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.