Part 38

A body acquires heat in the exact proportion that the adjacent substances become cold, and when heat is absorbed by a body it becomes an expansive force at the expense of those around that contract, but it is not lost. In chemical action at a distance the principle of the conservation of force is maintained, for a chemical action may be produced miles away from an electro-magnet, perfectly equivalent to the dominant chemical action in the battery. The two electricities are developed in equal proportions, which may be combined so as to produce many changes in their respective relations, yet the sum of the force of one kind can never be made in the smallest degree either to exceed or to come short of the sum of the other. Experimental research proves that the conservation of force is an unalterable law of nature—“a principle in physics as large and sure as that of the indestructibility of matter or the invariability of gravity. No hypothesis should be admitted, nor any assertion of a fact credited, that denies this principle. No view should be inconsistent or incompatible with it. Many of our hypotheses in the present state of science may not comprehend it, and may be unable to suggest its consequences, but none should oppose or contradict it.”

Having thus expressed his conviction of the truth of this great principle, Dr. Faraday considers the case of gravity, and concludes that “the definition of gravity as an attractive force between the particles of matter varying inversely as the square of the distance, while it stands as a full definition of the power, is inconsistent with the principle of the conservation of force.” For while in this definition the principle is maintained of the constancy of the force _at the same distance_, it implies a creation of force to an enormous amount when the distance is diminished, and an equal amount annihilated when the distance is increased,—“an effect,” he says, “which is equal in its infinity and its consequences with creation, and only within the power of Him who creates.” He continues, “It will not be imagined for a moment that I am opposed to what may be called the _law of gravitating action_, that is, the law by which all the known effects of gravity are governed; what I am considering is the _definition_ of the _force_ of gravitation. That the result of _one_ exercise of a power may be inversely as the square of the distance, I believe and admit; and I know that it is so in the case of gravity, and has been verified to an extent that could hardly have been within the conception of Newton himself when he gave utterance to the law; but that the _totality_ of a force can be employed according to that law I do not believe either in relation to gravitation, or electricity, or magnetism, or any other supposed form of power. That there should be a power of gravitation existing by itself, having no relation to the other natural powers, and no respect to the law of the conservation of force, is as little likely as that there should be a principle of levity as well as gravity. Gravity may be only the residual part of the other forces of nature, as Mossotti has tried to show; but that it should fall out from the law of all other forces, and should be outside the reach either of farther experiment or philosophical conclusions, is not probable. So we must strive to learn more of this outstanding power, and endeavour to avoid any definition of it which is incompatible with the principles of force generally, for all the phenomena of nature lead us to believe that the great and governing law is one. Thus gravitation can only be considered as part of a more general force whose law has yet to be discovered.”

The definition of the gravitating force immediately suggests the question of how it is transmitted; the full force of that question was felt by Newton himself when, in his third letter to Bentley, he wrote, “That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance, through a _vacuum_, without the mediation of anything else by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophic matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent, acting constantly according to certain laws; but whether this agent be material or immaterial I have left to the consideration of my readers.”

Since Newton’s time the continual decrease in the periodic times of the comets belonging to our system, and the undulatory theory of light and heat, have proved the existence of an extremely rare elastic medium filling space even to the most distant regions of which we are cognizant. But, rare as it may be, it has inertia enough to resist the motion of comets, and therefore must be material, whether considered to be ether or, according to Mr. Grove, the highly attenuated atmospheres of the celestial bodies. Professor William Thomson of Glasgow has computed that in the space traversed by the earth in its annual revolution, a cube whose side is 1000 miles would contain not less than a pound weight of the ethereal medium, and that the earth, in moving through it, would not displace the ·250th part of that pound of matter. Yet that is enormously more dense than the continuation of the earth’s atmosphere would be in interplanetary space, if rarefied according to Bayle’s law. But whatever be the density or nature of the ether, there is every reason to believe that it is the medium which transmits the gravitating force from one celestial object to another, or possibly it may possess a higher attribute with regard to gravity than its mere transmission.

Dr. Faraday, who discovered the magnetism of the atmosphere, is led to believe that the ethereal medium too is magnetic by the following experiment. Three solutions of the protosulphate of iron, _l_, _m_, _n_, the first of which contained 4 grains of the salt dissolved in a cubic inch of water, the second 8 grains, and the third 16 grains—these were respectively enclosed in three glass globules, all of which were attracted by the pole of a magnet. A quantity of the mean solution _m_ was then put into a vessel, and the globule containing the strongest solution _n_ was immersed in it, which was attracted as before, but the globule _l_, containing the weakest solution, was repelled when plunged into the same liquid. Here there was a diamagnetic phenomenon, although the glass globules and the liquid in which they were immersed contained iron. The effect was evidently differential, for when the liquid was less attracted than the globule, the globule approached the pole, and when the liquid was more attracted than the globule, the latter appeared to recede from the pole. In fact, the effect is the same as that of gravity on a body immersed in water; if it be more forcibly attracted than the water, it sinks; if less forcibly attracted, it rises, the effect being the same as if it were repelled by the earth. Hence the question, are all magnetic phenomena the result of a differential action of this kind, and is the ethereal medium less strongly attracted than soft iron, and more strongly attracted than bismuth, thus permitting the approach of the iron, but causing the bismuth to recede from the pole of a magnet? If such a medium exist, that is, if the ethereal medium be magnetic, then diamagnetism is the same with paramagnetism, and the polarity of the magnetic force in iron and bismuth is one and the same.

The ethereal medium may be presumed to transmit the gravitating force; it transmits the magnetism of the solar spots, its undulations constitute light, heat, and all the influences bound up in the solar beam; and the most perfect vacuum we can make is capable of transmitting mechanical energy in enormous quantities, some of which differ but little from that of air or oxygen at an ordinary barometric pressure; and why not thus admit, says Mr. Thomson, the magnetic property, of which we know so little that we have no right to pronounce a negative?

Mr. Waterstone is also of opinion that it would be taking too narrow a view if we limited the function of the luminiferous ether to the conveying of physical pulses only. The atmosphere also conveys physical pulses, but that is the least important of its functions in the economy of nature. There is nothing that should hinder us attributing to the media concerned in the radiation of light and heat the higher functions of electrical polarity and gravitation. The special dynamic arrangements by which this is effected may ever elude our research; but as there is no limit to the vis viva (N. 222) which such media may conserve in their minutest parts, so there is no physical impossibility in that vis viva being suddenly transferred to the molecules of ordinary matter in the proportion and sequence required to carry out the order and system of nature.

The fundamental principle of action in such media must be in accordance with _elastic impact_, for upon that the dynamic theory of heat and conservation of force rests as a foundation. The statical and dynamical characteristics of gravitation and transfusion of force conform to it, so that all the forces that hold the molecules of bodies together must also be in subjection to it.[20]

SECTION XXXV.

Ethereal Medium—Comets—Do not disturb the Solar System—Their Orbits and Disturbances—M. Faye’s Comet probably the same with Lexel’s—Periods of other three known—Acceleration in the mean Motions of Encke’s and Biela’s Comets—The Shock of a Comet—Disturbing Action of the Earth and Planets on Encke’s and Biela’s Comets—Velocity of Comets—The Comet of 1264—The great Comet of 1343—Physical Constitution—Shine by borrowed Light—Estimation of their Number.

IN considering the constitution of the earth, and the fluids which surround it, various subjects have presented themselves to our notice, of which some, for aught we know, are confined to the planet we inhabit; some are common to it and to the other bodies of our system. But an all-pervading ether must fill the whole visible creation, since it conveys, in the form of light, tremors which may have been excited in the deepest recesses of the universe thousands of years before we were called into being. The existence of such a medium, though at first hypothetical, is proved by the undulatory theory of light, and rendered certain by the motion of comets, and by its action upon the vapours of which they are chiefly composed. It has often been imagined that the tails of comets have infused new substances into our atmosphere. Possibly the earth may attract some of that nebulous matter, since the vapours raised by the sun’s heat, when the comets are in perihelio, and which form their tails, are scattered through space in their passage to their aphelion; but it has hitherto produced no effect, nor have the seasons ever been influenced by these bodies. The light of the comet of the year 1811, which was so brilliant, did not impart any heat even when condensed on the bulb of a thermometer of a structure so delicate that it would have made the hundredth part of a degree evident. In all probability, the tails of comets may have passed over the earth without its inhabitants being conscious of their presence; and there is reason to believe that the tail of the great comet of 1843 did so. M. Valz observed that the light of a brilliant comet was eclipsed as it passed over a star of the 7th magnitude, whence M. Babinet computed that the light of the comet must have been sixty times less than that of the star, and that matter so attenuated could not penetrate the earth’s atmosphere, but the constitution of these bodies is still a matter of conjecture.

The passage of comets has never sensibly disturbed the stability of the solar system; their nucleus, being in general only a mass of vapour, is so rare, and their transit so rapid, even when they had a solid part, that the time has not been long enough to admit of a sufficient accumulation of impetus to produce a perceptible action. Indeed, M. Dusejour has shown that, under the most favourable circumstances, a comet cannot remain longer than two hours and a half at a less distance from the earth than 10,500 leagues. The comet of 1770 passed within about six times the distance of the moon from the earth, without even affecting our tides. According to La Place, the action of the earth on the comet of 1770 augmented the period of its revolution by more than two days; and, if comets had any perceptible disturbing energy, the reaction of the comet ought to have increased the length of our year. Had the mass of that comet been equal to the mass of the earth, its disturbing action would have increased the length of the sidereal year by 2^h 53^m; but, as Delambre’s computations from the Greenwich observations of the sun show that the length of the year has not been increased by the fraction of a second, its mass could not have been equal to the 1/5000th part of that of the earth. This accounts for the same comet having twice swept through the system of Jupiter’s satellites without deranging the motion of these moons. M. Dusejour has computed that a comet, equal in mass to the earth, passing at the distance of 12,150 leagues from our planet, would increase the length of the year to 367^d 16^h 5^m, and the obliquity of the ecliptic as much as 2°. So the principal action of comets would be to alter the calendar, even if they were dense enough to affect the earth.

Comets traverse all parts of the heavens; their paths have every possible inclination to the plane of the ecliptic, and, unlike the planets, the motion of more than half of those that have appeared has been retrograde, that is, from east to west. They are only visible when near their perihelia; then their velocity is such, that its square is twice as great as that of a body moving in a circle at the same distance: they consequently remain but a very short time within the planetary orbits. And, as all the conic sections of the same focal distance sensibly coincide, through a small arc, on each side of the extremity of their axis, it is difficult to ascertain in which of these curves the comets move, from observations made, as they necessarily must be, near their perihelia (N. 227). Probably they all move in extremely excentric ellipses; although, in most cases, the parabolic curve coincides most nearly with their observed motions. Some few seem to describe hyperbolas; such, being once visible to us, would vanish for ever, to wander through boundless space, to the remote systems of the universe. If a planet be supposed to revolve in a circular orbit, whose radius is equal to the perihelion distance of a comet moving in a parabola, the areas described by these two bodies in the same time will be as unity to the square root of two, which forms such a connexion between the motion of comets and planets, that, by Kepler’s law, the ratio of the areas described during the same time by the comet and the earth may be found; so that the place of a comet may be computed at any time in its parabolic orbit, estimated from the instant of its passage at the perihelion. It is a problem of very great difficulty to determine all the other elements of parabolic motion—namely, the comet’s perihelion distance, or shortest distance from the sun, estimated in parts of the mean distance of the earth from the sun; the longitude of the perihelion; the inclination of the orbit on the plane of the ecliptic; and the longitude of the ascending node. Three observed longitudes and latitudes of a comet are sufficient for computing the approximate values of these quantities; but an accurate estimation of them can only be obtained by successive corrections, from a number of observations, distant from one another. When the motion of a comet is retrograde, the place of the ascending node is exactly opposite to what it is when the motion is direct. Hence the place of the ascending node, together with the direction of the comet’s motion, show whether the inclination of the orbit is on the north or south side of the plane of the ecliptic. If the motion be direct, the inclination is on the north side; if retrograde, it is on the south side.

The identity of the elements is the only proof of the return of a comet to our system. Should the elements of a new comet be the same, or nearly the same, with those of any one previously known, the probability of the identity of the two bodies is very great, since the similarity extends to no less than four elements, every one of which is capable of an infinity of variations. But, even if the orbit be determined with all the accuracy the case admits of, it may be difficult, or even impossible, to recognize a comet on its return, because its orbit would be very much changed if it passed near any of the large planets of this or of any other system, in consequence of their disturbing energy, which would be very great on bodies of so rare a nature.

By far the most curious and interesting instance of the disturbing action of the great bodies of our system is found in the comet of 1770. The elements of its orbit, determined by Messier, did not agree with those of any comet that had hitherto been computed, yet Lexel ascertained that it described an ellipse about the sun, whose major axis was only equal to three times the length of the diameter of the terrestrial orbit, and consequently that it must return to the sun at intervals of five years and a half. This result was confirmed by numerous observations, as the comet was visible through an arc of 170°; yet this comet had never been observed before the year 1770, nor has it ever again been seen till 1843, though very brilliant. The disturbing action of the larger planets affords a solution of this anomaly, as Lexel ascertained that in 1767 the comet must have passed Jupiter at a distance less than the fifty-eighth part of its distance from the sun, and that in 1779 it would be 500 times nearer Jupiter than the sun; consequently the action of the sun on the comet would not be the fiftieth part of what it would experience from Jupiter, so that Jupiter became the primum mobile. Assuming the orbit to be such as Lexel had determined in 1770, La Place found that the action of Jupiter, previous to the year 1770, had so completely changed the form of it, that the comet which had been invisible to us before 1770 was then brought into view, and that the action of the same planet, producing a contrary effect, has subsequently to that year removed it from our sight, since it was computed to be revolving in an orbit whose perihelion was beyond the orbit of Ceres. However, the action of Jupiter during the summer of 1840 must have been so great, from his proximity to that singular body, that he seems to have brought it back to its former path as he had done in 1767, for the elements of the orbit of a comet which was discovered in November 1843, by M. Faye, agree so nearly with those of the orbit of Lexel’s comet that the two bodies were supposed to be identical; by the subsequent computation of M. le Verrier, it appears, however, that they are not the same, that they were both brought to our system by Jupiter’s attraction, and that they have been in it more than a century, and have frequently come near the earth without having been seen. From the smallness of the excentricity of Lexel’s comet, the orbit resembles those of the planets, but this comet is liable to greater perturbations than any other body in the system, because it comes very near the orbit of Mars when in perihelion, and very near that of Jupiter when in aphelion; besides, it passes within a comparatively small distance of the orbits of the minor planets; and as it will continue to cross the orbit of Jupiter at each revolution till the two bodies meet, its periodic time, now about seven years, will again be changed, but in the mean time it ought to have returned to its perihelion in the year 1851. This comet might have been seen from the earth in 1776, had its light not been eclipsed by that of the sun. There is still so much doubt with regard to Lexel’s comet that during the present year, 1858, M. le Verrier has constructed a table of all the orbits in which the comet may have moved after leaving Jupiter in 1770, which will enable astronomers to recognise the comet even should the elements of its orbit be much altered. He thinks it possible that its path may have become hyperbolic, but that it is more likely an augmentation of its periodic time may have taken place. It is quite possible that comets frequenting our system may be turned away, or others brought to the sun, by the attraction of planets revolving beyond the orbit of Neptune, or by bodies still farther removed from the solar influence.

Other comets, liable to less disturbance, return to the sun at stated intervals. Halley computed the elements of the orbit of a comet that appeared in the year 1682, which agreed so nearly with those of the comets of 1607 and 1531, that he concluded it to be the same body returning to the sun at intervals of about seventy-five years. He consequently predicted its reappearance in the year 1758, or in the beginning of 1759. Science was not sufficiently advanced in the time of Halley to enable him to determine the perturbations this comet might experience; but Clairaut computed that, in consequence of the attraction of Jupiter and Saturn, its periodic time would be so much shorter than during its revolution between 1607 and 1682, that it would pass its perihelion on the 18th of April, 1759. The comet did arrive at that point of its orbit on the 12th of March, which was thirty-seven days before the time assigned. Clairaut subsequently reduced the error to twenty-three days; and La Place has since shown that it would only have been thirteen days if the mass of Saturn had been as well known as it is now. It appears, from this, that the path of the comet was not quite known at that period; and, although many observations were then made, they were far from attaining the accuracy of those of the present day. Besides, since the year 1759, the orbit of the comet has been altered by the attraction of Jupiter in one direction, and that of Saturn, Uranus, and Neptune in the other; yet, notwithstanding these sources of uncertainty, and our ignorance of all the possible causes of derangement from unknown bodies on the confines of our system, or in the regions beyond it, the comet appeared exactly at the time, and not far from the place assigned to it by astronomers; and its actual arrival at its perihelion a little before noon on the 16th of November, 1835, only differed from the computed time by a very few days, which was probably owing to the attraction of Neptune.

The fulfilment of this astronomical prediction is truly wonderful, if it be considered that the comet is seen only for a few weeks during its passage through our system, and that it wanders from the sun for seventy-five years to twice the distance of Uranus. This enormous orbit is four times longer than it is broad; its length is about 3420 millions of miles, or about thirty-six times the mean distance of the earth from the sun. At its perihelion the comet comes within nearly fifty-seven millions of miles of the sun, and at its aphelion it is sixty times more distant. On account of this extensive range it must experience 3600 times more light and heat when nearest to the sun than in the most remote point of its orbit. In the one position the sun will seem to be four times larger than he appears to us, and at the other he will not be apparently larger than a star (N. 228.)