CHAP. LXIX.

CURIOSITIES RESPECTING VARIOUS PHENOMENA, OR APPEARANCES IN NATURE.--(_Concluded._)

_The Aurora Borealis._

--------------Silent from the north A blaze of meteors shoots: ensweeping first The lower skies, they all at once converge High to the crown of heav'n, and all at once Relapsing quick, as quickly reascend, And mix and thwart, extinguish and renew, All ether coursing in a maze of light _Thomson._

THE AURORA BOREALIS, sometimes called Streamers, is an extraordinary meteor, or luminous appearance, shewing itself in the night time in the northern part of the heavens; and most usually in frosty weather. It is generally of a reddish colour, inclining to yellow, and sends out frequent corruscations of pale light, which seem to rise from the horizon in a pyramidical undulating form, and shoot with great velocity up to the zenith. The Aurora Borealis appears frequently in form of an arch, chiefly in the spring and autumn, after a dry year. The arch is partly bright, partly dark, but generally transparent: and the matter of which it consists, is also found to have no effect on rays of light which pa.s.s through it. Dr. Hamilton observes, that he could plainly discern the smallest speck in the Pleiades through the density of those clouds which formed the Aurora Borealis in 1763, without the least diminution of its splendour, or increase of twinkling.

[Ill.u.s.tration: AURORA BOREALIS.--Page 684.

This is an extraordinary appearance of the Aurora Borealis, observed by Captain Parry in his expedition to the Arctic regions.]

[Ill.u.s.tration: AURORA BOREALIS.--Page 684.

This is an aspect of the Aurora Borealis sometimes observed in Scotland.

The view embraces a portion of Loch Leven, with the island and the castle in which the unfortunate Mary Queen of Scots was imprisoned.]

This kind of meteor, which is more uncommon as we approach towards the equator, is almost constant during the long winter, and appears with the greatest l.u.s.tre in the polar regions. In the Shetland isles, the "Merry Dancers," as the northern lights are there called, are the constant attendants of clear evenings, and afford great relief amidst the gloom of the long winter nights. They commonly appear at twilight, near the horizon, of a dun colour, approaching to yellow; they sometimes continue in that state for several hours, without any perceptible motion; and sometimes they break out into streams of stronger light, spreading into columns, and altering slowly into ten thousand different shapes, and varying their colours from all the tints of yellow, to the most obscure russet. They often cover the whole hemisphere, and then exhibit the most brilliant appearance. Their motions at this time are most amazingly quick; and they astonish the spectator with the rapid changes of their form. They break out in places where none were seen before, skimming briskly among the heavens, are suddenly extinguished, and are succeeded by a uniform dusky tract. This again is brilliantly illuminated in the same manner, and as suddenly left a dark s.p.a.ce. In some nights, they a.s.sume the appearance of large columns, on one side of the deepest yellow, and on the other, gradually changing, till it becomes undistinguished from the sky. They have generally a strong tremulous motion from one end to the other, and this continues till the whole vanishes.

As for us, who see only the extremities of these northern phenomena, we can have but a faint idea of their splendour and motions. According to the state of the atmosphere, they differ in hue; and sometimes a.s.suming the colour of blood, they make a dreadful appearance. The rustic sages who observe them, become prophetic, and terrify the spectators with alarms of war, pestilence, and famine. Nor, indeed, were these superst.i.tious presages peculiar to the northern islands: appearances of a similar nature are of ancient date; and they were distinguished by the appellations of "phasmata," "trabes," and "balides," according to their forms and colours.

In old times they were either more rare, or less frequently noticed: they were supposed to portend great events, and the timid imagination formed of them aerial conflicts.

In the northern lat.i.tudes of Sweden and Lapland, the Aurorae Boreales are not only singularly beautiful in their appearance, but they afford travellers, by their almost constant effulgence, a very beautiful light during the whole night. In Hudson's Bay the Aurora Borealis diffuses a variegated splendour, which is said to equal that of the full moon. In the northeastern parts of Siberia, according to the description of Gmelin, these northern lights are observed to "begin with single bright pillars, rising in the north, and almost at the same time in the north-east, which, gradually increasing, comprehend a large s.p.a.ce of the heavens, rush about from place to place with incredible velocity, and, finally, almost cover the whole sky up to the zenith, and produce an appearance as if a vast tent were expanded in the heavens, glittering with gold, rubies, and sapphire. A more beautiful spectacle cannot be painted; but whoever should see such a northern light for the first time, could not behold it without terror. For, however fine the illumination may be, it is attended, as I have learned from the relation of many persons, with such a hissing, crackling, and rushing noise through the air, as if the largest fire-works were played off. To describe what they then hear, they make use of the expression, 'The raging host is pa.s.sing.' The hunters, who pursue the white and blue foxes in the confines of the Icy Sea, are often alarmed in their course by these northern lights. Their dogs are then so much frightened, that they will not move, but lie obstinately on the ground, till the noise has pa.s.sed. Commonly, clear and calm weather follows this kind of northern lights. This account has been confirmed by the uniform testimony of many, who have spent part of several years in these northern regions, and inhabited different countries from the Yenisei to the Lena; so that no doubt of its truth can remain. This seems, indeed, to be the real birth-place of the Aurora Borealis."

A person who resided seven years at Hudson's Bay, confirms M. Gmelin's relation of the fine appearance and brilliant colours of the northern lights, and particularly of their rushing noise, which he affirms he has frequently heard, and he compares it to the sound produced by whirling round a stick swiftly at the end of a string. A similar noise has likewise been noticed in Sweden. Mr. Nairne also, being in Northampton at the time when the northern lights were remarkably bright, is confident he heard a hissing or whizzing sound. Mr. Belknap, of Dover, in New Hampshire, North America, testifies to this fact. M. Cavallo says, that the cracking noise is distinctly audible, and that he has heard it more than once. Similar lights, called Aurorae Australes, have been long since observed towards the south pole, and their existence has been lately ascertained by Mr.

Forster, who a.s.sures us, that in his voyage round the world with Captain Cook, he observed them in high southern lat.i.tudes, though attended with phenomena somewhat different from those which are seen here.

On February 17, 1773, in south lat.i.tude 58, "a beautiful phenomenon (he says) was observed during the preceding night, which appeared again this and several following nights. It consisted of long columns of a clear white light, shooting up from the horizon to the eastward, almost to the zenith, and gradually spreading on the whole southern part of the sky. The columns were sometimes bent sideways at their upper extremities; and though in most respects similar to the northern lights (Aurora Borealis) of our hemisphere, yet they differed from them in being always of a whitish colour, whereas ours a.s.sume various tints, especially those of a fiery and purple hue. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point."

The periods of the appearance of these northern lights are very inconstant. In some years they occur very frequently, and in others they are more rare; and it has been observed, that they are more common about the time of the equinoxes than at other seasons of the year. Dr. Halley (see Philos. Trans. No. 347, p. 406,) has collected together several observations, which form a kind of history of this phenomenon. After having particularly described the various circ.u.mstances which attended that observed by himself, and many others, in March, 1716, and which was singularly brilliant, he proceeds with informing us, that the first account of similar phenomena recorded in the English annals, is that of the appearance noticed January 30, 1560, and called, Burning Spears, by the author of a book ent.i.tled, "A Description of Meteors," by W. F. D.D.; reprinted at London, in 1654. The next appearance of a like kind, recorded by Stow, occurred on October 7, 1564. In 1574, as Camden and Stow inform us, an Aurora Borealis was seen for two successive nights, viz. on the 14th and 15th of November, with appearances similar to those observed in 1716, and which are now commonly noticed. The same phenomenon was twice seen in Brabant, in 1575, viz. on the 13th of February, and the 28th of September; and the circ.u.mstances attending it were described by Cornelius Gemma, who compares them to "spears, fortified cities, and armies fighting in the air." In the year 1580, M. Masline observed these phasmata, as he calls them, at Baknang, in the county of Wirtemberg, in Germany, no less than seven times in the s.p.a.ce of twelve months; and again at several different times, in 1581. On September 2d, 1621, the same phenomenon was seen over all France; and it was particularly described by Ga.s.sendus, in his "Physics," who gave it the name of Aurora Borealis. Another was seen all over Germany, in November, 1623, and was described by Kepler. Since that time, for more than eighty years, we have no account of any such phenomenon, either at home or abroad. In 1707, Mr. Neve observed one of small continuance in Ireland; and in the same year, a similar appearance was seen by Romer, at Copenhagen; and during an interval of eighteen months, in the years 1707 and 1708, this sort of light had been seen no less than five times.

Hence it should seem, (says Dr. Halley,) that the air or earth, or both, are not at all times disposed to produce this phenomenon, though it is possible it may happen in the day-time, in bright moonshine, or in cloudy weather, and so pa.s.s un.o.bserved. Dr. Halley further observes, that the Aurora Borealis of 1716, which he described, was visible from the west of Ireland to the confines of Russia, and to the east of Poland; extending at least near thirty degrees of longitude, and from about the fiftieth degree of north lat.i.tude, over almost all the north of Europe; and in all places at the same time, it exhibited appearances similar to those which he observed in London. He regrets, however, that he was unable to determine its height, for want of contemporary observations at different places.

Father Boscovich has determined the height of an Aurora Borealis, observed on the 16th of December, 1737, by the Marquis of Poleni, to have been eight hundred and twenty-five miles; and Mr. Bergman, from a mean of thirty computations, makes the average height of the Aurora Borealis to be seventy-two Swedish, or (supposing a Swedish mile to be about six and a half English miles) four hundred and sixty-eight English miles. Euler supposes the height to be several thousands of miles; and Mairan also a.s.signs to these phenomena a very elevated region, the far greater number of them being, according to him, about two hundred leagues above the surface of the earth. Dr. Blagden, speaking of the height of some fiery meteors, (Phil. Trans. vol. lxxiv. p. 227,) says, "that the Aurora Borealis appears to occupy as high, if not a higher region, above the surface of the earth, as may be judged from the very distant countries to which it has been visible at the same time:" he adds, that "the great acc.u.mulation of electric matter seems to lie beyond the verge of our atmosphere, as estimated by the cessation of twilight." But as it is difficult to make such observations on this phenomenon as are sufficient to afford a just estimate of its alt.i.tude, they must be subject to considerable variation, and to material error.

Dr. Blagden informs us, that instances are recorded, in which the northern lights have been seen to join, and form luminous b.a.l.l.s, darting about with great velocity, and even leaving a train behind them like the common fire-b.a.l.l.s. This ingenious author, however, conjecturing that distinct regions are allotted to the electrical phenomena of our atmosphere, a.s.signs the appearance of fire-b.a.l.l.s to that region which lies beyond the limits of our crepuscular atmosphere; and a greater elevation above the earth, to that acc.u.mulation of electricity in a lighter and less condensed form, which produces the wonderfully diversified streams and coruscations of the Aurora Borealis.

CHAP. LXX.

CURIOSITIES RESPECTING GALVANISM.

"Nature, exhaustles still, has power to warm, And every change presents a novel charm."

GALVANI, a professor of anatomy in the university of Bologna, was one day making experiments on electricity. In his laboratory, near the machine, were some frogs that had been flayed; the limbs of which became convulsed every time a spark was drawn from the apparatus. Galvani, surprised at this phenomenon, made it a subject of investigation, and discovered that metals, applied to the nerves and muscles of these animals, occasioned powerful and sudden contractions, when disposed in a certain manner. He gave the name of Animal Electricity to this order of new phenomena, from the a.n.a.logy that he considered existing between these effects and those produced by electricity.

The name, Animal Electricity, has been superseded, notwithstanding the great a.n.a.logy that exists between the effects of electricity and of Galvanism, in favour of the latter term; which is not only applicable to the generality of the phenomena, but likewise serves to perpetuate the memory of the discoverer.

In order to give rise to galvanic effects, it is necessary to establish a communication between two points of one series of nervous and muscular organs. In this manner a circle is formed, one arch of which consists of the animal parts, rendered the subject of experiment, while the other arch is composed of exciting instruments, which generally consists of those animal parts called supporters; others, destined to establish a communication between the latter, are called conductors. To form a complete galvanic circle, take the thigh of a frog, deprived of its skin; detach the crural nerve, as far as the knee; put it on a piece of zinc; lay the muscles of the leg on a piece of silver; then finish the exciting arch, and complete the galvanic circle by establishing a communication by means of the two supporters, by iron or copper wire, pewter, or lead. The instant that the communicators touch the two supporters, a part of the animal arch formed by the two supporters will be convulsed. Although this disposition of the animal parts, and of galvanic instruments, be most favourable to the development of the phenomena, yet the composition of the animal and excitatory arch may be much varied. Thus contractions are obtained, by placing the two supporters under the nerve, and leaving the muscle out of the circle; which proves that nerves essentially const.i.tute the animal arch.

It is not necessary for nerves to be entire, in order to produce contractions. They take place whether the organs be tied or cut through, provided there exists a simple contiguity between the divided ends. This proves that we cannot strictly conclude what happens in muscular action, from that which takes place in galvanic phenomena; since, if a nerve be tied or divided, the muscles on which the energy is distributed lose the power of action.

The cuticle is an obstacle to galvanic effects; they are always feebly manifested in parts covered by it. When it is moist, fine, and delicate, the effect is not entirely interrupted. Humboldt, after having detached the cuticle from the posterior part of the neck and back, by means of two blisters, applied plates of metal to the bare cutis, and, at the moment of establishing a communication, he experienced sharp p.r.i.c.kings, accompanied with a serosanguinous discharge.

If a plate of zinc be placed under the tongue, and a flat piece of silver on its superior surface, on making them touch each other, an acerb taste will be perceived, accompanied with a slight trembling.

The exciting arch may be constructed with two or three metals, or even one metal only; with alloys, amalgams, or other metallic or mineral combinations, carbonated substances, &c. It is observed, that metals, which are in general the most powerful exciters, induce contractions so much the more as they have an extent of surface. Metals are all more or less excitants; and it has been noticed that zinc, gold, silver, and pewter, are of the highest rank; then copper, lead, nickel, antimony, &c.

Galvanic susceptibility is exhausted by too long-continued exercise, and is recruited by repose. Immersion of nerves in alkohol and opiate solutions diminishes, and even destroys, this susceptibility; in the same manner, doubtless, as the immoderate use of these substances in the living man, blunts, and induces paralysis in muscular action. Immersion in oxygenated muriatic acid, revests the fatigued parts, in being acted on by the stimulus. Animals killed by the repeated discharge of an electric battery, acquire an increase of galvanic susceptibility; and this property subsists unchanged in animals destroyed by submersions in mercury, pure hydrogen gas, azote, and ammoniac; and finally, it is totally annihilated in animals suffocated by the vapour of charcoal.

Galvanic susceptibility is extinct in the muscles of animals of warm blood, in proportion as vital heat is dissipated; sometimes even when life is terminated in convulsions, contractibility cannot be put into action, although warmth be not completely gone, as though the vital property were consumed by the convulsions amidst which the animals had expired. In those of cold blood, on the contrary, it is more durable. The thighs of frogs, long after being separated from every thing, and even to the instant of incipient putrefaction, are influenced by galvanic stimuli; doubtless, because irritability, in these animals, is less intimately connected with respiration, and life more divided among the different organs, which have less occasion to act on each other for the execution of its phenomena. The galvanic chain does not produce sensible actions (that is, contractions) until the moment it is completed, by establishing a communication with the parts const.i.tuting it. During the time it is complete, that is, throughout the whole s.p.a.ce of time that the communication remains established, every thing remains tranquil; nevertheless, galvanic influence is not suspended; in fact, excitability is evidently increased or diminished, in muscles that have been long continued in the galvanic chain, according to the difference of the reciprocal situation of the connecting metals.

If silver has been applied to the nerves, and zinc to the muscles, the irritability of the latter increases in proportion to the time they have remained in the chain. By this method, the thighs of frogs have been revivified in some degree, and afterwards became sensible to stimuli that before had ceased to act on them. By distributing the metals in an inverse manner, applying zinc to the nerves, and silver to the muscles, an effect absolutely contrary is observed; and the muscles that possessed the most lively irritability when placed in the chain, seem to be rendered entirely paralytic if they remain long in this situation.

This difference evidently depends on the direction of the galvanic fluid, determined towards the muscles or nerves, according to the manner in which these metals are disposed; and this is of some importance to be known for the application of galvanic means to the cure of diseases.

M. Volta's apparatus is as follows:--Raise a pile, by placing a plate of zinc, a flat piece of wet card, and a plate of silver, successively; then a second piece of zinc, &c. until the elevation is several feet high; for the effects are greater in proportion to its height; then touch both extremities of the pile, at the same instant, with one piece of iron wire: at the moment of contact, a spark is excited from the extremities of the pile, and luminous points are often perceived at different heights, where the zinc and silver come into mutual contact. The zinc end of this pile appears to be negatively electrified; that formed by the silver, on the contrary, indicates marks of positive electricity.

If we touch both extremities of the pile, after having dipped our hands into water, or, what is better, a saline solution, a commotion, followed by a disagreeable p.r.i.c.king in the fingers and elbow, is felt.

If we place, in a tube filled with water, and hermetically closed by two corks, the extremities of two wires of the same metal, which are in contact at the other extremity, one with the summit, the other with the base of the pile; these ends, even when separated only by the s.p.a.ce of a few lines, experience evident changes at the instant the extremities of the pile are touched: the wire in contact with that part of the pile composed of zinc, becomes covered with bullae of hydrogen gas; that which touches the extremity formed by silver, becomes oxydated. Fourcroy attributes this phenomenon to the decomposition of water by the galvanic fluid, which abandons the oxygen to the iron that touches the positive extremity of the pile; then conducts the other gas invisibly to the end of the other wire, there to be disengaged.

From the numerous experiments of Mr. Davy, many new and important facts have been established, and Galvanism has been found to be one of the most powerful agents in chemistry. By its influence, platina wire has been melted; gold, silver, copper, and most of the metals, have easily been burnt! the fixed alkalis, and many of the earths, have been made to appear as consisting of a metallic base and oxygen; compound substances, which were before extremely difficult to decompose, are now, by the aid of Galvanism, easily resolved into their const.i.tuent.

CHAP. LXXI.

CURIOSITIES RESPECTING MAGNETISM.

Almighty Cause! 'tis thy preserving care That keeps thy works for ever fresh and fair: Hence life acknowledges its glorious Cause, And matter owns its great Disposer's laws; Hence flow the forms and properties of things; Hence rises harmony, and order springs.

Thy watchful providence o'er all intends; Thy works obey their great Creator's ends.

Thee, Infinite! what finite can explore?

Imagination sinks beneath thy power.

Yet present to all sense that power remains, Reveal'd in nature, Nature's Author reigns.

_Boyse._

The obedient steel with living instinct moves, And veers for ever to the pole it loves, So turns the faithful needle to the pole, Tho' mountains rise between, and oceans roll.

_Darwin._