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.
FOOTNOTES:
[Footnote 45: _The New Astronomy_, p. 197.]
[Footnote 46: _American Journal of Science_, vol. xi., p. 421, June,
1901.]
[Footnote 47: Carbon does not liquefy under ordinary conditions. In the
production of his artificial diamonds M. Moissan employed tremendous
pressure and great heat; and, although the genuineness of his products
has been denied (Combes, _Moniteur Scientifique_, November, 1903), his
methods at least seem to have approximated to those by which Nature
fabricates her most authentic crystals.]
[Footnote 48: Sir R. Ball, _The Earth's Beginnings_, p. 243.]
[Footnote 49: _Proceedings of the Royal Society_, vol. xlv., p. 4.]