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MODERN COSMOGONIES




By the Same Author


A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY

 FOURTH EDITION.
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PROBLEMS IN ASTROPHYSICS

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THE SYSTEM OF THE STARS

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 '_The world's a prophecy of worlds to come._'--Young

 MODERN
 COSMOGONIES

 BY

 AGNES M. CLERKE

 HON. MEM. R.A.S.

 AUTHOR OF

 'A POPULAR HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY,'
 'THE SYSTEM OF THE STARS,' 'PROBLEMS IN ASTROPHYSICS,'
 AND OTHER WORKS

 LONDON

 ADAM AND CHARLES BLACK

 1905




PREFACE


Of the sixteen chapters constituting this little work, thirteen have
been published as a series, begun in _Knowledge_ and continued in
_Knowledge and Illustrated Scientific News_, and to the proprietors of
those journals, for their courteous permission to reprint them, I offer
my sincere thanks. Three additional chapters, equivalent to, though
not identical with, those that now appear, formed an integral part
of the original plan of the book, now presented to the public in the
hope that it will enable general readers to follow, with the profound
interest it should inspire, the course of modern inquiries regarding
the origin of the world. Their advance is by no means smooth or facile.
Many difficulties and perplexities are encountered in the attempt to
get back towards the beginning of things. Some of the old tracks, too,
have been torn up by the pioneers of twentieth century science, and
the process of constructing new ones, which shall lead further into
the unknown fore-time, is slow and laborious. But the rail-head in the
desert is a peculiarly suggestive place of pilgrimage, and several such
outlying posts and temporary halting-places are more or less vaguely
localized in the following pages.

 London, _November, 16, 1905_.




CONTENTS


 CHAPTER                                             PAGE

 I. FROM THALES TO KANT                                1

 II. THE NEBULAR HYPOTHESIS                           21

 III. CRITICISMS OF THE NEBULAR HYPOTHESIS            39

 IV. THE NEBULAR HYPOTHESIS VARIED AND IMPROVED       60

 V. TIDAL FRICTION AS AN AGENT IN COSMOGONY           83

 VI. THE FISSION OF ROTATING GLOBES                  100

 VII. WORLD-BUILDING OUT OF METEORITES               118

 VIII. COSMOGONY IN THE TWENTIETH CENTURY            135

 IX. PROTYLE: WHAT IS IT?                            150

 X. UNIVERSAL FORCES                                 166

 XI. THE INEVITABLE ETHER                            183

 XII. THE FORMS OF NEBULÆ                            199

 XIII. THE PROCESSION OF SUNS                        216

 XIV. OUR OWN SYSTEM                                 232

 XV. REMNANTS AND SURVIVALS                          250

 XVI. LIFE AS THE OUTCOME                            265

 INDEX                                               283




MODERN COSMOGONIES




CHAPTER I

FROM THALES TO KANT


Very few even of the most savage tribes are content to take the world
just as it is without speculating as to how it came to be. For time
has three dimensions--past, present, and future--and we can no more
restrict our thoughts within one of them than we can exist corporeally
in Flatland. We are, indeed, told that the Abipones and Esquimaux
refuse to trouble themselves with questions of origin, on the ground
that the hard facts of life leave no room for otiose discussions;
but even they feel obliged to justify their incuriosity. In easier
circumstances they, too, would claim the entirely human privilege of
'looking before and after,' as their forgotten progenitors may have
done. It is, indeed, difficult to think at all about the framework of
nature without attempting to divine, were it only by a crude surmise,
the process of its construction. We are instinctively convinced that
there is no such thing as fixity of condition. So far, Heracleitus was
in the right.

Experience tells us of continual change in ourselves and whatever
surrounds us. Reason teaches us that its minute momentary effects, if
pursued backward for an indefinite time, must sum up to a prodigious
total. No limit, that is to say, can be put to the difference
between what is and what was. Yet the machinery of modification
must somehow have been set going. An initial state is prescribed by
logical necessity. And the start was made on certain terms--it was
'conditioned.' But the conditioned implies the absolute; ordinances, an
enactive power. The inevitableness of the connection has been more or
less obscurely perceived wherever men have tried to establish some kind
of accord between phenomena and intuition, with results legible in the
wavering outlines of many primitive cosmogonies. Only, however, in the
Hebrew Scriptures has the idea of Creation been realized in all its
fulness and freedom; elsewhere the gods invoked to bring the world into
existence themselves demanded a birth-history, a theogony being the
usual and necessary prelude to a cosmogony.

Nevertheless 'picture-thoughts' (it has been well said),[1] and nothing
more, were represented by these prefatory genealogies. Night and
darkness loomed into personal shape, and from the obscurity of their
union the creatures of light radiantly sprang, and proceeded, according
to a predetermined law of order, to sort out the elements of chaos and
dispose them into cosmical harmony.

This mythical phase of thought terminated in Greece with the rise
of the Ionian School of Philosophy. Immemorial legends, discredited
by the advent of a new wisdom, took out a fresh lease of life under
the guise of folk-lore; Orphic fables were left to the poets and the
people; and the sage of Miletus set on foot a speculative tradition,
maintained by a long succession of metaphysicians down to the very
threshold of the recent scientific epoch. All were what we should call
evolutionists--Thales of Miletus no less than Descartes and Swedenborg;
their main object, in other words, was to find a practicable mode of
evoking a systematic arrangement of related parts from the monotony
of undifferentiated confusion. Now, in essaying this enterprise they
encountered two distinct problems. One was concerned with the nature
of the primeval world-stuff; the other with the operations to which it
had been submitted. Modern theorists have made it their primary object
to expound the mechanism of cosmic growth--the play of forces involved
in it, the transformations and progressive redistribution of energy
attending it. But questions of this kind could only in the scantiest
measure be formulated by early thinkers, who accordingly devoted their
chief attention to selecting an appropriate material for the exercise
of their constructive ingenuity.

Thales asserted all things to have been derived from water, and
water is still among unsophisticated tribes the favourite 'Urstoff.'
Anaximenes substituted air. Heracleitus gave the preference to the
mobile and vital element (as he thought it) of fire. Anaximander,
on the other hand, might put forward a colourable claim to priority
over Sir William Crookes in the invention of 'protyle.' He imagined
as the matrix of the world a boundless expanse of generalized matter,
containing potentially all the chemical species, which, separating out
by degrees through the affinity of like for like, formed, by their
contrasts and conjunctions, the infinitely varied sum of things.
The successors of Anaximander had recourse to spontaneously arising
condensations and rarefactions as the mainspring of development; but
all these vague principles were quickly crowded into oblivion by the
definite and intelligible doctrine of the 'four elements' enunciated
by Empedocles, which, guaranteed by the imprimatur of Plato, took a
place unchallenged for nearly two millenniums among the fundamentals
of science. Erroneous and misleading though it was, it yet served as
a means of regulating appearances and guiding vagrant ideas--it was a
track to follow in the absence of any better method of orientation.

Leucippus and his more famous disciples, Democritus and Epicurus, were
the first who ventured to trace the mechanical history of the cosmos.
Their primordial atoms were endowed with weight, and it was weight
or gravity which ultimately determined their spacial arrangement and
mutual relations. Rectilinear in the first draft of the scheme, their
movements were somewhat arbitrarily deflected by Epicurus; and the
gyrations thence ensuing eventually became, so to speak, authentic and
precise in the Cartesian vortices and in Swedenborg's solar maelstrom.
Kant's _Natural History_ of the universe was another, though an
entirely separate branch of the atomistic stock. The Democritean atoms,
however, and in a lesser degree the Kantian atoms, differed essentially
from the ultimates of chemical analysis postulated by Dalton. They were
a scratch lot--an incongruous assortment of fragments, rather than of
elementary portions of matter, indefinitely various in size, shape, and
mass.

Nor was this diversity created as a mere play of fancy. It was
strictly necessary to the plan of action adopted. For, apart from
heterogeneity, there could obviously be no development. Absolute
uniformity involves absolute permanence. Change can originate only
through inequality. There must be a tilt of level before the current
will begin to flow; some cause of predominance is needed to set it
going in a given direction. Here, of a surety, is the initial crux of
all cosmogonists. They usually surmount it by assuming the occurrence
of casual condensations, secure against disproof, while incapable of
verification. The expedient thus begs the question.

Theories of world-history made an integral part of antique philosophy.
Each founder of a school aimed at establishing a complete system of
knowledge, co-extensive with phenomena, embracing all things, from
the _primum mobile_ overhead to the blade of grass underfoot, and
rationalizing the past, present, and future of the comprehensive whole.
Modern science is less ambitious. Aspiring to no such vast synthesis,
it is content to make laborious acquaintance with the facts of nature,
to ponder their implications, and, if possible, to reconstruct on the
basis supplied by them the condition of things in the 'dim backward' of
unmeasured time. By no such means, it is true, can their beginning in
any real sense be arrived at; the weapons of induction become blunted
long before they strike home to the heart of that mystery; yet the
recognition of their inadequacy brings compensation in a fuller mastery
over their properly adapted use. Science, so called, was, indeed, down
to the Baconian era, a turbid mixture of physics with metaphysics. The
solution, it might be said, was attempted of an insoluble material
which refused to dissolve and was hindered from precipitating.

The Greek view of nature was essentially pantheistic. The Ionian
speculators appear to have presumed without expressly insisting upon
its self-regulating power. Aristotle alone emphatically rejected the
doctrine of cosmic vitality or sub-conscious tendencies. But Plato
accepted and magnified the Oriental tradition; the conception of a
'World-Soul' owed to him its vague splendour and perennial fascination.
The function of the Platonic vice-creator (for such the World-Soul
must be accounted) was that of moulding brute matter into conformity
with the archetypal ideas of the Divine mind; this was not, however,
accomplished once for all, but by a progressive spiritualizing of what
in its nature was dead and inanimate. The spiritual agent, becoming
incorporated with the universal frame, lent to it a semblance of life,
an obscure sensitiveness, and even some kind of latent intelligence;
and so the _anima mundi_ was shaped into existence, and continued
century by century to be the subject and source of imaginings beyond
measure wild and fantastic.

One great thought--that of the unity of nature--lay behind them, but
its significance was lost amid the phantasmagoria of Neo-Platonist
exaltations. Hence the Bacchic fervours of Giordano Bruno took their
inspiration; here was the groundwork of Spinoza's pantheism. Shelley's
Demiorgon, felt as 'a living spirit,' seen as 'a mighty darkness,'
descended lineally from that strange essence--formless, inarticulate,
devoid of individual self-consciousness--which animated the submerged
philosophy of Neo-Pagan times with the barren ardours of mysticism. The
doctrine, in its original and more sober version, obtained memorable
expression in Virgil's melodious hexameters:

 'Principio cœlum, ac terras, camposque liquentes,
 Lucentemque globum lunæ, Titaniaque astra,
 Spiritus intus alit, totamque infusa per artus
 Mens agitat molem, et magno se corpore miscet.'

In Conington's rhymed version they run as follows:

 'Know first, the heaven, the earth, the main,
 The moon's pale orb, the starry train,
                   Are nourished by a soul,
 A bright intelligence, whose flame
 Glows in each member of the frame,
                   And stirs the mighty whole.'

Kepler was no cosmogonist, but he aspired to found a 'physical
astronomy,' and in his gropings for a mechanical power that might
suffice to regulate the movements of the heavenly bodies, he stumbled
upon a mode of action highly appropriate for the explanation of their
growth. His ignorance of the laws of motion precluded him from the
conception of velocities persistent in themselves, and merely deflected
from straight into curved paths by a constant central pull. Hence he
was driven to the twofold expedient of creating a whirling medium for
maintaining the revolutions of the planets, and of supposing the sun
to exercise a 'magnetic influence,' by which they were drawn into
closed orbits. Here, then, central forces made a definitive entry on
the astronomical stage, although with scarcely a discernible promise
of their brilliant future. But it was otherwise with the clumsy
machinery they helped to animate. Kepler's simple _modus operandi_,
adopted, or more probably re-invented by Descartes, was published as
an epoch-making discovery in his _Principia Philosophica_ (1644), and
sprang under its new aspect into swift notoriety. The wide acceptance
of the theory of vortices was at least in part due to the impressive
largeness of its framework. Descartes left nothing out. The spacious
scope of his speculations embraced all that was knowable--nature,
animate and inanimate, life and time:

 'Planets and the pale populace of heaven,
 The mind of man, and all that's made to soar.'

A philosophy, a metaphysic, and a cosmogony were linked together in a
single plan. Its author distinguished in matter three gradations of
fineness. The coarsest kind was that composing the earth and other
opaque bodies; the more sublimated materials of the sun and stars
came next; finally, there was the ethereal substance of the skies, so
delicately constituted as to be luminous or luminiferous. This last
variety was regarded as of subordinate origin. It represented, in fact,
a kind of celestial detritus. Interstellar space had gradually become
filled with intangible dust, the product of molecular attrition among
originally angular solar and stellar particles. Ether was thus supposed
to bear to the subtlest description of ordinary matter very much the
same sort of relationship that ions presumably do to atoms.

Enough has been said to show that the Cartesian universe was based on
crude atomism. Its mode of construction, moreover, evinced a total
disregard of mechanical principles. Yet some acquaintance with the
laws of motion was by that time easily within reach. The first of the
three, at any rate, had been unmistakably enounced by Galileo in 1632,
and Descartes himself strongly championed its validity. Yet he thought
it necessary, in order to keep the planets moving, to immerse them in
one great self-gyrating vortex centred on the sun, each being further
provided with a similar subordinate whirlpool for the maintenance
of its domestic system. Comets were left in a singularly anomalous
position. They circulated freely on the whole, their exemption from
planetary restrictions being tacitly recognized; nevertheless, they
took advantage of every encountered swirl to help themselves on towards
their destination.

Among the fables of pseudo-science Delambre declared that, had the
choice been offered to him, he would have preferred the solid spheres
of Aristotle to the _tourbillons_ of Descartes. 'The spheres,'
he added,[2] 'have proved helpful both for the construction of
planetariums representing in a general way the celestial movements, and
for their calculation by approximate rules deduced from them; but the
system of vortices has never served any purpose whatsoever, whether
mechanical or computative.'

Its vogue had, nevertheless, been brilliant and sustained. Advanced
thinkers in the time of Louis Quatorze piqued themselves upon being
Cartesians. The vortical hypothesis was novel--it seemed daring; and
though it might not be true, it had plausibility enough for fashionable
currency. Nor did it deserve the unmitigated contempt with which it
was treated by Delambre. A glance at the skies makes us pause before
condemning it to scornful oblivion. Just two centuries after its
promulgation the first spiral nebula was identified in Canes Venatici.
That the heavens swarm with analogous objects is certain, and their
status as partially developed systems is visible in every line of their
conformation. Our own planetary world may, or may not, have traversed
the stage they so copiously illustrate; but in any case they prove
beyond question that vortices variously conditioned are prevalent
among the forms assumed by cosmic masses advancing towards an orderly
arrangement.

Mystical cosmogonies belong to the period of ethnic infancy. They
have not ceased to be current. World-fables must be invented wherever
the obscure wonder of savage communities is excited by the mysterious
spectacle of Nature's apparently designed operations and irresistible
power. But they were superseded among peoples in the van of progress
by philosophic cosmogonies at the epoch when Thales began to diffuse
throughout Ionia the wisdom of the Egyptians and Chaldeans. Schemes,
however, such as he and his successors elaborated result from the
discourse of reason unfettered by any close attention to facts.
They have been mostly wrought out by men who, in Delambre's words,
'Dissertaient à perte de vue, sans jamais rien observer, et sans jamais
rien calculer.'

The insubstantial fabrics reared by them were then fatally
discredited by Baconian methods and the Newtonian reign of law; they
survived--forms of thought die slowly--but insecurely, with noticeably
undermined foundations. Swedenborg was the last eminent reactionary,
and his restoration in 1734 of the Cartesian gyrating medium as the
motive power of the solar machine was a palpable failure. It could not
be otherwise, since its inceptive idea had grown superannuated. The
modern era of scientific cosmogony was at hand.

It was preceded by some remarkable attempts at sidereal generalization.
Cosmology is the elder sister of cosmogony. What _is_ must be studied
before what _was_ can be inferred. Precedent states remain visionary
unless they can be closely linked to actual and observable conditions.
Now about the middle of the eighteenth century an intelligible plan of
the stellar universe, so far as the telescope had then disclosed it,
began to be a desideratum. And the enterprise of supplying the need was
undertaken independently by two men of obscure origin and imperfect
education--one English, the other German.

Thomas Wright, of Durham, was the son of a carpenter at Byer's
Green, where he was born September 22, 1711. His life was one of
many vicissitudes, but ended happily. Having struggled hard for a
livelihood--now at sea, then again on shore as a clock and almanac
maker, a teacher and lecturer--he finally attained, somewhat
unaccountably, to distinction and affluence, built himself a handsome
house hard by his native shanty, and prosperously and reputably
inhabited it during a quarter of a century. He died February 25,
1786, just one year after Herschel had described to the Royal Society
the outcome of his first experiments in 'star gauging.' As the
originator of the 'cloven disc' theory of the Milky Way, Wright is
still deservedly remembered, for although that majestic structure is
assuredly otherwise designed, it was no mean achievement to have
initiated the science of its architecture.

Heinrich Lambert was a still more adventurous speculator than his
unknown English rival. His father was a poor tailor at Mühlhausen,
then in Swiss territory, and he worked as his apprentice. But his
irrepressible talents brought him into notice, and he died, in 1777,
through the favour of the second Frederick, a Berlin Academician. His
_Cosmological Letters_, published in 1761, were entirely original; they
were composed in ignorance of what Wright and Kant had already written.
In some respects he overtopped them both. He had splendid intuitions,
and just touched the confines of greatness. And if his performances
fell short of the very highest, it may have been rather through
abridgment of opportunity than through lack of capacity. The Milky Way
marked, to his apprehension, a sidereal ecliptic, and he coincided with
Wright in regarding it as a disc of aggregated stars, but with breaches
and gaps indicating a multiplicity of systems circulating, he thought,
round a common centre. Nor did he doubt the existence of other
Milky Ways--numberless, remote, unseen--grouped into a combination
of a higher order; while beyond, and still beyond, stretched further
hierarchies of systems on an ascending scale of magnitude and grandeur.

Our knowledge of the structural facts of the universe can never be made
exhaustive; in the middle of the eighteenth century, before Herschel
had opened his sidereal campaign, it was barely elementary. Wright
and Lambert were accordingly on a stint of material--they had to make
bricks with very little straw. Yet they did their best with what was
at hand. Both paid profound attention to the stellar heavens; they
earnestly sought the true interpretation of the appearances presented
by them, holding it possible, as we, despite accumulating difficulties,
still do, to harmonize countless detached phenomena in one vast
synthetic plan.

It was this purpose of fidelity to Nature which gave value to their
work, and made it a new thing in cosmological history. This alone
lent it impulsive force, and caused the meditations of two lonely
thinkers to become effective in stimulating fresh attempts, favoured
by improved conditions, to comprehend what actually exists, and to
infer thence, with rational confidence, its sources in the vague but
undeniable past.

FOOTNOTES:

[Footnote 1: Zeller, _History of Greek Philosophy_, translated by S.F.
Alleyne, vol. i., p. 86.]

[Footnote 2: Quoted by R. Wolf, _Handbuch der Astronomie_, Bd. II., p.
593.]




CHAPTER II

THE NEBULAR HYPOTHESIS


Immanuel Kant was, in 1751, still in the plastic stage. His period of
'pure reason' was remote, and might have appeared improbable. Such
as they were, his distinctions had been won in the field of concrete
science, and the world of phenomena invited his speculations more
seductively than the subtleties of logic. A seed was accordingly thrown
into fertile soil by his reading of Thomas Wright's _New Theory of the
Universe_, as summarized in a Hamburg journal. It set him thinking,
and his thoughts proved to be of the dynamic order. Wright regarded
the heavens under a merely statical aspect. He laid down the first
definite plan of their construction, showing that the stars were not
scattered at random, but aggregated by method; and this was much for
one necessitous human being to have accomplished unaided.

But the young professor of Königsberg could not rest satisfied with
the idle contemplation of any subsisting arrangement. His mind was
incapable of acquiescing in things simply as they presented themselves;
it craved to know further how they came to stand to each other in just
such mutual relations. He was, moreover, permeated with Epicurean
doctrines. Not in any reprehensible sense. He could not be reproached
either as a hedonist or as an atheist. His pleasures were intellectual,
his morals austere, his convictions orthodox. Behind the veil of
material existence he divined its supreme immaterial Originator, and
his perception of the activity in Nature of an ordering First Cause
remained equally vivid, whether its disclosures were taken to be by
immediate creation or through tedious processes of modification and
growth. His large and luminous view embraced besides the ethical
significance which such processes adumbrate. The following sentence
shows an appreciation of the place of man in Nature truer and more
profound than was attained perhaps by any other of his philosophical
contemporaries: 'The cosmic evolution of Nature,' he wrote in memorable
words, 'is continued in the historic development of humanity, and
completed in the moral perfection of the individual.'[3]

Nevertheless, he owned to a community of ideas with Democritus as to
the origin of the universe. Lucretius had cast over him the spell
of his lofty diction, and captured his scientific adhesion by the
stately imagery of his verse. With reservations, however. Docile
discipleship was not in his line. He availed, then, of the Democritean
atoms, but by no means admitted their concourse to be fortuitous.
Chaos itself, as he conceived it, half concealed, half revealed the
rough draft of a 'perfect plan.' His postulates were few. He demanded
only a limitless waste of primordial matter, animated by no forces
save those of gravitation and molecular repulsion, and undertook to
produce from it a workable solar system. The attempt was no more than
partially successful. Retrogressive investigations lead at the best
to precarious results, and this one, in particular, was vitiated by
a fundamental error of principle. Its author clearly perceived that
planetary circulation must be the outcome of a vortical swirl in the
nebulous matrix; but he failed to see that no interaction of its
constituent particles could have set this swirl going.

Systems cannot of themselves add to their 'moment of momentum.' No
changes of internal configuration avail to increase or diminish the
sum of the products obtained by multiplying the mass of each of the
connected bodies into its areal velocity projected on a common plane.
The sum is of the algebraic kind. Equal and opposite motions cancel
each other, the total representing only the aggregate excess of speed
in either direction. A system with all its parts in rapid motion might
then conceivably be devoid of moment of momentum. And if this were its
state to begin with, it should be its state to the end of time, unless
external force were applied to alter it. But the possibility may be
dismissed as ideal. The establishment of so nice a balance as it would
require is not practically feasible. In the actual world one side of
the velocity account would be sure to exceed the other, albeit very
slightly, and the smallest predominance would suffice to set on foot an
eventual rotation of the system.

Had Kant been better acquainted with mechanical principles, he
might then have safely trusted to the minute beginnings supplied by
aboriginal inequalities of movement and dissymmetry of arrangement for
the development in his colossal dust-cloud of the wheeling movement
necessary for his purpose; and he would thus have escaped stumbling
at the threshold of his daring inquiry. Rightly averse to employing
arbitrary expedients, he piqued himself on the simplicity of his
postulates, and was thus misled into substituting an imaginary for
a real cause. The hypothesis adopted by him was that the particles
forming the initial inchoate mass fell together by gravity, but were
deviated from rectilinear courses through the effects of unequal
resistance. And he derived from the combination of these multitudinous
encounters a common axial rotation for the entire agglomeration. The
futility of this mode of procedure was adverted to by M. Faye in
1885.[4] The deviations in question would, in fact, exactly balance one
another, there being no reason why movement in one sense should prevail
over movement in the opposite; consequently a general rotatory movement
could not even begin to affect the seething mass, which would condense
in sterile rigidity. Kant should then, as Laplace did when his turn
came, have assumed the gyration indispensable to his purpose. He asked
too little from Nature on one side, and too much on the other, with the
result of arresting the machinery he designed to set going.

Kant made the germ of the future sun to consist in an aggregation
of atoms at the core of the nebula, which, growing by successive
innumerable accessions, provided the motive power for the machinery
of planetary construction. For it was, as we have seen, the jostling
of the particles drawn towards the gradually preponderating centre of
attraction which set on foot, it was supposed, the whirl eventually
transformed into the tangential velocities of the sun's attendant
bodies. They were formed, like the sun, by the perpetuation and
increase of subordinate nuclei sure to arise in the elemental tumult.
They were formed, not under the guidance of a definite law, but just
where chance--or what seemed like chance--favoured an accretion.

The progressive increase of planetary distances noted by Titius
and Bode could never have arisen in the Kantian system. Nor could
the Kantian planets have had a direct rotation.[5] Under the given
conditions retrograde systems should have originated. This would
have necessarily ensued from the incoherence of their materials.
Particles revolving independently one of the other have smaller
velocities the more remote they are from the focus of movement. Should
they agglomerate into a globe, the inner flights must, as being
the swiftest, determine the direction of its rotation, which will
consequently reverse the direction of its orbital revolution. Hence,
it depends upon the nature of their generating stuff no less than upon
the advance of central condensation whether planets, in their domestic
arrangements, contravene or obey the larger law of circulation
prevailing in the system to which they belong, and Kant's nebula was
undoubtedly such as to involve its contravention.

Yet his scheme, with all its deficiencies, bore the authentic stamp of
genius--of genius imperfectly equipped with knowledge, but original,
penetrative, divinatory. The very entitling of the work, _A Natural
History of the Heavens_ was an audacity implying a radical change of
conception. It was in this remarkable treatise that 'island universes'
made their definitive appearance. Wright, it is true, had, five years
previously (in 1750), thrown out the idea that 'cloudy spots' might
represent 'external creations,' but as a mere vagary of the scientific
imagination. Kant unhesitatingly laid hold of it, classed nebulæ
as so many separate galaxies, and regarded them as combining with
our own into a revolving system on a surpassing scale of grandeur.
Kant was also the first to take into account the effects on their
development of the plasticity of the heavenly bodies. He published in
1754, in a Königsberg paper, by way of preliminary to his forthcoming
_Natural History_, an outline of the workings of tidal friction in the
earth-moon system. He saw clearly that it had acted in the past to
reduce our satellite's rotation to its present minimum rate, and that
it even now, by very slow degrees, tended to retard the spinning of
the earth. This brilliant forecast remained unnoticed for well-nigh a
century.

The assertion, however, that Kant's cosmogony was an anticipatory
'Meteoritic Hypothesis' lacks foundation. It is only true in the
sense that his building materials were pulverulent, not 'fluid.'
Laplace's primitive nebula was a coherent mass. It rotated as a whole;
it divided only under considerable strain; its separated parts had
individual unity--they held together with, so to speak, a purpose of
concentration. Kant's elemental matter, on the contrary, was a loose
aggregate of independent particles, each pursuing its way, disturbed,
indeed, by its neighbours, but essentially isolated from them. They
were, in short, genuine Lucretian atoms, intended to stand for the
irreducible minima of Nature. The chaos that they formed was in nowise
a 'meteoritic plenum,' unless the phrase be emptied of all distinctive
meaning. Meteorites, so far from being primordial units, have the
show and semblance of advanced cosmical products. They raise special
questions in chemistry, mineralogy, geology, and physics, claiming to
be dealt with by experts in each branch. Before serving for explanatory
purposes, in fact, they themselves need to be explained.

Laplace enounced his hypothesis in 1796, and republished it with
supplementary details in 1808. Herschel had meanwhile ascertained the
retrograde movement of the Uranian satellite-system, a circumstance
highly damaging to the validity of the adopted line of reasoning; yet
its author was content to leave it in jeopardy. He must, to be sure,
have regretted that Nature had seen fit to mar the admirable symmetry
indicative of her presumed plan of action, running counter thereby to
the plainest teachings of the doctrine of probabilities. But he kept
his own counsel on the subject, preferring that it should be discussed,
as it has been in full detail, by posterity; and posterity has, at
any rate, learned that the seeming caprices of Nature are often more
instructive than her most harmonious regularity, and has derived a
warning from her frequent breaches of continuity against the undue
extension of apparently well-grounded inferences.

Nevertheless, the constructive scheme handed on by the eighteenth to
the nineteenth century has not, up to the present, been consigned
to the limbo of vanities. It accorded too profoundly with undoubted
realities to be thus summarily disposed of. No one then living had
studied the mechanism of the solar system so attentively, or was so
intimately acquainted with its workings, as Pierre Simon Laplace.
None knew better how admirable, yet how far from inevitable, were the
adjustments by which its stability was secured. Long meditation upon
their poise and plan persuaded him that the subsisting congruities
of arrangement must have had their source in a community of origin.
He thus acquired the settled conviction that the sun engendered his
cortège, or was together with it engendered from one parent-mass.
And this virtually new truth (for Kant's speculation had attracted a
negligible amount of notice) was set forth by him with a directness
and lucidity which won for it an immediate place among the permanent
acquisitions of the human intellect. Few, perhaps, any longer believe
that planetary formation took the precise course laid down for it in
the _Système du Monde_, but fewer still doubt that the entire ambit of
the solar system was once occupied by an inchoate sun, and that its
component bodies came into being incidentally to that sun's progressive
contraction.

In favour of this view Laplace could allege no clinching argument; it
recommended itself to him solely through its inherent probability.
Unexpected confirmation has, none the less, been afforded to it by the
modern theorem of the conservation of energy, applied by Helmholtz with
widely illuminative effect to solve the problem of the maintenance of
solar heat. Laplace assumed an enormously high initial temperature. It
was the only way open to him, and he took it. But a transcendentally
hot nebula is not easily conceivable; an exalted thermal state seems,
and probably is, incompatible with a high degree of attenuation.
The key to the enigma was given by the demonstration that a diffuse
mass, although actually cold, might contain vast stores of potential
heat. There was then no need to postulate a primitive 'fire-mist'; the
surrendered energy of position amply sufficed to meet the requirements
of the case. The temperature of the nebula necessarily rose as it
contracted through gravitational stress; shrinkage and heat-evolution
proceeded together; and they in all likelihood proceed together still.
Our existence depends in part, or wholly, upon the collapse of the sun.
If its particles ceased to descend, their incandescence would become
less intense, and terrestrial vitality would be seriously compromised.

Their number, however, being finite, the store of energy they can
supply in falling even from an infinite distance is also finite. The
process of solar sustentation is then terminable; it had a beginning,
and it will assuredly come to an end. Now the _terminus ad quem_
is of a calculable remoteness: it can be located (unless shifted
by radio-active processes) within certain limits of time. But the
_terminus a quo_ depends upon too many conditions to be satisfactorily
defined. It is only certain that the sun is to-day slightly more
condensed than it was a year ago. It might a few millenniums back have
been measurably larger, had modern micrometrical methods been available
in the Stone Age; while, looking into the geological past, we discern a
continually more diffuse globe, filling the orbit of Mercury when the
earth was perhaps still red-hot, then successively ampler spheres, out
to, and beyond, that of Neptune. And just such a vastly diffused sun
realizes the nebula of Laplace. The state of things he imagined can be
reached accordingly, either by tracing forward the development of a
tenuous rotating mass, or by pursuing backward the surely indicated,
unceasing, and inevitable distension of the sun. Hence, no sooner
was it acknowledged that energy may be transformed, but cannot be
destroyed, than the nebular cosmogony assumed a new and authoritative
aspect.

But here a _caveat_ has been entered by the latest inquirers--a
_caveat_ not to be ignored, though based upon modes of action still
exceedingly obscure. Radio-activity is a fledgling science; its
capabilities, though immense, are vaguely outlined. Until they more
fully approve themselves, it would be unwise to admit conclusions which
they may eventually enforce. Subversive ideas are in the air; the
theory of atomic dissociation goes to the very root of things, and it
insistently claims assent. Its verification, by disclosing the presence
in the universe of a measureless store of unsuspected energy, would
overthrow all the calculations of cosmic time heretofore attempted, and
might protract indefinitely the radiative span of the sun.

Mr. W. E. Wilson pointed out in 1903[6] that its entire thermal output
could be supplied by the spontaneous liberation of energy from 3·6
grammes of radium in each cubic metre of its volume; and although we
have no evidence of the actual existence of radium in the sun, the
possibility that chromospheric helium represents the decay of solar
radio-active elements[7] must be taken into consideration. The ground
here is undermined with pitfalls. We can only see that although
Helmholtz's gravitational rationale of the sun's long life-history
remains true, the results derived from it may be profoundly modified
by co-ordinate processes, variously efficacious according to
circumstances, perhaps knowable, but as yet unknown.

The scope of the nebular hypothesis had widened prodigiously by the
time Helmholtz took it in hand. Five years before its promulgation at
Paris, Herschel gave at Slough the first hint of a corresponding scheme
of sidereal evolution. The discovery of a nebulous star in Taurus
(N.G.C. 1514) set him pondering; and he found himself, as the upshot of
his meditations, reduced to the dilemma either of concluding nucleus
and _chevelure_ to be alike stellar, though composed of stars differing
enormously in real magnitude, or of admitting the possession by the
star of a voluminous appendage constituted of a peculiar and unknown
'shining fluid.' He chose the latter alternative, adding the pregnant
remark: 'The shining fluid might exist independently of stars,' and
'seems more fit to produce a star by its condensation than to depend on
the star for its existence.'[8]

Thus tentatively, and under the compulsion of phenomena rather than
by the deliberate choice of its inventor, the universal theory of
the genesis of stars from nebulæ took its rise. Herschel shaped it
definitively in 1811 and 1814 into a formal plan for the interpretation
of celestial appearances, but in a large and general way. He made no
attempt to realize the particularities of a _modus operandi_ vaguely
conceived of as involving growth by absorption or assimilation. He and
Laplace thought out their separate schemes quite irrespectively one
of the other. There is no evidence of their having exchanged views
personally or by correspondence, nor does their mutual influence appear
to have been appreciable.[9] Yet Laplace needed as the raw material for
his solar system precisely the 'shining fluid' elaborated, one might
say, by Herschel, partly through the revelations of his telescopes,
partly as the outcome of his reasonings concerning the _chevelure_ of
the star in Taurus. Halley, it is true, had, by a sagacious intuition,
surmised the composition of nebulæ out of a 'lucid medium.' But the
ineffectual phrase remained stranded in the pages of the _Philosophical
Transactions_, and has only of late been set floating on the stream of
scientific literature.

Down to the end of the eighteenth century world-building had been a
purely speculative undertaking. It lacked actuality; it was concerned
with operations thought of as belonging exclusively to a past order
of things, now over and done with, and lying wholly outside the
range of experience. Through Herschel's synthesis, however, those
dimly apprehended operations were brought into view as variously
progressing even now in different parts of the cosmos, as incipient
in some regions, far advanced in others, the rubbish of the workshop
here half masking the rising edifice, while elsewhere signs of
decay and exhaustion give legible presage of an appointed end. And
this stupendous vision of a forming universe has not vanished on
critical scrutiny. It is no dream-tissue; it cannot dissolve into
airy nothingness; it is based upon a firm substratum of reality.
The immeasurable purposes of creative wisdom are still only in
part fulfilled. It has become the strange privilege of humanity to
contemplate from its little shoal of time the oceanic flow of their
development. Thus, in the swing of the ages, Laplace's thought was
caught up and vitalized. He himself was scarcely sensible of their
movement. He recognised very imperfectly, if at all, his obligations to
Herschel's nebulous star. His means were inadequate; his field of view
narrow; his knowledge, though co-extensive with that of his time, fell
short of what his boundless task demanded. In some respects his mode
of procedure was faulty; his forecasts have been belied; the behaviour
imputed by him to a nebula such as he devised is questionable, if
not impossible. But with the instinct of consummate intelligence he
hit off the 'psychological moment,' and, divining the genetic import
of harmonies of construction obvious to perception, but arduous of
interpretation, he laid down with masterly simplicity the ground-plan
of a structure likely to maintain its substantial integrity despite
innumerable additions and rectifications.

FOOTNOTES:

[Footnote 3: Quoted by Dr. Hastie in the preface to his translation of
Kant's _Cosmogony_, Glasgow, 1900.]

[Footnote 4: _Sur l'Origine du Monde_, 3e éd., p. 136.]

[Footnote 5: This also was pointed out by M. Faye, _loc. cit._, p. 150.]

[Footnote 6: _Nature_, July 9, 1903.]

[Footnote 7: Rutherford, _Radio-activity_, p. 342.]

[Footnote 8: _Philosophical Transactions_, vol. lxxxi., p. 85.]

[Footnote 9: Herschel met Laplace during a visit to Paris in July,
1801, but what passed between them is unrecorded. In the sixth edition,
however, of the _Exposition du Système du Monde_, Laplace referred to
Herschel's observations of nebulæ as confirmatory of his own genetic
scheme.]




CHAPTER III

CRITICISMS OF THE NEBULAR HYPOTHESIS


Laplace's theory was a perfectly definite conception. In this lay its
distinctive merit; in this also its special susceptibility to attack.
Here was no question of condensation round nuclei arising at discretion
amid the large possibilities of boundless elemental confusion; but of
an orderly succession of occurrences, rendered inevitable by the steady
operation of mechanical laws, and harmonizing, in their outcome, with
the array of ascertained phenomena visible in the planetary system.
These accordingly ceased to be regarded as arbitrary or casual; they
became linked together in the present, and with the past, as joint
products of one grand scheme of development. The mode of origin of
the bodies exhibiting them accounted, its inventor claimed to have
shown, simply and entirely for them all; and at least the fundamental
propositions laid down by him could not be gainsaid.

Clearly, the unanimity of planetary movements is no result of chance;
it represents quite obviously a survival of the general swirl of an
inchoate mass, occupying primitively the whole recognised sphere of
solar influence. Ambiguities set in only when details come to be
considered. The engendering nebula devised by Laplace was provided
with a vast endowment of heat and a slow movement of rotation; hence
cooling, contraction, and acceleration advanced _pari passu_, the last
as a consequence of the mechanical law by which the algebraic sum
of the areas described by any number of bodies round a given axis,
multiplied by their several masses and projected upon a single plane,
remains constant to the end of time. In other words, to repeat what has
been stated a few pages back, the moment of momentum of a congeries
of particles can neither increase nor diminish through the effects of
their mutual interactions, however varied and prolonged.

The nebula then quickened its pace until a stage was reached at
which centrifugal speed could no longer be controlled by gravity;
separation became inevitable, and an equatorial ring was abandoned,
which thenceforward revolved on its own account in the period conformed
to by the undivided mass at the epoch of its secession. This was the
first of many subsequent crises of instability, each eventuating in
the detachment of a nebulous ring. These rings, however, were regarded
as merely transitional forms. They survived, just for illustrative
purposes, in the Saturnian system; elsewhere they broke up into
fragments, which ultimately coalesced into globes, and the globes were
embryo planets. There was, indeed, a hitch in the line of argument
which did not escape the acumen of the French geometer. The direction
of the axial movement imparted to the members of the solar family
depended essentially upon the relative velocities of the portions of
matter brought together for their construction. If the inner sections
of the self-shaping mass moved faster than the outer, the resulting
rotation should have been retrograde; if slower, direct rotation would
have ensued. Now, in a ring like that of Saturn, composed of discrete
particles, linear speed decreases continuously outward, each of its
minutest constituents obeying independently Kepler's law of periods and
distances. Such a formation, since it would necessarily have yielded
backward-spinning planets, would have been unfit for the purpose in
view, and Laplace accordingly substituted an annulus endowed with a
considerable amount of cohesion, and capable of rotating, like a solid,
in a single period. It is true that such unanimity of movement was
incompatible with the other postulated conditions; but the anomaly
escaped notice for above half a century.

Professor Darwin has moreover pointed out[10] that a ring of matter
distributed with any approach to uniformity must concentrate, if at
all, round its own centre of gravity. It should accordingly collapse
upon, and become re-absorbed by, the parent-body. If markedly
unsymmetrical and ill-balanced, its materials might certainly collect
at an interior point more or less remote from the centre; but in no
case could the focus of condensation be situated in any part of the
annular circumference, where it was located by Laplace.

Whether workable or not, the genetic plan traced out by him was a
strictly regulated one; its steps were marked with characteristic
precision. Yet by this very determinateness it gave hostages to the
future. It challenged the application of tests which designs more
vaguely sketched might have evaded. The primary criterion of its truth
was the prevalence of concordant motion throughout the solar domain.
Counter-currents were formally excluded; their possibility was not
even contemplated. Hence, the discovery of the retrograde systems of
Uranus and Neptune flatly contravened its pretensions to unconditional
acceptance. With less evidence, but equal certainty, Laplace's
hypothesis, strictly interpreted, involves the consequence that each
planet circulates in the identical time occupied by the rotation of the
undivided nebula just before instability toppled over into separation.
Each of the planetary periods should accordingly bear a certain ratio,
prescribed by inexorable mechanical law, to the actual period of the
sun's rotation. In point of fact, however, the periods in question
are much shorter than comports with the necessity for the conservation
from age to age of the system's moment of momentum. The discrepancy was
adverted to nearly half a century ago by M. Babinet.[11] He showed in
March, 1861, that the axial movement of the solar mass, when distended
to fill the sphere of Neptune, should have been, by the law of areas,
so excessively slow that more than 27,000 centuries would have been
needed for the completion of a single rotation; while the period, even
when the shrinking nebula had come to be bounded by the terrestrial
orbit, must still have been protracted to 3,181 years. Under these
circumstances, centrifugal force would never have overbalanced central
attraction; no rings could have separated, and no planets could have
been formed.

Quite recently, Mr. F. R. Moulton, of Chicago,[12] has reconsidered
the subject in the course of a careful and candid discussion of the
difficulties besetting the nebular cosmogony as viewed from the
standpoint of modern science, and he comes to essentially the same
conclusion. His calculations, though founded on data expressly chosen
so as to give the classic theory the benefit of every doubt, made it
perfectly clear that the moment of momentum of the embryo planetary
system should have exceeded its present value no less than 213 times
if, when it extended to the distance of Neptune, it rotated in what
is now the period of Neptune. But moment of momentum is a constant.
The lapse of millions of years makes no difference to it; it is not,
like energy, subject to 'dissipation'; it can neither have gained nor
lost value since the sky was first flecked with the 'breath-stain'
appointed to condense into our sun, which, in this respect at least,
must at every stage of its subsequent evolution have maintained
immutability. On the other hand, this being so, its primeval wheeling
motion would have been much too leisurely to permit the occurrence
of accesses of instability. Gravity would have steadily kept its
supremacy over the forces tending to disruption until the nebula had
contracted to less than the compass of the Mercurian sphere, and its
overthrow at that epoch would have been too late for the origination
of any of the sister orbs of the earth. These results, it is true,
depend in part upon the mode of variation in density ascribed to the
progressively shrinking nebula; but the law adopted by Mr. Moulton
has a consensus of authorities in its favour. Nor could its deviation
from exactitude--if it be inexact--possibly suffice to account for the
enormous discrepancies which calculations based upon it have brought to
light.

The nebular hypothesis stipulates further that satellites must revolve
more slowly than their primaries rotate. The reason is patent. In
the periodic time of a body detached by centrifugal acceleration the
rate of gyration of the original mass is, if the theory be valid,
perpetuated. Subsequent contraction tends to quicken, and very
greatly to quicken, the rotation of the planet, while the period of
the satellite survives unaltered as a standing record of what the
joint period was. This relation may indeed be modified by the effects
of tidal friction, but it is more than doubtful whether it can ever
be reversed. It is, then, a characteristic feature of the mode of
evolution described by Laplace that no month--so to call it--can be
shorter than the corresponding day. And the rule is conformed to in
nearly every part of the solar system. Nevertheless, two flagrant
violations of it have lately obtruded themselves upon notice, and
can scarcely be explained away by supplementary hypotheses. The
first ascertained anomaly of the kind was met with in the swift
circulation of Phobos, the inner satellite of Mars, which completes
three revolutions and enters upon a fourth while the planet attended
by it wheels once on its axis. The fact is most perplexing, and the
confident persuasion that solar tidal friction would avail to remove
the difficulty has not proved well grounded. Solar tidal friction, it
may be remarked, acts as an external force upon subordinate systems
submitted to its influence. Within their precincts moment of momentum
may be destroyed by it; it tends, so far, to abrogate the law of
conservation; and the supposition was hence feasible that the rotation
of Mars had, in the course of ages, greatly slackened through the
retarding effect of sun-raised tides. But the agency was demonstrably
inadequate to the task assigned to it.

The reduction of the rotational moment of Mars to about one
twenty-fifth its primitive amount[13] would have brought other
consequences in its train, at least one of which did clearly not ensue.
At an early stage of the process Phobos should have been re-engulfed
in the mass of its primary.[14] For the pull of the small tidal wave
raised by it on the surface of that body would have been backward from
the instant that the balance of periods became inclined, through solar
compulsion, in a direction contrary to that it would have naturally
taken; and the ensuing loss of velocity must have entailed the descent
of the little satellite along a spiral path towards an inevitable doom.
Its continued existence, then, closes this way of escape from the
difficulty raised by the shortness of its period. M. Wolf had recourse
to a different explanatory subterfuge.[15] He believed that Phobos
might have owed its origin to one of Roche's 'elliptic sheddings' of
nebulous matter dropped downward from near the polar regions of the
distended Martian spheroid, and rotating, owing to its low rate of
linear speed, in the immediate vicinity of the cooling planet. The
explanation, though ingenious, is too recondite to be satisfactory. The
mind takes no grip of it; it evades distinct apprehension.

The Saturnian system exhibits a case of the same kind, but still more
perplexing to speculative prepossessions. Saturn's ring-system has
always appealed to thinkers as a striking object-lesson in nebular
development. It forcibly arrested Kant's attention, and he sketched its
birth-history on lines anticipatory of those adopted by Laplace for the
solar system in its entirety. Laplace himself regarded the formation
as the one surviving relic of the annular stage of planet-building--as
a witness from the dim past to a condition of things elsewhere
transitory. Yet the witness has turned king's evidence, and betrayed
the whole situation. The innermost Saturnian ring has a period far
too short to be compatible with the requirements of theory. For its
meteoric constituents, known on spectroscopic testimony to revolve each
on its own account, complete their circuits in between five and six
hours, while the planet needs just ten hours and a half for its axial
rotation. Moreover, tidal friction is here far less available than on
Mars; yet no other retarding agency has been invented. The deadlock
appears final and hopeless.

An objection quite as formidable, and even more fundamental, was raised
by Kirkwood in 1869. The nebulous material of the uncondensed sun must
have been, at the outset, of the utmost tenuity. Atmospheric air is,
by comparison, a dense and massive substance. Yet no reasonable person
could ascribe to aerial matter the least power of resisting strain.
We know perfectly that a rotating globe of air, and, _à fortiori_,
a globe of matter thousands of times less compact than air, would
unintermittently disintegrate at the surface with the progress of
acceleration. The disturbance and restoration of equilibrium would be
virtually simultaneous. There could be no accumulation of internal
stress, and consequently no definitely separated epochs of instability.
At the first solicitation, at the first instant that centrifugal
velocity gained the upper hand over gravity, nebulous wisps would
have become detached, and their detachment would have gone on without
pause. Space would have been strewn with the débris of the condensing
nebula, and there should have resulted a vast cloud of cosmic dust, not
a majestic array of revolving spheres.

Further, the possibility of their emergence from pre-existent annuli
is by no means assured. Even if the nebulous material had possessed
the fabulous cohesion indispensable for its division into voluminous
rings with wide intervening empty gaps, their ultimate agglomeration
into planetary globes would probably never have been effectually
accomplished. Kirkwood long ago questioned the feasibility of the
process. Mr. Moulton has gone far towards demonstrating that it must
have had an abortive outcome. Professor Darwin pronounces its very
inception, apart from very special conditions, to be impracticable.

Another grave objection to Laplace's scheme is founded on the
marked deviations visible in the solar system, from conformity to a
fundamental plane of motion. Unless acted on by influences difficult to
imagine or explain, all the planets should circulate along the level
of the sun's equator, and rotate on axes perpendicular to it. How far
this is from being realized in nature we have only to look around us
to perceive. We owe the changes of our seasons to the tilted fashion
of the earth's spinning. Yet it is by no means easy to understand how
the pole of its equator comes to be situated in the tail of Ursa Minor,
while the pole of the ecliptic is involved in the folds of Draco. They
should have coincided if the simple rules of the nebular prescription
had been followed in the making and modelling of the planets. Nor are
the terrestrial arrangements exceptional. The Saturnian equator and the
Saturnian rings have a still higher inclination; while in the systems
of Uranus and Neptune--if we may thus interpret their retrograde
revolutions--the angle exceeds the limit of a quadrant. These and other
similar discrepancies prove the solar mechanism to have originated by
a more complex method than that imagined by Laplace, and an hypothesis
which invokes the aid of a multitude of auxiliary devices for its
extrication from accumulating embarrassments falls thereby under the
suspicion of not being worth the trouble of extricating. It forfeits,
at any rate, all claim to commendation for directness and simplicity.

The cosmogony turned out at Paris has thus proved vulnerable on a
number of points; but all the blows aimed at it have not told with such
deadly effect as those just referred to. Some have fallen harmlessly,
or glanced aside. One hostile argument in particular, which for a time
seemed irresistible, has been completely overthrown by the logic of
facts, and deserves mention only as a historical curiosity. Towards the
middle of the nineteenth century the progress of sidereal astronomy
seemed to take the direction of showing all nebulæ indiscriminately to
be of stellar composition. With Lord Rosse's great reflectors a good
many such objects were genuinely, and some besides were deceptively,
resolved into stars, the illusory effects being confirmed by Bond's
observations with the deservedly celebrated 15-inch refractor then
recently built by Merz for Harvard College. Hence the rash inference
was drawn that resolution was wholly a question of optical power,
and that no real distinction existed between the stellar and the
nebular realms. Herschel's 'shining fluid' assumed a mythical air;
'island-universes' came into popular vogue; and all but a few careful
thinkers held nebulæ and clusters to be differentiated merely by
degrees of remoteness. But if space contained only full-grown stars
and no stars in the making--no star-spawn, no star-protoplasm--then
the imagined evolutionary history of our system was left in the air,
destitute of even the most fragile prop of observed fact.

From this precarious position it was rescued, partly by the cogent
reasonings of Whewell and Herbert Spencer, finally and triumphantly
by Sir William Huggins's spectroscopic discovery of the cosmic gas
'nebulium.' Since August, 1864, there has been no possibility of
denying that the heavens contain ample stores of just the kind of
material Laplace wanted, though whether it played just the part he
assigned to it in the manner that he supposed is a question to be
answered with profound and growing reserve.

An objection of late urged against the nebular theory from the
standpoint of the kinetic doctrine of gaseous constitution is of much
speculative interest. A gaseous nebula equal in mass to the sun and
planets, and distended sufficiently to fill the orbit of Neptune, would
have been, supposing the prevalent opinion correct, subject to a rapid
leakage into space of its lighter ingredients. Of hydrogen and helium,
we are told, it should infallibly have become depleted; yet there is no
lack of either in the sun of the twentieth century. Their retention,
it must be admitted, is, on the hypothetical conditions, difficult to
account for. The 'critical velocity' at the limiting surface of the
supposed nebula would have been 4·8 miles a second. This is, in fact,
at the distance of Neptune, parabolic speed. The planet itself, if it
could attain to it, would break the bonds that bind it to the sun,
and seek its fortunes under some different allegiance. Similarly, any
particle of the primitive nebula thus accelerated should have become an
irreclaimable vagrant.

Now, the velocity of hydrogen molecules at the zero of Centigrade is,
in the mean, about 1-1/6 miles a second, but attains in the extreme
to above seven miles. Hydrogen could not then have been permanently
retained by the solar nebula, and the escape of helium would have
more slowly ensued. Yet these results, though seemingly inevitable,
did not actually come to pass, either because the generating body
was differently constituted from what has been supposed, or because
countervailing influences were brought to bear. It is, for instance,
amply possible that the dynamical condition of gases may be essentially
modified by rarefaction carried to a degree transcending the range of
experimental enquiries. The progress of science affords many warnings
against trusting implicitly to the rule of continuity. Curves of change
seldom preserve indefinitely a uniform character. Their unexplored
sections may include quite unlooked-for peculiarities of flexure, and
the possibility seriously undermines confidence in inferences depending
upon 'extrapolation.' The presence of hydrogen and helium in our system
cannot, then, be ranked among facts incontestably contradictory of the
nebular hypothesis.

The concerted advance of mathematical astronomy during the eighteenth
century was effected with the confident serenity of irresistible
power. One after another the obstacles barring its path went down
before repeated and skilful onslaughts, the unbroken succession of
which lends a certain exultant sameness to the story of the heroic age
of analysis. The _Mécanique Céleste_ attested 'victory all along the
line.' There were no more worlds to conquer that Laplace knew of; the
reign of gravitational law was firmly established throughout the solar
dominions; menaced revolts had been appeased; anomalies removed; no
extant observations any longer impaired the perfect harmony between
what was and what had been foreseen. Nature for the moment submitted
readily to the trammels put upon her by human thought; her intricacies
had apparently ceased to defy unravelment; her modes of procedure
looked straightforward and intelligible. As they were judged to be in
the present, so they might be presumed to have been in the past; and
the temptation was irresistible to adventure backward speculation,
inferring initial conditions from the elaborated product laid open to
scrutiny.

It was an epoch of peremptory renewals. The formula of equality
promised to regenerate society; a political panacea had been found
by the creation of a republic 'one and indivisible'; and the success
of the guillotine in securing its supremacy was almost outdone by
the triumphs of the calculus in vindicating the unimpeded sway of
gravitation.

Humanity had made a fresh start; science should do likewise. The
sanguine spirit of a rejuvenated world animated all forms of human
endeavour. It has long since evaporated. The buoyant hopes of a century
back have been crushed; the future of civilization looks dim; and its
uncertainty compromises the future of knowledge. But we, at any rate,
no longer delude ourselves with the idea that he who runs may read
the secrets of the universe. We have learned by convincing experience
how much, and how variously, 'the subtlety of nature transcends the
subtlety of sense and intellect'; we are vividly aware that there is no
single and simple recipe for the 'cosmification' of chaos.

That devised by Laplace has ceased to be satisfactory. Its simplicity,
at first sight so seductive, leaves it at a disadvantage compared with
the intricacy of the effects it was designed to elicit. The relations
claiming explanation have multiplied with the progress of research.
Those of the dynamical order were alone attended to by the geometers of
the eighteenth century, and even they have grown recalcitrant; while
those of a physical and chemical kind have proved wholly unmanageable.
It has, indeed, become abundantly clear that the series of operations
described by Laplace could scarcely, under the most favourable
circumstances, have been accomplished, and in a thin nebulous medium
would have been entirely impossible. The nebular cosmogony has not,
then, stood 'Foursquare to all the winds that blew.'

Its towers and battlements have crumbled before the storms of adverse
criticism. It survives only as a wreck, its distinctive features
obliterated, although with the old flag still flying on the keep. In
the next chapter we shall attempt a survey of the works set on foot for
its reconstruction.

FOOTNOTES:

[Footnote 10: Presidential Address to the British Association,
Johannesburg, August 30, 1905.]

[Footnote 11: _Comptes Rendus_, tom. lii., p. 481.]

[Footnote 12: _Astrophysical Journal_, vol. xi., p. 103.]

[Footnote 13: Moulton, _Astrophysical Journal_, vol. xi., p. 110.]

[Footnote 14: Nolan, _Nature_, vol. xxxiv., p. 287.]

[Footnote 15: _Bulletin Astronomique_, tom. ii., p. 223.]




CHAPTER IV

THE NEBULAR HYPOTHESIS VARIED AND IMPROVED


'Restorations' often go very far. Things may be improved beyond
recognition, nay, out of existence. So it has happened to the nebular
hypothesis. _Stat nominis umbra._ The name survives, but with
connotations indefinitely diversified. The original theme is barely
recalled by many of the variations played upon it. Entire license of
treatment prevails. The strict and simple lines of evolution laid
down by Laplace are obliterated or submerged. Some of the schemes
proposed by modern cosmogonists are substantially reversions to Kant's
_Natural History of the Heavens_; the long-discarded and despised
Cartesian vortices reappear, with the éclat of virtual novelty, in
others; nor are there wanting theories or speculations reminiscent
even of Buffon's cometary impacts. Moreover, the misleading fashion
has come into vogue of bracketing Kant with Laplace as co-inventor
of the majestic and orderly plan of growth commonly designated the
'nebular hypothesis.' This has been, and is, the source of much hurtful
confusion. Save the one fundamental idea--and that by no means their
exclusive property--of ascribing unity of origin to the planetary
system, Kant's and Laplace's evolutionary methods had little in common.
Their postulates were very far from being identical; they employed
radically different kinds of 'world-stuff'; and the 'world-stuff' was
subjected, in each case, to totally dissimilar processes.

Yet it is often tacitly assumed that to defend or refurbish one
scheme is to rehabilitate the other. Under cover of the intellectual
vagueness thus fostered, a backward drift of thought is, indeed,
discernible towards the view-point of the Königsberg philosopher. It
is recommended, not so much by the favourable verdict of science as
by the wide freedom of the prospect which it affords. The imperative
guidance of Laplace, reassuring at first, led to subsequent revolts.
But Kant is highly accommodating; one can deviate widely from, without
finally quitting, the track of his conceptions; they are capacious and
indefinite enough to comport with much novelty both of imagination and
experience, and hence lend themselves with facility to the changing
requirements of progress.

A noteworthy attempt was made, in 1873, by the late Édouard Roche of
Montpellier to reconstruct, without subverting, Laplace's hypothesis.
This remarkable man lived and died a provincial. Only a few scattered
students have made acquaintance at first hand with his works; his fame,
always dim, now already begins to seem remote. Yet a score of years ago
he was still lecturing at the Lycée of his native town. The waters of
oblivion have grown, perhaps, more turbid than of yore. Anyhow, Roche
of Montpellier is only vaguely remembered, and that by a specially
educated section of the public, as having fixed a limit within which
a satellite cannot revolve intact.[16] Nearer to the ruling planet
than 2·44 of its mean radii, it could not--setting aside improbable
conditions of density--maintain a substantive globular status under the
disruptive strain of tidal forces. In point of fact, all the moons so
far discovered in the solar system circulate outside 'Roche's limit';
and Saturn's rings, which lie within it, owe to that circumstance,
it may plausibly be asserted, their pulverulent condition. Professor
Darwin accordingly regards knowledge of that condition as dating from
1848, the year in which Roche published the law involving it as a
corollary.[17]

Roche was the precursor of Poincaré and Darwin in those profound
investigations of the figures of equilibrium of rotating fluid bodies
which have opened up new paths and disclosed untried possibilities
in evolutionary astronomy. His researches, moreover, into the origin
of the solar system[18] constituted a reinforcement of first-rate
importance to the strength of Laplace's position. He was perhaps its
most effective and timely defender; he came to the rescue just when its
safety was seriously compromised, repaired its breaches, and threw up
skilfully constructed outworks. Adopting the same premisses, he drew
virtually the same conclusions as Laplace, ingeniously modifying them,
however, so as to evade certain objections, and temporarily to silence
the less obstinate cavillers. His results were, indeed, almost as
difficult to disprove as they had been to attain. They were arrived at
laboriously, legitimately, by long-drawn analytical operations; and the
reasonings survive in full credit, even although the initial conditions
they started from now wear an aspect of unreality. Thus, the invention
of _trainées elliptiques_ not only usefully met an argumentative
emergency, but still remains as a supplementary adjunct to cosmic
processes. Undeniably, polar annulation may have played a part in
planetary formation; the possibility cannot be gainsaid.

The 'ellipsoidal trains' investigated at Montpellier were huge
nebulous strata detached from the polar regions of the primitive
spheroid, which, bringing with them the low rotational velocity proper
to that situation, tended, some to constitute interior equatorial
rings, others to become agglomerated with the central mass. But their
incorporation should have had as its consequence--since the 'law of
areas' is inviolable--a quickening of angular rotation throughout the
nebula. The 'law of areas,' it may be explained, is merely a short
title for the 'law of conservation of moment of momentum,' which
prescribes--as we know--that the sum total of the areas described in a
given time on a given plane by the members or constituent particles of
a rotating system, multiplied by their several masses, remains constant
under all conceivable circumstances of re-arrangement or mutual
disturbance. Hence, approach towards the centre, because it narrows
the circle, must quicken the speed of rotation. A short line having
to sweep over the same space as one of greater length, its moving end
must proportionately hurry its pace. An engulfment, accordingly, by the
embryo sun of one of Roche's 'elliptic trains' would have occasioned
an immediate shortening of the period of revolution of both nucleus
and atmosphere, an accession of centrifugal force producing sudden
instability, and, as a consequence, the separation of an equatorial
ring.

By this subtly devised expedient Roche sought to explain away the
difficulty connected with the wide intervals between the planets.
For they originated, he conceived, not in the regular course of
condensation, but through complications arising abruptly and
exceptionally. What he called the 'limiting surface' of the nebula
might also be described as the atmospheric limit. It corresponds to
the widest possible extension of a true atmosphere. Its boundaries
are fixed at the distance just outside of which a satellite could
freely circulate in the axial period of its primary. Now the
limiting surface, if contraction had proceeded equably, should have
retreated continuously, as axial movement quickened, its withdrawal
being attended by the shedding of slender rivulets of superfluous
matter. But by the introduction of 'elliptic trains,' stability,
artificially maintained (so to speak) throughout long spells of time,
was overthrown only by catastrophic downrushes from the shoulders of
the nebulous spheroid, when, with the prompt abridgment of the axial
period, the limiting surface as promptly shrank inward, and there was
left, outstanding and self-subsistent, the tenuous ring destined to
coalesce into a planet. A singular and unexplained felicity of Roche's
analysis consisted in the symmetry of time-relations established
by it. The successive births of his planets followed each other at
equal intervals. A species of translation of Bode's law of distances
(extended by him to satellite-systems) in terms of the nebular
hypothesis thus appeared to be rendered feasible.[19]

That hypothesis, in its original form, as explained in the last
chapter, produced planets with retrograde rotation--that is, spinning
in an opposite sense to that of their circulation. For the purpose
of abolishing the anomaly, Kirkwood, in 1864,[20] had recourse to
solar tidal friction, and he was followed, doubtless independently,
by Roche, and by Roche's interpreter, C. Wolf of Paris. Objections to
any particular mode of planetary formation, on the ground that its
outcome must have been inverted axial movement, lost their validity,
they remarked, through the consideration that solar tidal friction
would have availed to redress the incongruity. For its retarding
action would have ceased only when synchronism with the revolutionary
period was attained--that is, when the planet wheeled in its orbit,
as Mercury seems to do, turning always the same face inward; and then
already direct rotation would have set in, and, becoming accelerated by
contraction, should permanently retain the direction impressed upon it
by the friction of sun-raised tides. A certain air of plausibility is
given to this view by the fact that the only two retrograde planetary
systems are situated entirely beyond the possible range of any such
manner of influence, and may accordingly be supposed to have preserved
unaltered their primitive fashion of gyration.

The late M. Faye was less loyal to tradition than the savant of
Montpellier. The appearance in 1884 of his work, _Sur l'Origine du
Monde_, gave the signal for renewed activity and a larger license in
cosmological speculation. Conservative opinions on the subject are now
rarely held; the old groove has been by most definitively quitted;
inquiry becomes continually more individual and less constrained by
tradition. Faye's reform, however, was not avowedly of a revolutionary
character. He did not make a clean sweep of the work of his great
predecessor, by way of preliminary to setting forth his own more
perfect plan. Yet his emendations of it went very deep.

Laplace's nebula was of a gaseous consistence, and it stood in a
genuine atmospheric relation to the central condensation--that is to
say, its strata gravitated one upon the other; they were subject to
hydrostatic pressure. Faye ruled things otherwise. The nebulous matrix
which he postulated was a vast congeries of independently moving
particles, forming a system governed by a single period, in which both
gravity and velocity increased in the direct ratio of the distance from
the centre. Now, globes formed by the method of annulation (admitting
its practicability) out of materials thus conditioned, should have
possessed, _ab initio_, a direct rotation; their axial spinning would
have been in the same sense as their orbital circulation. And this it
was which recommended to Faye the adoption of a meteoric structure for
the inchoate solar system. But the simple law of force regulating it at
first would, by degrees, have undergone essential alteration. That of
inverse squares, familiarized to ourselves by long habits of thought,
would have begun to supersede it so soon as a sun, properly so called,
could be said to exist. The retrograde planets, Uranus and Neptune,
must, however, by Faye's supposition have taken shape under the modern
regimen; they were formed subsequently to the earth and all the rest
of her sister orbs. This unexpected inversion of the recognised order
of planetary age involved the further consequence that the ante-natal
offspring of the sun--thus paradoxically to designate them--must have
drawn closer to him as his attractive power developed, Uranus and
Neptune alone among the entire cortège preserving the original span of
their orbits.

Faye's scheme, if it did not meet all the arduous requirements of the
problem it confronted, served, at any rate, to illustrate very forcibly
the devious variety of tracks by which nebular evolution might advance
towards its goal. The particular one chosen was certainly not clear
of impediments. In his preoccupation with the removal from Laplace's
hypothesis of the flaw relating to planetary rotation, M. Faye had
discarded its cardinal merit of explaining secessions of material by
the growth of centrifugal force. He alleged no sufficient reason, and
none could be alleged why the remodelled nebula should have separated
into rings.[21] The process implies definite and special conditions;
it testifies to a rhythmically acting cause. Laplace brought such a
cause into play. Faye abolished it, and his annuli, accordingly, wear
a fictitious aspect. It is, indeed, true that an annular structure
is commonly visible in nebulæ, but it is begging a most arduous
question to assume that nebular spires have anything in common with
planet-forming rings.

These would probably never have been heard of save for the Saturnian
example. A pattern is easily copied; an idea palpably feasible is
tempting to adopt; a demonstration on the _solvitur ambulando_
principle cannot but prove convincing. But how if the rings cannot be
made to coalesce into globes? And the difficulty of the transformation
becomes more apparent the more clearly its details are sought to be
realized. Reversed in direction, it might better find a place in the
order of Nature. 'Analysis seems to indicate,' Kirkwood wrote in
1884,[22] 'that planets and comets have not been formed from rings, but
rings from planets and comets.' Nor is this mode of procedure merely
possible according to theory; it is also vividly illustrated by facts.
Meteoric swarms can be observed, decade by decade, to disperse under
the scattering influence of the sun and planets, and unmistakably tend
to become more or less uniformly distributed along the entire round of
their orbits. Their advance is directed, not towards condensation, but
towards disaggregation; and they pursue it with surprising rapidity.

Faye's theory was disfigured by a still more glaring incongruity.
Nothing in the planetary economy seems more evident than that the
zone of asteroids marks a division between two strongly dissimilar
states of the solar nebula. It is a visible halting-place. One series
of events came to an end, and there was an interlude before the next
began. During that interlude, during the partial suspension of activity
which ensued upon the production of the Ajax among the planets, the
crowd of planetoids were launched to fill the blank space. Here, if
anywhere, Nature changed her hand and tried a fresh method. Faye's
shifting of the scene of change to trans-Saturnian regions is then, as
M. Wolf justly perceived, non-natural, and undermines the credit of a
plan to which the device is essential.

On the other hand, it had the merit of being elastic enough to include
the great cometary family. Kant had also, although in an unsatisfactory
manner, made room for them; but Laplace had no choice save to regard
them as casual intruders from space, the admission of which as natives
of his well-ordered domain would have led to the subversion of all
its harmonious regulations. Modern inquiries, however, prove comets
decisively to be no such stray visitors as Laplace supposed, but to
be of the same lineage--however remotely traceable--with the planets,
and to own the same allegiance. Drifting with the sun, they form part
of its escort on the long, irrevocable voyage it is engaged upon, and
cannot, save by accidents of perturbation, be driven finally to part
from its company. The problems of planetary and cometary origin are
then inseparable; the two classes of body are fellow-citizens of one
kingdom. Comets become only by compulsion cosmopolitan wanderers from
star to star.

There was yet another motive and semblance of justification for Faye's
reform of the nebular hypothesis. The discovery of the conservation
of energy supplemented, as we have seen, very happily the mechanics
of a condensing nebula by satisfactorily solving the enigma of
solar radiation. Helmholtz was thus able, in 1871, to sketch cosmic
development as, in its essence, a thermodynamic process on the grandest
scale. Yet the alliance entered into, fruitful and fortifying though
it was, had an attendant embarrassment. Time had now to be reckoned
with. In the cosmogonies of Kant, Herschel, and Laplace the allowance
of æons was unstinted. Because the rate of change was indeterminate,
they might be permitted to elapse _ad libitum_. But it was otherwise
when the driving-power came to be defined. 'Conservation of force'
implies the measurableness of force. Equivalence cannot be ascertained
where no limits are determinable. Knowledge, accordingly, regarding the
source of the sun's heat brought with it the certainty that the source
was by no means inexhaustible. The stock of energy rendered available
by shrinkage from a primitively diffuse to its present compact state
was enormous, but not boundless. The task then became incumbent
upon cosmogonists of proving its sufficiency, or of eking out its
shortcomings.

The problem is both retrospective and prospective. We look back towards
the birth of the sun, we look forward to its demise; and each event
has, if possible, to be located on our time-scale. Helmholtz assigned
terms of twenty-two millions of years in the past and seventeen
millions in the future for the shining of our luminary with its actual
intensity. Geologists and biologists, however, claimed a much more
extended leisure for the succession of phenomena on this globe, and
efforts on the part of physicists to meet their demands barely availed
to tone down without removing the discrepancy. M. Faye then came to
the rescue. His suggestion that the earth took separate form while the
sun was still nebulous was designed to conciliate the demands of those
who needed all but eternity for the slow accumulation into specific
differences of infinitesimal variations. In this way a start was
gained upon the sun; the preparations for vitality on our planet were
going forward long before the lavish radiative expenditure designed
to nurture its development had begun. The earth, in fact, was shaping
itself for its destiny in advance of the epoch when time began to count
for the sun.

This supposed relation of precedence cannot, indeed, be insisted upon;
it was imagined to save a difficult situation, and intimates a design
more or less academic. Yet the expedient was significant as regards the
effect of the introduction into modern thought of the principle of the
conservation of energy. It gave definiteness and a kind of solidity
to speculation by widening the basis upon which it was made to rest.
At the same time it necessitated adjustments between the exigencies
of the various sciences, and brought into prominent view apparent
incompatibilities only to be removed by prolonged investigations of
wide scope and intricate bearings. Modern cosmogony, in short, while
disposing of enlarged means, has to meet multiplied requirements. Quite
lately, nevertheless, some authoritative exponents of geological and
biological science manifest a satisfactory disposition to 'hurry up
their phenomena,' quite independently of the inadequate age of the
sun.[23] On neither side, accordingly, are the irreconcilable claims of
the past any longer insisted upon, and a compromise has become easily
possible.

A theory of planetary evolution marked by some novel features was
ably expounded by M. du Ligondès in 1897.[24] Designed to improve, by
simplifying, Faye's plan, it reduced postulates to a minimum, and left
the freest possible play to 'original indetermination.'[25] The embryo
world of M. du Ligondès was a tumultuous mêlée of particles moving
anyhow. Their jostlings, however, did not, and could not, exactly
balance, and the inequality, small though it might be, sufficed to
afford a basis for harmonious growth. Motion became regularized by
collisions; counter-currents of velocity were gradually eliminated; and
the particles pursuing eccentric or retrograde courses, brought sooner
or later to a stand, fell towards the centre and accumulated into the
sun, while the remnant that travelled in the prevalent direction along
circular paths finally constituted the planets. They were formed, not
at haphazard, but through the medium of zones of maximum density,
due to the variations of gravity within the disc towards which the
primitive spheroid finally collapsed; and each, as it took shape,
became a source of perturbative influence on its subsequently developed
neighbours, by which the inclinations of their orbital planes and of
their axes of rotation were in various ways altered. The planetary
zones, too, contracted with the advance of condensation, so that the
matured planets occupied positions much nearer to the sun than those
assigned to their inchoate materials. The _modus operandi_ employed, in
short, adapted itself with praiseworthy readiness to the diversities of
nature.

Sir Robert Ball is at one with M. du Ligondès in regarding the origin
of the solar system chiefly under its mechanical aspect. Like Helmholtz
and Faye, he chooses pulverulent materials to work with; his nebula is
a 'white nebula.' But looking still further back, he discerns as its
parent an irregular 'green' nebula, the confused movements of which
falling into a settled order as the result of encounters, it slowly
flattened down into the 'plane of maximum areas'--the fundamental plane
conformed to more and more closely as the energy of a system inevitably
wastes. He dispenses with the troublesome process of annulation, and
starts his planets virtually by Kant's method of accidental nuclear
condensation.[26] A spiral structure, moreover, would be imparted to
the entire nebula by the gradual propagation outward of the central
acceleration due to contraction.

But would it have contracted? It had, by supposition, reached the stage
of approximate unanimity in movement. The great bulk of its constituent
bodies circulated in the same direction, in nearly the same plane,
and presumably in orbits not deviating much from circularity. Their
aggregate condition might then be regarded as permanent and stable. The
central mass would, accordingly, no longer be fed by the engulfment
of particles brought to rest by their mutual impacts; motion being
unimpeded, heat could not be evolved; and the imagined transformation
of a disc-like meteoric formation into a sun and planets would fail to
come to pass.

What, then, we may ask ourselves, is the upshot of these various
efforts at reconstruction? They establish, certainly, the unassailable
unity of the solar world; and the solar world must be understood to
embrace comets and cometary meteors. The arguments favouring this
unity have gained enormously in cogency through modern discoveries.
For those depending upon structural coincidences and harmonies
of movement have been reinforced by others of a totally different
nature, furnished by the doctrine of the conservation of energy and
the teachings of spectrum analysis. The sun is hot because it was
anciently expanded; the energy of position formerly belonging to its
particles incontestably provided a large part, if not the sum total,
of its present thermal energy, and this amounts to saying that a
sphere indefinitely great was once filled by our inchoate system. The
conclusion that it arose from an undivided whole through the gradual
differentiation of its parts is further ratified by the identity of
solar and terrestrial chemistry. The earth is thus strongly averred to
have once made an integral part of the substance of the sun, and what
is true of the earth is no less true of its sister planets.

Regarding the mode and manner of cosmic change there is, nevertheless,
no consensus of opinion. Faye made a noteworthy effort to elaborate a
process that might endure modern tests of feasibility, yet his theory
has been well-nigh torn to pieces by adverse criticism. M. du Ligondès
escapes some, but not all, of the objections which proved fatal to
his predecessor. That there was in the beginning a solar nebula, all
are agreed; but whether it was gaseous or pulverulent, whether it
shone with interrupted or continuous light, how it became ordered and
organized, how it collected into spheres, leaving wide interspaces
clear, the wisest are perplexed to decide.

Mr. Moulton concludes, from his careful examination of the subject,
that 'the solar nebula was heterogeneous to a degree not heretofore
considered as being probable, and that it may have been in a state'
resembling that exhibited in recent photographs of spiral nebulæ.[27]
But, even if all the facts do not chime in with this tempting analogy,
there can be little reason to dissent from his intimated opinion that
'the Laplacian hypothesis is only partially true, and that we do not
yet know the precise mode of the development of the solar system.'

FOOTNOTES:

[Footnote 16: _Mémoires de l'Académie de Montpellier_, tom. i.]

[Footnote 17: _The Tides_, p. 327.]

[Footnote 18: _Mémoires de l'Académie de Montpellier_, tom. viii.]

[Footnote 19: C. Wolf, _Bulletin Astronomique_, tom. i., p. 596.]

[Footnote 20: _American Journal of Science_, vol. xxxviii., p. 3.]

[Footnote 21: G. H. Darwin, _Nature_, vol. xxxi., p. 506.]

[Footnote 22: _Proceedings of the American Philosophical Society_, vol.
xxii., p. 109.]

[Footnote 23: De Vries, _Die Mutationstheorie_, Bd. II., p. 714.]

[Footnote 24: _Formation Mécanique du Système du Monde._ See also _Le
Problème Solaire_, by the Abbé Th. Moreux, p. 63 _et seq._]

[Footnote 25: _Revue des Questions Scientifiques_, January, 1904.]

[Footnote 26: _The Earth's Beginnings_, p. 247.]

[Footnote 27: _Astrophysical Journal_, vol. xi., p. 130.]




CHAPTER V

TIDAL FRICTION AS AN AGENT IN COSMOGONY


The effects of tidal friction are of almost infinite complexity. How it
will act in each particular case cannot be predicted offhand; it is a
matter for detailed inquiry. Mutually countervailing influences have to
be taken into account, nor is the balance easy to strike. The manner of
its inclination may, indeed, often depend upon qualities and relations
of the bodies concerned which lie outside the range of what can be
distinctly ascertained. All that may be hoped for, then, is to arrive
at estimates neither misleading by their ostensible precision, nor yet
so vague as to be wholly uninstructive, of the part played by tidal
forces in moulding the history of connected globes.

The assumption that they attract one another as if the mass of each
were collected at its centre, is one of those convenient fictions
without which the advancing feet of science would be impeded by tangled
thickets of illusory refinements and superfluous elaborations. The
fiction would correspond with fact only if the globes were truly
spherical, and they could be truly spherical only if they were ideally
rigid. Cosmic bodies, however--suns and planets alike--are actually
plastic spheroids; they can, to be sure, be treated without sensible
error as attractive points when their distances are very great
relatively to their diameters; but upon a closer approach inequality
of action supervenes. The component parts of the gravitating masses
respond, each individually, and in a measure independently, to the
graduated pulls exercised upon them, and tidal strains begin variously
to take effect.

Their historical significance was in part divined by Kant. His
penetration of so recondite a secret is truly astonishing. A struggling
young pedagogue in a remote Prussian province, profoundly learned,
though no more than half skilled in technical acquirements, saw by
intuition what escaped the acumen of all the great geometers of the
eighteenth century--namely, that the moon turns one perpetual face
towards the earth, because its primitive rotation was stopped by the
friction of earth-raised tides. He perceived besides that a reciprocal
action of the same kind must affect the earth, and will continue to
affect it until the day coincides in length with the month. Nor did he
fail to point out that, in a molten state of the globes, the process
would advance with comparative rapidity. To one solitary thinker, then,
it became apparent, already in 1754,[28] that oceanic tides are, in
cosmogony, of negligible importance compared with bodily tides.

There is no substance in nature that will not change its shape through
prolonged stress, and the more readily the nearer it approaches to the
fluid condition. The heaping-up of the waters on the earth's surface at
the bidding of the moon is thus a differential effect. Continents heave
and subside as well as oceans, though not nearly to the same extent.
The measurable rise of water serves to gauge the relative mobilities
of the solid globe and of its liquid envelope. If the former did not
yield at all to the pull so readily obeyed by the latter, the tides
would, in fact, be greater than they actually are in the proportion of
about three to two, the ratio indicating for the earth an effective
rigidity at least equal to that of steel.[29] Were there no discrepancy
in rigidity between the various parts of our terraqueous world, tides
would fail to be perceptible. The ocean and the bed of the ocean would
rise and fall together, and to the same extent. In the far past there
_was_ no discrepancy. The viscous earth took, as a whole, the form
momentarily impressed upon it by the unequal attractions of the sun and
moon on its variously distant sections, with the upshot of bringing the
year, month, and day into relations so familiar as to appear inevitable.

Tidal friction does not merely act as a check upon rotational speed.
One element of motion in a system cannot be altered without some
counter-change in the others. They are coupled up together like a train
of geared wheels. From the principle of the conservation of moment of
momentum, we know with certainty that a loss in one direction must
be compensated by a gain in some other. Tidal friction had, then,
reactive consequences. They were first adverted to by Julius Robert
Mayer in 1848,[30] and were brought prominently into view in the series
of investigations begun by Professor Darwin in 1879. The rotational
momentum removed from the earth by the drag of a circulating wave of
deformation must assuredly have reappeared in some other part of the
system. It was restored, all but the percentage wasted as heat, by the
widening of the lunar orbit.[31] Concomitantly with the slackening of
the earth's axial rate, the moon retreated from its surface, pulled
forward by the tidal crest continually in advance of its position. This
redressed the balance by augmenting orbital momentum, while at the same
time diminishing the moon's linear velocity. The importance of this
secondary frictional effect in the history of the earth-moon system was
the virtual discovery of Professor Darwin.

That system occupies a critical situation in the solar cortège. The
planets interior to it have no satellites; the planets exterior to it
(Neptune making probably only an apparent exception to the rule) have
two or more. The earth alone is truly binary; and the moon is not only
its solitary companion, but it is by far the largest companion-body,
relatively to the mass of its primary, to be found within the precincts
of the solar domain. These circumstances are certainly not disconnected
one from the other, and they obviously depend upon a single cause.
Solar tidal friction was here the determining factor. The apportionment
of satellites to the various planets was, beyond doubt, in great
measure prescribed by the degrees of retarding power exerted on their
axial movement through the agency of sun-raised tides in their still
plastic bodies. Hence, the disruptive rate of spinning needed for
the separation of satellites was never attained by either Mercury or
Venus; they remained moonless for all time, and exposed, through the
cutting down of their rotational velocity, to uncompensated extremes
of temperature. How the earth was to fare in both respects long hung
in the balance. Rightly to forecast its destiny would, indeed, have
demanded no common perspicuity in an intelligent onlooker from some
other sphere. Although the solar brake acted upon terrestrial rotation
with no more than one-eleventh the power brought to bear upon that
of Venus, it nevertheless sufficed during uncounted ages to hinder
acceleration from reaching the pitch involving instability.

Our embryonic planet had long ceased to be nebulous, and had, in fact,
shrunk by cooling nearly to its present dimensions before the die was
cast. Then, at last, the hurrying effects of contraction prevailed
over the slowing down due to tidal resistance, axial speed overbore
equilibrium, and the spheroid divided. Now globes thus far advanced
in condensation are apt to split less unequally than globes in a more
primitive stage; and the moon, because late-born, was of large size.
Its mass is 1/81 that of the earth; the masses of Titan and Saturn are
as 1 to 4,600; while Jupiter's third and greatest satellite contains
only 1/11300 part of the matter englobed in the parent-body. Moreover,
Professor Darwin has made it clear that the satellites of Jupiter and
Saturn revolve now in orbits not widely remote from those at first
pursued by them; while the moon, on the contrary, started on its career
almost, if not quite, from grazing contact with its primary. Owing to
these two exceptional circumstances--its considerable relative mass
and its close initial vicinity--the moon wielded over the earth tidal
influence incomparably more powerful than that exerted by any of its
compeers in the sun's realm.

The lunar-terrestrial system offers, accordingly, an example unique
among those in solar subordination of a pair of globes, the mechanical
relations of which have been settled on their present basis by the
predominating agency of bodily tides. It holds forth, too, the one
case in which origin by fission was possible. Professor Darwin's
communication to the Royal Society in 1879 occasioned on this point a
remarkable diversion of ideas. Saturn's rings were at last, through
the reasonings contained in it, perceived to be illustrative of only
one among many feasible modes of cosmic growth. It became clear that a
single cut-and-dried method would not answer all the infinitely varied
purposes of creative design. Annulation might have served its turn,
but there were alternatives. A fresh standpoint was virtually attained,
and the wide prospect commanded by it begins already to spread out
invitingly before the gaze of investigators.

But whether the moon emerged from the earth as a protuberance, or was
abandoned by it as an irregular equatorial ring, it was revolving,
when our theoretical acquaintance with it begins, in a period of
not less than two and not more than four hours, quite close to the
earth's surface; while the nearly isochronous rotation of the earth
was conducted with all but disruptive rapidity. The situation is so
suggestive that it needs only a short and tolerably safe leap in the
dark to reach the conclusion that the two masses had very recently
been one. With their division, at an epoch estimated to have been
about sixty million years ago, the process began by which the moon was
pushed back along a widening spiral course to its present position,
the vanished rotational momentum of the earth cropping up again in the
augmented orbital momentum of the moon. And the transformation is, at
least in theory, still going on.

Tidal friction has further capabilities. The transference of momentum
from one part of a system to another is only the most obvious among
the crowd of its results. Scarcely an element of movement escapes its
influence. It increases, as a rule, orbital eccentricity. The smallest
initial deviation from circularity develops, through the inequality of
accelerative action thence ensuing, into pronounced ovalness. That of
the moon's path can in this way be accounted for. Moreover, its plane
was, in all probability, shifted simultaneously and under compulsion
of the same power, from its original coincidence with the earth's
equatorial plane to the level it now occupies. The obliquity of the
ecliptic, too, is partially explicable on the same principle. 'The
present motion of the two bodies' (to quote Professor Darwin's words),
are 'completely co-ordinated by the theory that tidal friction was the
ruling power in their evolution.' Holding this clue, we are enabled to
trace them back to the start of their dual existence, and to follow
the insensible modifications by which their state was moulded to its
actual form.

In no other satellite-system is this possible. No moon besides our own
possesses a stock of orbital momentum large enough to intimate for it
an analogous history. Planetary attendants elsewhere travel nearly in
their original tracks; the fluid ripples raised by them on the surfaces
of their primaries lacked power to displace them sensibly. Their own
rotation, indeed, seems to have been completely destroyed. Destroyed,
that is, relatively to the destroying body. There is a certainty that
some, there is the strongest likelihood that all, of the Jovian and
Saturnian satellites turn unchangingly the same face towards their
primaries. They rotate in the period of their several revolutions,
just as our moon does, and as a consequence of the same cause. Tidal
friction, however, appears to have been otherwise of subordinate
importance in shaping their dynamical relations.

The agency will not, then, serve in all cases for a _deus ex machinâ_.
It is not indiscriminately efficacious. The modes of its action have,
in each of the systems considered, to be delicately distinguished.
The stage of development arrived at by the bodies affected, their
degree of viscosity, their comparative mass and bulk, their modes of
motion, all avail profoundly, and it may be incalculably, to modify the
outcome. The facility of error in estimates of the kind is illustrated
by Professor Darwin's remark that the magnitude of the tide-raising
force is only one factor of the product.[32] The other is relative
movement. Now, in the case of the moon the former continually augmented
retrospectively, while the latter fell off. Tidal generative power
varies inversely as the cube of the distance; in antique times, then,
when the earth and moon revolved contiguously, the bodily distortions
they mutually produced were beyond question on an extremely large
scale. Yet, because of the near coincidence of the periods of the
globes, they must have been almost inoperative for frictional purposes.
The travelling of the piled-up matter over their surfaces was too slow
to lend it much power as a friction-brake. The insignificant waves
raised by the sun were, we are led to believe, because of their swift
relative motion, more influential at that early epoch in checking
terrestrial rotation than the colossal, but nearly stationary waves due
to the moon.

Numerical calculations, where they are practicable, afford the only
safe guide to this intricate field of inquiry. It does not suffice to
show that tidal action would have been of the kind required--would
have taken the right direction--for bringing about some apparently
anomalous result. Proof must, besides, be forthcoming that the action
would have been of adequate power. Plausible guesses on the subject
may be entirely fallacious. The machine, even if properly constructed
for the end in view, may work too feebly for its attainment. We are,
for instance, assured that no difficulty connected with the sense of
planetary rotation need impede acceptance of the theory of planetary
origin from separated rings, since even if the embryo globes gyrated
the wrong way at the outset, solar tidal friction would promptly have
reduced them to conformity with the general current of movement. This
is true in principle, but will it bear quantitative investigation?
Many promising hypotheses have broken down under the weight of
figures; whether this particular one is strong enough to survive their
application remains to be seen. We are, indeed, sure of its validity as
regards Mercury, but the efficacy of tidal friction decreases as the
sixth power of increasing distance, and the actual rotation of Venus
furnishes an enigma sufficiently perplexing to discourage scrutiny of
its dimly discerned antecedent conditions. As regards the earth and the
exterior planets, the question could only be answered with the help of
information which is not forthcoming.

The unexpected circumstance that the newly-discovered ninth Saturnian
moon circulates from east to west can thus be no more than tentatively
explained by invoking this agency of change. Admitting (as we seem
bound to do) that satellites are the offspring of the planets they
attend, there is no evading the conclusion that the small body under
discussion was thrown off from a primary endowed with a rotation
opposite to that now possessed by it. And the reversal must have been
completely brought to pass before the eighth satellite, Japetus, came
into existence. The crux is most arduous; there is no other resource
for meeting it but to consider the effects on planetary rotation of
solar tides, and this Professor W. H. Pickering, the discoverer of
Phœbe, has done.[33] But a cause may be true without being sufficient;
and close calculation will be needed to determine, in this instance,
how the matter stands.

Professor Darwin's researches were fruitful just because they were
definite. They demonstrated, once for all, the diverse faculties of
tidal friction as a cosmogonic agency, and indicated clearly the
departments of cosmogonic change in which its competence lay. They
availed, moreover, to determine for the earth-moon system the amount of
work actually done by tidal friction in these several departments, and
to prove its large excess over the corresponding output in any other
sub-system falling within the sphere of observation. This memorable
result suggests that our terrestrial home may be singular, not only in
its evolutionary history, but in the innumerable adjustments fitting it
to be the abode of life.

The relations of the earth and moon adumbrate, and scarcely more
than adumbrate, the physical influences mutually exerted upon each
other by numerous twin-globes in stellar space. Tidal friction is of
maximum power in systems formed of equal masses; and those of double
stars are seldom widely disparate. Most, if not all of them, were,
besides, primitively very near neighbours, so that their symmetry must
have been marred by conspicuous tidal deformations. The results upon
their development have been expounded in detail by Dr. See. One of
the most remarkable is the high average eccentricity of their orbits.
Visual binaries, with few exceptions, travel in considerably elongated
ellipses, while spectroscopic binaries as a rule pursue approximately
circular paths. Dr. See's argument that the eccentricity of the more
spacious systems was acquired under the influence of tidal friction,
during the long course of progressive separation, is well-nigh
irresistible.

True, this line of explanation is not wholly clear of obstacles and
incongruities. Yet they may probably be described as of a complicating,
rather than of a contradictory kind. The theory of tidal friction
is not a universal solvent of the difficulties encountered in the
study of double stars. That the mode of action it deals with had a
contributory share towards regulating their mechanical arrangements
may, nevertheless, be regarded as certain, while the potency and
perhaps even the manner of its operation varied extensively from system
to system. What precisely it effected in each lies beyond our range of
determination. For the data available regarding the viscosity, density,
and axial movements of embryo star-pairs must always be too scanty and
insecure to provide a basis for rigorous computations. The mystery of
the fore-time can never be entirely dissipated. Enough if we can look
at it through a glass which darkens, without distorting, the objects
presented in its field of view.

FOOTNOTES:

[Footnote 28: _Sämmtliche Werke_, Bd. VI., pp. 5-12, 1839.]

[Footnote 29: G. H. Darwin, _Encyclopædia Britannica_, article on
'Tides.']

[Footnote 30: _Dynamik des Himmels_, p. 49.]

[Footnote 31: Darwin, _Philosophical Transactions_, vol. clxxii., p.
528.]

[Footnote 32: _Philosophical Transactions_, vol. clxxi., p. 876.]

[Footnote 33: _Harvard Annals_, vol. liii., p. 58.]




CHAPTER VI

THE FISSION OF ROTATING GLOBES


Few people need to be told that a rotating fluid mass is shaped very
much like an orange. It assumes the form of a compressed sphere. And
the reason for its compression is obvious. It is that the power of
gravity, being partially neutralized by the centrifugal tendency due
to axial speed, decreases progressively from the poles, where that
speed has a zero value, to the equator, where it attains a maximum.
Here, then, the materials of the rotating body are virtually lighter
than elsewhere, and consequently retreat furthest from the centre. The
'figure of equilibrium' thus constituted is a spheroid, a body with
two unequal axes. In other words, its meridional contour--that passing
through the poles--is an ellipse, while its equator is circular.

Now we know familiarly, not only that a spinning sphere becomes a
spheroid, but that the spheroid grows more oblate the faster it spins.
The flattened disc of Jupiter, for instance, compared with the round
face of Mars, at once suggests a disparity in the rate of gyration.
But there must be a limit to the advance of bulging, or the spheroid,
accelerated _ad infinitum_, would at last cease to exist in three
dimensions. Clearly this unthinkable outcome must be anticipated; at
some given point the process of deformation must be interrupted. A
breach of continuity intervenes; the train is shunted on to a branch
line. Nor is it difficult to divine, in a general way, how this comes
to pass. Equilibrium, beyond doubt, breaks down when rotation attains a
certain critical velocity, varying according to circumstances, and the
spheroid either alters fundamentally in shape or goes to pieces.

So much plain common-sense teaches, yet the precise determination of
the course of events is one of the most arduous tasks ever grappled
with by mathematicians. M. Poincaré essayed it in 1885;[34] it was
independently undertaken a little later by Professor Darwin;[35]
and the subject has now been prosecuted for eighteen years, chiefly
by these two eminent men, with a highly interesting alternation of
achievement, one picking up the thread dropped by the other, and each
in turn penetrating somewhat further into the labyrinth. The results,
nevertheless, are still to some extent inconclusive; they indicate,
rather than indite, the genetic history of systems. A strong light is,
indeed, thrown upon it; but in following its guidance, the limitations
of the inquiry have to be borne in mind. The chief of these are, first,
that the assumed spheroid is liquid; secondly, that it is homogeneous.
Neither of these conditions, however, is really prevalent in nature,
so that inferences based upon them can only be accepted under reserve.
They were adopted, not by choice, but through the necessities of the
case. There was no possibility of dealing mathematically with bodies
in any other than the liquid state. The equilibrium of gaseous globes
defies treatment, except under arbitrary restrictions.[36] Nor is
it possible to cope with the intricacies of calculation introduced
by variations of interior density. Cosmical masses, as they actually
exist, are nevertheless strongly heterogeneous, so that at the utmost
only an approximation to the genuine course of their evolution can
be arrived at by the most skilful analysis. Yet even an approximate
solution of such a problem is of profound interest. We can here only
attempt briefly to specify its nature.

The course of change by which the equilibrium of a rotating liquid
spheroid is finally overthrown has, at any rate, been satisfactorily
tracked. When its spinning quickens to a disruptive pitch, it acquires
three unequal axes instead of two. The equator becomes elliptical like
the meridians. A 'Jacobian ellipsoid' is constituted. To this new form,
it would seem, a long spell of stability must be attributed; only its
major axis becomes more and more protracted as cooling progresses,
and with cooling, contraction, and with contraction the increase of
axial velocity. Then at last a crisis once more supervenes; there
is a collapse of equilibrium, and its re-establishment involves the
sacrifice of the last vestige of symmetry. An 'apioid,' or pear-shaped
body, replaces the antecedent ellipsoid; and its apparent incipient
duality suggested to M. Poincaré that the furrow unequally dividing it
might deepen, with still accelerated gyration, into a cleft, splitting
the primitively single mass into a planet and satellite. But this
eventuality, he was careful to note, had no direct bearing on Laplace's
hypothesis, which dealt with a nebula condensed towards the centre,
while the fissured apioid was liquid and homogeneous.[37]

Professor Darwin followed out the conditions of this remarkable
pear-shaped body to a closer degree of approximation than its original
investigator had done, and succeeded in virtually demonstrating its
conditional stability. But his analysis tended to smooth away the
characteristic peculiarities of its shape, and, so far, to diminish
the probability of its ultimate disruption. Mr. Jeans, on the other
hand, from an elaborate study of a series of cigar-shaped figures
which in theory follow a parallel course of development to that
pursued by ellipsoids, derived, by strict mathematical reasoning, the
actual separation of a satellite from one end of a parent-cylinder.
The representative figures reminded Professor Darwin 'of some such
phenomenon as the protrusion of a filament of protoplasm from a mass
of living matter.' 'In this almost life-like process' he saw 'a
counterpart to at least one form of the birth of double stars, planets,
and satellites.'[38]

But the resemblance, when examined dispassionately, seems shadowy and
evasive, especially when we confront it with the case of double stars.
Here, indeed, an entirely different set of conditions comes into play
from that postulated by Poincaré and Darwin, since stars are certainly
not liquid bodies.[39] They are most likely gaseous to the core,
though the indefinite diffusiveness incident to gaseity is restricted
by their condensed photospheric surfaces. This circumstance intimates
the possibility that the results arrived at for liquid globes by
mathematical analysis may, with qualifications, be extended to stars;
but the necessary qualifications, unfortunately, are vague and large;
for too little is known regarding the physical condition of stellar
spheres to warrant assumptions that might provide a secure basis for
research.

The evolution of binary stars can then only be treated of
inferentially, not rigorously; and we must, at the outset, discard
the idea that it is illustrated by the phenomena of double nebulæ.
Many such objects thought to supply clinching visual arguments for the
actual effectiveness of slow cosmic fission proved, on the application
to them of the late Professor Keeler's searching photographic methods,
to be knots on spiral formations. Their mutual relations are then
entirely different from what had been supposed by telescopic observers;
they are, in fact, still structurally connected, and the mode of their
origin, however inviting to conjecture, scarcely comes within the scope
of definitely conducted inquiries. Their future destiny is no more
accessible to it than their past history, and only by a daring flight
of imagination can we see in spiral nebulæ the prototypes of double
stars.

Questions as to the mode of genesis of these latter systems have, in
recent years, acquired extraordinary interest. Conclusive answers
cannot, indeed, at present be given to them, because the terms in which
they are couched lack distinctness, owing to our lack of knowledge;
but probable answers may legitimately supply their place, at least _ad
interim_, above all when their probability is heightened almost to
certainty by the accumulation of circumstantial evidence.

Observations and investigations of stellar eclipses have created a
new department of astrophysics, and have vastly widened the domain of
cosmogony. They have brought to notice a number of systems, not merely
in a primitive, but seemingly in an inchoate stage of development. The
periods of occulting stars are nearly all of them less than seven days,
although one extending to thirty-one has lately been recognised; and
the comparative length of the intervals of obscuration shows them to
be produced by the circulation in narrow orbits of distended globes.
These are characteristic symptoms of juvenility, for, as we have seen,
orbits widen and periods lengthen with the efflux of time through the
frictional power of bodily tides.

Now the class of stars which obviously and certainly undergo eclipses
has some outlying members of a still dubious nature. And their marginal
position serves greatly to enhance the present, the prospective, and
the retrospective interest attaching to them. These remarkable objects
vary in light continuously. Their phases are not, like those of Algol,
mere interruptions to a regular course of steady shining. They progress
without a moment's sensible pause; they are represented graphically by
a smoothly-flowing, symmetrical curve. The eclipses by which they are
occasioned--if they are so occasioned--must, accordingly, succeed each
other in a strictly unbroken series. No sooner has one terminated than
the next commences. One star passes first behind, then in front of its
companion, and their combined brightness is seen undimmed only during
the few moments of actual maximum. This means that they revolve in
contact; they are separated by no sensible gap of space.

Goodricke's variable, β Lyræ, is held to be thus constituted. The
possibility, at least, of employing the 'satellite-theory' to account
for its changes was demonstrated some years ago by Mr. G. W. Myers, of
Indiana.[40] He found the system to be composed of two barely separated
ellipsoids, circulating in the visual plane, and producing, by their
successive transits, two unequal eclipses in the course of each period
of 12·91 days. The joint mass of the pair is just thirty times that of
our sun, but their mean density has the almost incredibly small value
of 1/1200 that of water. Their real existence is conditional upon the
possibility that masses much more tenuous than atmospheric air should
radiate with the intensity of true suns. Spectroscopic observations
are not wholly unfavourable to Mr. Myers's hypothesis, but their
interpretation is hampered by discrepancies so numerous and perplexing
that no secure inference can be derived from them. Moreover, the star
supposed to be alone presented to view at the principal minimum is
that giving the bright-line spectrum; yet it is compulsorily assumed,
in order to meet the exigencies of the situation, to be much more
massive, while much less intrinsically bright, than its companion. This
is disquieting, but nearly everything connected with β Lyræ _is_ more
or less disquieting.

A variable of the same type, but much fainter, was made the subject
of a similar inquiry by Mr. Myers in 1898.[41] U Pegasi never attains
ninth magnitude; hence, spectroscopic complications equally with
spectroscopic verification remain at present out of sight. The star,
nevertheless, excites keen interest, and claims sustained attention.
Its light-curve has been laid down with exquisite accuracy at Harvard
College, and shows two slightly unequal minima to be comprised within a
period of nine hours, signifying, on the adopted theory, the occurrence
of alternating eclipses at intervals of four and a half hours. The
distance from centre to centre of the occulting stars, the smaller of
which is of about eight-tenths the brightness of the larger, 'does not
materially differ,' Mr. Myers tells us, 'from the sum of their radii,
suggesting the probable existence of the "apioidal" form of Poincaré.'
If they do not actually coalesce, the component bodies revolve in
contact, and rotate synchronously. Thus, it is hard to say whether U
Pegasi should be accounted as a single pear-shaped mass spinning in the
time of light-change, or as a close couple circulating freely in that
identical period. The mean density of the system appears to lie between
one-third and one-fourth that of the sun.

Another specimen of the 'dumb-bell' system is possibly met with in R2
Centauri. The narrow range of its variation makes it a delicate object
to observe; but Mr. A. W. Roberts, who first noticed its peculiarity
in 1896, has since accumulated an extensive series of wonderfully
accurate visual determinations of its fluctuating brightness, and has
besides rendered them the basis of an able and exhaustive theoretical
discussion.[42] The double period of R2 Centauri is restricted to
fourteen hours thirty-two minutes. Within this brief span quadruple
phases are included--that is to say, two evenly balanced maxima and
two slightly disparate minima. These result, Mr. Roberts concludes,
from the mutual eclipses of interpenetrating ellipsoids, one somewhat
more luminous than the other, revolving--if they can properly be said
to revolve--in an orbit inclined 32° to the visual plane. They are of
just one-third the solar density, and the forms satisfying photometric
requirements by the varying areas of luminous surface presented to
sight in different sections of their path show a surprising agreement
with the bi-prolate figure given by Professor Darwin's analysis as the
shape of a body on the verge of disruption through accelerated rotatory
movement. The inference is, then, almost irresistible that R2 Centauri
really exemplifies the nascent stage of binary stars. To establish
this completely, however, spectroscopic data are needed; and they are
difficult to procure for a star below the seventh magnitude.

No such obstacle impedes the investigation of the analogous, but much
brighter object, V Puppis. Detected as a spectroscopic binary by
Professor Pickering in 1895, this star traverses so wide an orbit in
the short period of thirty-five hours as to imply--if the published
details are correct--that the pair possess no less than 348 times the
gravitational power of the sun. They are, nevertheless, according
to Mr. Roberts, fifty times more tenuous, and each globe should have
a diameter of about 16-1/2 million miles; yet nothing of all this is
incredible. The light-curve of V Puppis, as traced by Mr. Roberts,
is closely modelled upon that of U Pegasi. And he postulates similar
conditions of eclipse. It rests with the spectroscope to determine
whether those conditions are realized or not.

Probably all short-period variables are binaries, with coincident
orbital- and light-cycles. But all are not occulting binaries. There
are some--we are still ignorant of their proportionate numbers--which
undergo a course of light-change, apparently compatible with an
occulting hypothesis, yet certainly escape eclipse. Professor
Campbell has made it unmistakably clear that ζ Geminorum is thus
constituted.[43] Two stars are present, but their plane of motion
is inclined at an unknown angle to the line of sight; it does not
approximate to coincidence with it. Now the possibility is not
excluded that V Puppis belongs to the same class. Mr. Roberts's
assumptions are, indeed, in themselves plausible, and they may at any
moment be proved, by a few well-timed spectrograms, to be undeniably
true.

The one conclusive test of their truth is the cessation of radial
movement at epochs of minimum. Evidently, if the diminution in lustre
be due to an eclipse, the eclipsing and eclipsed bodies must be
crossing the line of sight just when the obscuration is deepest. There
is no evading this geometrical requirement, and it must be rigorously
complied with in the circular orbits traversed by bodies revolving in
contact. Before, then, Mr. Roberts's theory of V Puppis can be accepted
with implicit confidence, it has to be ascertained whether a zero of
radial speed is reached concurrently with the photometric minima. If
so, these may be unhesitatingly set down as eclipse-phenomena; if,
on the contrary, the decline in brightness prove to be unrelated to
a slackening of speed, then the supposition that it accompanies and
indicates a transit must be peremptorily discarded. Moreover, the
spectroscopic verdict as regards V Puppis can safely be applied to
stars with similar light-curves, especially to R2 Centauri and U
Pegasi, and may serve to clear away some of the intricacies connected
with the exceptional system of β Lyræ. The measurement of a single
spectrographic plate might thus, by deciding the test-case of the
binary in the poop of Argo, be made essentially to supply the lack of
desirable, but at present unattainable determinations as regards a
considerable number of analogous objects.

The existence of stellar systems of the 'dumb-bell' type would violate
no mechanical law. 'Roche's limit' does not apply to globes comparable
in size. The range of disparity within which it holds good has not,
indeed, been theoretically established; but it may be said, in general
terms, to concern the relations of planets and satellites (to use
a purposely vague phrase), not those of double stars. What the law
asserts is that a subordinate small body cannot revolve intact at
a less distance than 2·44 radii of its primary from that primary's
centre, if their mean density be the same. For satellites of slighter
consistence the limit should be extended. Our own moon, for instance,
could never have circulated, without being rent in pieces by tidal
strains, in an orbit less than 22,000 miles in diameter.[44]

Bodies of co-ordinate mass are, however, exempted from the prohibitive
rule against mutual approach. No analytical veto is imposed upon the
origin by fission of double stars, or upon the subsistence of stellar
Siamese twins. The inequalities of their mutual attractions avail to
distort, not to disrupt, such embryo globes. Their individuality,
therefore, once created, is in a manner indestructible. It tends, in
fact, to become more pronounced as the orbital span gradually widens
through the reactive effects of tidal friction. The 'dumb-bell'
condition may then be regarded as in a manner transitory. Nor can we
be assured of its actuality otherwise than by the peculiar nature of
the eclipses attending upon it, taken in connection with correlated
spectroscopic observations proving eclipses of the kind veritably to
take place. The disclosure, by such means, of systems so strangely
conditioned promises to afford a deeper insight than would else have
been possible into the cosmical order, and fills a blank page in the
marvellous history of sidereal birth and growth.

FOOTNOTES:

[Footnote 34: _Acta Mathematica_, vol. vii., Stockholm, 1885.]

[Footnote 35: _Proceedings Royal Society_, vols. xlii., lxxi.;
_Philosophical Transactions_, vols. cxcviii., cxcix., Series A.]

[Footnote 36: J. H. Jeans, _Philosophical Transactions_, vol. cxcix.,
Series A, p. 1.]

[Footnote 37: _Figures d'Équilibre d'une Masse Fluide_, p. 172.]

[Footnote 38: _Proceedings Royal Society_, vol. lxxi., p. 183.]

[Footnote 39: J. H. Jeans, _Astrophysical Journ._, vol. xxii., p. 93.]

[Footnote 40: _Astrophysical Journal_, vol. vii., p. 1.]

[Footnote 41: _Astrophysical Journal_, vol. viii., p. 163.]

[Footnote 42: _Monthly Notices_, vol. lxiii., p. 627.]

[Footnote 43: _Astrophysical Journal_, vol. xiii., p. 90; _Science_,
July 27, 1900.]

[Footnote 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.

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.]




CHAPTER VIII

COSMOGONY IN THE TWENTIETH CENTURY


Prospective and retrospective inquiries into physical conditions stand
very much on the same footing. The same degree of uncertainty attaches
to results of both kinds; the same qualifications need to be applied
to them; a similar reserve is understood to accompany our admission of
them. The reserve grows more marked as science unfolds to our surprised
apprehension the multiplex possibilities of Nature. The time has gone
by when 'men of light and leading' could draw cheques for unlimited
amounts on the bank of public credulity. Not that the balance has
diminished, but that it is reserved for other uses. Most of us in these
days, have learnt to 'look before and after' for ourselves, and we
instinctively mix the proverbial grain of salt with what is told to
us, even on the highest authority. Ideas are on the move; dim vistas
are opening out; much that lies beyond the verge of actual experience
is seen to be possible, and sedate reasoning may at any moment suffer
outrage by fantastic discovery. Hence, finality of assertion is out of
date.

The secular parallax affecting men's views of the universe is nowhere
more strongly apparent than in the trend taken by speculations as to
its origin. They have become more subtle, more far-reaching, yet less
confident. They have ramified in unexpected directions, but rather
tentatively, than with the full assurance of attaining absolute truth.
Laplace considered only the solar system, from which he arbitrarily
excluded comets. On the vast sidereal world he bestowed barely casual
attention. Sir William Herschel, on the other hand, occupied himself
exclusively with the growth-processes of nebulæ, relegating the details
of planetary evolution to a position of secondary importance. Later,
the spectroscope having become available for discriminating generic
differences among the suns in space, their relative ages, the order
of their succession, their mutual affinities, claimed predominant
attention. Just now, however, the flood of speculation is too high to
be restrained within separate channels; cosmogonists look far afield;
they aim at obtaining a general survey of relations bewildering in
their complexity. To some extent they have succeeded; parts are
beginning to find their places in a great whole; links are seen to
connect phenomena at first sight seemingly isolated; on all sides
analogies are springing into view. The unwearied circling of the moon
and its imperturbable face remind us how a sun may have been born;
the flash of every meteor suggests the mode by which suns die. The
filmy traceries of comets intimate the nature of the force acting in
nebulæ; the great cosmic law of spirality is remotely hinted at by the
antipodal disturbances of the sun. Thus, one set of facts dovetails
into the next; none can be properly considered apart from the rest.

The limitations of the human mind impose, nevertheless, restrictions
of treatment. Individual efforts cannot grapple with the whole of
the known and the knowable, and the larger part of both is included
in the scope of modern cosmogony. It deals with all that the skies
hold, visibly or invisibly; draws unstintingly on time past and time
to come; concerns itself equally with gradual transformations and
sudden catastrophes, with the dissipation and concentration of energy,
with the subtle interplay of matter and force, with physical and
ultra-physical, chemical and electrical modes of action. But let us
consider a little more particularly how things actually stand, so as to
collect some definite ideas regarding the lines of advance practicable
and promising for the immediate future.

To begin with our domestic circle. The insecure state to which
Laplace's scheme has been reduced by the assaults of numerous objectors
has found compensation in the development of the tidal theory. Much
light has thereby been thrown upon planetary pre-history. The relations
of planets to the sun, and of satellites to planets, have been rendered
comparatively intelligible. Noticeable above all is the discovery
thence ensuing of the earth's suggestive position, just outside the
boundary of the region where planetary rotation was destroyed by
sun-raised tides, and with it the prospects of planetary vitality.

Moreover, the dubious state of the inchoate terrestrial spheroid,
consequent upon its intermediate situation, accounts for the peculiar
mode of birth of the moon, and the distinctively binary character
of the earth-moon system; while the variety perceptible in the
circumstances of the different planets precludes the employment
of any single recipe for their development from a primal vortex.
The forces concerned, we can now see, acted in a far more complex
manner than could formerly have been supposed, and their balance was
proportionately more delicate. To which side it would have inclined in
a given case must then often be incalculable, or calculable only with
the guidance of the known result. The strict bonds of reasoning have
thus become somewhat relaxed, and difficulties that looked formidable
have, in the long run, proved not to be insuperable. But conviction has
also grown faint. The old, imposing façade of theory remains erect;
the building behind it has been, for the most part, pulled to pieces,
and the architect has yet to be found who can reconstruct it to our
satisfaction.

On one point we have, nevertheless, acquired certainty. It is now known
that comets with their dependent trains of meteors are aboriginal in
the solar system. They are no unlicensed intruders, but collateral
relations of the planetary family. Possibly they represent waste scraps
of world-stuff which escaped the action of the formative machinery;
and if so, they exemplify its primitive texture. Not that their
composition need be, on this supposition, identical with that of the
planets. A sifting of elements would have been likely to accompany the
processes of cooling and contraction. Comets were, perhaps, made (to
speak illustratively) of the white of the nebulous egg, planets of
its yolk. But in any case we may safely regard the glimmering fabrics
of acetylene and cyanogen that occasionally illuminate our skies as
shearings from a wide-spreading, fleecy haze, flung aside before
'the starry tides' had as yet begun to 'set towards the centre.' In
one respect the quality of these relics is a surprise. They show no
chemical affinity with nebulæ. Their spectra are radically different
from nebular spectra, gaseous or continuous. They accordingly lend no
countenance, although not fatally adverse to the view that the sun was
once, in the distinctive sense, a nebulous star.

The grand topic of sidereal succession is no longer abandoned to
fruitless surmises. Broad lines have been laid down, along which,
so far as we can at present see, progress must inevitably have been
conducted. And one fact of overwhelming significance in this connection
is entirely of recent discovery. The multitudinous existence of obscure
bodies in space had, indeed, been foreseen as a logical necessity long
before Bessel founded the 'Astronomy of the Invisible'; but its strong
substantiation is almost wholly due to the use of modern spectrographic
methods. Decrepit or dusky suns are assuredly no imaginary product,
but a potent reality, though it would be too much to assert that all
have sunk to extinction by the same road. Nor is it absolutely certain
that their present state is uniformly the outcome of prolonged decay.
Circumstances connected with many of them suggest rather a congenital
incapacity for shining.

We stand, too, on firmer ground than our predecessors in respect to the
history of stellar systems. That its course is mainly prescribed by
the influence of tidal friction has been ably demonstrated by Dr. See.
Telescopic double stars can be led back by the aid of this clue to an
initial stage, when they revolved close together, very much like the
earth and moon in Professor Darwin's theory; and it was owing to their
voluminousness and the unequal attractions it engendered that their
orbits became enlarged and elongated to the degree generally observed.

This theoretical inference has been confirmed with singular aptness by
the discovery of spectroscopic binaries. Pairs circling in orbits too
narrow for visual discernment are the natural complement of pairs just
divisible with the telescope; the first class represent the unseen,
early stages of the second. The two together form an unbroken sequence
of stellar systems, for spectroscopic binaries include couples fully
separated, and still separating, as well as others barely divided, and
revolving almost in contact. Nay, they include specimens, we are led
to believe, of globes conjoined into the apioidal figure theoretically
investigated by Darwin and Poincaré, which may be regarded as
preparatory to the development by fission of two mutually revolving
stars from one primitive rotating mass. Some of these supposed
dumb-bell systems are variable in light; and if the eclipse-rationale
of their obscurations be confirmed by the spectroscope, there is no
gainsaying the inference that each flickering object is composed of two
stars actually contiguous, if not confluent.

Now, compound stars are by no means of exceptional occurrence. Their
relative abundance has been found to augment rapidly with every advance
in our knowledge of the heavens. From the measures of stellar radial
velocity lately carried out at the Yerkes Observatory by Professors
Frost and Adams, it appears that the proportion of binary to single
stars considerably exceeds Professor Campbell's earlier estimate.
If those giving helium-spectra are alone considered, there are most
probably as many of one kind as of the other. But why the distinction?
it may be asked. The answer is not far to seek. Helium stars are the
most primitive, and form the closest and most readily apparent systems.
The companions of more fully developed stars would be likely to give
less striking spectroscopic signs of their presence. A physically
double star must always remain such. There is no law of divorce by
which it can put away its companion, although their relations must
alter with time. But their alteration tends continually to enhance the
difficulty of their detection. For as the members of a pair are pushed
asunder by tidal friction their velocity slackens, and the tell-tale
swing of their spectral lines diminishes in amplitude, and finally, by
its minuteness, evades observation.

And since the majority of spectroscopic satellite-stars are very
imperfectly luminous, their eventual telescopic discovery, when far
enough away from their primaries to be optically separable from them,
would rarely ensue. It must then be concluded that half the stars in
the heavens (let us say) broke up into two or more bodies as they
condensed. What follows? Well, this. Half the stars in the heavens
were, from the first, incapacitated from becoming the centres of
planetary systems. To our apprehension, at least, it appears obvious
that a binary condition must have inhibited the operations of planetary
growth. These innumerable systems are doubtless organized on a totally
different principle from that regulating the family of the sun. The
nebular hypothesis, even in its most improved form, has no application
to them; the meteoritic hypothesis still less. Mathematical theories
of fluid equilibrium, combined with a long series of changes due to
tidal friction, afford some degree of insight into the mode of their
origin and the course of their development. Yet the analogy with the
earth-moon couple, which irresistibly suggests itself, is imperfect,
and may be misleading, owing to the wide difference in state between
plastic globes approaching solidification, and sunlike bodies radiating
intensely and probably gaseous to the core.[50]

The world of nebulæ confronts us with entire cycles of evolutionary
problems, which can no longer be treated in the offhand manner perforce
adopted by Herschel. The objects in question are of bewildering
variety; yet we can trace, amid their fantastic irregularities, the
underlying uniformity of one constructive thought. Nearly all show,
more or less markedly, a spiral conformation, and a spiral conformation
intimates the action of known or discoverable laws. Their investigation
must, indeed, be slow and toilsome; its progress may long be impeded by
the interposition of novel questions, both in physics and mechanics;
nevertheless, the lines prescribed for it seem definite enough to
give hope of its leading finally to a clear issue. And when at last
something has been fairly well ascertained regarding the past and
future of nebulous spirals, no contemptible inroad will have been made
on the stupendous enigma of sidereal relationships.

Its aspect, if we venture to look at it in its entirety, is vast
and formidable. Not now, as in former times, with a mere fragment
of creation--a single star and its puny client-globes, one of which
happens to be the temporary abode of the human race--but with the
undivided, abysmal cosmos, the science of origin and destiny concerns
itself. The obscure and immeasurable uncertainties of galactic history
invite or compel attention. We know just enough to whet our desire to
know a great deal more. The distribution of stars and nebulæ is easily
seen to be the outcome of design. By what means, we cannot but ask
ourselves, was the design executed? How were things ordered when those
means began to be employed? How will they be ordered when all is done?
For an ultimate condition has, presumably, not yet been reached. And
if not, agencies must be at work for the perfecting of the supreme
purpose, which are not, perhaps, too subtle for our apprehension.
Meanwhile, facts bearing on sidereal construction are being diligently
collected and sifted, and we shall do well to suspend speculation until
their larger import is made known.

The inquisitions of science do not cease here. They strive to penetrate
a deeper mystery than that of the scattering in space of stars and
nebulæ. What are they made of? is the further question that presents
itself. What is the nature of the primal world-stuff? Whence did
it obtain heat? By what means was motion imparted to it? If it be
urged that such-like topics elude the grasp of finite intelligence
and belong to the secrets of creative power, we may reply that we are
not entitled, nor are we able, to draw an arbitrary line, not to be
transgressed by our vagrant thoughts. The world has been, by express
decree, thrown wide to their excursions, and it is not for us to
restrict their freedom. We need not fear getting too near the heart of
the mystery; there is no terminus in the unknown to which we can travel
by express; in a sense, we are always starting, and never get nearer to
our destination. But that is because it retreats before us. We do, in
truth, advance; and as we advance the mists clear, and we see glimpses
beyond of imperishable order, of impenetrable splendour. Our inquiries
need not then be abandoned in despair at the far-reaching character
they have spontaneously assumed.

From the earliest times there has been a tendency to regard varieties
of matter as derivative. They have been supposed to be procured by
supramundane agency, or by the operation of inherent law, from some
universal undifferentiated substance. We moderns call that substance
'protyle,'[51] and believe ourselves to be in experimental touch
with it. The implications of this view we shall consider in the next
chapter.

FOOTNOTES:

[Footnote 50: _Cf._ Jeans, _Astrophysical Journal_, vol. xxii., p. 93.]

[Footnote 51: A term signifying 'first matter,' constructed from
corresponding Greek words by Roger Bacon, and revived by Sir William
Crookes.]




CHAPTER IX

PROTYLE: WHAT IS IT?


The notion of a primordial form of matter meets us at every stage
of cosmogonical speculation. It is the outcome of an instinctive
persuasion that, if we could only 'lift the painted veil' of phenomena,
the real business of the universe would be found to be proceeding in
the background, 'without haste or rest,' on a settled plan everywhere
the same; that uniformity is fundamental, diversity only incidental;
and that the transformations of the one simplified substance might be
represented by a single formula, the discovery of which would place
in our hands the master-key to the locked secrets of the universe.
Among untutored thinkers some familiar kind of matter, idealized and
generalized, commonly stood for the typical world-stuff. Water was the
first favourite. Thales, the 'wise man' of Miletus, procured his cosmos
by precipitation from an aqueous solution, and many savage tribes
have devised analogous expedients. Anaximenes regarded air as the
universal solvent; Heracleitus replaced it with fire, and set on foot
a scheme of what is now often designated 'elemental evolution.' From
the perpetual 'flux of things' he conceived that the four substances
selected by Empedocles as the bases of nature were not exempt, and a
fragment of his scheme survived in Francis Bacon's admission of the
mutual convertibility of air and water. In the main, however, the
author of the 'Novum Organum' adhered to the Paracelsian doctrine of
an elemental triad,[52] although he regarded the association of salt
as a fundamental 'principle' with sulphur and mercury as inept and
unnecessary.[53]

These twilight fancies faded in the growing light of chemical science;
yet the mental need that they had temporarily appeased survived,
and had somehow to be satisfied. An 'Ur-Stoff' was still in demand,
but the nineteenth century characteristically attempted to supply it
by weight and measure. Dalton's combining equivalents afforded the
warrant for Prout's hydrogen hypothesis. The problem to be faced was
to find a unit-atom by the varied combinations of which all the rest
of the chemical atoms might be formed. The condition indispensable to
be fulfilled was that their weights should be exact multiples of that
of the unit, and it came near to fulfilment by the hydrogen-atom or
semi-atom. It was, nevertheless, a case in which approximate agreement
was of no avail; the adverse decision of the balance finally became
unmistakable; and Prout discreetly fell back, in 1831,[54] upon the
expedient of deriving hydrogen itself from some body lower in the
scale. His hypothesis, in short, dissolved into a conjecture. It had
only emphasized the stipulation that the 'protyle' of the ancients must
be such as would serve not only for the physical unification of the sum
of things, but likewise as the substratum of all the chemical species.

Meanwhile, the theoretical search for it had been carried on in
widely different fields of inquiry. Laplace's speculations, no less
than Herschel's observations, had led to the conception of some
kind of 'fire-mist' as the genuine star-plasm. But its nature and
properties remained indefinite, or were assigned at the arbitrary
choice of adventurous cosmogonists. So the 'shining fluid' of space was
'everything by turns and nothing long,' until Sir William Huggins, in
1864, gave it spectroscopic individuality. The 'recognition-mark' of
nebulium is a vivid green ray, by the emission of which it is known to
have a concrete existence. Yet the little that has besides been learned
about it discountenances its identification with the _materia informis_
of antique philosophy. This we should expect to be the subtlest of all
substances. Professor Campbell, however, has gathered indications that
nebulium is denser than hydrogen. Its luminosity, at least, which is
invariably associated with that of hydrogen, spreads less widely in the
same formations; it is confined to a lower level. The nebulium-atom is
not, then, the chemical or the cosmical unit.

This evasive entity, or something that curiously simulates it, has
proved to be of less recondite origin. Sir William Crookes is amply
justified in claiming the venerable designation of protyle for the
'radiant matter' first produced in his vacuum-tubes nearly thirty years
ago. The discovery was astonishing and unsought, and its significance
has not yet been measured. Matter assumes the 'fourth state,' in which
it is neither solid, liquid, nor gaseous,[55] under the compulsion
of an electric discharge in high vacua. At an exhaustion of about
one-millionth of an atmosphere the manner of its transit abruptly
alters. Conduction gives way to convection. Luminous effects are
abolished. The tubes cease to glow with brilliant, parti-coloured
striæ; the poles are no longer marked by shimmering halos or
brushes; only a green phosphorescence is seen where the glass walls
of the receptacle are struck by the stream of projected particles.
They come, with half the velocity of light, exclusively from the
negative pole, the positive pole remaining inert. Hence the name
'cathode-rays,' bestowed by Goldstein on the carriers of electricity in
highly-exhausted bulbs.

These mysterious, sub-sensible agents possess certain very definite
properties. Their paths are deflected in a magnetic field; they can
traverse metallic films; and their investigation in the open, thereby
rendered feasible, has shown them to possess photographic efficacy,
and the faculty of breaking down electrical insulation; moreover, they
transport a negative charge of fixed amount, and have a determinate
momentum. They are, then, assuredly no mere pulsations of the ether;
unless our senses 'both fail and deceive us,' their quality is
material. Material, yet not quite with the ordinary connotation of the
term. The most essential circumstance about the cathode-rays is that
they remain unmodified by the chemical diversities of the originating
gases.[56] A hydrogen tube yields identically the same radiant matter
as an oxygen or a nitrogen tube. Here, then, at last we have within
our grasp undifferentiated substance--matter not yet specialized,
neither molecular nor atomic, matter destitute of affinities, exempt
from the laws of combination--matter in its inchoate, and perhaps
ultimate form; in a word, the far-sought protyle.

Already, in 1879, Sir William Crookes conjectured the infinitesimal
missiles propelled from the cathode to be the 'foundation-stones
of which atoms are composed.'[57] And in 1886 he pronounced them
more decisively to be the raw material of atoms, which, to Sir John
Herschel's apprehension, bore the unmistakable stamp of a 'manufactured
article.' Nor did his recent commentator refrain from attempting
distantly to divine the method of their construction, or from laying
his finger on the by-products and residues associated with it,[58]
although he felt compelled to relegate the cosmic factory to the edge
of the world, where inconceivable things may happen. All this, indeed,
seemed, in the late Victorian era, like mounting the horse of Astolfo
for a trip to the moon; and sane common-sense pronounced it fantastic
enough to 'make Democritus weep and Heracleitus laugh.'[59] But we have
since learned from Nature herself some tolerance of audacities.

Step by step the new order of ideas has irresistibly come to the front.
It owed its origin to Sir William Crookes's skill in producing high
vacua, and the consequent development in his tubes of radiant effects.
Then, in 1879, universal importance was claimed for them, and matter
in the 'fourth state,' by a revival of the dreams of the ancients,
expanded into a kind of visionary protyle. Philipp Lenard made the next
advance towards its actualization by slipping it, in 1894, through an
aluminium window, and watching its behaviour towards ordinary matter.
Two years later Röntgen rays made their entry on the scene; and before
the end of 1896 Becquerel, hurrying along the track of novelties, came
upon the momentous discovery of radio-activity.

A revision of ideas has ensued. Some time-honoured assumptions have had
to be discarded; so-called laws have been found to need qualification;
the old system of physics is consequently out of gear, and much time
and patient labour must be expended upon the adjustment of the new and
improved system destined to replace it. The leading and indisputable
fact of the actual situation is that a number of hitherto unsuspected
modes of energy have been disclosed as widely operative in nature.
All are of a 'radiant' character. They travel in straight lines with
enormous speed; they start from a material base, and produce their
several effects on reaching a material goal. Now, these effects are
closely alike, notwithstanding that the rays themselves are radically
dissimilar. Those of the cathodic kind are corpuscular. They consist
of streaming particles, each, according to Professor J. J. Thomson,
of about one-thousandth the mass of the hydrogen atom. Others, the
noted 'alpha rays,' are atomic; they are supposed to aggregate into
helium. Finally, the Röntgen variety are ethereal; they are composed
of light-vibrations reduced in scale, and augmented correspondingly in
frequency.

What is most remarkable is that these various forms of activity give
rise, by different means, to very much the same results. They are, in
fact, distinguishable only by careful observation. They possess in
common, though not to the same degree, the faculties of penetrating
opaque matter, of impressing sensitive plates, of evoking fluorescence;
while under the impact of cathode and Röntgen rays, as well as of
ultra-violet light, insulated electric charges leak away and evanesce.
There is, however, one clear note of separation between cathodic and
X rays in the sensibility of the former, and the indifference of
the latter, to magnetic influence. Thus alone, it would appear, is
electrified matter set apart from what we call ether. A magnet acts
only upon bodies carrying an electric charge; so that, if flying
corpuscles could be obtained in a neutral condition, the only tangible
distinction between the various kinds of rays would vanish. But this
is evidently impracticable. Indeed, advanced physicists abolish the
material substratum of the corpuscle, and assign its attributes to
the associated atom of electricity. It is, at any rate, undeniable
that the electrical relations of matter become more intimate as our
analysis of its constitution goes deeper. Ether, electricity, matter,
all seem to merge together in the limit; their differences ultimately
evade definition. So animal and vegetable life appear to coalesce in
their incipient stages, and strike apart with advance towards a higher
perfection.

The various branches of inorganic nature, too, possibly spring from
a common stock. Our powers of discrimination fail to separate them
as we trace them downward; but that may be because of the inadequacy
of the guiding principles at our command. A larger synthesis is
demanded for the harmonizing of multitudinous facts, at present
grouped incongruously, or left in baffling isolation; and it is
rendered increasingly difficult of attainment by the continual growth
of specialization. Year by year details accumulate, and the strain of
keeping them under mental command becomes heavier; yet what _can_ be
known _must_, in its essentials, be known as a preliminary to extending
the reign of recognised law in Nature.

Sooner or later, nevertheless, the wealth of novel experience recently
acquired will doubtless be turned to the fullest account. Just now,
we can grasp only tentatively its far-reaching import. That it bears
profoundly on the hoary problem of the genesis of visible things is
sufficiently obvious. The questions of what matter is, and of how it
came to be, have been cleared of some of the metaphysical cobwebs
involving them _ab antiquo_, and insistently crave definite treatment
by exact methods. We should, indeed, vainly aspire to reach--or to
comprehend, even if we could reach--an absolute beginning. To quote
Clerk Maxwell's words: 'Science,'[60] he wrote, 'is incompetent to
reason upon the creation of matter itself out of nothing. We have
reached the utmost limit of our thinking faculties when we have
admitted that, because matter cannot be eternal and self-existent, it
must have been created.' The discovery that atoms disintegrate into
corpuscles does not, then, bring us any nearer to the heart of the
mystery; but it is eminently suggestive as regards secondary processes.

Acquaintance with ultra-atomic matter, begun within the narrow
precincts of 'Crookes's tubes,' has advanced rapidly since 'radiology'
took its place among the sciences. For, from the time when Becquerel
first saw a plate darkened by the photogenic projectiles of uranium,
and Madame Curie sifted radium from the refuse of the mines of
Joachimsthal, the lines of proof steadily converged towards the
conclusion that chemical atoms are not only divisible, but that their
decay progresses spontaneously, irresistibly, in fire, air, earth, and
water, as part of the regular economy of Nature.

To explain further. Radio-active bodies are composed--according to
Rutherford's plausible hypothesis--of atoms in unstable equilibrium.
The gradual changes incidental to their own internal activities
suffice to bring about their disruption. And their explosive character
is obviously connected with unwieldy size, since uranium, thorium,
and radium, the three substances pre-eminent for radio-activity,
possess the highest atomic weights known to chemistry. The precarious
balance, then, of each of these complex, though infinitesimally
small systems is successively overthrown, regardless of external
conditions or environment, their constituent parts being hurled abroad
with the evolution of an almost incredible amount of energy. Their
products include cathode-rays; matter in the 'fourth state,' matter
a thousand times finer than hydrogen, is ejected in torrents from
the self-pulverized atoms of radium. Moreover, the issuing rays are
equivalent to currents of negative electricity. Each corpuscle bears
with it an electron, or is itself an electron, for the choice between
the alternatives is open. In either case, we are confronted with
matter apparently in its ultimate form; and to that form ordinary,
substantial bodies tend to become reduced. Electrons may fairly be
called ubiquitous. They occur in flames, near all very hot masses,
wherever ultra-violet light impinges on a metallic surface;[61] they
are freely generated by Röntgen and cathode-rays; they are the agents
of electrical transmission in conductors.

Everywhere throughout the universe, atoms are thus in course of
degradation into corpuscles. But no information is at hand as to
the scene or mode of their reconstitution. The waste and decay are
patent; the processes of compensation remain buried in obscurity.
Indeed, Sir William Crookes anticipates the complete submergence,
at some indefinitely remote epoch, of material substance in protyle,
the 'formless mist' of chaos. He assumes an identity between the past
state and the future, leaving, however, the present unexplained. The
break-up of matter, in fact, does not render its construction the
more intelligible. Running-down is an operation of a different order
from winding-up. It is an expenditure of a reserve of force. It needs
no effort; it accomplishes itself. But to create the reserve for
expenditure demands foresight and deliberate exertion; it implies a
designed application of power.

Now each atom is a storehouse of energy, representing the force
primitively applied to reduce some thousands of free electrons to the
bondage of a harmoniously working system. Its disruption is accompanied
by the dissipation of the energy previously accumulated in it; and that
atomic systems are not calculated for indefinite endurance is one of
the most surprising of modern discoveries. The secret of their original
construction is, none the less, impenetrable. That they are composed
of protyle--that their clustering members are corpuscles moving under
strong mechanical control--is more than probable. And the law of order
adumbrated by what are called the 'periodic' relations of the chemical
elements shows that their concourse was very far from being fortuitous.
But beyond this point there is no holding-ground for distinct thought.
We are ignorant, too, whether the process of building matter out of
protyle is at present going on, or was completed once for all in the
abysmal fore-time, decay being now definitive. Nor is it likely that
we shall ever succeed in capturing with recognition a brand-new atom
freshly minted for cosmical circulation.

FOOTNOTES:

[Footnote 52: First introduced by Basilius Valentinus. See Fowler's
edition of the _Novum Organum_, p. 576, note.]

[Footnote 53: Thus recurring, as Mr. Fowler remarked (_loc. cit._), to
Geber's earlier view.]

[Footnote 54: _Dictionary of National Biography_, vol. xlvi., p. 426.]

[Footnote 55: Crookes, _Philosophical Transactions_, vol. clxx., p.
163.]

[Footnote 56: J. J. Thomson, _The Discharge of Electricity through
Gases_, p. 195; _Philosophical Magazine_, vol. xliv., p. 311.]

[Footnote 57: _Science_, June 26, 1903.]

[Footnote 58: _Proceedings of the Chemical Society_, March 2, 1888.]

[Footnote 59: _Times_, March 30, 1888.]

[Footnote 60: _Encyclopædia Britannica_, article 'Atom.']

[Footnote 61: Fleming, _Proceedings Royal Institution_, vol. xvii., p.
169.]




CHAPTER X

UNIVERSAL FORCES


We find it equally impossible to conceive of matter without force,
as of force without matter. The two modes of action, or of being,
are inseparable. Yet our minds strongly distinguish between them;
they impart a dual aspect to the world. Phenomena are not simple
manifestations of disembodied energy, if such a thing could be; they
have a substantial basis which, nevertheless, eludes apprehension,
and seems to slip away into nothingness if we try to empty it of its
immaterial contents. Nor do these energize in the void. They and the
bodies they animate are knowable only in combination, and exist, to our
apprehension, only on the condition of mutual dependence. All we can do
towards discriminating them is to fix our attention predominantly on
one or the other side of things, and so facilitate thought by drawing
ideal lines of demarcation.

Just as there are many forms of matter--all springing, we are led to
believe, from an undifferentiated, fundamental world-stuff--so there
are various kinds of force, reducible, possibly without exception, to
one universal principle. Their correlation, indeed, has been already
in large measure demonstrated; heat, light, electricity, and kinetic
power are known to be equivalent and interchangeable; but there
are outstanding activities, which resist assimilation, and seem to
originate under different conditions from the rest. Forces manifest
themselves chiefly through attractive and repulsive effects, varying
in accordance with their natures and the modification of attendant
circumstances. The minute particles of matter, for instance, cohere;
they cling together tenaciously; yet no pressure avails to bring them
into actual contact; at a certain point of mutual approach, they
develop an invincible power of resistance to any further encroachments
upon their separate molecular domains. And it is this faculty which
gives to matter its distinctive property of hardness. It is rendered
tangible to sense just by its recalcitrance to constraint.

Neither the mode of operation nor the nature of the forces by which
molecules are organized into masses is known; while the power acting on
the masses thus organized, and regulating by its action the mechanism
of the universe, is fully as baffling to comprehension. Wonder at
its results is blunted by familiarity; presented to us as novelties,
they should be pronounced to outrage reason. The relations of gravity
are of the utmost simplicity; and they are, on that very account,
supremely perplexing. They are governed by one steadfast law, the same
everywhere, and under all varieties of conditions within the range of
experiment or precise observation. It governs impartially every kind
and quality of matter, taking no notice of its states or combinations,
ignoring its subjection to chemical, thermal, magnetic, or electrical
influences. Gravity is not only indifferent, but inevitable and
inexorable; there is no resisting its sway; no screen serves as a
shelter against its persuasions; it spreads equably in all directions,
becoming enfeebled, like wave-motion, in the strict proportion of its
diffusion from a centre over successive spherical surfaces. Its most
singular peculiarity, however, is its apparent unconcern with time. The
gravitational pull is virtually instantaneous; its transmission--if
it be transmitted--takes place millions of times faster than that of
light; the finest tests have, so far, failed to elicit symptoms of
delay. These would be found in minute discrepancies between calculated
and observed perturbational effects in the heavens.

The action of gravity, if propagated with finite velocity, should
differ for bodies preserving an invariable distance from its source,
and for bodies travelling towards or away from it. Their movement would
modify the law of attraction.[62] Yet, up to the present, it has proved
impossible to detect the slightest deviation from the plain rule of
inverse squares. Again, the penetrative faculty of this strange force
seems absolute and unlimited. We know by ordinary experience that we
cannot diminish the weight of an object by interposing any kind of
shield between it and the earth; and no refinement in experimentation
avails to alter this result. That it is so, is a fortunate circumstance
for the harmony of the world. We can dimly imagine the riot of
confusion that would ensue if a transiting globe could intercept the
attraction, as it does the light, of a central governing mass. But
from the wonderfully adjusted universal order gravitational eclipses
are excluded; nor does the densest body throw even the faintest
gravitational shadow.

The nature of a power so singularly conditioned is almost
inconceivable; yet attempts have not been wanting to fathom the mystery
that surrounds it. Professor Osborne Reynolds, in the Rede Lecture for
1902, claimed to have arrived at 'a complete, quantitative, purely
mechanical explanation of the cause of gravitation,' based on the
'dilatancy' of a granular medium in close piling. But his working model
of the universe will probably be remembered only as a lesson in the
'inversion of ideas,' showing that with skill and ingenuity a fairly
concordant outcome of phenomena may be derived from antagonistic
hypotheses. In this author's view matter is equivalent to a deficiency
of mass, the spaces where his cosmic grains are relatively few,
because their arrangement is out of gear, being driven towards one
another by the pressure of the surrounding medium, in which they are
compactly stowed, and therefore numerous. Thus, the acting forces in
nature are made to depend upon the compression by the denser medium
of interspatial tracts of rarer consistence, forming what we call
matter. The theory is difficult, if not impossible of acceptance, not
because it involves the overthrow of conceptions which may be rooted in
habitual modes of thought, rather than in absolute truth, but because
of its startling postulates and large vacuities. To be valid, it should
be complete; and there are obvious chasms in the vast expanse of ground
which it covers with surprising, though only partial success.

The _multa renascentur_ of the poet is verified by the revolutions no
less of thought than of speech. Flights of minute material particles
have served the turn of theorists often, and in more ways than one,
and have as frequently been consigned to discredited oblivion; but
they are in vogue once more. George Louis Lesage, of Geneva, devoted
sixty-three of the seventy-nine years of his life, which came to
an end in 1803, to the elaboration of a mechanical rationale of
gravity, first given to the world in the _Transactions_ of the Berlin
Academy of Sciences for 1782, and with details of amplification in
his _Traité de Physique Mécanique_, edited by Pierre Prévost in
1818.[63] The explanation it offered of molar attractions was by the
supposed unceasing impacts of 'ultramundane corpuscles,' speeding in
countless numbers and at fabulous velocities, from nowhere everywhere,
and thus enforcing the mutual approach of masses of gross matter.
This involved the supposition of an infinitesimal screening effect,
producing a small inequality in the strength of the bombardment on
the sheltered and unsheltered sides of the bodies exposed to it. This
inequality, in fact, was taken to be the _causa causans_ of gravity.
Yet its production encountered a difficulty. There was required for
it a trifling degree of opacity in every kind of matter, while
perfect gravitational transparency is asserted by the most delicate
observations. Lesage, then, reduced the arrests laid upon his particles
to a minimum; one in ten thousand, for instance, might at the utmost
be intercepted by the terrestrial globe.[64] Even this insignificant
minority, however, would suffice, through the surrender of their
momentum to the impeding bodies, to endow them with the noted property
of gravitation.

Clerk Maxwell urged the objection that the accompanying loss of kinetic
energy by the corpuscles should, if transformed into heat, render all
gravitating bodies white-hot. But Professor J. J. Thomson holds that
the transferred corpuscular energy might, instead of reappearing in
thermal form, be converted into some highly penetrative species of
radiation capable of escaping unperceived into surrounding space.[65]
'A simple calculation,' he adds, 'will show that the amount of kinetic
energy transformed per second in each gramme of the gravitating body
must be enormously greater than that given out in the same time by
one gramme of radium.' This consequence of Lesage's theory takes
one's breath away; the 'fables of the Talmud' seem, by comparison,
easy of belief; nevertheless, Lord Kelvin[66] declared it in 1873 to
be more complete in the expository sense, and not more arduous in its
assumptions, than the kinetic theory of gases.

Its fundamental postulate, at any rate, has been curiously verified in
the course of recent researches into the arcana of physics. Entities
in some degree corresponding to the ultramundane corpuscles of the
Genevese philosopher do actually exist. Electrons are being continually
expelled from bodies in all parts of the universe; they issue forth
under all conceivable conditions and in unlimited numbers. Space is
perhaps thronged with them; no material object can be exempt from their
multitudinous buffetings, which are beginning to be taken account of
in many cosmical speculations, and cannot certainly be ignored in
efforts to solve the most obvious to superficial apprehension, the most
intricate on a profound consideration, of all cosmical problems. But
there is one fatal objection to an electronic theory of gravitation.
The agency appealed to travels too slowly to be available for the
required purpose. The velocity of light, there is reason to believe,
sets a limit impossible to be surpassed or even attained by the
velocity of electrons; yet it is incomparably smaller than the rate of
gravitational transmission.

Tisserand estimated at six million times the quickness of luminous
travel the minimum speed at which the sun's attraction must be
propagated in view of the imperceptibility in planetary observations
of effects corresponding to a time-inequality;[67] and this value
may be taken as authentic. So colossal a discrepancy excludes any
kind of impact-rationale of the mutual pull of heavy masses; Lesage's
corpuscles remain 'ultramundane'; their identification with known
atoms or sub-atoms appears to be precluded; no products of ionic
disintegration possess the qualities necessarily to be ascribed to them.

We turn, then, inevitably to the menstruum of mysteries, the bank of
the insolvent in speculation, to the all-serviceable ether. Ethereal
radiations exercise an impulsive power; light-pressure has secured
a recognised status among cosmic agencies; and every vibrational
system of the luminous type undoubtedly shares the faculty by which
light tends to drive minute particles forward along the lines of its
propagation. Professor J. J. Thomson, accordingly, considered that but
for the drawback of their insufficient velocity, 'very penetrating
Röntgen rays' might with advantage be substituted for corpuscular
streams as the cause of gravity.[68] They would, in some slight
degree, be absorbed by encountered masses, to which they would impart
a proportionate amount of momentum. Two bodies mutually shadowing one
another would, under such circumstances, be drawn together with a force
varying as the inverse square of distance; and if further they absorbed
the impinging rays strictly in the measure of their density (as
observation shows to be approximately the case), the attraction would
increase in the same ratio as the product of their masses. But Röntgen
rays travel with the precise velocity of light; they are, in truth,
ultra-invisible light; and they must hence be regarded as hopelessly
incompetent to explain an influence transmitted at least six million
times more rapidly.

This was fully admitted by Dr. H. A. Lorentz,[69] who, five years
ago, weighed the vibrational hypothesis of gravity in the balance of
rigorous calculation, and found it wanting. His equations yielded the
unexpected result that, if its postulates were granted, the noted
attractions between massive bodies could subsist only on the terms
of an incessant waste of electro-magnetic energy. But this is of
course inadmissible. The theory involving such a consequence stands
self-condemned, to say nothing of the wholly inadequate rate of
propagation afforded by it.

Impulsion hypotheses, whether by corpuscles or rays, being hopelessly
discredited, Dr. Lorentz reverted to a half-forgotten speculation
by Mosotti, which, though sixty years old, struck him as capable of
being adapted to modern requirements. It was of an electrical nature,
and, in the novel shape given to it, supposed gravitational action to
depend upon strains of the ether due to the disturbing effects of the
positive and negative ions constituting ordinary matter. These 'states'
of the medium are distinct in kind; they cannot neutralize one another;
and the familiar law of attraction represents their resultant effect.
To bring it about much has to be taken for granted; yet the hypothesis
can lay claim to one singular prerogative. Although the disturbances
invoked by it traverse the ether with no more than the standard speed
of light, it appears from Dr. Lorentz's investigation that, owing to
certain modifications in the properties of the medium produced by
moving matter, the planetary perturbations betraying loss of time in
gravitational transmission would, on the electrical theory, be so
small as to evade detection. As regards this crucial point, the Dutch
physicist has hit upon a felicity of explanation entirely original,
and, as it were, unsought.

An 'undulatory theory' of gravity, adumbrated, rather than advanced
by Mr. Whittaker in 1902,[70] excited hopes that the ideal aim of
science--a complete unification of the forces of Nature--might at
last be within reach. Based upon a striking mathematical research,
it exhibited the attraction between masses as, in a manner, the
integration of innumerable wave disturbances, propagated at a rate
not strictly definable, but perhaps immensely surpassing that of
gravity. No suggestion was made as to the primary mode or cause of
agitation, yet it seemed much to learn that the medium we are cognizant
of in space might be capable of transmitting the pull of gravity.
Unfortunately, however, the physical foundation of this reassuring
congruity proves to be weak or unsound. The mathematical mill works
magnificently, but the grist put into it is of dubious quality.
Stripping Mr. Whittaker's result of its purely analytical form, Dr.
Johnstone Stoney showed that an assumption of extreme improbability lay
concealed in his equations, which could not, he concluded, be seriously
taken to correspond with the reality of things.[71]

There would then seem to be no alternative but to accept _ad interim_
the electro-dynamical view of the nature of gravity. If not true, it
is at least not obviously false. Through its subtlety it escapes direct
confutation. And the method of exclusions, by eliminating competitors,
has left it in virtual possession of the field.

Nothing is more curious in the history of recent science than the
continual and irresistible growth which it records in the importance
of electrical phenomena. All others tend to become merged in them; the
most varied data of experience claim to be translated into electrical
terminology. They are not, assuredly, rendered more intelligible by
the process, but it at any rate abolishes the confusion incidental
to multitudinous points of view. Thus, in the last resort, we find
electrical forces (if they may be so designated) swaying the world.
What they essentially consist in, we cannot tell; the utmost that may
reasonably be hoped for is to arrive at a clear conception of modes
of action reduced to antagonistic stresses, by which the play and
counterplay of the universe may be kept up. And to this extent we find
it possible to understand how electricity works the ethereal machinery.
It is strongly dualistic. The nearer we get to the foundations of
nature, the more sharply positive and negative charges appear to be
differentiated.

The opinion is nevertheless held by some inquirers that negative
electricity is the only substantive kind, and that its complement is
ordinary matter deprived of some of its negative particles. This, in
fact, revives Franklin's 'one fluid theory,' only with the substitution
of negative for positive electricity as the active principle.[72] But
we are met by the doubt whether 'ordinary matter' can be said to exist
in and by itself. If it do, the mode of its existence becomes more and
more baffling to comprehension, as the association of mass with charge
makes its way into the foreground of thought.[73] Moreover, a charge
is, or produces, a 'state of the ether' (to use the unsatisfactory
current phrase); and the ether being capable of opposite distortions,
the effects upon it of opposite charges are contrary and similar,
though perhaps not equivalent; whence the 'two fluid theory' obtains
a _primâ facie_ recommendation as the simplest, though a crude
interpretation of electrical phenomena. These are ubiquitous; destitute
though we are of sense-organs for their perception, we still indirectly
recognise their presence on every side. If the unification of the
forces in nature be attainable, the unifying formula will doubtless
be derived from them. Electricity is the _mot de l'énigme_; yet it is
itself the most inscrutable of enigmas.

FOOTNOTES:

[Footnote 62: Lorentz, _Proceedings Amsterdam Academy_, 1900, p. 565.]

[Footnote 63: Sir W. Thomson, _Philosophical Magazine_, vol. xlv.,
fourth series, 1873. Many hints have been taken, in what is above
written, from this valuable paper.]

[Footnote 64: Sir W. Thomson (Lord Kelvin), _loc. cit._, p. 323.]

[Footnote 65: _Electricity and Matter_, p. 159.]

[Footnote 66: _Loc. cit._, p. 331.]

[Footnote 67: _Traité de Mécanique Céleste_, tome. iv., p. 495.]

[Footnote 68: _Electricity and Matter_, p. 160.]

[Footnote 69: _Proceedings Amsterdam Academy of Sciences_, 1900, p.
559.]

[Footnote 70: _Monthly Notices_, vol. lxii., p. 619; vol. lxiii., p.
258.]

[Footnote 71: _Monthly Notices_, vol. lxiii., p. 424.]

[Footnote 72: J. J. Thomson, _Electricity and Matter_, p. 88.]

[Footnote 73: _Ibid._, p. 47.]




CHAPTER XI

THE INEVITABLE ETHER


Ether is the fundamental postulate of physics. Its existence, nowise
apparent, is in all manner of ways implied. The properties that must
be assigned to it are certainly arduous of conception. We need the
aid of forced analogies to enable us to realize, even imperfectly and
indistinctly, the mode in which it discharges functions obviously
somehow discharged. But in the last resort everything is obscure;
if our thought-borings go deep enough, they always reach the
incomprehensible.

The original ether was the 'quintessence' of the ancients--a kind of
matter vaguely imagined as pure and incorruptible enough to serve for
the raw material of the heavenly bodies, the four common elements being
reserved exclusively for sublunary use. The distinction, however,
eventually broke down. All the spheres, from the _primum mobile_ to
the very surface of our low earth, are pervaded by a subtle mode of
action, demanding for its transmission machinery of a finer kind than
could be constructed out of gross everyday matter. The phenomena of
light, when they came to be attentively studied, imperatively required
a medium, universally diffused, evasive to sense, accessible only by
processes of reasoning. Hooke and Newton accordingly brought the ether
through the Horn Gate of dreamland into a region of reality, where it
became a subject of legitimate speculation to men of science. The task,
nevertheless, of definitely specifying its qualities was not taken
seriously in hand until the beginning of the nineteenth century, when
the establishment of the undulatory theory of light supplied tangible
holding-ground for ideas regarding the vehicle of propagation, and
rendered the ether a fixture of thought.

A great deal is demanded from it. We cannot afford to set up an
establishment of ethers; one factotum must suffice. Incongruous offices
are devolved upon it. It has to be Atlas and Mercury in one. It is
the universal supporter and the universal messenger. Whatever kind of
influence or form of energy can pass from world to world is conveyed
by its means. If 'action at a distance' be inadmissible (as Newton
himself held it to be), the pull of gravity must be exerted through
a medium; and common-sense insists upon its identification with the
transmitting medium of light, as well as upon the identification of
that with the transmitting medium of electricity. A genuine conformity
to these demands of reason is vouched for, not only by Hertz's
discovery that an electrical explosion starts an undulatory disturbance
indistinguishable, except in scale, from luminous waves; but also by
Dr. Lorentz's indicated conclusion that strains of the same ethereal
essence bear the all-pervading mandates of gravity. The unity of the
medium may, then, be regarded as finally ascertained; the complex
interactions of sundry different 'fluids' need no longer be taken into
account. To provide one with the capabilities implied by the services
we perceive it to render is, indeed, a sufficiently formidable task.

In popular apprehension the ether of space figures as a finer kind of
air. No idea could be more misleading. The elasticity by which air
transmits the longitudinal waves of sound is totally different from the
elasticity by which ether propagates the transversal waves of light.
Air yields to pressure; disturbance hence produces in it undulatory
condensations due to oscillations of the gaseous molecules along the
line in which the audible commotion travels. Ether, on the contrary,
appears to be entirely incompressible; it conveys no vibrations
directed lengthwise. Now this is extremely perplexing. We have no
experience of a kind of matter absolutely rigid to pressure, while
yielding, albeit with intense reluctance, to distortional stresses.

A jelly-like solid makes the nearest, though a very distant, approach
to fulfilling the indispensable conditions; and a solid ether was
accordingly in vogue until long past the middle of the nineteenth
century. For a solid it had very peculiar qualities; that, for
instance, of offering no resistance to motion. It was, in truth,
obviously a mere temporary expedient--a scientific fiction which
might pass muster until replaced by something corresponding less
distantly with the fundamental fact. At last, on the advent of the
electro-magnetic theory of light and the modified conceptions which it
brought in its train, the solid ether withdrew behind the scenes. Its
properties, though inconsistent and unconvincing, had not been chosen
arbitrarily; they were imposed by the necessities of the situation;
and when these varied, speculators naturally had recourse to fresh
inventions.

The most plausible is that of a medium neither solid, liquid, nor
gaseous in the ordinary sense, but in the ideal state of a 'perfect
fluid.' Out of such an ether Lord Kelvin, with exquisite ingenuity,
constructed his 'vortex-atoms,' which 'had their day and ceased to be.'
Other ideas now prevail. 'The present tendency of physical science,'
the late Mr. Preston wrote in 1890,[74] 'is to regard all the phenomena
of Nature, and even matter itself, as manifestations of energy stored
in the ether.' The more closely we look into the things around us,
the more strongly the persuasion is forced upon us that what we call
ether, electricity, and matter are really varied forms of one primal
substance.

Two comprehensive schemes of molecular physics, resting upon the basis
of this unifying thought, have lately been elaborated--one by Dr.
Larmor, the other by Professor Osborne Reynolds. They have nothing in
common beyond the largeness of their synthesis. In every respect they
are radically unlike, save in regarding the intangible ether as the
one material reality. Dr. Larmor, however, is not quite confidently
explanatory. He presents no cut-and-dried theory of the universe; its
haunting mysteries are not ignored in his efforts to rationalize them.
He is vividly aware of the difficulties besetting the endowment of the
ether with the type of elasticity which it is recognised to possess. He
can only surmise that it results from particular modes of motion--from
'kinetic stability' ensuing upon a special dynamical state. The medium
may thus be thought of as pervaded by 'a structure of tangled or
interlaced vortex filaments, which might resist deformation by forming
a stable configuration.'[75] But the details of any such scheme of
action are evidently far too intricate to be easily unravelled; what
concerns us here is to point out that no simple structureless fluid can
avail to maintain cosmical communications.

Reduced to its lowest terms, Dr. Larmor's conception of the ether is
that of a 'rotationally elastic medium.'[76] In other words, it resists
being turned round an axis. The forces continually acting upon it are
nevertheless of a gyratory nature; and hence arise strains, betrayed
to our apprehension by electrical manifestations. Each 'electron' is
held to be the nucleus of some kind of distortion or displacement,[77]
and carries with it, as it moves, a field of force. Out of these 'point
charges' material atoms are variously built up. They are 'structures in
the ether,' encompassed by 'atmospheres of ethereal strain,' not--as
they were formerly taken to be--'small bodies exerting direct action at
a distance on other atoms according to extraneous laws of force.'[78]
Obviously the new view brings to the front extremely subtle questions
regarding the nature of 'dynamical transmission'[79]--what the
propagation of energy essentially consists in, and by what mechanism
it is effected; and they are, for the present, unanswerable.
Electricity is, on the theory we are attempting to sketch, positive
or negative according to the direction of the originating strain.
A positive electron might be imagined to resemble a spiral nebula
of the right-handed sort, a negative one a left-handed spiral, or
_vice versâ_. The analogy is, perhaps, fanciful; yet it helps towards
obtaining a mental picture of objects which, insignificant and elusive
though they appear, may be the initials and ultimates of this strange
world.

The forces, at any rate, by which it is at present kept going are
evoked _ad libitum_ by the pioneers of modern research from the
ethereal plenum. The actualities of matter are potentialities in the
ether. 'All mass,' in Professor J. J. Thomson's opinion, 'is mass of
the ether, all momentum, momentum of the ether, and all kinetic energy,
kinetic energy of the ether.[80] Only if this be so,' he adds, 'the
density of the ether must be immensely greater than that of any known
substance.'

The condition is startling, but in dealing with such subjects we must
be prepared to meet with anomalies. They come, as the ghosts appeared
to Odysseus in Hades, at first one by one, then in an awe-inspiring
swarm. Yet, in spite of the perplexities they occasion, we can discern,
with growing sureness of insight, the amazing reality of the universal
medium. It is, in a manner, the only reality. For what is manifest to
sense is subject to change. We can conceive that the visible framework
of material existence might crumble and dissolve, like 'the baseless
fabric of a vision,' into seeming nothingness. But a substance that is
inapprehensible is, to our limited ideas, imperishable. The ether is
assuredly the seat of intense activities, which lie at the root, most
likely, of all the processes in Nature. An absolutely uniform medium,
however, can scarcely be imagined to energize or react. Some kind of
heterogeneity it must possess; and the heterogeneity produced, in Dr.
Larmor's view, by strains, is associated, in Professor Reynolds's
theory, with intrinsic texture.

The 'Sub-Mechanics of the Universe' are here made to depend upon the
fitting together of ineffably small, ideally rigid grains. A misfit
gives rise to matter, which might hence be defined as 'ether out of
joint'; and the misfit can be propagated endlessly from one range of
granules to the next. This propagation through the ether of an abnormal
arrangement of its constituent particles, without any transference of
the particles themselves, explains the phenomena of matter in motion. A
concrete existence belongs to the ether alone. It is composed of round
aboriginal atoms, the diameters of which measure the _seven hundred
thousand millionth part_ of the wave-length of violet light.[81] They
are packed closely together, yet not so closely but that free paths are
left to them averaging in length the _four hundred thousand millionth
part_ of their diameters.

This inconceivably small relative motion suffices, nevertheless, to
render the medium elastic; is, indeed, 'the only cause of elasticity
in the universe, and hence is the prime cause of the elasticity of
matter.' The medium so formed is ten thousand times denser than
water; it exerts a mean pressure of 750,000 tons on the square inch;
the coefficient of its transverse elasticity is 9 + 10^{24} (in C.G.S.
units); which gives a velocity of transmission identical with that
of light for vibrations of the same type, while longitudinal waves
are propagated 2·4 times more rapidly. The scheme further includes a
plausible rationale of gravity and of electrical effects; so that there
is much to warrant the claim of its author to have excogitated 'the one
and only conceivable purely mechanical system capable of accounting for
all the physical evidence, as we know it, in the universe.'

The machine, to be sure, lacks motive power; but that is a want which
no human ingenuity can supply. Its source is obscured in the primal
mystery of creation. And as regards the preliminary assumptions
required for the constitution of an atomic ether, inclined though we
might be to cavil at them, we should, perhaps, act more wisely in
following Dr. Larmor's advice by abstaining from attempts to explain
'the simple group of relations which have been found to define the
activity of the ether. We should rather rest satisfied,' he tells us,
'with having attained to their exact dynamical correlation, just as
geometry explores or correlates, without explaining, the descriptive
and metric properties of space.'[82] Yet one cannot help remarking
that the properties of space are not ordinarily modified to suit the
needs of demonstration, while those of the ethereal medium are varied
at the arbitrary discretion of rival cosmogonists. In the future, when
they come to be more clearly ascertained, they will, perhaps, form
the basis of a genuine new science. Already, the study of ethereal
physics excites profound interest and attention. Nor is it possible to
ignore the gathering indications that it will impose qualifications
upon principles consecrated by authority and hitherto regarded as
fundamental.

The grand modern tenet of the conservation of energy, for example,
may need a gloss; it may prove to be admissible only with certain
restrictions. The second bulwark of the scientific edifice is even more
seriously undermined. For the 'strain theory' of atomic constitution
necessarily includes the conception of opposite distortions
corresponding to positive and negative electricity. And the further
inference lies close at hand that these, by combining, may neutralize
one another. The coalescence, then, of a positive and negative electron
should result in the smoothing out of the complementary strains they
stand for; and there would ensue the annihilation of a pair of the
supposed ultimates of matter. The event might be called the statical
equivalent of the destruction of light through interference. That
its possibility should be contemplated even by the most adventurous
thinkers is a circumstance fraught with meaning as to the subversive
tendencies of recent research.

Already, in May, 1902, Professor J. A. Fleming[83] pointed out that 'if
the electron is a strain-centre in the ether, then corresponding to
every negative electron there must be a positive one. In other words,
electrons must exist in pairs of such kind that their simultaneous
presence at one point would result in the annihilation of both of
them.' The consequence thus viewed in the abstract finds concrete
realization, if Mr. Jeans's suggestion be adopted,[84] in the processes
of radio-activity, which possibly consist 'in an increase of material
energy at the expense of the destruction of a certain amount of matter.
There would, therefore, be conservation neither of mass nor of material
energy.'

No longer ago than at the opening of the present century such notions
would have been scouted as extravagant and paradoxical; now there
is no escape from giving them grave and respectful consideration.
Scientific reason has ceased to be outraged by hypotheses regarding
the disappearance of mass and the development of energy. Mass and
energy may, after all, be interchangeable; they are, at any rate, kept
less rigidly apart in our meditations than used formerly to be the
case. Nor can we assert with any confidence that partial subsidences
into or emergences from the surrounding medium are for either a sheer
impossibility; the universal framework, on the contrary, presents
itself to us in the guise of an iridescent fountain leaping upward
from, and falling back towards, the ethereal reservoir.

To the very brink of that mysterious ocean the science of the twentieth
century has brought us; and it is with a thrill of wondering awe that
we stand at its verge and survey its illimitable expanse. The glory
of the heavens is transitory, but the impalpable, invisible ether
inconceivably remains. Such as it is to-day, it already was when the
_Fiat Lux_ was spoken; its beginning must have been coeval with that
of time. Nothing or everything, according to the manner in which it
is accounted of, it is evasive of common notice, while obtrusive to
delicate scrutiny. Its negative qualities are numerous and baffling.
It has no effect in impeding motion; it does not perceptibly arrest,
absorb, or scatter light; it pervades, and may even share in the
displacements of gross matter; yet its motion (if it do move), is
without effect on the velocity of light.

Looking, however, below the surface of things, we find the
semi-fabulous quintessence to be unobtrusively doing all the world's
work. It embodies the energies of motion; is, perhaps, in a very real
sense, the true _primum mobile_; the potencies of matter are rooted in
it; the substance of matter is latent in it; universal intercourse is
maintained by means of the ether; cosmic influences can be exerted only
through its aid; unfelt, it is the source of solidity; unseen, it is
the vehicle of light; itself non-phenomenal, it is the indispensable
originator of phenomena. A contradiction in terms, it points the
perennial moral that what eludes the senses is likely to be more
permanently and intensely actual than what strikes them.

FOOTNOTES:

[Footnote 74: _Theory of Light_, second edition, p. 28.]

[Footnote 75: _Encyclopædia Britannica_, vol. xxv., p. 106.]

[Footnote 76: _Report of the British Association_, 1900, p. 626.]

[Footnote 77: _Æther and Matter_, p. 26.]

[Footnote 78: _Nature_, vol. xlii., p. 453.]

[Footnote 79: Larmor, _Report British Association_, 1900, p. 625.]

[Footnote 80: _Electricity and Matter_, p. 51.]

[Footnote 81: _The Structure of the Universe_, Rede Lecture, June 10,
1902, p. 14.]

[Footnote 82: _Nature_, June, vol. lxii., p. 451.]

[Footnote 83: _Proceedings of the Royal Institution_, vol. xvii., p.
177.]

[Footnote 84: _Nature_, vol. lxx., p. 101.]




CHAPTER XII

THE FORMS OF NEBULÆ


Sir William Herschel's celestial surveys first made the classification
of nebulæ practicable. Until he began grinding specula at Bath very
few such objects were known, and those too imperfectly for the
effectual discrimination of their differences. Arrangement presupposes
comparison, and comparison some variety of specimens to be compared,
which became available only through Herschel's scrutiny. The rapidity
and penetrative power of his observations in this field almost passes
belief. He detected with discernment. Discovery and enrolment did not
satisfy him; he was, besides, keen to note analogies and contrasts,
likenesses and dissimilitudes. He could not see without at the same
time setting in order what he saw; and the law of order that commended
itself to him was founded on an evolutionary principle. The contents
of the heavens seemed to fall spontaneously, as he regarded them,
into genetic sequences; and the nebulæ with particular facility. The
criterion adopted was that of progressive condensation. Development
must clearly, he judged, be attended by contraction and local
brightening. Diffused milky tracts represented cosmic formations in
their most rudimentary form; they assumed, through the unremitting
action of gravity in drawing their particles together, a more compact
texture, more definite shapes, and a heightened lustre.

But things have changed somewhat in aspect during the last hundred
years. Herschel's simple regulative plan, although of unquestioned
validity, needs to be supplemented and controlled. Much auxiliary
knowledge has been acquired since it was formulated. In attempting
to estimate the comparative antiquity of nebulæ, we no longer depend
exclusively upon one set of indications. The conclusions drawn from
their immediate inspection can at least be checked by the study of
their spectra and distribution.

The Milky Way might be figuratively described as the nursery-garden
from which the parterres of the universe are stocked. A primitive
condition is usually assigned, not without good reason, to any class
of objects markedly tending to collect in its plane. And this is the
case with gaseous or 'green' nebulæ. Moreover, their materials appear
to be in a highly elementary state (if it be permissible to speak of
one kind of matter as more elementary than another); their spectra
including no rays due to metallic incandescence, but mainly those
of nebulium, hydrogen, and helium. These substances, inconceivably
attenuated, constitute the vast irregular formations placed by
Herschel at, or near, the start of cosmical development. And so far
he has been justified by the outcome of modern research. But he has
not been justified in his description of planetary nebulæ as 'very
aged, and drawing on towards a period of change or dissolution.' For,
despite their determinate shape and definite boundaries, they do not
appreciably differ in composition from nebulæ of the irregular class,
and must be reckoned as, in a manner, coeval with them.

There is, on the whole, a concurrence of evidence that gaseous nebulæ
are at a very early stage of growth. They are the least elaborated of
sidereal objects; they seem, many of them, barely to have crossed the
threshold of creation. Yet their mutual relations in time are by no
means obvious. They cannot easily be disposed in any kind of rational
sequence. Each of the great nebulæ, at any rate, exhibits features and
occupies a position shared by none of its fellows. The most discerning
cosmologist cannot pretend to say that the Argo nebula, say, is of
greater or less antiquity than the Orion or the 'America' nebula.
They are individual growths, simultaneous, not successive. The line
of development suggested by their relationships is rather towards the
formation of star-clusters than of diverse nebular species. Thus, the
Pleiades illustrate not improbably the future condition of the Orion
nebula, the contained stars having gained predominance over their misty
envelopments, though fragmentary swaddling-bands, later, presumably, to
be shaken off, still adhere to many of them.

Planetary nebulæ have much more in common than irregular nebulæ, and
their minor varieties might, with some plausibility, be associated with
differences in relative age. They are marked chiefly by the character
of the nuclear star which, in nearly all such objects, appears to act
as the pivot of the surrounding vaporous structure. The supposition
lies close at hand that it is designed as a provision for the
nourishment of the star--that the star gains in mass and light at the
expense of the nebula, which it is eventually destined wholly to absorb
and supersede. On this view, planetaries like the green glow-lamp at
the pole of the ecliptic (N.G.C. 6,543) should be regarded as the
most advanced, while Webb's planetary in Cygnus (N.G.C. 7,027) would
exemplify an inchoate condition. In the former the central star is of
9·6 magnitude, and sharply stellar; in the latter it is double and
diffuse,[85] perhaps a wide binary system in embryo.

The question is, however, still open as to the real nature of the
connection between planetaries and their central stars. The pabulum
theory is a promising conjecture; but no facts with which we are
acquainted stringently enforce it. Ideas on the subject will need
complete revision if the traces of spirality noted from time to time in
some of these peculiar objects prove to be of radical significance. The
_oculi_, distinctive of the 'Owl nebula' (N.G.C. 3,587), as originally
shown by the Parsonstown reflector, consisted of luminous traceries
coiled round _two_ interior stars,[86] but the appearance was either
due to illusion, or became effaced by change, since the camera has
refused to endorse it as genuine. The 'helical' planetary in Draco[87]
is doubtless essentially a spiral conformation;[88] and Professor
Schaeberle, by means of exposures with a 13-inch reflector of 20
inches focus, has compelled, not only the Ring nebula in Lyra,[89] but
the Dumb-bell in Vulpecula, to betray the surprising secret of their
whorled structure. Both these nebulæ give a spectrum of bright lines,
and inventiveness is at a loss to devise means for building up gaseous
materials into edifices of strongly characterized architectural forms.
The materials, indeed, may not be wholly gaseous;[90] or we possibly
see (as Professor Darwin long ago suggested) merely illuminated
stream-lines of motion furrowing an obscure mass. But if this be indeed
so, there is the further question to be asked: What direction does the
motion take? Do the tides set inward or outward?

Our spontaneous impressions are all in favour of concentrative
tendencies. We cannot easily shake off centripetal prejudices. Our
lives are passed under a regimen of central attraction, and we
naturally incline to universalize our experience. Herschel's scheme
of sidereal evolution invites accordingly at first sight ready
acceptance. Stars seem as if they could not act otherwise than as
foci of condensation in nebulæ; the lucid stuff involving them must,
apparently, with the lapse of ages, settle down towards their surfaces,
and become absorbed into their substance. Such processes, indeed,
belong, unless counteracted by different modes of action, to the
inevitable order of nature; but these may, and probably do, exist.
From sundry quarters the conviction is pressed upon us that cosmic
bodies can drive out matter as well as draw it in. Repulsive forces
insist upon recognition, and their effects become more palpable the
more attentively they are considered. Under certain conditions they get
the better of gravity; and stars may possibly, like cocoon-spinning
insects, expend their organic energies in weaving themselves faintly
lucent envelopes, the products of subtle and unaccountable activities.

The example of Nova Persei is fresh in every mind, but we make
no pretension to decide the controversy it raised. A dogmatic
pronouncement is unadvisable where the unknown elements of the question
obscure and outweigh those that are known. A less slippery foundation
for reasoning is afforded by the permanently visible spiral nebulæ, and
features charged with an emphatic meaning have been revealed in them by
photographic means.

Looking at the entire contents of the nebular heavens, we find the
spiral type very largely predominant. It claims more specimens, and
emerges more distinctly with each development of delineative power. Its
chief prevalence is among 'white' nebulæ, showing continuous spectra.

They are vastly numerous. Gaseous nebulæ are reckoned by the score,
white nebulæ by tens of thousands. Moreover, they collect near the
poles of the Milky Way,[91] while the gaseous variety crowd towards
its plane, both branches of the family thus manifesting galactic
relationships, though of an opposite character. Now, these facts of
distribution have some bearing on the question of relative age. There
is, as already remarked, a consensus of opinion that objects showing
a marked preference for the Milky Way are in a more primitive state
than those withdrawn from it, and the inference is supported by the
circumstance that nebulæ situated in high galactic latitudes shine with
continuous light, those near the galactic equator with vivid lines. Yet
it would be rash to assume that any individual nebula traverses these
successive stages. The series would be satisfactorily established only
if we could point to a number of intermediate instances, which seem
to be almost wholly lacking. We cannot trace in nebular as we can in
stellar growth the insensible gradations of progressive change. They
are perhaps complicated in nebulæ by influences of a different kind
from those which have gained the ascendancy in stars. Diffusive effects
may in them be more conspicuous than concentrative effects;[92] or a
balance may be temporarily struck between antagonistic tendencies.

Spiral conformation is the real crux of nebular cosmogony. The
conditions from which it arises are met with only in the sidereal
heavens, but are there widely prevalent. Though remote from our
experience, they are fundamental in the realms of space. If we could
define and comprehend them, we should be in a better position for
determining the cosmical status of nebulæ.

The choice is open between two rival theories of nebulous spirals.
The first is the more obvious, and readily falls in with admitted
mechanical principles. Sir Robert Ball has adopted and ingeniously
advocated this view.

A globular collection of promiscuously revolving particles inclines,
if left to itself, to flatten down into a disc. The reason is this: In
a system of the kind moment of momentum is invariable, while energy
constantly diminishes. To render the contrast intelligible we have
only to consider that moment of momentum is the algebraic sum of all
the products of mass and motion in the aggregation, reduced to, or
projected upon, its 'principal plane,' while energy is independent of
the varied directions of velocity. Collisions consequently involve no
diminution of moment of momentum, but combine with radiative waste to
produce a steady loss of energy. Inevitably, then, the system will
assume the form in which it possesses the minimum of energy that is
consistent with the maintenance of its original momentum; and it is
that of a disc extended in the principal plane. Retrograde movements
will by the time this shape is definitively arrived at have become
eliminated; the constituent particles circulate unanimously in one
direction; and Sir Robert Ball adds that their circulation, owing to
the more rapid rotation of the central mass, is along spiral paths.[93]
They would accordingly present the twisted conformation so commonly
observed in the heavens, and might even include subordinate centres of
attraction, fitted to ripen and strengthen into a full-blown retinue
of planets. Such are spiral nebulæ regarded in their direct mechanical
aspect. Spherical nebulæ are their immediate progenitors; suns, with or
without trains of dependent worlds, their lineal descendants.

Let us, however, consult some autographic records and weigh attentively
what these peculiar objects tell us about themselves. We see at once
that their curving lines, far from being laid down at the dictate of
chance, follow a strictly defined plan. Spiral nebulæ are not formed
like watch-springs by the windings of a single thread. They are always
two-branched. From opposite extremities of an elongated nucleus issue
a pair of nebulous arms, which enfold it in double convolutions.
Their apparent superposition and interlacements occasion, in the Lyra
nebula, the noted effect of a fringed and ruptured annulus, and it is
of profound interest to perceive that even in gaseous masses the same
constructive rule prevails as in the great Whirlpool in Canes Venatici.

Yet this circumstance is well-nigh irreconcilable with the hypothesis
that an influx of material is in progress.[94] Falls due to gravity
could not be limited to two narrow areas on the central body. Matter
ejected from it might, on the other hand, quite conceivably follow this
course. Interior strain could easily be supposed to cause yielding
along a given diameter, and nowhere else. Solar disturbances partially
and dimly illustrate such a kind of activity. Diametrically opposite
prominences are not unknown. They indicate the action of an explosive
force right across the solar globe. Similarly, the formation of a
spiral nebula cannot be rightly apprehended otherwise than as the
outcome of long-continued, oppositely-directed eruptions.

The history of the heavens involves the law of spirality. The scope
of its dominion continually widens as research becomes intensified.
The Huygenian 'portent' in the Sword of Orion now figures as merely
the nucleus of the 'great winding Nebula' photographed by Professor
W. H. Pickering in 1889. That the vast nebulosity encompassing the
Pleiades is an analogous structure seems eminently probable, though
the brilliancy of the enclosed stellar group obliterates most traces
of its ground-plan. The magnitude of the mixed system, we are told by
Professor Barnard,[95] who detected it in 1893 by means of a ten hours'
exposure with the Willard lens, transcends our powers of realization.
It covers 100 square degrees of the sky with intricate details. Again,
some four minutes of arc to the north-west of the Ring in Lyra lies
a small nebula discovered visually by Professor Barnard in 1893, and
photographically resolved by Keeler into a delicate spiral. It is a
two-branched, left-handed spiral, as the large adjacent object has also
proved to be. One is, in fact, the miniature of the other, and they
are now shown, by Professor Schaeberle's short-focus reflector, to be
linked together by curving folds of nebulosity into a compound spiral
system. The Dumb-bell is held, on the same authority, to be similarly
conditioned, and the analogy frequently noted between its aspect and
that of the Ring nebula has thus become incalculably widened in scale.

The galactic relations of the Magellanic Clouds are not easily defined.
They are within the Milky Way, yet not of it. Enigmatical excrescences
upon the universe, they suggest an origin from gigantic eddies in
the onflowing current of sidereal arrangement. Their miscellaneous
contents are, to all appearance, disposed along eddying lines. Mr. H.
C. Russell's photographs[96] rendered this, in 1890, to some extent
manifest, and their indications were ratified by the Arequipa plates,
from the study of which Professor Pickering gained the conviction that
the great Looped Nebula, 30 Doradûs, is the structural nucleus of the
Nubecula Major. 'It seems,' he wrote,[97] 'to be the centre of a great
spiral, and to bear the relation to the entire system that the nebula
in Orion bears to the great spiral nebula which covers a large part of
that constellation.'

On all sides, in the sidereal heavens, we can discern the signs of the
working of a law of convolution. Sometimes they are patent to view;
sometimes half submerged; but they can generally, with attention,
be disentangled from overlying appearances. They are exhibited by
stars no less than by nebulæ, as the late Dr. Roberts pointed out
from convincing photographic evidence; the 'hairy' appendages of
globular clusters betray them by their curvilinear forms; they meet
us in every corner of the wide nebular realm. Many investigators
recognise in the Milky Way itself the stamp of spirality. Stephen
Alexander, of New Jersey,[98] regarded the majestic galactic arch as
a four-branched spiral, resulting from catastrophic breaches in a
primitive, equatorially loaded spheroid, the currents of matter ejected
by which should, owing to their lower angular rotation, lag behind as
they retreated from the nucleus, and thus flow along helicoidal lines.
R. A. Proctor subsequently devised convoluted galactic streams, which,
however, corresponded imperfectly with what the sky showed. And Dr.
Easton[99] has designed by way of simple illustration an elaborate
series of spires, originating possibly from a central galactic
condensation, the projection of which upon the sphere may, he thinks,
account for the known peculiarities of the Milky Way.

Our interior situation, nevertheless, makes it extremely difficult to
determine the real relations in space of the star-streams circling
around it. The observed facts are, perhaps, equally compatible with
many other structural schemes besides those based on the idea of
spirality; and it will be prudent to adopt none, for the present,
with settled conviction. We can, however, gather one sufficiently
definite piece of information regarding the history of the Cosmos.
All the inmates of the heavens, stellar and nebular, represent quite
evidently the débris of a primitive rotating spheroid. Its equator is
still marked by the galactic annulus, its poles by a double canopy
of white nebulæ. The gyrating movement which it once possessed as a
whole doubtless survives in its parts, but ages must elapse before the
fundamental sidereal drift can be elicited.

FOOTNOTES:

[Footnote 85: Keeler, _Lick Publications_, vol. iii., p. 214.]

[Footnote 86: Rosse, _Transactions Royal Dublin Society_, vol. ii., p.
93.]

[Footnote 87: First detected as such by Holden and Schaeberle in 1888,
_Monthly Notices_, vol. xlviii., p. 388.]

[Footnote 88: Deslandres, _Bulletin Astronomique_, February, 1900.]

[Footnote 89: _Astronomical Journal_, Nos. 539, 547.]

[Footnote 90: Maunder, _Knowledge_, vol. xix., p. 39.]

[Footnote 91: Dr. Max Wolf places the point of nebular concentration in
R.A. 12h 53m, D. +61° 20´, that assigned to the galactic pole being
in R.A. 12h 49m, D. +62°. _Königstuhl Publ._, Bd. I., p. 174.]

[Footnote 92: T. J. J. See, 'Repulsive Forces in Nature,' _Popular
Astronomy_, No. 100, December, 1902.]

[Footnote 93: _The Earth's Beginnings_, pp. 243-247.]

[Footnote 94: _Cf._ Moulton, _Astrophysical Journal_, vol. xxii., p.
165.]

[Footnote 95: _Monthly Notices_, vol. lx., p. 259.]

[Footnote 96: See _Knowledge_, vol. xiv., p. 50.]

[Footnote 97: _Harvard Annals_, vol. xxvi., p. 206.]

[Footnote 98: _Astronomical Journal_, vol. ii., p. 100, 1852.]

[Footnote 99: _Astrophysical Journal_, vol. xii., p. 158.]




CHAPTER XIII

THE PROCESSION OF SUNS


Phenomena are functions of time, and the form of the function has to
be determined in each particular case. That is what the historical
method comes to, and its use is prevalent and almost compulsory. We can
no longer be satisfied with a simple bird's-eye view of the universe;
our thoughts are irresistibly driven to grope into its past, and to
divine its future. Statical conceptions sufficed for our intellectual
forefathers. They aimed at establishing the equilibrium of things,
while we see them in a never-ending flux. One aspect of them calls up
the next, and that another, and so on _ad infinitum_; we cannot, if we
would, balance our ideas on the pivot of the transient present.

The immutable heavens of the ancients strike us to-day as the
invention of a strange race of beings. We see them, on the contrary,
with Shelley as a 'frail and fading sphere,' a 'brief expanse,' the
seat and scene of change. The 'fixed' stars long ago broke away from
their moorings, and began to flit at large through space. Of late a
less obvious, more intimate kind of mobility has been attributed to
them. Grooves of individual development have been assigned to them,
along which they appear to shift as the tardy ages go by; and since
everything that grows must decay, the orbs of heaven, too, incur
the doom of mortality. But modern science has done much more than
extend to them the dismal philosophy of the phrase, 'Tout passe,
tout casse, tout lasse.' The grandiose enterprise has been not
unsuccessfully essayed of tracing in detail the progress of sidereal
evolution, and of marshalling the vast stellar battalions in order of
seniority. This has been rendered feasible by the disclosures of the
spectroscope. Apart from their guidance, the track might have been
seen by elusive glimpses, but could never have been laid down with any
approach to definiteness. Herschel found for it a _terminus à quo_
in nebulæ of various forms, but attempted to pursue it no further.
We do not hesitate to run it on, from station to station, right down
to the _terminus ad quem_. Not, it is true, without the perception
of outstanding difficulties and insecurities, which yet seem to be
outweighed by a certain inevitableness of self-arrangement in the
related facts.

The argument from continuity is that mainly relied upon. An unbroken
succession of instances is strongly persuasive of actual transition,
provided only that a principle of development (so to call it) may
reasonably be assumed as influential. A series of mineralogical
specimens, however finely differenced, does not suggest the progressive
enrichment of one original mass of ore. In the stars, on the other
hand, a species of vitality may be said to reside. They are not
finished-off products, but spontaneously-acting machines. They are
centres of energy, which they dispense gratis, supplying the cost out
of their own funds. And the process is not only obviously terminable,
but must be accompanied by constitutional alterations, which might be
traceable by subtle methods of inquiry. They _are_ traceable, unless
we are deceived by illusory appearances.

Secchi's classification of the stars was unwarped by any speculative
fancy. It was purely formal; it aimed only at providing distinct
compartments for the convenient arrangement of a multitude of
differently characterized items of information. Then, by degrees,
the closeness of the gradations between one class and the next came
to be noticed; partitions melted away; the methodized array showed
itself to be in movement; and the bare framework took shape, under the
auspices of Zöllner and Vogel, as a cosmic pedigree. The white stars
were set forth as the progenitors of yellow, yellow of red stars; and
the insensibly progressive reinforcement of the traits of relationship
between the successive types went far towards demonstrating some
partial, if not a complete, correspondence of the indicated order
with the truth of things. It has since been found necessary to divide
the first stellar class into helium and Sirian stars; and here,
too, essential diversity shades off imperceptibly into likeness
approximating to identity. All the groups hang together; the entire
scheme is on an inclined plane of change. Helium stars, as they
condense, pass into Sirian, these into solar stars, which finally,
reddening through the increase of absorption, exhibit the badge of
post-meridional existence in fluted spectra. The finality of the red
stage is, indeed, very far from being absolute, but what lies beyond is
matter of conjecture.

There are several good reasons for taking helium stars to be the
'youngest' or most primitive of the amazing assemblage that sparkle in
the vault of heaven. The first is their affinity with nebulæ. Every
star, perceived to be involved in folds or effusions of shining haze,
has yielded--if bright enough for profitable examination--a spectrum
of helium quality. Further, they are remarkably tenuous bodies. It has
been ascertained with approximate certainty, from the investigation of
stellar eclipses, that helium stars are commonly, perhaps invariably,
of far slighter consistence than the sun. Radiation, however, is
maintained by contraction; hence, orbs at the outset of their course
must be, on the whole, the most diffuse. A third note of youth is
membership of embryo systems, and this is affixed very markedly to
helium stars. One-third certainly, probably one-half of those lately
submitted to trial by Professors Frost and Adams proved to have
spectroscopic companions. They are pairs believed to have been recently
divided by the fission of a single parent-globe. And this is an
operation which must, we should suppose, be undergone early, or not at
all.

The spectra of helium stars are peculiar and suggestive. Those
belonging to Miss Maury's earliest groups--many of them visibly
nebulous--bear next to no traces of metallic absorption, showing
instead lines of oxygen, of nitrogen, and of hydrogen in all its
three series. The conditions, accordingly, needed to produce the
'cosmic' modification of hydrogen are realized in these inchoate
bodies. What those conditions actually are we cannot tell, yet it may
be confidently surmised that they will prove to be of an electrical
nature. Hydrogen resembles the metals in being electro-positive; it
collects at the negative pole during the electrolytic decomposition
of water. There is, however, an unmistakable tendency in primitive
sidereal objects to display absorption rays of electro-negative
rather than of electro-positive elements. It is conceivable that
hydrogen may be capable of altering its behaviour in this respect,
and that the molecules radiating the Pickering and Rydberg series, in
addition to the more familiar Huggins series, have, in fact, through
some corpuscular re-arrangement, assumed the electro-negative quality
properly characterizing a non-metallic substance. The association of
this form of hydrogen with oxygen and nitrogen in early helium stars
would thus be naturally related to the simultaneous quasi-disappearance
from them of the spectral badges of metals.

The helium-line most distinctive of this stellar family is situated
well up in the blue. It appertains to the same vibrational sequence
with D3, which is also represented in Rigel, one of the more
'advanced' Orion stars. In Rigel, too, we meet a fairly prominent
magnesium ray, lying below the blue helium emanation, while as yet iron
is unapparent. Numerous fine, faint streaks, due to its absorption,
only emerge when the Sirian type is fully reached, and they are mostly
of the 'enhanced' kind. When the spark discharge is substituted for
the arc as the source of illumination, certain lines in the resulting
spectrum brighten relatively to the others, and these have been
distinguished by Sir Norman Lockyer as 'enhanced.' Now, the rule is
strikingly prevalent that the absorption rays in white stars are of
this class; yet it can no longer be interpreted as indicating for them
an excessively high temperature. Rather, it would seem that electrical
conditions still imperfectly defined are in question, and their
gradual removal or subsidence is, beyond doubt, largely instrumental
in bringing about the transition to the solar stage. The effacement of
helium-absorption is even more perplexing. No sooner does iron begin to
show than it vanishes. There is still a faint trace of its 'blue' line
in Vega; none survives in Sirius.

In spectra of the solar type two great bars of violet light are stopped
out by calcium; otherwise metallic arc-lines predominate, while those
of hydrogen are no longer so powerfully emphasized as in white stars.
Moreover, the whiteness of the unveiled Sirian photospheres has become
tinged with yellow owing to the development of a shallow envelope
partly impermeable to blue rays. For this reason the comparative
extension of their ultra-violet spectra affords, for stars of different
types, no secure criterion of relative temperature. Sound in principle,
it becomes inapplicable when the unknown factor of general absorption
comes into play. The energy-curve of the solar spectrum, as it is, can
be determined; the energy-curve of the solar spectrum, as it would
be if unaffected by general absorption, has to be constructed from
inference. But only photospheres bare to space give comparable results.
Hence, there are no valid grounds for asserting that Sirius is hotter
than the sun, or the sun than Betelgeux. It may be so, but the evidence
at present available is inconclusive. The appearances expounded in this
sense may bear quite different meanings.

The reasons for holding that solar mature into Antarian stars are of
the same character, and of equal cogency with those tending to prove
their own development from luminaries of Sirian type. There is a
similar continuity of specimens. They can be ranged one after another
in an unbroken series, in which, as we run down the line, primrose
shades into orange, and orange into red; general absorption arrests an
increasing percentage of the blue radiations, while specific absorption
becomes strengthened by dusky channellings of titanium. Carbon stars
are less easily located. Dr. Vogel regards them as co-ordinate with
the Antarian class. The two varieties of red stars with banded spectra
descend, in his opinion, from the common stock exemplified by our sun.
Professor Hale also favours this view, some attendant difficulties
notwithstanding. His photographs have certainly established for carbon
stars links of relationship both with the Antarian and the solar
families; yet the fact remains indisputable that the carbon type is,
to a great extent, isolated from all the rest. Tokens of a genuine
migration towards it are few and obscure.

The ultimate fate of both tribes of red stars can only be conjectured.
Most of the objects constituting them vary in brightness, some to
the verge of periodical extinction; and variability may be a symptom
of interior dilapidation. But the organization of such bodies is
profoundly enigmatical. They are exceptionally remote, and offer slight
holding-ground for inquiry. No indications have been gathered as to
their density or intrinsic light-power. Very little is known about
their movements. They rarely form binary combinations, and those that
they do form are almost always relatively fixed. No red star travels
in a computed orbit; only one, η Geminorum, occurs on the long list of
spectroscopic binaries. The revolutions of this curious system ought
to prove, when thoroughly investigated, replete with interest and
instruction.

Coupled stars present special opportunities to students of cosmogony.
They are obviously contemporaries; they have started fair in the
evolutionary race; identical influences have acted upon them; hence,
differences in their standing can only result from dissimilarities
in mass or composition. It is commonly taken for granted that a body
containing less matter than its fellow must develop faster, and incur
the final quenching sooner. But Sir William and Lady Huggins have
adverted to the probability of the very opposite being the case.
Powerful surface-gravity may, they consider, serve to hasten the
transition from a Sirian to a solar spectrum; and we should then
have giant suns like Capella, advanced in type while at a very early
stage of condensation. This perhaps explains the remarkable spectral
relations of contrasted stellar pairs. Always, so far as we yet know,
the Sirian spectrum is yielded by the lesser star, the mass of which,
judging by analogy, must be even smaller than would be indicated by the
proportion of its faintness. It is true that the distribution of mass
in binary systems is often widely different from what might have been
anticipated. Certain purplish satellites, for instance, of undetermined
spectral quality exercise a gravitative sway of surprising force. Some
results of this kind lately obtained by Mr. Lewis and others are likely
to prove of fundamental importance to theories of stellar evolution.

What we know of 'dark stars' has been mainly derived from the
observation of stellar systems. They are assumed to be the denizens of
a stellar Hades, dim wanderers amid the shades, who 'have had their
day, and ceased to be' as suns. In the 'cold obstruction' of these
viewless orbs the grand cosmical procession is held to terminate.
Their presence attests the downward progress of decay, and gives
logical completeness to the argument for development. Yet there are
circumstances warning us against too full an assurance that their
status is really that of skeletons at the feast of light. They are very
frequently found to be in close attendance upon brilliant white stars.
Thus intimately, if incongruously, coupled, they circulate and compel
circulation in brief periods, as members of systems just, it might be
said, out of the shell. What are we to think, for instance, of the
obscure body spectroscopically discovered to control the revolutions of
the chief star in the Orion trapezium? It is evidently comparable in
mass with that imperfectly condensed luminary. Is it credible that it
has already traversed all the stages of stellar existence, and cooled
down to planetary rank? So violent an assumption should at least not
be made without due consideration; and we may more prudently hold our
judgment in suspense as to whether globes so circumstanced--and they
abound--should be regarded as effete, or as abortive suns.[100]

Speculations on the exhaustion of stellar vitality have lately become
inextricably involved with the complex problem of elemental evolution.
A dim inkling has been acquired of the activity in the universe of
obscure forces, availing, we can just see, to falsify many forecasts.
The theory, among others, of the dissipation of energy needs to be
revised or qualified. Nor was it propounded by Lord Kelvin with
dogmatic certainty. He carefully noted the possibility that in 'the
great storehouses of creation' reserves of energy might be provided
by which the losses incurred through radiation could be, wholly or in
part, made good.[101] The anticipated possibility is perhaps realized
in the phenomena of radio-activity. But if we inquire how, we are met
at the threshold by difficulties connected with the origin of helium.
Helium appears to result from the disintegration of radium, its
generation being accompanied by the setting free of enormous quantities
of energy. Its copious presence, then, argues long-continued and lavish
expenditure of heat and light. Yet it is as a constituent of highly
primitive orbs that it is chiefly conspicuous. Gaseous nebulæ, too,
include immeasurable supplies of it, while it is incompatible with
whatever we seem to know about them to suppose that radium at any time
entered into their composition.

The genesis of the elements has, in truth, not yet been made the
subject of coherent speculation. Current ideas regarding it imply a
double course of change, by aggregation first, and subsequently by
disintegration. And this should give us a twofold series of elements.
On one side there should be fixed survivals from the advancing process,
on the other, products of decomposition, continuously evolved, and
even now accumulating. If the claim of helium to take rank among these
last should be finally established, our conceptions of the nature and
history of nebulæ might have to undergo a strange inversion; but the
outcome of the researches in progress is still uncertain, and may be
far off.

It is, nevertheless, quite clear that the electronic theory of matter
supplies no genuine explanation of the source of energy in the
universe. What is given out when the atoms go to pieces must have been
stored up when they were put together. Whence was it derived? This is
the fundamental question which underlies every discussion concerning
the maintenance of the life of suns. It is unanswered, and probably
unanswerable.

FOOTNOTES:

[Footnote 100: It must at the same time be borne in mind that their
total darkness is not proved. All that is certain is that their spectra
are not bright enough to leave any impression on the exposed plates.]

[Footnote 101: Thomson and Tait, _Natural Philosophy_, Appendix E, p.
494, edition 1890.]




CHAPTER XIV

OUR OWN SYSTEM


Our sun is clearly middle-aged. It bears none of the marks associated
with juvenility in stars, while its decrepitude is in the distant
future. It is crossing, most likely, a level tract where recuperation
so nearly balances expenditure that radiation can be maintained for an
indefinite time at a high and fairly uniform standard. Stars of the
solar type pursue the even tenor of their way with particularly few
interruptions. They show little tendency to intrinsic variability.
Their periodicity, when it exists, is due to the presence of a
companion. Variables, in other words, belonging to the spectral family
of our sun, are binary systems; and they are usually, if not always,
non-eclipsing binaries, on the pattern of δ Cephei. Light changes can
thus be impressed upon sunlike stars by external influence; they do
not conspicuously arise through native instability.

Our planet, accordingly, is attached to a safe and steady luminary,
one subject, not to destructive spasms, but to vicissitudes so mild as
to evade distinct meteorological recognition.[102] It is, moreover,
governed by a polity settled on a broad basis of tranquillity and
permanence. All this is as it should be. The conditions specified were
a pre-requisite to the unfolding of human destinies. Nor can it be
confidently asserted that they have been realized anywhere else. Our
system may be unique; while, on the other hand, replicas of it might,
imperceptibly to us, be profusely scattered through the wide realms of
space. It is certain that a telescopic observer on Sirius or α Centauri
would see our sun unattended; not even Jupiter could be brought into
view by optical appliances in any degree comparable to those at our
disposal.

There are, nevertheless, strict limitations to the possible diffusion
of planetary worlds like those that wander amid the zodiacal
constellations. We have become aware of incapacitating circumstances,
by which a multitude of stars are precluded from maintaining retinues
of subordinate globes. Spectroscopic discoveries have compelled a
revision of ideas as to cosmical arrangements. Especially the large
proportion established by them of binary to single stars makes it
impossible any longer to regard the solar system as a pattern copied at
large throughout the sidereal domain. We cannot, then, compare it with
any other; the mechanism of which the earth forms part must, perforce,
be studied in itself and by itself, and it may, for aught that appears,
be the outcome of special and peculiar design.

The machine in question is self-sustaining and self-regulating; no
extraneous power noticeably affects its working. This immunity from
disturbance is the fortunate consequence of its isolation. A great
void surrounds it. The span of Neptune's orbit is but a hand-breadth
compared with the tremendous unoccupied gulf outside--unoccupied, that
is to say, by bodies of substantial mass. The feebleness of starlight
relatively to sunlight affords some kind of measure of the impotence of
stellar attractions to compete with the over-ruling gravitational power
that sways the planetary circulation. This it is which gives to it such
remarkable stability. The incomparable superiority of the sun over his
dependent orbs not only safeguards them against foreign interference,
but reduces to insignificance their mutual perturbations. Hence the
strong concentration of force exemplified in our system--the absolutely
despotic nature of the authority exercised--makes for a settled order
by excluding subversive change.

The organization of the solar kingdom, as disclosed by modern research,
is greatly more varied and complex than Laplace took it to be. His
genetic scheme was, indeed, no sooner promulgated than deviations from
the regularity and unanimity of movement upon which it was based began
to assert their inconvenient reality. They have since multiplied; and,
emerging to notice under the most unlikely aspects, they occasion
incongruities which tax, for their explanation, all the resources and
audacities of the most inventive cosmogonists. Let us briefly consider
their nature.

The swarm of asteroids that bridge the gap between Mars and Jupiter
revolve, it is true, with the general swirl of planetary movement; but
they use a large license as regards the shape and lie of their orbits.
And their partial exemption from the rules of the road becomes entire
for comets and meteors, which have nevertheless proved themselves
to be aboriginal in our system by their full participation in its
proper motion. Finally, several of the major planets set convention at
defiance in the arrangement of their several households, and thereby
intimate departures from the supposed normal course of development so
frequent and so considerable as to shake belief even in its qualified
prevalence. Thus, the anomalously short period of the inner satellite
of Mars, besides throwing doubt over its own mode of origin, tends
to obscure the history of its more sedately circulating associate.
Deimos cannot have been thrown off from its primary under conditions
materially different from those attending the birth of Phobos.

The sub-systems of Uranus and Neptune exhibit, moreover, eddies of
retrograde movement suggesting primitive disturbances of a fundamental
kind; while the surprising disclosures connected with Saturn's
firstborn and furthest satellite, photographically detected by
Professor W. H. Pickering in 1898, have added one more knotted thread
to the tangled skein we would fain unravel. Until acquaintance was made
with Phœbe, counterflows of revolution within the same satellite-family
were unknown, and, if contemplated at all, would have been scouted
as impossible. One ternary star, to be sure--ξ Scorpii--had been
recognised as probably owning an immediate and a more remote attendant,
in oppositely directed orbital movement;[103] but the cases are in many
ways disparate, and the analogy, though instructive, is imperfect.

If the ninth Saturnian moon is to be regarded as sprung from the
condensing mass of the planet, a total change in the state of the
parent body must have supervened during the long interval between
its separation and that of its successor, Iapetus. The change, in
Professor W. H. Pickering's opinion,[104] was nothing less than a
reversal of axial movement. The nebulous spheroid destined to develop
into the wonderful Saturnian system had, presumably, when Phœbe
became detached from it, a diameter of sixteen million miles, and
gyrated tranquilly from east to west, in a period of about a year and
a half. But the action of sun-raised tides availed first to destroy
and finally to invert this movement; for the natural outcome of tidal
friction is synchronism, and this implies agreement, both in period and
direction, between the rotation and revolution of the body acted upon.
Acceleration through contraction did the rest; and by the time another
satellite was ready to separate, the originating globe span normally in
seventy-nine days, the actual revolutionary period of Iapetus. The view
that such was the course of events is plausible at first sight; yet the
doubt remains whether the cause alleged was adequate to the effect
produced. At the distance of Saturn, solar tidal friction exerts only
about 1/20000 its power on the earth;[105] its efficacy would, on the
other hand, be greatly enhanced by the distension of the mass subjected
to it; but approximately to what extent, our powers of calculation are
impotent to determine.

This is not all. Exhaustive photographic research promises to unfold
intricacies of construction in secondary systems demanding the
patient industry of many generations for their complete unravelment.
The families of the great planets will perhaps be found to include
crowds of inferior members which pay slight heed in their circulatory
arrangements to the trammels of convention. In those of both
Jupiter and Saturn the phenomenon has lately been brought to light
of 'asteroidal' satellites, as they may be termed, minute bodies
travelling round their primaries at nearly the same mean distances,
each group evidently representing the unagglomerated materials of a
single full-sized satellite. The pigmy components of such groups
doubtless exist in multitudes; each great planet, most likely,
is encompassed by at least one zone of moonlets; but so far only
specimen-objects have been picked up. The tenth Saturnian satellite,
discovered, like its predecessor, by Professor W. H. Pickering, is thus
associated, by its period and locality, with Hyperion, the seventh and
least prominent of Saturn's visual train, the apparent insignificance
of which suggested to Sir John Herschel that it might have many
co-occupants of the wide gap between Titan and Iapetus.[106] But the
surmise had to await verification until methods were intensified beyond
what seemed possible in the middle of the nineteenth century.

The corresponding Jovian pair found by Professor Perrine circulate
far outside the boundaries of the original Galilean realm, in
orbits which interlock as a consequence of their marked difference
in eccentricity.[107] They are mutually inclined at an angle of
27 degrees, nor are they supposed actually to intersect, so that
collisions are apparently out of the question. Direct movement is
indicated, but cannot yet be claimed to belong quite certainly to both
objects. We are only beginning to make acquaintance with the submerged
populations of the Saturnian and Jovian kingdoms; they are perhaps
multitudinous; they are certainly peculiar, and we await impatiently
and curiously the further developments of their remarkable behaviour.

The one certain inference derivable from the diversity of facts
ascertained within the last hundred years is that our world is not (so
to speak) machine-made. The _modus operandi_ employed to disengage
the planets from their nebulous matrix was not of cast-iron rigidity;
it was adaptable to circumstances; it left room for the display of
boundless inventiveness in details. This was made, nevertheless, to
consist with the perfect preservation of the main order, both in design
and operation. The general plan is broadly laid down and unmistakable;
the springs of the machine are undisturbed in their free play, and for
the primary reason that departures from regularity, which might, in any
way, prove a menace to stability, affect bodies of negligible mass.
The great swing of settled movement goes on irrespectively of them. 'De
minimis non curat lex.'

So the erratic procedure of comets is harmless only because of their
insignificance. If imitated by substantially attractive masses, it
could not fail to jeopardize the planetary adjustments. Even the
asteroids would be unsafe neighbours but for their impotence; and it
is remarkable that Mercury, by far the smallest of the major planets,
circulates along a track of the asteroidal type. It would seem as if an
important size carried with it an obligation to revolve in an orbit of
small eccentricity, inclined at a low angle to the principal plane of
the system. The reason why this should be so is not obvious; but were
it otherwise the equilibrium, now so firmly established, would subsist
precariously, or not at all.

The assertion, indeed, that it is firmly established can only be
made under reserve. We are ignorant of any causes tending towards
its overthrow; yet they may supervene, or be already imperceptibly
active. One such lurking possibility is the presence of a resisting
medium in interplanetary space. Waifs and strays of matter must,
beyond doubt, be encountered there--outlawed molecules, self expelled
from the gaseous envelopes of feeble globes; thin remnants of cometary
paraphernalia, driven off amid the fugitive splendours of perihelion;
products of ionic dissociation set flying by the impact of ultra-violet
light--and all disseminated through an ethereal ocean, which 'is cut
away before and closes from behind' as moving bodies traverse it. That
its indifference is shared by ordinary material substances, when in the
last stage of attenuation, is a plausible but unverified conjecture. It
is only safe to say that retardation of velocity in what may pass for
empty space is insensible or null.

There may, nevertheless, be springs of decadence in the solar system.
Some of them have been discussed by M. Poincaré,[108] whose confidence
in the reassuring demonstrations of Laplace and Lagrange is inversely
proportional to the magnitude of the terms they were forced to neglect.
They dealt with fictitious globes, devoid of appreciable dimensions,
and swayed by the strict Newtonian law. But the real planets and
their satellites are acted on by other forces as well, frictional,
magnetic, radio-repulsive, the joint effects of which may not be
wholly evanescent. The tidal drag on rotation undoubtedly occasions
a small but irretrievable loss of energy. The moon, for instance, as
M. Poincaré states, now gains, by the reactive consequences of tidal
friction in widening its orbit, no more than 1/28 the _vis viva_ of
which the earth is deprived by the infinitesimal slowing down of its
rotation; and the remaining 27/28, being dissipated abroad as heat, are
finally abstracted from the system.

The ultimate state, we are told, towards which the planetary
mechanism tends is that of the synchronous revolution, in a period
of about twelve years, of all its members. This might, apart from
the possibility of a resisting medium, have indefinite permanence;
otherwise precipitation to the centre would gradually ensue, and
one solitary sphere, cold, stark, and unilluminated, would replace
the radiant orb of our cerulean skies, with its diversified and
exquisitely poised cortège. Unsecured drafts upon futurity, however,
are not among the most valuable assets of science, and a consummation
so immeasurably remote may be anticipated by a score of unforeseen
contingencies. What can be and has been ascertained is the relative
durability of the scheme with which the visible destinies of the human
race are so closely connected. It will unquestionably last long enough
for their accomplishment. Curiosity that would seek to pierce the
ulterior darkness is likely to remain ungratified.

But there is a further outlook. Other and incalculable items remain
to be taken into account. The sun, although an autocrat within his
own dominion, is himself subject to external influences. As a star,
he is compelled to follow whithersoever the combined attractions of
his fellow-stars draw him; nor can we thoroughly interpret the summons
which he obeys. The immediate upshot in the transport of the solar
system towards the constellation Lyra has, it is true, been determined,
but the eventual scope and purpose of the journey remain profoundly
obscure. The pace is to be reckoned as leisurely: twelve miles a
second is little more than half the average stellar speed. We should,
however, probably suffer no inconvenience from being whirled through
the ether in the train of such a stellar thunderbolt as Arcturus. Only
the excessive velocities of any adventitious bodies we might happen to
pick up would betray to ordinary experience the fact of our own swift
progress. As it is, our sweepings from space appear to be scanty.

If shreds from inchoate worlds, or dust of crumbled worlds, strewed
the path of our system, they should be annexed by it in its passage,
temporarily or completely, and we should then expect to find the apex
of the sun's way marked, if no otherwise, by the predominant inflow
from that quarter of comets and meteors. Yet there is no trace of such
a preference in the distribution of their orbits. Hence the enforced
conclusion that the sun has attached to him, besides the members of his
immediate household, an indefinite crowd of distant retainers, which,
by their attendance upon his march, claim with him original corporate
unity. To this rule there may be a few exceptions. An occasional
aerolite probably enters the earth's atmosphere with hyperbolic
velocity, and takes rank accordingly as, in the strictest sense, a
foreign intruder; but the broad truth can scarcely be challenged that
the sun travels through a virtual void.

We can, however, see no necessity why he should for ever continue to do
so. Widely different conditions seem to prevail near the centre and out
towards the circumference of the sidereal world. What may be designated
the interior vacuity of the Milky Way is occupied mainly by stars of
the solar type, including one to our apprehension super-eminent over
the rest; they are separated by vast, apparently clear intervals; they
are non-nebulous, and of stable constitution. This secure habitat is
ours for the present, although it may at some future time be exchanged
for one less exempt from disturbance. The shape and size of the sun's
orbit are utterly unknown; the changes of environment, accordingly,
that will accompany the description of it defy conjecture. Our actual
course is inclined at a small angle to the plane of the Milky Way. It
will presumably become deflected, but perhaps not sufficiently to keep
our system clear of entanglement with the galactic star-throngs. In
our ignorance of their composition no forecast of the results can be
attempted: they are uncertain and exorbitantly remote. Moreover, the
comparative slowness of the sun's motion in a manner guarantees the
permanence of his subsisting cosmical relations. For anything that
science can tell, they may ultimately be subverted by some preordained
catastrophe; but the possibility lies outside the sphere of rational
forecast.

The universe, as reflected in the mind of man, gains extent as the
mirror acquires polish. Early astronomers conceived of but one solar
system and one 'dædal earth,' upon which the 'pale populace of heaven'
rained influences sinister or propitious. Later, human egotism took
another form. The whole universe was assimilated to our particular
little settlement in it. Terrestrial conditions were universalized.
None divergent from them were counted admissible or profitable. But one
answer seemed possible to the perpetual _Cui bono?_ with which restless
thought assailed the heavens. But one purpose was regarded as worthy
of fulfilment, that of multiplying, in distant sidereal climes, copies
of our own planet, and of providing suitable locations for myriads
of intellectual beings, as little alien to ourselves as might be
compatible with the minimum of diversity in their material surroundings.

The spread of this astral philanthropy has been in some measure
checked by the advance of knowledge. Our position and circumstances
have been shown by it to be, if not quite peculiar, at any rate very
far from inevitable. It has reduced, by a process of exclusions, to
a relatively limited number the class of stars that can fairly be
regarded as possible centres of vitality; it has immensely widened the
scope of discernible variety in cosmical arrangements, and held out
warnings against errors of exposition due to inborn prejudices. And we
shall surely not wander from the truth by recognising our inability to
penetrate all the depths and complexities of Infinite Design.

FOOTNOTES:

[Footnote 102: The uncertainty affecting the best attainable
results in weather-cycle investigation is rendered strikingly
apparent by a comparison of the able and laborious papers by H. W.
Clough (_Astrophysical Journal_, vol. xxii., p. 42), and C. Easton
(Petermann's _Geogr. Mittheilungen_, 1905, Heft VIII., and _Proceedings
Amsterdam Academy of Sciences_, June 24, 1905).]

[Footnote 103: R. T. A. Innes, _Reference Catalogue_, p. 155A.]

[Footnote 104: _Harvard Annals_, vol. liii., p. 61, where, however, the
reversal is explained by a shifting of the plane of rotation.]

[Footnote 105: G. H. Darwin, _Philosophical Transactions_, vol.
clxxii., p. 526; Moulton, _Astrophysical Journal_, vol. xi., p. 110.]

[Footnote 106: _Monthly Notices_, vol. ix., p. 91.]

[Footnote 107: F. E. Ross, _Lick Bulletin_, No. 82.]

[Footnote 108: _Annuaire du Bureau des Longitudes_, 1898.]




CHAPTER XV

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.

The counterglow, first remarked by Pezénas in 1730,[112] soon fell into
oblivion, and had to be rediscovered after six-score years by Brorsen,
who bestowed upon it its current title of the 'Gegenschein.' Of late
it has been pretty constantly observed, particularly by Professor
Barnard, to whom it presented itself, owing to the scantiness of
the available records, as a surprising novelty.[113] Surprising it
certainly is. The appearance of the Gegenschein is that of a large
elliptical patch of diffuse light, measuring about 12 by 9 degrees,
and situated diametrically opposite to the sun.[114] Now and again,
though somewhat rarely, it is perceived to be united to the cone by
the 'zodiacal band,' a strip of evanescent luminosity nearly following
the line of the ecliptic. We cannot, then, be mistaken in recognising
the great pyramidal beam centred on the sun, with the counterglow and
its linking band, as sections of a single formation, constituting in a
manner the substratum of the solar system. A recent observation made
by Professor Newcomb under unique conditions proves it to be much less
exclusively 'zodiacal' than had been supposed. Looking north from the
summit of the Rothhorn, at midnight, on July 29, 1905, he perceived a
well-marked glow spreading 35 degrees from the sun's place.[115] It was
the light in its thwartwise aspect, which had never before been seen,
or even looked for; and we learn from it the remarkable fact that the
sun is enclosed in a vast, dimly luminous sphere, with a girth not much
smaller than the orbit of Venus, and indefinitely diffused along the
equatorial plane.

Notwithstanding its dim indefiniteness, neither the spectroscope nor
the camera is wholly ineffective for the scrutiny of this extraordinary
appurtenance. We have learned positively that its radiance is of the
continuous sort, the origin of which through the reflection of sunlight
from small solid bodies seems more than probable. The whole structure
must accordingly be of a pulverulent or meteoric nature; it consists
of independently moving particles. But to the further question, Under
what regimen do these particles circulate? no decisive answer is as
yet forthcoming. M. Hansky[116] and others hold the light to be a
true solar appendage, an extension of the corona, in which case it
would have a formal, but no material permanence. It would represent
the continually changing aggregate of multitudinous minute bodies
issuing from or repelled by the sun, and in large proportion falling
back towards his surface. Yet some difficulty is raised to this view
by the vast dimensions of the problematical glow. That it extends far
beyond the earth's orbit is rendered patent by the phenomena of the
Gegenschein and the band. True, the scope of the sun's repulsive action
cannot be limited; still, we might naturally expect its products to
become too attenuated for recognition beyond a radius of perhaps fifty
million miles.

Admitting, on the other hand, the residual character of the zodiacal
light, we should attribute to it a constitution analogous to that of
Saturn's rings. Each one of the cosmic atoms collected in it would
revolve round the sun on its own account, scarcely disturbed by
its neighbours. Nor need we despair of determining with reasonable
certainty which way the truth lies in this matter. The rival hypotheses
may be tried by a criterion the application of which is by no means
remotely feasible. It is furnished by the geometrical relations of the
zodiacal light. Evidently, if the sun can claim organic connection
with it, its axis should coincide with the plane of the solar
equator; while, if it represent wastage from the Kantian nebula, it
should stretch along the principal plane of our system--the plane of
maximum moment of momentum--the plane towards which the primitive
agglomeration of revolving particles collapsed as it condensed. The
question of planes is, then, crucial. Is the zodiacal effluence placed
symmetrically as regards the solar equator, or does it appertain
properly to the ecliptic, which deviates very slightly from the
fundamental plane of the solar system? The evidence is, unfortunately,
contradictory. Most observers have located the dim equinoctial
cone right along the pathway of the sun; some, under exceptionally
favourable circumstances, have perceived in it a marked departure from
the track of the Signs.

M. Marchand's determinations from the Pic du Midi, for instance,
indicated a probable coincidence between the solar equatorial plane and
the axis of the light;[117] and Dr. Max Wolf succeeded, in 1889, in
getting a photographic impression which, though partial and imperfect,
tended to corroborate Marchand's inference.[118] Again, on November
16, 1904, when the cone showed a remarkable lustre, it was distinctly
perceived at Königstuhl to sheer off and separate from the ecliptic as
it mounted the sky. Now, however, that a beginning has been made in
photographing this enigmatical tenant of the sphere (the feat has been
performed at Flagstaff as well as at Heidelberg), we may confidently
expect a speedy reconcilement of inconsistent statements regarding
its whereabouts. Until then we cannot venture to assert that it is in
actual reality what it appears to be, a nebulous survival.

FOOTNOTES:

[Footnote 109: Ligondès, quoted by l'Abbé Moreux, _Le Problème
Solaire_, p. 67.]

[Footnote 110: Moreux, _Le Problème Solaire_, p. 133; Ledger,
_Nineteenth Century_, March, 1905.]

[Footnote 111: Humboldt, _Cosmos_, vol. iv., p. 563 (Otté's
translation); Maunder, _Journal of the British Astronomical
Association_, vol. viii., p. 174; Max Wolf, _Königstuhl Report_ for
1904.]

[Footnote 112: _Paris Memoirs_, 1731, quoted by R. Wolf, _Geschichte
der Astronomie_, p. 695.]

[Footnote 113: _Popular Astronomy_, vol. i., p. 337.]

[Footnote 114: Nijland, _Astr. Nach._, No. 4,008.]

[Footnote 115: _Astrophysical Journal_, vol. xxii., p. 209.]

[Footnote 116: _Comptes Rendus_, 1905, No. 6; _Nature_, February 23,
1905.]

[Footnote 117: _Comptes Rendus_, tom. cxxi., p. 1134.]

[Footnote 118: _Sitzungsberichte_, Munich, Bd. XXX., p. 197.]




CHAPTER XVI

LIFE AS THE OUTCOME


The making of worlds, we are assured, was not purposeless, and its most
obvious purpose to our minds is the preparation of suitable abodes
for organic life. No other seems of comparable importance; no other,
indeed, comes within the full grasp of our apprehensive intelligence.
Yet its limitations must not be forgotten. The human standpoint is not
the only one from which the sum of things may be surveyed; and although
we be unable to quit it, we can still admit that the view obtainable
from it is probably not all-embracing. We only know with certainty that
the end which appears to us supreme has, in one case, been successfully
attained; how far it was sought to be compassed elsewhere must always
remain a matter of speculation.

On our own globe the presence of life is none the less mysterious for
being profuse and familiar. We can trace the strange history of its
slow unfolding, but the secret of its initiation baffles our utmost
scrutiny. The cooled rind of a once molten globe serves as the stage
for the drama; beneath it primeval heat still reigns. Temperature
rises steadily with descent into the interior of the earth; at a
depth of about two miles it must reach the boiling-point of water at
the sea-level. This temperature, which is absolutely prohibitive of
vitality, was formerly, beyond question, that of the surface. At some
long past epoch, accordingly, our future oceans hung suspended as a
prodigious envelope of vapour above a hot crust of slag and lava; our
teeming planet lay barren; it harboured no promise, no potency, no
visible possibility of life.

So it should have remained had the law of continuity been rigidly
enforced; but there came a time for a new beginning, and a new
beginning was made. A momentous alteration took place; inert Nature
was quickened; what had been sterile became all at once fruitful; an
immeasurable gulf was bridged, and movement was started along an
endless line of advance. That the advance was set on foot and directed
by an intelligent Will is the only inference derivable from a rational
survey of the known facts.

Life can be studied in its manifestations, not in itself. Attempts
to define it have served only to show our inability to 'lift the
painted veil.' We can, however, see that its presence is attended by
characteristic effects, brought about in harmony with the laws of
inorganic nature, although not in blind submission to them. Their
operation is somehow restrained, and appears to be subtly though
securely guided towards determinate ends prescribed by the vital
needs of each animal or plant. This modifying principle unmistakably
regulates the economy of every living organism; the cessation of its
activity means death.

Science has made no real progress towards solving the enigma of
vitality. Its evasiveness becomes, on the contrary, more apparent as
inquiry is rendered more exact. Under a laxer discipline of thought
the contrast between life and death seemed less glaring. It was
easily taken for granted that creeping things were engendered by
corruption, aid being invoked, if required, from the _virtus cœlestis_
of the eighth sphere. Thus, the birth of mice from the damp earth
was, in the ninth century, held to be signified by the word _mus_
(=humus);[119] and Van Helmont, at the height of the revival of
learning, published without misgiving a recipe for the creation of the
same animals.[120] Yet there was already better knowledge to be had
for the asking; and Francesco Redi, in 1668, crystallized Harvey's
opinion in the celebrated maxim, '_Omne vivum ex vivo._' Its truth is
incontrovertible. Challenged and tested again and again, it has as
often been vindicated, and may now be said, despite certain anomalous
effects of radium on veal broth, to stand outside the legitimate range
of debate. 'That life is an antecedent to life,' Lord Kelvin declared
in 1871, 'seems to me as sure a teaching of science as the law of
gravitation.'[121]

But the succession is not easy to start within the terms of a strictly
uniformitarian convention. The expedient is tempting, if scarcely
satisfactory, of demanding from the past what we dare not claim from
the present. Two and a half millenniums ago it was already in vogue.
Herodotus dismisses a genealogical embarrassment with the remark,
γένοιτο δ'άν πᾶν έν τῶ μακρῶ χρόνω, which may be freely translated, 'In
the long run of time anything may happen.' Conditions, we are apt to
think, may have been more elastic long ago. The proven impossibility
of to-day becomes vaguely thinkable seen through the mist of uncounted
yesterdays. 'If it were given to me,' Professor Huxley said,[122] 'to
look beyond the abyss of geologically recorded time to the still more
remote period when the earth was passing through physical and chemical
conditions which it can no more see again than a man can recall his
infancy, I should expect to be a witness of the evolution of living
protoplasm from non-living matter.' To these first vital compounds he
attributed a fungoid nature and mode of growth, and the choice deprived
his speculation of any plausibility that might otherwise have belonged
to it. Fungi are not self-supporting; they cannot supply themselves
with nourishment from the raw materials of the mineral world; they
depend upon the hospitality of differently organized beings. They
were, then, certainly not among the 'first mercies of nature.' Mr.
Herbert Spencer, too, was inclined to regard spontaneous generation
as a superannuated process. The leap from the non-vital to the vital,
now admitted by the saner kind of biologists to be impracticable,
might have been taken, it seemed to him, when 'the heat of the earth's
surface was falling through those ranges of temperature at which the
higher organic compounds are unstable.' But the 'reason why' is to
seek. A sterilized solution is precisely one which has cooled from
a high thermal grade; a baked brick is similarly circumstanced. Why
should the appearance of life in primeval times have been favoured by a
state of things fatal to it here and now?

The essence of the biological crux resides in 'protoplasm.' The word
was coined by Von Mohl in 1846, with the object of emphasizing the
importance of the substance it signified, which indeed forms the bulk
of every organism, animal and vegetable, man, mushroom, and amœba.
Huxley rightly termed it 'the physical basis of life,' adding, however,
the infelicitous conjecture that its production might have been one
of the lucky hits of nature. It would have been a hit of incalculable
moment, but of incalculable improbability. 'Odds beyond arithmetic'
were against that particular throw coming out of the Lucretian
dice-box. The 'primal slime' (to use Oken's phrase) is composed of
oxygen, nitrogen, hydrogen, and carbon, with minute percentages of
phosphates and other salts. But these constituents are put together in
a highly artificial manner. Eight or nine hundred elementary atoms,
in fact, go to the making of one molecule of protoplasm, forming a
structure of extreme complexity, most delicately balanced and eminently
unstable. It results, accordingly, from the employment of specially
directed forces, and stores, for the benefit of the producing organism,
the energy expended in its construction. Left to itself, it promptly
goes to pieces, and yields back its component particles to their native
inorganic sphere. The laws there ruling are in truth adverse to the
existence of protoplasm; abandoned to their unmitigated action, it
perishes. We should then as reasonably suppose that in the geological
past rivers flowed uphill as that inorganic nature stumbled blindly
upon this wonderful postulate and product of life.

Professor Huxley affirmed life to be 'a property of protoplasm,' the
inevitable outcome of 'the nature and disposition of its molecules,'
and he sought to cover the absurdity of the dictum by claiming as
analogous a case wholly disparate. Water, he argued, has qualities
totally unlike those of oxygen or hydrogen, and protoplasm may
similarly, by mere intricacy of arrangement, and the evoking of latent
affinities, become endowed with the transcendant powers connected
with animated existence. 'What better philosophical status, then,' he
exclaimed, 'has _vitality_ than _aquosity_?'[123] 'True,' he added,
'protoplasm can only be generated by protoplasm, in a manner that
evades our intelligence, but does anybody quite comprehend the _modus
operandi_ of an electric spark which traverses a mixture of oxygen and
hydrogen?' The illustration is inapt. The electric spark fulfils no
constructive function. It simply agitates the molecules so as to bring
their native affinities into play. It acts like a mechanical blow on
dynamite. Further, water is a stable compound, because its formation is
attended by loss of energy; it represents a plane permanently occupied
because reached by a steep descent; but protoplasm is, in this respect,
the antitype of water. It needs force for its composition; water needs
force for its decomposition. Protoplasm needs force plus a suitable
apparatus; it can be turned out only by an artfully adapted machine
with a head of steam on. It is thus continually manufactured by plants
under the stimulus of light. They provide the apparatus, sunbeams the
energy. If the supply of power is cut off, the machinery comes to a
halt, protoplasm ceases to be generated, the organism dies of inanition.

Many German biologists find themselves compelled, by the impossibility
of explaining vital activities in terms of chemistry or physics, to
associate protoplasm with some kind of psychical activity.[124]
Individuality, at least, implies an ultra-material principle, and it
asserts itself at the very base of the animal creation. An amœba is the
simplest of living beings. Formerly called the 'Proteus animalcule,' it
is 'everything in turn, and nothing long.' It can be round or radiated,
spherical or lenticular, as momentary convenience prescribes. Organs it
has none, its limbs are conspicuous by absence, it is 'sans everything'
that belongs to the ordinary outfit of an animated creature. Yet
such-like nucleated globules of protoplasm have flourished exuberantly
during countless ages. Adaptability insured survival. An amœba is at
home in almost any environment. What it has not ready-made, it can
supply at a moment's notice. Out of any part of its substance it can
improvise feelers and tentacles for the capture of its prey, as well as
a stomach for its digestion, and it thus effectively goes through the
full round of animal economy. Some varieties, too, are noted builders.
Those called Foraminifera have the faculty of secreting carbonate of
lime from sea-water, and construct with it fairy dwellings, perforated
in all directions to allow of the protrusion of exploratory filaments.
Fossil chambered shells of this type are extraordinarily abundant.
Their dense conglomeration in the chalk elicited Buffon's exclamation
that 'the very dust had been alive!'[125] The _calcaire grossier_ of
which Paris is built consists mainly of them, and to this day, in
oceanic depths, the materials of future capitals are in course of
preparation by the monumental industry of these unpretending creatures.

Such as they are, they maintain a status incomparable with that of
non-living things. Incomparable, for instance, as regards the water in
which they float. The contrast is startling despite its familiarity. An
amœba incarnates a purpose; it embodies a spark of personal existence,
unconsciously swaying the forces of inorganic nature towards the
ends of its own well-being. The subordination is most real, though
profoundly mysterious. In the organic and the inorganic worlds the same
laws hold good; the same ultimate atoms exert their preferences in
each; in neither is an uncaused effect possible. A bullet can no more
be fired from a gun that has no charge than a man can lift a finger
without a corresponding outlay of food-products. Accordingly, while
plants store and animals expend energy, plants and animals are equally
incompetent for its origination. What they can do is to appropriate
and specifically apply it; and herein resides the essence of life. 'It
would seem,' Sir George Stokes wrote in 1893,[126] 'to be something
of the nature of a directing power, not counteracting the action of
the physical forces, but guiding them into a determined channel.'
What the power is in itself it would be futile to seek to define. We
are only sure of its being extra-physical. Matter cannot evolve a
principle which disposes of it as its master. Evolution means only the
unfolding into self-evidence of something already obscurely present.
The 'latent process' (to use a Baconian term) of the hatching of an egg
is typical and instructive. Yet it is not the less recondite for being
daily conducted before our eyes. A concourse of suns, indeed, fails to
impress us with the unutterable wonder of the 'flower in the crannied
wall' apostrophized by the last great poet of the nineteenth century.

The two wide kingdoms of life lack a 'scientific frontier.' The
boundary-line is ill-marked and irregular. So much so that a few
naturalists have set up a neutral zone, or no man's land, inhabited
by creatures of mixed or uncertain nature, by plant-animals, or
zoophytes in the literal sense of the word. But the expedient avails
to shelter ignorance rather than to advance knowledge. For it seems
probable that there is no organism so imperfectly characterized as to
be genuinely incapable of giving a categorical answer to the question,
'Under which king, Bezonian?' Fungi might, perhaps, on a superficial
view, be taken for hybrids. They share the nature of animals so far
as to be unable to elaborate their own food, while appearing in other
respects to be authentic vegetables. They are, in fact, parasites
and scavengers. Not the smallest reason exists for supposing them to
constitute a genetic link between the two chief hierarchies. These
are, in all likelihood, fundamentally distinct. Only by a gratuitous
hypothesis can they be credited with a common ancestor. Each seeks
a different kind of perfection; their ideals, so to speak, follow
divergent tracks. That the tracks were marked out from the beginning
may be safely affirmed; and this implies radical separation. Plants
came first, since animals pre-suppose and imperatively require them;
the antecedence having quite possibly been by a vast interval of time.
On this point, geological evidence, though inconclusive, is suggestive.
The Laurentian beds, which are among the very earliest stratified
formations, contain no recognisable fossils. They were once supposed to
enshrine the remains of a lowly organism dubbed _Eozöon Canadense_; but
the markings that simulated animal forms are now known to be of mineral
origin. Laurentian graphite, on the other hand, occurs plentifully;
and graphite may be described as coal at a more advanced stage of
mineralization. Such deposits, we are led to believe, consist of
altered vegetable substances; and it seems to follow that these hoary
rocks are the burying ground of a profuse succession of virgin forests.
That they flourished beneath the sea--were, in fact, composed of
algæ--was the opinion of Professor Prestwich,[127] and it is not easily
gainsaid.

Primitive animal life was unquestionably marine, and the Huronian
strata, which overlie the Laurentian, afford traces of it in a few
sponge-spicules, the cast of an annelid, and such-like scanty leavings.
Higher up, the Cambrian series swarms with oceanic invertebrates;
fishes, the first type of vertebrates, came upon the scene in Silurian
times; and so, by a various and surprising progression, life advanced
through the ages, until the ascending sequence culminated with a being
cast in a diviner mould, who walks the earth, even now, with face
uplifted to the stars.

 'Natus homo est; illum mundi melioris origo
 Finxit in effigiem moderantûm cuncta deorum.'

In the vegetable kingdom the vital law of development has wrought with
less conspicuous effect. The superiority of recent to ancient floras
is more significant than striking. A tree-fern or a sigillaria bears
comparison with an oak much better than a trilobite or a plesiosaurus
with an eagle, horse, or lion. The geological variations of plants,
however, have unmistakably tended to make them more serviceable to
man--more serviceable to his material needs, and also more gratifying
to his æsthetic instincts. For him, flower-petals were painted and
perfumes distilled; for him, the grasses of the prairie laid up stores
of sustaining nutriment; in preparation for his advent, choice fruits
ripened and reddened under Tertiary sunshine; while the barren and
sombre vegetation of the Carboniferous epoch had already done its part
by dying down into seams of coal for the eventual supply of power for
human industry and warmth for human comfort.

It would be an abuse of our readers' patience to discuss the futile
conjecture of an extra-terrestrial origin for life on our globe. The
agency, in this connection, of germ-laden aerolites was first invoked
by Richter of Dresden; and Lord Kelvin gave currency to the notion by
an incidental reference to it in 1871 from the Presidential Chair of
the British Association. Its adoption would oblige us to regard the
denizens of our planet, fauna and flora alike, as salvage from the
wreck of some unknown world in space. _Credat Judæus Apella._ To our
minds, the adventures of Baron Munchausen appear more credible than
the pre-natal history of the primal organism implied by this 'wild
surmise.' Inquiry into the nature of the supposed organism serves
to draw closer the web of embarrassment. The remarkable aridity of
meteorites excludes the possibility of its having had an aquatic
habitat. Members of the fungoid order are unsuited to act as pioneers,
owing to their helplessness in the matter of commissariat; and the
spores or lichens or mosses could scarcely be expected to survive
the vicissitudes of such a journey as they must have performed if
meteor-borne to terrestrial shores. The immigration hypothesis,
moreover, even if it were plausible, could not be made useful.
Difficulties do not vanish on being pushed into a corner; the problem
of life is as formidable in one world as in another; we should not
expect to find it easier to square the circle in Mars than Deinostratos
found it in Greece; matter, we are convinced, has no more psychical
initiative in the system of Arcturus than can be ascribed to it in the
system of the sun. Profitless conjectures may then be dismissed; they
do not help us out of the slough of intellectual impotence.

This need not, indeed, be absolute. The determination to regard things
mechanically alone renders them unintelligible. Science becomes
unscientific when it refuses to be guided by experience; and we have
the plainest testimony of consciousness to the working in ourselves of
originative faculties independent of, and irrepressible by, physical
agencies. Here we hold the clue to the labyrinth. The intimation
conveyed is distinct of a Power outside nature, continually and
inscrutably acting for order, elevation, and vivification.

FOOTNOTES:

[Footnote 119: Hewitt, _Problems of the Age_, p. 105.]

[Footnote 120: Pasteur, _Annales de Chimie et de Physique_, tome xliv.,
p. 6, 1862.]

[Footnote 121: _Popular Lectures and Addresses_, vol. ii., p. 198.]

[Footnote 122: _Report British Association_, 1870, p. 84.]

[Footnote 123: _Collected Essays_, vol. i., p. 153]

[Footnote 124: Neumeister, _Betrachtungen über das Wesen der
Lebenserscheinungen_, 1903.]

[Footnote 125: Owen, _Palæontology_, pp. 11, 14.]

[Footnote 126: _Gifford Lectures_, p. 46.]

[Footnote 127: _Geology_, vol. ii., p. 22.]




INDEX


 Alexander, structure of the Milky Way, 214

 Amœba, versatile organization, 274-5

 Anaximander, generalized matter, 5

 Anaximenes, air as the 'Urstoff,' 4, 151

 Aristotle, conception of nature, 8;
   solid spheres, 13

 Asteroids, orbital relations, 236


 Babinet, objection to Laplace's cosmogony, 44

 Bacon, elemental triad, 151

 Ball, Sir Robert, mechanics of a condensing nebula, 79, 209-10

 Barnard, discoveries of nebulæ, 212;
   observations of the Gegenschein, 259

 Becquerel, detection of radio-activity, 157, 162

 Bessel, astronomy of the invisible, 141

 Bode's law of planetary distances, 26

 Brorsen, the zodiacal counterglow, 259

 Bruno, a platonic pantheist, 9

 Buffon, cometary impacts, 61;
   fossil foraminifera, 275


 Campbell, system of ζ Geminorum, 113;
   number of spectroscopic binaries, 143;
   density of nebulium, 153

 Chladni, meteoric cosmogony, 118

 Comets, position in Cartesian system, 12-13;
   in that of Laplace, 136;
   origin, 73-4, 252-4;
   meteoric relations, 122-4, 133;
   solar dependants, 133, 140, 236, 251;
   nebular affinities, 137, 254-5;
   insignificant mass, 242

 Cosmogony, primitive, 1-3, 14;
   philosophical, 3-7;
   scientific, 7, 15;
   of Descartes, 11-13, 60;
   Kant's, 21-28, 61-2, 252, 254;
   Laplace's, 29-33, 38-53, 57-61, 138, 235;
   Herschel's, 35-7, 136, 199-201;
   Faye's, 68-76, 81, 129;
   Lockyer's, 125-134;
   sidereal, 105-7, 145-7, 217-230

 Crookes, Sir William, protyle, 5, 149;
   radiant matter, 154, 156-7;
   future of the world, 163

 Curie, Madame, discovery of radium, 162


 Dalton, combining equivalents, 152

 Dark stars, 141, 226-7

 Darwin, condensation of nebulous rings, 42, 51;
   history of earth-moon system, 87-92, 94, 97, 142;
   equilibrium of rotating globes, 102, 104-5, 112, 143;
   fluid pressure on meteor-swarms, 129, 131;
   spiral nebulæ, 205

 Delambre, on cosmic speculations, 14, 15

 Democritus, mechanical theory of the world, 5, 22

 Descartes, theory of vortices, 11-13

 Dissipation of energy, 229

 Double stars, genesis and development, 98-9, 106-7, 142-5, 226-7;
   occulting, 108-113;
   variable, 113-115, 143;
   'dumb-bell' type, 115, 116, 143;
   frequency of occurrence, 144, 234


 Earth, effective rigidity, 86;
   critical situation, 87;
   antique disruption, 89, 91;
   tidal effects upon, 90, 244;
   meteoric intakes, 119-120, 133;
   internal heat, 266

 Easton, the galactic spiral, 214

 Eclipses, stellar, 107-8;
   questionable occurrence, 109-113;
   criterion of reality, 114

 Electricity, discharge through vacua, 154;
   relations with disintegrated matter, 155, 159-160, 163;
   gravity explained by, 177-8, 179;
   dual properties, 180-181;
   fundamental in nature, 182;
   ethereal transmission, 185

 Electrons, copious production, 174;
   strain-centres, 189-190, 195;
   ultimates of matter, 231

 Elements, ancient and mediæval ideas regarding, 5, 151;
   genesis, 152, 230-231

 Empedocles, four elements, 5, 151

 Ether, in Cartesian philosophy, 12;
   evasive character, 159, 183, 197-8;
   relation to gravity, 175, 185;
   properties, 184, 191, 193-4

 Evolution, of solar system, 22, 241, 250;
   of stars, 35-6, 217-225;
   of stellar systems, 98-9, 106-7, 142-5, 226-7;
   of nebulæ, 145-6, 200-211;
   of the chemical elements, 151, 230-231


 Faye, objections to Laplace's cosmogony, 25;
   modified scheme, 68-73, 81, 129;
   duration of the sun, 76

 Fleming, nature of electrons, 195

 Frost and Adams, spectroscopic binaries, 143, 221


 Galileo, first law of motion, 12

 Goldstein, cathode-rays, 155

 Gravitation, mode of action, 168, 185;
   velocity of transmission, 169, 175, 177;
   explanatory hypotheses, 170-179


 Hale, relationships of carbon stars, 225

 Halley, nature of nebulæ, 36

 Hansky, status of zodiacal light, 261

 Harvey, life from life, 268

 Helium, present in the sun, 55;
   origin by atomic decay, 158, 230;
   absorption in stars, 220, 222-3;
   a constituent of nebulæ, 230

 Helmholtz, maintenance of solar heat, 31, 34, 35, 74-5

 Helmont, spontaneous generation, 268

 Heracleitus, elemental evolution, 151

 Herodotus, omnipotence of time, 269

 Herschel, Sir John, nature of atoms, 156;
   Saturn's satellites, 240

 Herschel, Sir William, Uranian system, 29;
   sidereal evolution,
  35-6, 37, 136;
   observations of nebulæ, 199;
   history of their growth, 200-201

 Huggins, Sir William, discovery of gaseous nebulæ, 54, 153

 Huggins, Sir William and Lady, stellar development, 226

 Huxley, origin of life, 269, 271, 272

 Hydrogen, molecular velocity, 55;
   the unit-atom, 152;
   tenuity, 153;
   absorption in stars, 221-2


 Island-universes, 54


 Jeans, figures of equilibrium, 104-5;
   nature of radio-activity, 196

 Jupiter's system, 240-1


 Kant, cosmological speculations, 20-28, 60-62, 252, 254;
   tidal effects, 27, 84-5;
   Saturn's rings, 49;
   status of comets, 73

 Kelvin, Lord, Lesage's theory of gravitation, 174;
   vortex-atoms, 187;
   dissipation of energy, 229;
   origin of life, 268, 280

 Kepler, physical astronomy, 10

 Kirkwood, objection to nebular hypothesis, 50, 51;
   effects of solar tidal friction, 67


 Lambert, sidereal construction, 17-18

 Langley, mass of meteoric infalls, 119

 Laplace, hypothesis of planetary origin, 25, 29-31, 33, 36-42,
 52-3, 58-9, 60-62, 235;
   Saturn's ring-system, 49;
   _Mécanique Céleste_, 57;
   status of comets, 73, 136

 Larmor, scheme of molecular physics, 188;
   definition of ether, 189, 193

 Lenard, cathode-rays, 157

 Lesage, rationale of gravity, 172-3, 175

 Lewis, masses of double stars, 226

 Ligondès, scheme of planetary growth, 77-8, 81, 252;
   comets as survivals, 253, 256

 Lockyer, Sir Norman, meteoritic hypothesis, 125-7;
   enhanced lines, 223

 Lorentz, electrical hypothesis of gravity, 177-8, 185


 Magellanic clouds, spiral conformation, 213

 Marchand, plane of the zodiacal light, 263

 Mars, rotation of, 47-8

 Maxwell, origin of matter, 161;
   corpuscular theory of gravitation, 173

 Mayer, J. R., effects of tidal friction, 87

 Mercury, action upon of solar tides, 88, 96;
   eccentric orbit, 242

 Meteoritic formation of planets, 28-9, 119-122, 125, 128, 131-3;
   of comets, 122-4;
   of nebulæ, 126-7, 134

 Meteors, abundance, 119, 133;
   mineralogy, 121-2, 134;
   hyperbolic orbits, 133;
   attendance on the sun, 236

 Meteor-swarms, progressive dispersal, 72, 124;
   constitution, 122-4, 128-131;
   primitive existence, 134

 Milky Way, theories regarding, 16-17, 215;
   undeveloped condition, 201, 207;
   relation to sun's movement, 247

 Mohl, protoplasm, 270

 Moment of momentum, 23, 40, 44, 45, 65, 209

 Moon, origin, 87-92, 97, 139, 145;
   retreat from the earth, 244

 Moulton, nebular cosmogony, 44, 51, 82

 Myers, systems of β Lyræ and U Pegasi, 109-110


 Nebulæ, gaseous composition, 37, 153, 201, 205;
   illusory resolution, 53;
   spiral structure, 79-80, 106, 146, 190, 204-208, 211-214;
   classification, 199;
   development, 200-202, 205, 207-211;
   relation to stars, 202-3, 205;
   helium ingredient, 230, 255

 Nebular hypothesis, propounded by Kant, 22-8;
   by Laplace, 29-31, 38, 52-3, 61;
   difficulties, 39-52, 54-6;
   amendments, 60-82

 Nebulium, a constituent of nebulæ, 54, 153, 201

 Neptune, retrograde system of, 43, 52, 237

 Newcomb, transverse observation of the zodiacal light, 260

 Newton, transmission of gravity, 184, 185

 Nova Persei, nebulous appurtenance, 206


 Perrine, new Jovian satellites, 240

 Pezénas, notice of the counterglow, 259

 Phobos, anomalous revolution, 47-8, 237

 Phœbe, retrograde motion, 96, 237-8

 Pickering, E. C., binary nature of V Puppis, 112;
   structural relations of the Looped Nebula, 213

 Pickering, W. H., origin of Phœbe, 97, 238;
   photograph of a great nebula, 212;
   satellite discoveries, 237, 240

 Plato, conception of a world-soul, 8

 Poincaré, rotational equilibrium, 101, 105, 110, 143;
   destiny of the solar system, 243-4

 Prestwich, Laurentian graphite, 279

 Proctor, structure of the Milky Way, 214

 Protoplasm, its properties, 270-273

 Protyle, supposed discovery, 149, 154-7;
   function and qualities, 152;
   cosmic relations, 164-5

 Prout, unity of matter, 152


 Radio-activity, possible effects, 33-4, 229;
   discovery, 157;
   nature and cause, 158-9, 162-3, 196

 Red stars, affinities, 224-6

 Redi, transmission of life, 268

 Reynolds, explanation of gravity, 170;
   molecular physics, 188, 192-3

 Richter, germ-laden aerolites, 280

 Roberts, A. W., systems of R2 Centauri and of V Puppis, 111, 113

 Roberts, Isaac, spiral arrangements of stars, 214

 Roche, planet-making expedients, 48;
   reconstruction of Laplace's hypothesis, 62-67

 Roche's limit, 63, 115-6

 Röntgen rays, discovery, 157;
   ethereal nature, 158, 176;
   electronic effects, 163;
   suggested connection with gravity, 176

 Rothmann, early notice of the zodiacal light, 256

 Russell, photographs of Magellanic Clouds, 213

 Rutherford, atomic disintegration, 162


 Satellites, periods of revolution, 46;
   anomalies of movement, 47-8, 52, 96, 236-9;
   effects on of tidal friction, 93-4, 96, 97, 238;
   origin by fission, 105;
   'asteroidal,' 239-40

 Saturnian system, an example to cosmogonists, 41, 71;
   anomalies, 49, 52, 96, 237-240

 Schaeberle, photographs of nebulous spirals, 204, 212

 Secchi, star-classification, 219

 See, tidal friction in stellar systems, 98, 142

 Shelley's Demiorgon, 9

 Solar system, origin, 22, 39-42, 69-82, 241;
   anomalies, 29, 43-52, 236-241;
   mechanism, 30, 234-5;
   stability, 235, 242;
   springs of decadence, 243-5;
   translation, 245-8

 Spectroscopic binaries, 98, 112-114, 142-4, 221

 Spencer, status of nebulæ, 54;
   spontaneous generation, 270

 Spinoza, pantheistic views, 9

 Stars, relations to nebulæ, 35-6, 202-3;
   distribution, 147;
   relative ages, 217-221, 225;
   spectra, 221-5

 Stokes, Sir George, nature of life, 276

 Stoney, criticism of gravitational hypothesis, 179

 Sun, mode of development, 25, 31;
   maintenance of heat, 31-5;
   helium-constituent, 55-6;
   stellar relationships, 224-5;
   actual standing, 232-3;
   motion in space, 245-8

 Swedenborg, cosmic evolution, 4, 6, 15


 Thales, ideas of cosmic evolution, 4, 14, 151

 Thomson, J. J., mass of corpuscles, 158;
   gravitational effects, 173;
   Röntgen rays and gravity, 176;
   ether and matter, 190

 Tidal friction, effects in earth-moon system, 28, 87-92, 94, 97, 244;
   in other sub-systems, 48, 50, 93-7, 238-9;
   on binary stars, 98-9, 108, 142;
   modifying power, 46-7, 67-8, 83-7

 Tisserand, transmission-rate of gravity, 175


 Uranian system, retrograde motion, 43, 52, 70, 237


 Variable stars, 108-115

 Venus, mode of rotation, 96

 Vogel, stellar development, 219, 225


 Whewell, status of nebulæ, 54

 Whittaker, undulatory theory of gravity, 178-9

 Wilson, radium in the sun, 34

 Wolf, C., motion of Phobos, 48;
   solar tidal friction, 67;
   criticism of Faye's cosmogony, 73

 Wolf, Max, photographs of the zodiacal light, 263

 World Soul, function in cosmogony, 8-9

 Wright, theory of the Milky Way, 16-17, 18, 20


 Zodiacal light, origin, 256-7, 261, 264;
   triple aspect, 257-60;
   transversal dimensions, 260;
   constitution, 262;
   plane of extension, 262-3


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