Part 80

On February 17, 1773, in south latitude 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 assume 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 circumstances 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 entitled, "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 circumstances 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 space 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 Gassendus, 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 pass unobserved. 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 latitude, 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 assigns 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 accumulation 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 altitude, 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 balls, darting about with great velocity, and even leaving a train behind them like the common fire-balls. This ingenious author, however, conjecturing that distinct regions are allotted to the electrical phenomena of our atmosphere, assigns the appearance of fire-balls to that region which lies beyond the limits of our crepuscular atmosphere; and a greater elevation above the earth, to that accumulation 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 analogy that he considered existing between these effects and those produced by electricity.

The name, Animal Electricity, has been superseded, notwithstanding the great analogy 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 constitute 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 prickings, 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 constituting it. During the time it is complete, that is, throughout the whole space 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 pricking 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 space 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 bullæ 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 constituent.

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._

MAGNETISM is supposed to have been first rendered useful about the end of the twelfth, or at least very early in the thirteenth, century, by John de Gioja, a handicraft of Naples, who noticed the peculiar attraction of metals, and iron in particular, towards certain masses of rude ore; the touch of which communicated to other substances of a ferruginous nature, especially iron or steel bars, the property of attraction: these touched bars he observed to have a peculiar and similar tendency towards one particular point; that when suspended in equilibrio, by means of threads around their centres, they invariably turned towards the same point; and that, when placed in a row, however adversely directed, they soon disposed themselves in perfectly parallel order. In this instance he improved upon the property long known to, but not comprehended or applied to use by, the ancients, who considered the loadstone simply as a rude species of iron ore, and curious only so far as it might serve to amuse.

Gioja being possessed of a quick understanding, and of a strong mind, was not long in further ascertaining the more sensible purposes to which the magnet might be appropriated. He accordingly fixed various magnets upon pivots, supporting their centres in such a manner as allowed the bars to traverse freely. Finding that, however situated within the reach of observation and comparison, they all had the same tendency, he naturally concluded them to be governed by some attraction, which might be ultimately ascertained and acted upon. He therefore removed into various parts of Italy, to satisfy himself whether or not the extraordinary impulse which agitated these bars, that had been magnetized by friction, existed only in the vicinity of Naples, or was general. The result of his researches appears to be, that the influence was general, but that the magnets were rendered extremely variable, and fluctuated much, when near large masses of iron. The experiments of Gioja gave birth to many others, and at length to a trial of the magnetic influence on the surface of the water. To establish this, a vessel was moored out at sea, in a direction corresponding with that of the magnet; and a boat, having a magnet equipoised on a pivot at its centre, was sent out at night in the exact line indicated thereby; which, being duly followed, carried them close to the vessel that was at anchor. Thus the active power of attraction appeared to be established on both elements, and in the course of time the magnet was fixed to a card, marked with thirty-two points, whereby the mariner's compass was presented to us. The points to which the magnet always turned itself, being generally in correspondence with the meridian of the place where it acted, occasioned the extremities of the bars to be called poles. Succeeding experiments proved, that the magnetic bar never retained an exactly horizontal position; but that one of its poles invariably formed an angle with any perfect level, over which it was placed: this was not so very measurable in a short bar, but in one of a yard in length was formed to give several degrees of inclination. This, which is called "the dip of the needle," (or magnet,) seems to indicate that the attracting power is placed within the earth. What that attracting power is, we cannot determine; some consider it to be a fluid, while others conjecture it to be an immense mass of loadstone, situated somewhere about the north pole. The difficulty is, however, considerably increased by the known fact of the needles of compasses not always pointing due north; but in many places varying greatly from the meridional lines respectively; and from each other at different times and places.

The facility with which a meridional line may be drawn by solar observation, and especially by taking an azimuth, fortunately enables navigators to establish the variation between the true northern direction, and that indicated by the magnet attached to the card of the compass. Nevertheless, we have great reason to believe, that, for want either of accurate knowledge of the prevalent variations, or from inattention thereto, many vessels, of which no tidings were ever heard, have been cast away; it being obvious, that a false indication of the northern point, in many places amounting to nearly the extent of twenty-five degrees, must produce so important an error in a vessel's course, as to subject her to destruction on those very shoals, rocks, &c. of which the navigator unhappily thinks he steers perfectly clear. To obviate such danger, as far as possible, all modern sea-charts have the variations of the compass in their several parts duly noted down; and in reckoning upon the course steered by the compass, an allowance is usually made for the difference between the apparent course by the compass, and the real course, as ascertained by celestial observation. Under circumstances so completely contradictory, the principle of magnetism must remain unknown: we know not of any hypothesis which strikes conviction on our minds, or which seems to convey any adequate idea of the origin, or _modus operandi_, of this wondrous influence. All we can treat of is, the effect; also of the appearances which guide our practice, and of the manner in which the attractive power may be generated and increased. In regard to the latter point, namely, the generation and increase of the magnetic attraction, we shall endeavour to give a brief but distinct view of what relates thereto: observing, that where volcanic eruptions are frequent, and in those latitudes where the Aurora Borealis is distinctly seen, the needle or magnet is sensibly affected.

Previously to earthquakes, as well as during their action, and while the northern lights are in full display, no reliance can be placed on the compass; the card of which will appear much agitated. This has given rise to the opinion held by some, that the power is a fluid: to this, however, there appear so many objections, that we are more disposed to reject than to favour it, although under the necessity of confessing, that we are not able to offer one that may account satisfactorily for the various phenomena attendant upon magnetism.