REMNANTS AND SURVIVALS
If the sun and planets were, in sober truth, wrought into their present shape out of a primordial nebula, the comparatively void surrounding space should naturally be strewn with fragments of unappropriated material. For the process of englobement could hardly, one would think, be carried out with such neatness and precision as to leave no shreds or shavings lying about the great atelier. Residual stuff there must be, unless our preconceived ideas are grossly erroneous; nor have we far to look in order to find it. We find it, apparently, under two forms presenting curious dissimilarities, yet belonging fundamentally, we can scarcely doubt, to the same order of things. These two kinds of waste product may be identified in the innumerable army of comets and in the strange, pale cone of the zodiacal light.
One of the most important and secure additions to knowledge in the department of cosmogony made during the nineteenth century was the establishment of comets in a position of entire, perennial, and aboriginal dependence upon the sun. That is to say, a vast majority, if not the whole of them, attend him on his sidereal journey. They are, accordingly, and have immemorially been his clients, and they can lose that status only through the effects of violent disturbance compelling them to depart irrevocably from their closed orbits along hyperbolic tracks. A trifling leakage of comets from our system is thus possible, which may or may not be compensated by annexations of adventitious members of the class, similarly banished from the precincts of remote stars. But this is a secondary consideration; the essential point to be borne in mind is that comets are native-born subjects of the sun, that they make an integral part of his cortège, that they own the same substantial origin, are dominated by his power, and must share his fortunes. Their study should then prove strongly illuminative as to the pre-history of our system, and for this especial reason, that they seemingly belong by right to that vanished world which it is the chosen task of cosmogony to reconstruct. They are, we can infer, the genuine primitives of the solar company; they retain something of prairie wildness, not having been broken in by steadily enforced gravitational discipline. Each perihelion passage is an adventure; between it and the next, fateful incidents may occur. Forces negligible on dense planetary globes act sensibly on their tenuous materials; they in part strikingly illustrate, and in part fantastically invert, the common modes of natural procedure. But it is their antiquarian significance that mainly concerns us here.
Admitting for the inchoate solar nebula such a constitution as that devised by Kant, and adopted with amendments by M. du Ligondès, we find ourselves confronted with the almost inevitable consequence of symptomatic survivals. Wisps of crude matter, in other words, which escaped being drawn into the vortices of embryo planets should continue to circulate, as they had from the first circulated, in all possible planes, and with no partiality for either a right-handed or a left-handed direction. These waifs and wastrels should, in fact, be indistinguishable from comets—'les seuls témoins,' according to the French cosmogonist, 'qui nous restent sur le mode de la circulation première.'[109] The identification is seductive to the imagination, and does not fall far short of convincing the reason.
There is clear evidence that what we may venture to call the native mode of cometary circulation is absolutely exempt from the rules which impress the movements of the planets with an unequivocal stamp of congruity. The few comets showing some degree of compliance with the general plan are those which have been subjected to manifold perturbations, and can hence no longer be called as unbiassed witnesses; while their untrained associates, left relatively free to follow the impulsion of their start, betray no geometrical preferences in their manner of travelling. They revolve indifferently with or against the course of the signs; their paths are inclined at every possible angle to the ecliptic; they approach the sun in sensibly equal numbers from all quarters of the sky; they agree only in pursuing ellipses so elongated as to verge towards the parabolic limit. But just in this way, and no otherwise, we should expect to find bodies circulating which, having been aggregated at random (as Kant supposed) in the beginning, had departed to the least possible extent from the initial conditions of their systemic union. A good primâ facie case can, then, be made out for regarding comets as samples of the used-up nebula, as superannuated constituents of an inconceivable chaos, which, evading the operation of laws of change, have floated down the stream of ages virtually intact and undisturbed.
Yet the question has other aspects besides this purely mechanical one. They should all be harmonized by truth, which cannot be more securely guaranteed than by consilient testimony; nevertheless, there are difficulties in effecting the accommodation. Comets are not, in a chemical sense, closely related to nebulæ. They are fundamentally of carbonaceous composition—free hydrogen makes no spectroscopic show in them—while they include metallic ingredients occasionally rendered glowing by the powerful excitement of a perihelion rush-past. But gaseous nebulæ shine mainly with the light of certain unknown substances, reinforced by rays of hydrogen and helium. Carbon flutings and metallic lines are alike alien to their spectra. Nor is there any community that we yet know of between the chemistry of white nebulæ and that of comets. The nebular hypothesis of cometary origin is thus discountenanced by the results of light-analysis. Still, there are possibilities of reconcilement. Spectral conditions must be subject to change. The quality of light emitted by a body of mixed composition cannot fail to alter with the inevitable alteration of physical state brought about by external influences or internal change.
Selective illumination is beyond doubt largely concerned in modifying the information we are able to obtain as to the composition of remote masses, and its modes of action seem capricious because they are very imperfectly understood. Hence, spectral modifications may take place merely through the substitution of some elements for others in carrying (let us suppose) an electric discharge, though all were from the first simultaneously present in unvaried proportions. Moreover, chemical immutability can no longer be taken for granted. We have learned of late that even elementary individuality breaks down under the battering-ram of time. Sooner or later the stamp, however apparently inviolable, will be defaced, transformations of species will ensue, and novel combinations of material will subtly accommodate themselves to the needs of a growing world. These things, it is true, are involved in much obscurity, but we have caught glimpses of instability clear enough to convey an emphatic warning against dogmatic interpretations of spectral characters. Physical science may then license M. du Ligondès' theory of comets with a provisional Nihil obstat.
The zodiacal light suggests a different set of considerations. Comets being of pre-planetary origin, the ecliptical glow must be supposed post-planetary. It belongs to a later epoch, being composed, according to an accepted opinion,[110] of superfluous materials left over from the construction of the train of globes to which our own belongs. It might be compared to whey from which the curd has been separated. All the good has been got out of it; we might be tempted to throw it aside upon the rising rubbish-heap of the skies, with the importunate asteroidal throng, a few dozen undistinguished comets, and some hundreds of ill-defined meteoric systems. But celestial refuse is always worth sifting, above all, for tokens of genealogical descent, and we should be unwise to neglect the chance of finding them in the peculiar relations of the zodiacal light.
A triple phenomenon, it consists, when completely seen, of a cone, a band, and a counterglow. The connection of these parts into a whole is obvious, though enigmatical. Usually, however, only the cone is visible. It appears about the time of the spring and autumn equinoxes, after sunset and before sunrise respectively, as a faint lenticular illumination, tapering upward from the sun's place below the horizon to an apex high up near the meridian. Under the name of the 'False Dawn' it was familiar, probably from an early age, to Oriental peoples. But they looked for it at the opposite end of the night from that favoured by European observers; nor did the phenomenon attract any particular notice here in England until 1660, when Joshua Childrey published a description of it in his Britannia Baconica. Yet it had been specifically observed about seventy years previously by Christoph Rothmann of Hesse, and must have been less intelligently perceived by numberless spectators, who most likely included it, with such miscellaneous objects as comets' tails, auroral beams, and meteor-trails, in the undefined class of appearances known from of old as trabes.
The light is ordinarily much feebler than the Milky Way, which it nevertheless on occasions unmistakably outshines.[111] Real fluctuations of brightness seem implicated in these changes; yet they follow no traceable law of periodicity, and are certainly independent of the sunspot cycle.