that if we excite theelectric spark in a mixture of azotic and oxygen gas, these sub-stances will result severally, according to (he relation which sub-sists between the two gases. And tliat these substances, underdifferent circumstances which take place in the aurora borealis,assume the colours observed in that phenomenon. lie also shewsthat the electric fluid restoring its equilibrium in the polar re-gions must pass through a mixture of azot and oxygen; andtherefore produce the gaseous fluids above-mentioned, givingbirth to red and volatile vapours which elevated in the atmo-sphere form the aurora borealis. Mr. Dalton is of opinion thatthe aurora borealis is a magnetic phenomenon.

Al’ RUM, Gold, a metal of a reddish-yellow colour, not tar-nished by the air, softish, and very tenacious; not sonorous;very malleable and ductile; soluble only in nitro-muriatic acid..Specific gravity 19,300. The largest piece of .native gold evtrknown was brought from W icklow in Ireland , and weighed 'J'Jounces, containing in 24 parts fine gold, 21 -j-j, tine silver 1 andcopper and iron alloy 0-J-.

Al-ru.m Fulminans, a compound of ammonia and oxide ofgold, which when heated explodes with great violence. It is a.very dangerous preparation,, and should be used with great cau-tion. A scruple ot this powder acts more forcibly than half apound of gunpowder a single grain laid on the point of a knife,and lighted at a candle, goes oil' with a greater, noise than a mus-quet.

Aurum Musi-uum, Mucicum, or Mosaicum, a substancecomposed of 60 parts white oxide of tin, and 40 parts of sulphur,,formed by heating gradually in a retort equal parts of each. It isused as a pigment, and for mixing with glass, to imitate the span-gles of the lapis lazuli.

Aurum Potabile, or tincture of gold; a medicine formerlyin great request, but at present rarely used.

Aurum Sophisticum, mimic gold, a chemical preparation,made as follows: take line distilled verdigrise eight ounces, crudeAlexandrian tultv four ounces, borax twelve ounces, salt-petreone ounce and a half; pulverise and mix them altogether, em-pering them with oil to the consistence of a plaster; then put aGerman crucible into a wind-furnace; heat it red hot, and puttingyour mass into it, let it be covered, the furnace filled with coalsover the crucible. When the mass is melted, let it cool of itself,then break the crucible, and you will find at the bottom a line re-gains, like gold, weighing about four ounces, which being malle-able, may be wrought into any form.

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