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.]