FOOTNOTES:
[34] Acta Mathematica, vol. vii., Stockholm, 1885.
[35] Proceedings Royal Society, vols. xlii., lxxi.; Philosophical Transactions, vols. cxcviii., cxcix., Series A.
[36] J. H. Jeans, Philosophical Transactions, vol. cxcix., Series A, p. 1.
[37] Figures d'Équilibre d'une Masse Fluide, p. 172.
[38] Proceedings Royal Society, vol. lxxi., p. 183.
[39] J. H. Jeans, Astrophysical Journ., vol. xxii., p. 93.
[40] Astrophysical Journal, vol. vii., p. 1.
[41] Astrophysical Journal, vol. viii., p. 163.
[42] Monthly Notices, vol. lxiii., p. 627.
[43] Astrophysical Journal, vol. xiii., p. 90; Science, July 27, 1900.
[44] G. H. Darwin, The Tides, p. 327.
[CHAPTER VII]
WORLD BUILDING OUT OF METEORITES
The idea is seductive that we see in every meteoric fire-streak a remnant of the process by which our world, and other worlds like or unlike it, were formed. It is not a new idea. Chladni entertained it in 1794; and it has since from time to time been revived and rehabilitated with the aid of improved theoretical knowledge and a larger array of facts. Survivals are tempting to thought. It costs less effort to realize differences in degree than differences of kind. The enhanced activity of familiar operations is readily imagined, while perplexity is apt to shroud the results of modes of working strange to experience. Hence the presumption in favour of continuity; nor can it be said, even apart from our own mental inadequacy, that the presumption is other than legitimate. Nature is chary of her plans, lavish of her materials. Her aims are characterized by a majestic unity, but she takes little account (that we can see) of surplusage or wreckage. Now, it seems likely that meteorites represent one or the other of these two forms of waste stuff. They are analogous, apparently, either to the chips from shaped blocks, or to the dust and rubbish of their destruction. Let us consider what it is that we really know about them.
It cannot be said that the sources of our information are scanty. Fully one hundred millions are daily appropriated by the earth as she peacefully spins through the ether. Their absorption leaves her unaffected. It produces no perceptible change in her internal economy, and makes no sensible addition to her mass. The hundred millions of small bodies taken up have, nevertheless, in Professor Langley's opinion, an aggregate weight of more than one hundred tons.[45] And this increment is always going on. Yet its accumulated effect is evanescent by comparison with the enormous mass of our globe. That it was more considerable in past ages than it is at present might be plausibly conjectured, but cannot reasonably be maintained. Geological deposits contain—unless by some rare exception—no recognisable meteoric ingredients. There is nothing to show that the earth was subject to a heavier bombardment from space during the Silurian era than in the twentieth century. Nor could the whole of its constituents have been, in any case, thus provided. Out of kiln-dried fragments, like the Mazapil iron or the 'thunder-stones' of Adare, a terraqueous planet could not have been formed. This objection, urged by Mr. O. Fisher,[46] is seemingly irrefutable.
Meteorites signify their existence to us, in general, only by the bale-fires of their ruin; but in a few cases their tangible relics come to hand. Those substantial enough to escape total disintegration through atmospheric resistance to their swift movements plunge into the sea or bury themselves in the earth, and in a certain proportion of cases find their way to museums and laboratories, where they are subjected to the searching investigation demanded by their exotic origin. Its results are scarcely what might have been expected. Aerolites—as these samples from space are distinctively called—are not chemically peculiar; they consist exclusively of the same elementary substances composing the crust of the earth; but their mineralogy is strongly characteristic. They are extremely complex structures, formed apparently in the absence of water, and with a short supply of oxygen; the further condition of powerful pressure is indicated with some probability, nay, with virtual certainty for those including small diamonds,[47] while prolonged vicissitudes of fracture and re-agglomeration are possibly recorded by the brecciated texture of many of these rocky trouvailles. Their aspect is thus anything but primitive; each fragment tacitly lays claim to an eventful history; they suggest a cataclysm, of which we behold in them the shattered outcome. The nature of such cataclysms is scarcely open to conjecture; only a hint regarding it may be gathered from the circumstance that the most profound terrestrial formations are those which approximate most closely to the mineralogical peculiarities of meteorites.
Nevertheless, the only ascertained relationships of meteorites are with comets. In every system of shooting stars the primary body most probably is, or at any rate was, a comet. Each appears to be the offspring of a cometary parent, and develops pari passu with its decay. The view has hence been adopted, and not without justification, that comets in their primitive integrity are simply 'meteor-swarms.' Assent may be given to it with some qualifications which we need not here stop to discuss. What immediately concerns us is the interesting question as to the constitution of meteor-swarms. What is the real meaning of the term? What does it convey to our minds? A meteor-swarm may be defined as a rudely globular aggregation of small cosmical masses, revolving under the influence of their mutual attraction, round their common centre of gravity. Each must revolve on its own account, though all have the same period; and their orbits may be inclined at all possible angles to a given plane, and may be traversed indifferently in either direction. From this tumultuous mode of circulation collisions should frequently ensue, but they would be of a mild character. They could not be otherwise in a system of insignificant mass and correspondingly sluggish motion. We are considering, it must be remembered, only cometary swarms, as being the only collections of the sort that come, even remotely, within our ken; and comets include the minimum of matter. This we are entitled to infer from the fact that none of those hitherto observed, whether conspicuous or obscure, newly arrived from space, or obviously effete, have occasioned the slightest gravitational disturbance to any member of our system.
A cometary swarm, if left to itself, might eventually shape itself into a reduced model of the 'Saturn' planetary nebula. Colliding particles should, owing to their loss of velocity, subside towards the centre, and accrete into a globular mass. A predominant current of movement would, through their elimination, gain more and more completely the upper hand; and it would finally, with the inevitable diminution of energy,[48] be restricted almost wholly to the principal plane of a system, composed essentially of a rotating nucleus encompassed by a wide zone of independently circulating meteorites. But this mode of evolution is not even distantly followed by comets. It would be possible only if they were isolated in space, and, in point of fact, their revolutions round the sun are of overwhelming importance to their destinies. The sun's repulsive energy causes them to waste and diffuse with expansion of splendid plumage. Under the sun's unequal attraction at close quarters they are subject to disruption, and the upshot of the tidal stresses acting upon them is the dispersal of their constituent particles along the wide ambit of their oval tracks.
We are, nevertheless, invited to look further afield. Cometary meteor-swarms may be only miniature specimens of the contents of space. Why should not remote sidereal regions be thronged with similar assemblages, colossal in their proportions, countless in number? And may they not supply the long-sought desideratum of a suitable 'world-stuff' for the construction of suns and planets? From some such initial considerations as these Sir Norman Lockyer developed, in 1887, a universal meteoritic hypothesis, designed on the widest possible lines, based on promising evidence, and professing to supply a key to the baffling enigma of cosmical growth and diversification. The meteoric affinities of comets formed its starting-point; comets were assimilated to nebulæ; and from nebulæ were derived, by gradual processes of change, all the species of suns accessible to observation. The view was of far-reaching import and magnificent generality, but its value avowedly rested on a body of facts of a special kind. In this it differed from the crowd of ambitious speculations regarding the origin of things by which it had been preceded. In this it attained an immeasurable superiority over them, if only the testimony appealed to could be established as valid. Indeed, it is scarcely too much to say that, whether it were valid or not, the mere circumstance of having called the spectroscope as a witness in the high court of cosmogony constituted an innovation both meritorious and significant.
The spectrum of the nebulæ was a standing puzzle. A theory which set out by making its meaning plain secured at once a privileged position. This was seemingly accomplished by Sir Norman Lockyer through the means of some simple laboratory experiments on the spectra of meteorites. Certain 'low temperature' lines of magnesium given out by the vapours of stony aerolitic fragments were shown to fall suspiciously close to the chief nebular lines previously classed as 'unknown.' The coincidences, it is true, were determined with low dispersion, and were published for what they were worth, but they looked hopeful. Their substantiation, had it been feasible, would have marked the beginning of a new stadium of progress. Nature, however, proved recalcitrant. The suggested agreements avowed themselves, on closer inquiry, as approximate only; magnesium light makes no part of the nebular glow, and nebulium, its main source, evades terrestrial recognition. The light of cosmic clouds is sui generis—it includes no metallic emissions; while the fundamental constituents of meteorites are metals variously assorted and combined.
The decipherment of the nebular hieroglyphics was the crucial test; its failure to meet it left the hypothesis seriously discredited; for coincidences between spectral rays, common to nearly all the heavenly bodies, naturally counted for nothing. Yet the investigation had its uses. The energy with which it was prosecuted, the ingenuity and resource with which it was directed, told for progress. There has been a clash of arms and a reorganization of forces. Thought was stirred, observation and experiment received a strong stimulus, fresh affluents to the great stream of science began to be navigated. Efforts to prove what had been asserted were fruitful in some directions, and the work of refutation had inestimable value in defining what was admissible, and establishing unmistakable landmarks in astrophysics.
The discussion, it must be admitted, threw very little light on the part played by meteorites in cosmogony. Their world-building function remains largely speculative. Doubts of many kinds qualify its possibility, and lend it a fantastic air of unreality. But this may in part be due to a defect of imaginative power with which the universe was not concerned. Waiving, then, preliminary objections, we find ourselves confronted with the fundamental question: Given a meteor-swarm of the requisite mass and dimensions, is there any chance of its condensing into a planetary system? Sir Norman Lockyer set aside this branch of his subject. His hypothesis was, in fact, 'pre-nebular.' He assumed that the small solid bodies with which it started would, in course of time, become completely volatilized by the heat of their mutual impacts, and that the resulting gaseous mass would thenceforward comport itself after the fashion imagined by Laplace. Professor Darwin regarded the matter otherwise. It seemed to him possible to combine the postulates of the meteoric and nebular theories in a system planned on an original principle. For this purpose it was necessary to excogitate a means of rendering the kinetic theory of gases available for a meteor-swarm. 'The very essence,' he wrote,[49] 'of the nebular hypothesis is the conception of fluid pressure, since without it the idea of a figure of equilibrium becomes inapplicable.'
M. Faye abandoned this idea; he built up his planets out of incoherent materials, thereby avoiding the incongruities, but forfeiting the logical precision of Laplace's stricter procedure. Professor Darwin consented to forfeit nothing; he stood forward as a syncretist, his object being to 'point out that by a certain interpretation of the meteoric theory we may obtain a reconciliation of these two orders of ideas, and may hold that the origin of stellar and planetary systems is meteoric, whilst retaining the conception of fluid pressure.' For the compassing of this end he adopted a bold expedient. Fluid pressure in a gas is 'the average result of the impacts of molecules.' Fluid pressure in a meteor-swarm might, he conceived, be the net product of innumerable collisions between bodies to be regarded as molecules on an enormously magnified scale. The supposition is, indeed, as Kepler said of the distances of the fixed stars, 'a big pill to swallow.' Between molecules and meteorites lies a wide unbridged gap. The machinery of gaseous impacts is obscure. It can be set in motion only by ascribing to the particles concerned properties of a most enigmatical character. These particles are, however, unthinkably minute; and in sub-sensible regions of research the responsibilities of reason somehow become relaxed. We are far more critical as to the behaviour of gross, palpable matter, because experience can there be consulted, and is not unlikely to interpose its veto.
Meteorites are, doubtless, totally dissimilar from molecules, however many million-fold enlarged; and they would infallibly be shattered by collisions which only serve to elicit from molecules their distinctive vibrations. Moreover, the advance of the shattering process would admittedly end the prevalence of fluid pressure. So that the desired condition, even if initially attained, would be transitory. There is, besides, a radical difference between a group of bodies in orbital circulation and a congeries of particles moving at haphazard, unconstrained by any predominant law of force. A meteoric swarm belongs to the first category; it is a community swayed in some degree by a central power; while the gaseous contents of a retort or a balloon obey purely individual impulses. The analogy looked for by Professor Darwin can then scarcely be said to exist, and his paper stands out as a monument of ingenious mathematical treatment applied to an ideal state of things.
An aggregation of revolving meteorites has no figure of equilibrium, and it is through the consequences necessarily resulting from this property that mathematicians are enabled to trace the progressive changes of a rotating fluid mass. In the absence of any such direct means of attack, their position regarding the problem presented by an assemblage of flying stones is not much better than that occupied by Kant, face to face with an evolving universe. It seems, nevertheless, clear that a meteor-swarm can be impelled to condense no otherwise than through the effects of collisions among its constituents. When the irregularities of movement upon which their occurrence depends are got rid of, the system must remain in statu quo. Order makes for permanence; a tumultuary condition is transient. The eventual state of the system can, however, be no more than partially foreseen. Bodies arrested in their flight should fall inward, hence a central mass would form and grow; but the production of planets would seem to be conditional upon the existence of primitive inequalities of density in the swarm. These might serve as nuclei of attraction for meteoric infalls, not yet completely exhausted, but plying with harmless fire one at least of the globes they helped to shape.
There could, indeed, on this showing, have been no such harmonious succession of events as constituted the predominant charm of Laplace's scheme. The planets should be supposed to have issued pell-mell out of a chaos; or, rather, the chaos should have contained from the beginning the seeds of a predestined cosmos. Its evolution would have been like that of the oak from the acorn, an unfolding of what was already essentially there. And it may be that at this stage of penetration into the past, the unaided human intellect meets its ne plus ultra. There is a vital heart of things which we cannot hope to reach. Thought instinctively pauses before the vision of the symbolical brooding dove.
To resume. Meteoric cosmogony has a rational basis. The modes of action it demands are still operative. Enfeebled almost to evanescence compared with the vigour they must have needed to be efficacious in world-building, they continue to make play in our nocturnal skies. They make play, it is true, with a very small quantity of material; but it may even now be distributed elsewhere in relatively enormous profusion, and in the solar system itself it presumably was much more abundant formerly than it now is. The earth has been raking up meteoric granules by hundreds of millions daily during untold ages, and her zone of space is still very far from being swept clean. The persistence of the supply, however, may be occasioned by the continual arrival of reinforcements from interstellar realms.
Comets appertain to, and travel with, the sun's cortège, and this is also inevitably true of comet-born meteors. But a multitude besides circulate independently of comets, and with much higher velocities. Their orbits are, then, hyperbolic; they belong to the category of 'irrevocable travellers,' and by their capture we are privileged to possess genuine shreds of sidereal matter. Universal space contains probably a vast stock of them, yet there is nothing to prove their collection into swarms. The spectroscope supplies no assurance to that effect; it has given its verdict against the meteoric constitution of nebulæ and temporary stars. And if we admit, through the persuasion of mineralogical testimony, that the aerolites so strangely landed on terrestrial soil are really the débris of ruined worlds, we can see for them no chance of restoration. Solitary they are, even if they occasionally pursue one another along an identical track, and solitary they must remain. Bodies do not of themselves initiate mutual circulation. Planetary or stellar outcasts cannot become re-associated into a gravitational system. Of a cosmic swarm, as of a poet, it may be said, Nascitur, non fit; and their birth-secret is undivulged.