CHAPTER I.

ON THE COSMICAL ORIGIN OF THE PLANETS OF THE SOLAR SYSTEM.

In this Chapter we propose to treat briefly of the probable formation of the various members of the solar system from matter which previously existed in space in a condition different from that in which we at present find it—_i.e._, in the form of planets and satellites.

It is almost impossible to conceive that our world with its satellite, and its fellow worlds with their satellites, and also the great centre of them all, have always, from the commencement of time, possessed their present form: all our experiences of the working of natural laws rebel against such a supposition. In every phenomenon of nature upon this earth—the great field from which we must glean our experiences and form our analogies—we see a constant succession of changes going on, a constant progression from one stage of development to another taking place, a perpetual mutation of form and nature of the same material substance occurring: we see the seed transformed into the plant, the flower into the fruit, and the ovum into the animal. In the inorganic world we witness the operation of the same principle; but, by reason of their slower rate of progression, the changes there are manifested to us rather by their resulting effects than by their visible course of operation. And when we consider, as we are obliged to do, that the same laws work in the greatest as well as the smallest processes of nature, we are compelled to believe in an antecedent state of existence of the matter that composes the host of heavenly bodies, and amongst them the earth and its attendant moon.

In the pursuit of this course of argument we are led to inquire whether there exists in the universe any matter from which planetary bodies could be formed, and how far their formation from such matter can be explained by the operation of known material laws.

Before the telescope revealed the hidden wonders of the skies, and brought its rich fruits into our garner of knowledge concerning the nature of the universe, the philosophic minds of some early astronomers, Kepler and Tycho Brahe to wit, entertained the idea that the sun and the stars—the suns of distant systems—were formed by the condensation of celestial vapours into spherical bodies; Kepler basing his opinion on the phenomena of the sudden shining forth of new stars on the margin of the Milky Way. But it was when the telescope pierced into the depths of celestial space, and brought to light the host of those marvellous objects, the nebulæ, that the strongest evidence was afforded of the probable validity of these suppositions. The mention of “nebulous stars” made by the earlier astronomers refers only to clusters of telescopic stars which the naked eye perceives as small patches of nebulous light; and it does not appear that even the nebula in Andromeda, although so plainly discernible as to be often now-a-days mistaken by the uninitiated for a comet, was known, until it was discovered by means of a telescope, in 1612, by Simon Marius, who described it as resembling a candle shining through semi-transparent horn, as in a lantern, and without any appearance of stars. Forty years after this date Huygens discovered the splendid nebula in the sword handle of Orion, and in 1665 another was detected by Hevelius. In 1667 Halley (afterwards Astronomer Royal) discovered a fourth; a fifth was found by Kirsch in 1681, and a sixth by Halley again in 1714. Half a century after this the labours of Messier expanded the list of known nebulæ and clusters to 103, a catalogue of which appeared in the “Connaissance du Temps” (the French “Nautical Almanac”) for the years 1783-1784. But this branch of celestial discovery achieved its most brilliant results when the rare penetration, the indomitable perseverance, and the powerful instruments of the elder Herschel were brought to bear upon it. In the year 1779 this great astronomer began to search after nebulæ with a seven-inch reflector, which he subsequently superseded by the great one of forty feet focus and four feet aperture. In 1786 he published his first catalogue of 1000 nebulæ; three years later he astonished the learned world by a second catalogue containing 1000 more, and in 1802 a third came forth comprising other 500, making 2500 in all! This number has been so far increased by the labours of more recent astronomers that the last complete catalogue, that of Sir John Herschel, published a few years ago, contains the places of 5063 nebulæ and clusters.

At the earlier periods of Herschel’s observations, that illustrious observer appears to have inclined to the belief that all nebulæ were but remote clusters of stars, so distant, so faint, and so thickly agglomerated as to affect the eye only by their combined luminosity, and at this period of the nebular history it was supposed that increased telescopic power would resolve them into their component stars. But the familiarity which Herschel gained with the phases of the multitudinous nebulæ that passed in review before his eyes, led him ultimately to adopt the opinion, advanced by previous philosophers, that they were composed of some vapoury or elementary matter out of which, by the process of condensation, the heavenly bodies were formed; and this led him to attempt a classification of the known nebulæ into a cosmical arrangement, in which, regarding a chaotic mass of vapoury matter as the primordial state of existence, he arranged them into a series of stages of progressive development, the individuals of one class being so nearly allied to those in the next that, to use his own expression, not so much difference existed between them “as there would be in an annual description of the human figure were it given from the birth of a child till he comes to be a man in his prime.” (_Philosophical Transactions, Vol. CI., pp. 271_, _et seq._)

His category comprises upwards of thirty classes or stages of progression, the titles of a few of which we insert here to illustrate the completeness of his scheme.

Class 1. Of extensive diffused nebulosity. (A table of 52 patches of such nebulosity actually observed is given, some of which extend over an area of five or six square degrees, and one of which occupies nine square degrees.) ” 6. Of milky nebulosity with condensation. ” 15. Of nebulæ that are of an irregular figure. ” 17. Of round nebulæ. ” 20. Of nebulæ that are gradually brighter in the middle. ” 25. Of nebulæ that have a nucleus. ” 29. Of nebulæ that draw progressively towards a period of final condensation. ” 30. Of planetary nebulæ. ” 33. Of stellar nebulæ nearly approaching the appearance of stars.

In a walk through a forest we see trees in every stage of growth, from the tiny sapling to the giant of the woods, and no doubt can exist in our minds that the latter has sprung from the former. We cannot at a passing glance discern the process of development actually going on; to satisfy ourselves of this, we must record the appearance of some single tree from time to time through a long series of years. And what a walk through a forest is to an observer of the growth of a tree, a lifetime is to the observer of changes in such objects as the nebulæ. The transition from one state to another of the nebulous development is so slow that a lifetime is hardly sufficient to detect it. Nor can any precise evidence of change be obtained by the comparison of drawings or descriptions of nebulæ at various epochs, with whatever care or skill such drawings be made, for it will be admitted that no two draughtsmen will produce each a drawing of the most simple object from the same point of view, in which every detail in the one will coincide exactly with every detail in the other. There is abundant evidence of this in the existing representations of the great nebula in Orion; a comparison of the drawings that have been lately made of this object, with the most perfect instruments and by the most skilful of astronomical draughtsmen, reveals varieties of detail and even of general appearance such as could hardly be imagined to occur in similar delineations of one and the same subject; and any one who himself makes a perfectly unbiassed drawing at the telescope will find upon comparison of it with others that it will offer many points of difference. The fact is that the drawing of a man, like his penmanship, is a personal characteristic, peculiar to himself, and the drawings of two persons cannot be expected to coincide any more than their handwritings. The appearance of a nebula varies also to a great extent with the power of the telescope used to observe it and the conditions under which it is observed; the drawings of nebulæ made with the inferior telescopes of a century or two centuries ago, the only ones that, by comparison with those made in modern times, could give satisfactory evidence of changes of form or detail, are so rude and imperfect as to be useless for the purpose, and it is reasonable to suppose that those made in the present day will be similarly useless a century or two hence. Since then we can obtain no evidence of the changes we must assume these mysterious objects to be undergoing, _ipso facto_, by observation of _one nebula_ at _various periods_, we must for the present accept the _primâ facie_ evidence offered (as in the case of the trees in a forest) by the observation of _various nebulæ_ at _one period_.

“The total dissimilitude,” says Herschel at the close of the observations we have alluded to, “between the appearance of a diffusion of the nebulous matter and of a star, is so striking, that an idea of the conversion of the one into the other can hardly occur to any one who has not before him the result of the critical examination of the nebulous system which has been displayed in this [his] paper. The end I have had in view, by arranging my observations in the order in which they have been placed, has been to show that the above mentioned extremes may be connected by such nearly allied intermediate steps, as will make it highly probable that every succeeding state of the nebulous matter is the result of the action of gravitation upon it while in a foregoing one, and by such steps the successive condensation of it has been brought up to the planetary condition. From this the transit to the stellar form, it has been shown, requires but a very small additional compression of the nebulous matter.”

Where the researches of Herschel terminated those of Laplace commenced. Herschel showed how a mass of nebulous matter so diffused as to be scarcely discernible might be and probably was, by the mere action of gravitation, condensed into a mass of comparatively small dimensions when viewed in relation to the immensity of its primordial condition. Laplace demonstrated how the known laws of gravitation could and probably did from such a partially condensed mass of matter produce an entire planetary system with all its subordinate satellites.

The first physicist who ventured to account for the formation of the various bodies of our solar system was Buffon, the celebrated French naturalist. His theory, which is fully detailed in his renowned work on natural history, supposed that at some period of remote antiquity the sun existed without any attendant planets, and that a comet having dashed obliquely against it, ploughed up and drove off a portion of its body sufficient in bulk to form the various planets of our system. He suggests that the matter thus carried off “at first formed a torrent the grosser and less dense parts of which were driven the farthest, and the densest parts, having received only the like impulsion, were not so remotely removed, the force of the sun’s attraction having retained them:” that “the earth and planets therefore at the time of their quitting the sun were burning and in a state of liquefaction;” that “by degrees they cooled, and in this state of fluidity they took their form.” He goes on to say that the obliquity of the stroke of the comet might have been such as to separate from the bodies of the principal planets small portions of matter, which would preserve the same direction of motion as the principal planets, and thus would form their attendant satellites.

The hypothesis of Buffon, however, is not sufficient to explain all the phenomena of the planetary system; and it is imperfect, inasmuch as it begins by assuming the sun to be already existing, whereas any theory accounting for the primary formation of the solar system ought necessarily to include the origination of the most important body thereof, the sun itself. Nevertheless, it is but due to Buffon to mention his ideas, for the errors of one philosophy serve a most useful end by opening out fields of inquiry for subsequent and more fortunate speculators.

Laplace, dissatisfied with Buffon’s theory, sought one more probable, and thus was led to the proposition of the celebrated _nebular hypothesis_ which bears his name, and which, in spite of its disbelievers, has never been overthrown, but remains the only probable, and, with our present knowledge, the only possible explanation of the cosmical origin of the planets of our system. Although Laplace puts forth his conjectures, to use his own words, “with the deference which ought to inspire everything that is not a result of observation and calculation,” yet the striking coincidence of all the planetary phenomena with the conditions of his system gives to those conjectures, again to use his modest language, “a probability strongly approaching certitude.”

Laplace conceived the sun to have been at one period the nucleus of a vast nebula, the attenuated surrounding matter of which extended beyond what is now the orbit of the remotest planet of the system. He supposed that this mass of matter in process of condensation possessed a rotatory motion round its centre of gravity, and that the parts of it that were situated at the limits where centrifugal force exactly counterbalanced the attractive force of the nucleus were abandoned by the contracting mass, and thus were formed successively a number of rings of matter concentric with and circulating around the central nucleus. As it would be improbable that all the conditions necessary to preserve the stability of such rings of matter in their annular form could in all cases exist, they would break up into masses which would be endued with a motion of rotation, and would in consequence assume a spheroidal form. These masses, which hence constituted the various planets, in their turn condensing, after the manner of the parent mass, and abandoning their outlying matter, would become surrounded by similarly concentric rings, which would break up and form the satellites surrounding the various planetary masses; and, as a remarkable exception to the rule of the instability of the rings and their consequent breakage, Laplace cited the case of Saturn surrounded by his rings as the only instances of unbroken rings that the whole system offers us; unless indeed we include the zodiacal light, that cone of hazy luminosity that is frequently seen streaming from our luminary shortly before and after sunset, and which Laplace supposed to be formed of molecules of matter, too volatile to unite either with themselves or with the planets, and which must hence circulate about the sun in the form of a nebulous ring, and with such an appearance as the zodiacal actually presents.

This hypothesis, although it could not well be refuted, has been by many hesitatingly received, and for a reason which was at one time cogent. In the earlier stages of nebular research it was clearly seen, as we have previously remarked, that many of the so-called nebulæ, which appeared at first to consist of masses of vapoury matter, became, when scrutinised with telescopes of higher power, resolved into clusters containing countless numbers of stars, so small and so closely agglomerated, that their united lustre only impressed the more feeble eye as a faint nebulosity; and as it was found that each accession of telescopic power increased the numbers of nebulæ that were thus resolved, it was thought that every nebula would at some period succumb to the greater penetration of more powerful instruments; and if this were the case, and if no real nebulæ were hence found to exist, how, it was argued, could the nebular hypothesis be maintained? One of the most important nebulæ bearing upon this question was the great one in the sword handle of Orion, one of the grandest and most conspicuous in the whole heavens. On account of the brightness of some portions of this object, it seemed as though it ought to be readily resolvable, supposing all nebulæ to consist of stars, but all attempts to resolve it were in vain, even with the powerful telescopes of Sir John Herschel and the clear zenithal sky of the Cape of Good Hope. At length the question was thought to be settled, for upon the completion of Lord Rosse’s giant reflector, and upon examination of the nebula with it, his lordship stated that there could be little, if any, doubt as to its resolvability, and then it was maintained, by the disbelievers in the nebular theory, that the last stronghold of that theory had been broken down.

But the truths of nature are for ever playing at hide and seek with those who follow them:—the dogmas of one era are the exploded errors of the next. Within the past few years a new science has arisen that furnishes us with fresh powers of penetration into the vast and secret laboratories of the universe; a new eye, so to speak, has been given us by which we may discern, by the mere light that emanates from a celestial body, something of the chemical elements of which it is composed. When Newton two hundred years ago toyed with the prism he bought at Stourbridge fair, and projected its pretty rainbow tints upon the wall, his great mind little suspected that that phantom riband of gorgeous colours would one day be called upon to give evidence upon the probable cosmical origin of worlds. Yet such in truth has been the case. Every substance when rendered luminous gives off light of some colour or degree of refrangibility peculiar to itself, and although the eye cannot detect any difference between one character of light and another, the prism gives the means of ascertaining the quality and degree of refrangibility of the light emanating from any source however distant, and hence of gaining some knowledge of the nature of that source. If, for instance, a ray of light from a solid body in combustion is passed through a prism, a spectrum is produced which exhibits light of all colours or all degrees of refrangibility; if the light from such a body, before passing through the prism, be made to pass through gases or certain metallic vapours, the resulting spectrum is found to be crossed transversely by numbers of fine dark lines, apparently separating the various colours, or cutting the spectrum into bands. The solar spectrum is of this class; the once mysterious lines first observed by Wollaston, and subsequently by Fraunhoffer, and known as “Fraunhoffer’s lines,” have now been interpreted, chiefly by the sagacious German chemist Kirchoff, and identified as the effects of absorption of certain of the sun’s rays by chemical vapours contained in his atmosphere. The fixed stars yield spectra of the same character, but varying considerably in feature, the lines crossing the stellar spectra differing in position and number from those of the sun, and one star from another, proving the stars to possess varied chemical constitutions. But there is another class of spectra, exhibited when light from other sources is passed through the prism. These consist, not of a luminous riband of light like the solar spectrum, but of bright isolated lines of coloured light with comparatively wide dark spaces separating them. Such spectra are yielded only by the light emitted from luminous gases and metals or chemical elements in the condition of incandescent vapour. Every gas or element in the state of luminous vapour yields a spectrum peculiar to itself, and no two elements when vapourized before the prism show the same combinations of luminous lines.

Now in the course of some observations upon the spectra of the fixed stars by Dr. Huggins, it occurred to that gentleman to turn his telescope, armed with a spectroscope, upon some of the brighter of the nebulæ, and great was his surprise to find that instead of yielding continuous spectra, as they must have done had their light been made up of that of a multitude of stars, they gave spectra containing only two or three isolated bright lines; such a spectrum could only be produced by some luminous gas or vapour, and of this form of matter we are now justified in declaring, upon the strength of numerous modern observations, these remarkable bodies are composed; and it is a curious and interesting fact that some of the nebulæ styled resolvable, from the fact of their exhibiting points of light like stars, yield these gaseous spectra, whence Dr. Huggins concludes that the brighter points taken for stars are in reality nuclei of greater condensation of the nebular matter: and so the fact of the apparent resolvability of a nebula affords no positive proof of its non-nebulous character.

These observations—which have been fully confirmed by Father Secchi of the Roman College—by destroying the evidence in favour of nebulæ being remote clusters, add another attestation to the probability of the truth of the nebular hypothesis, and we have now the confutation of the luminologist to add to that of the astronomers who, in the person of the illustrious Arago, asserted that the ideas of the great author of the “Mécanique Céleste” “were those only which by their grandeur, their coherence, and their mathematical character could be truly considered as forming a physical cosmogony.”

Confining, then, our attention to the single object of the universe it is our task to treat of—the Moon—and without asserting as an indisputable fact that which we can never hope to know otherwise than by inference and analogy, we may assume that that body once existed in the form of a vast mass of diffused or attenuated matter, and that, by the action of gravitation upon the particles of that matter, it was condensed into a comparatively small and compact planetary body.

But while the process of condensation or compaction was going on, another important law of nature—but recently unfolded to our knowledge—was in powerful operation, the discussion of which law we reserve for a separate Chapter.