THE PHENOMENA OF COMETS.
From the extracts thus cited we may form a fairly clear idea of the
phenomena which comets present, and these facts represent about all
that we know of these mysterious objects. They approach the sun
in a nearly radial direction, thus cutting the planetary orbits
transversely. They approach the sun from all directions and at
all angles, without reference to the common plane in which all the
planetary orbits lie. They have no rotation on their own axes, as the
planets have, but, like an aggregated mass of meteorites or cosmical
dust, rush inward from the exterior realms of space, so that their
course is diametrically opposite that of the planets and the other
cosmical bodies which constitute our solar system. Such a body as a
comet, in fact, would present in its approach to our solar system very
much the phenomena of an approaching exterior sun, corresponding far
more closely in appearance and behavior to our own sun than to any
of the planets. Such a body could not generate positive electricity,
as the planets do, but, on the contrary, must have an electrosphere
of negative, or at least neutral, polarity. On its approach to our
planetary system the batteries of all the planets would be at once
turned upon the intruder, and it would be rapidly thrown into the
same state of active electrical polarity as the sun. The aqueous
vapor condensed around its nucleus by gravity in its approach through
space, or buried among the meteoric particles constituting the
comet, would be necessarily decomposed into its constituent gases,
just as in the case of the sun, by the positive electrical currents
from the planetary electrospheres, and the disassociated hydrogen
would form the negative electrosphere of the comet, glowing with its
own luminosity, by gaseous incandescence. "We should then observe,
during its continued approach to the sun, phenomena similar to those
which we might expect to manifest themselves during the approach
of a minute solar body towards the sun, characterized by a rapid
increase of velocity, due to attraction of gravity, and tremendous
mutual repulsion between the solar and cometic electrospheres. We
should see the luminous hydrogen and associated gases boiling upward,
and thence drawn forward from the nucleus by the combined gravity
of the sun's mass, that of the planetary masses, and the opposite
polarity of the planetary electrospheres, while they would be, at
the same time, repelled backward by the enormous repulsive force of
the negative electrosphere of the sun. As a result, we should find
these gases in a state of ebullition, forced forward under great
excitement and disturbance, boiling, eddying about, driven to and fro
in all directions until the sun's repulsive force had overcome the
different attractions, when these luminous clouds or envelopes would
be swept swiftly off to the rear, as by a powerful current of wind,
around the margins of the nucleus, and they would be seen to stream
backward from the sun as an elongated envelope or tail. New volumes of
gas would pour to the front, attracted from deeper depths, and these,
on reaching the cometary electrosphere, would be again repelled by
the solar activity and driven to the rear, while the gases thus driven
backward, themselves similarly electrified, would mutually repel each
other as they streamed backward around the margins of the nucleus.
Let us now see what these gases are: if they are such as appear
in the sun's electrosphere, we will know that such must be their
action; if, on the contrary, they are such as appear in planetary
electrospheres, we will find any such attempted explanation to be
a failure. Quoting largely from Dr. Huggins, Professor Proctor, in
his "Cometic Mysteries," says, "The spectrum of the brightest comet
of that year was partly continuous, and on this continuous spectrum
many of the well-known Fraunhofer lines could be traced. This made it
certain that part of the comet's light was reflected sunlight, though
Dr. Huggins considers also that a part of the continuous spectrum of
every comet is due to inherent light. On this point some doubt may
be permitted. It is one thing for special bands to show themselves,
for some substances may become self-luminous under special conditions
at very moderate temperatures; it is quite another thing that the
solid parts of a comet's substance should become incandescent. I
venture to express my opinion that this can scarcely happen, except
in the case of comets which approach very near to the sun. Besides
the continuous spectrum with dark lines, the photograph showed
also a spectrum of bright lines. 'These lines,' says Dr. Huggins,
'possessed extreme interest, for there was certainly contained within
this hieroglyphic writing some new information. A discussion of
the position of these new lines showed them to be undoubtedly the
same lines which appear in certain compounds of carbon. Not long
before Professors Liveing and Dewar had found from their laboratory
experiments that these lines are only present when nitrogen is also
present, and that they indicate a nitrogen compound of carbon,--namely,
cyanogen. Two other bright groups were also seen in the photograph,
confirming the presence of hydrogen,--carbon and nitrogen.' It is
worthy of notice that only a few days later Dr. H. Draper succeeded
in obtaining a photograph of the same comet's spectrum. It appeared
to him to confirm Dr. Huggins's statements, except only that the dark
Fraunhofer lines were not visible, the photograph having probably
been taken under less favorable conditions.... But the latest comet
has brought with it fresh news. Its spectrum is not like that given
by the comets we are considering. The bright lines of sodium are
seen in it, and also other bright lines and groups of lines which
have not yet been shown to be identical with any belonging to the
hydrocarbon groups, but probably are so.... The cyanogen groups are
not seen.... But it is manifest that this comet underwent important
changes.... In April was found simply a faint continuous spectrum; in
May the three bands associated with carbon were present, though faint,
while there was no trace whatever of the sodium band. On the contrary,
in June the nucleus of the comet gave a very strong and extended
continuous spectrum with an excessively strong bright line in the
orange-yellow identical with the well-known double sodium line of the
solar spectrum. On this ... it is necessary to conclude that during
the last fortnight of May the spectrum of Wells's comet had changed
in a manner of which the history of science furnishes no precedent."
It should be observed that the elements carbon and hydrogen closely
resemble each other, not only in their multifarious chemical
affinities and reactions, but in their electric polarities, and the
hydrocarbon compounds, like their constituents, carbon and hydrogen,
are electrically similar to each other, an example of this similarity
of the elements being found in the identical action of the carbon
arc and hydrogen envelope in the heating and lighting experiments
with electrical currents hereinbefore described.
We have already seen that carbon follows quite a different law from the
other concrete elements, in the fact that its electrical resistance
diminishes as the temperature rises; it also differs widely from
the other solid elements in its atomic heat, which has a value much
less than one-half the mean constant, which is 6.4. Of this matter of
specific heat, Professor Fownes, in his work on chemistry (Bridges'
edition), says, "Dulong and Petit observed in the course of their
investigation a most remarkable circumstance. If the specific heats
of bodies be computed upon equal weights, numbers are obtained all
different and exhibiting no simple relations among themselves; but if,
instead of equal weights, quantities be taken in the proportion of the
atomic weights, an almost perfect coincidence in the numbers will be
observed, showing that some exceedingly intimate connection must exist
between the relations of bodies to heat and their chemical nature; and
when the circumstance is taken into view that relations of even a still
closer kind link together chemical and electrical phenomena, it is not
too much to expect that ere long some law may be discovered far more
general than any with which we are yet acquainted.... Nevertheless,
this law must not be understood as perfectly general, for there are
three elements--namely, carbon, boron, and silicon" [these form a
single group of elements in chemical classification]--"which exhibit
decided exceptions to it."
Organic chemistry is substantially based upon the almost infinitely
interchanging relations among carbon-hydrogen radicals, supplemented
by a few other elements. According to Professor Fownes, "Organic
chemistry is in fact the chemistry of carbon compounds." The position
of carbon among the elements is something like that of camphor among
the oils, the latter being a volatile oil, but concrete in form. With
a concrete element having the peculiar character of carbon we can
well understand its universal chemical and electrical relationship
with gaseous hydrogen in the grandest operations of nature.
Cyanogen is an electrically similar compound of carbon with the
addition of nitrogen. Of these elements it will be seen that nitrogen
and hydrogen are found to exist also in the gaseous nebulæ, and with
the probable addition there of oxygen; but in comets the quota of
active oxygen must be sought for in the correlated planetary, and
not in the cometic, atmospheres, as is the case with the sun. Of
the presence of the vapor of carbon in comets Professor Ball says,
"This is a very singular fact, when it is remembered that carbon is
one of the substances essentially associated with life in the forms in
which we know it." Professor Huggins says, "Since that time the light
from some twenty comets has been examined by different observers. The
general close agreement in all cases, notwithstanding some small
divergencies, of the bright bands in the cometary light with those
seen in the spectrum of hydrocarbons justifies us fully in ascribing
the original light of these comets to matter which contains carbon
in combination with hydrogen."
We may learn something further of the constitution of comets,
perhaps, by considering the chemical reactions which their spectra
seem to indicate. The following extract is from a recent article on
the manufacture of illuminating gas: "Ammonia contains 82.35 parts
of nitrogen and 17.65 of hydrogen. It is not produced by a direct
combination, for nitrogen can be caught and wedded only by a hot and
skilful wooing. In the gas retort, at a temperature of 2200 degrees
and in the presence of lime, soda, or potash, it will combine with
carbon and form cyanogen, and then further combine with the alkali
to form a cyanide. There is steam in the retort, and, as nearly
as the gas chemists can make out, the nitrogen promptly divorces
itself, gives up the carbon to the oxygen of the steam, and, taking
the hydrogen to itself, becomes, for the time at least, a fixed, if
volatile, substance, but ever ready to enter into new alliances." It
will be remembered that in the comets examined by Professors Huggins
and Draper the spectroscope revealed both cyanogen and the double
line of sodium. The function of the sodium is readily understood,
as by its presence it enables the nitrogen in the cometic atmosphere
to combine with a part of the carbon of the gaseous hydrocarbons
which constitute this atmosphere, and thus produce the cyanogen. But
to effect this combination requires in the retort a temperature of
2200 degrees. If the combining temperature around the nucleus of a
comet is the same, it will show that the temperature of this comet's
nucleus must be very high, and, while many times less than that of
the sun's photosphere, it still clearly illustrates the powerful
character of the impact of the planetary electrical currents upon
the comet, and its tremendous repulsion by the similarly electrified
solar electrosphere. The second one of the above reactions, that
from cyanogen to ammonia, is due to the steam or aqueous vapor in
the retort. But in the case of the comet all the aqueous vapor and
its constituent oxygen have disappeared by electrolytic decomposition
long before the combining temperature of cyanogen has been reached;
so that the sodium, the hydrocarbons, and the cyanogen alone appear,
and the oxygen compounds are missing. But on the reversal of polarity
of this comet by contact with a planetary electrosphere, should such
ever occur, and its consequent assumption of positive electricity,
the oxygen would again appear, and, if the temperature had not yet
receded below that of the reaction which produces ammoniacal vapors,
we might expect, should a fragment of this comet enter our atmosphere
as a meteorite, to find ammonia as well as sodium as a constituent
thereof; otherwise the ammonia would be replaced by carbonic oxide
and carbonic acid, by the action of oxygen upon the hydrocarbons,
and water by the action of oxygen upon the hydrogen of the same,
at much lower temperatures than would suffice for the generation of
ammonia. The cyanogen would then perhaps remain as cyanide of sodium,
unless decomposed by contact with the meteoric metallic iron at a
high temperature, as occurs in the operation known in the arts as
"case-hardening." The presence of microscopic diamonds in meteorites
may be accounted for by a somewhat similar reducing reaction under
heat and the active force of the planetary and solar voltaic arc.
In the popular view comets are always associated with tails, but,
in fact, comets without tails are far more numerous than those to
which these appendages pertain; the tails, when such exist, are the
direct result of the repulsive energy of the solar electrosphere,
and are only manifested when their proximity to the sun has aroused
sufficient activity to swiftly sweep backward from the sun with
inconceivable velocity the gaseous matter concentrated in and around
the nucleus. As these tails owe their formation to the sun's repulsive
energy, they must always extend radially outward from the sun, and by
the self-repulsive energy of the diverse constituents of the tails
themselves these will be broken occasionally into two, four, or six
lateral strands, and (possibly by the attraction of the different
planetary electrospheres) curvatures may be apparent along the sweep
of the comets' tails corresponding, in effect, with perturbations
produced by gravity in the orbit of the nucleus. Of these various
phenomena, Professor Proctor, in his article on comets, says, "A very
large number of comets have no visible tails. When first seen in the
telescope a comet usually presents a small, round disk of hazy light,
somewhat brighter near the center. As the comet approaches the sun the
disk lengthens, and, if the comet is to be a tailed one, traces begin
to be observed of a streakiness in the comet's light. Gradually a tail
is formed, which is turned always from the sun. The tail grows brighter
and larger, and the head becomes developed into a coma surrounding
a distinctly marked nucleus. Presently the comet is lost to view
through its near approach to the sun; but after a while it is again
seen, sometimes wonderfully changed in aspect through the effects of
solar heat. Some comets are brighter and more striking after passing
their point of nearest approach to the sun than before; others are
quite shorn of their splendor when they reappear." This change of
aspect is not due to solar heat, but to the energetic repulsion of
the solar electrosphere. The force of gravity irresistibly impels
the comet forward to the sun's electrical vortex, and the change of
aspect is due to the repulsion of its entire stock of free gaseous
matter into space in case its supply is small, or to its increased
development and pouring forth in case the supply is large. It is
like the volatilization by a heated atmosphere of ammoniacal gas,
for instance, absorbed in water. The ebullition is vastly increased
by the heat, but if the entire stock of ammonia has been driven off
in its passage through the heated medium, it will emerge with the
residual water quiescent; otherwise, in a state of increased agitation.
The same author, in "Cometic Mysteries," says, "Repulsion of the
cometary matter could only take place if this matter, after it has
been driven off from the nucleus, and the sun have both high electric
potentials of the same kind." His further guess, however, that it is
analogous to the aurora, is wide of the mark; it is due, in fact,
to the mutual repulsion of their similar negative electrospheres,
the cometic electrosphere, however, being so much smaller than that
of the sun that the latter shows no appreciable disturbance, as
is the case, under similar circumstances, with the electrospheres
of the earth and moon. In the article last quoted it is said,
"There is a dark space immediately behind the nucleus,--that is,
where the nucleus, if solid, would throw its shadow if there were
matter to receive the light all round so that the shadow could be
seen." This presents, it is stated, a great difficulty. The author,
by a happy guess,--almost an inspiration, in fact, of which this
splendid writer and observer was so full,--suggests in a foot-note a
possible explanation, which, while not in itself correct, suggests an
analogous process very like what we actually see. "If the particles
forming the envelopes are minute flat bodies, and if anything in the
circumstances under which these particles are driven off into the
tail causes them to always so arrange themselves that the planes in
which they severally lie pass through the axis of the tail (which,
if the tail is an electrical phenomenon, might very well happen),
then we should find the region behind the nucleus very dark or almost
black, for the particles in the direction of the line of sight there
would be turned edgewise towards us, whereas those on either side or
in the prolongation of the envelopes would turn their faces towards
the observer." As a matter of fact, the envelope streaming backward
from the nucleus forms a hollow tube, the opposite sides of which
exhibit the same mutual repulsion as both exhibit towards the sun;
hence the phenomenon would be similar to that exhibited by blowing
into a closed bag of porous material covered with wisps of cotton, for
example, and the gases, in addition to their rush backward from the
sun, would also exhibit a radial rush outward from the longitudinal
axis of the tail. This is what we actually observe, and sufficiently
accounts for the phenomenon, be it altogether or only partially real,
and not merely, as that author thinks it may be, apparent. It is said,
in the same article, that "Bredichen has shown that where there are
three tails to a comet their forms correspond with the theory that
the envelopes raised from the head are principally formed of hydrogen,
carbon, and iron; but this ... seems open at present to considerable
doubt." At all events, these separate tails are self-repulsive, or
they would be merged into each other by the sun's repulsive energy;
in fact, they occupy the resultant of the direction produced by the
line of the sun's repulsion and those of their own mutually repellent
force,--that is to say, radial or divergent.
It must not be supposed that these tails are of insignificant
proportions. "When we see the tail of a comet occupying a volume
thousands of times greater than that of the sun itself, the question
naturally suggests itself, 'How does it happen that so vast a body
can sweep through the solar system without deranging the motion of
every planet?' Conceding even an extreme tenuity to the substance
composing so vast a volume, one would still expect its mass to be
tremendous. For instance, if we supposed the whole mass of the tail of
the comet of 1843 to consist of hydrogen gas (the lightest substance
known to us), yet even then the mass of the tail would have largely
exceeded that of the sun. Every planet would have been dragged from
its orbit by so vast a mass passing so near. We know, on the contrary,
that no such effects were produced. The length of our year did not
change by a single second.... Thus we are forced to admit that the
actual substance of the comet was inconceivably rare.... From what we
have already seen, it will be manifest that the formation of comets'
tails is a process of a very marvellous nature, apparently involving
forces other than those with which we are acquainted. The tail, ninety
million miles in length, which was seen stretching from the head of
Newton's comet nearly along the path which the retreating comet had
to traverse, must, it would seem, have been formed by some force
far more active than the force of gravity. The distance traversed
by the comet in the last four weeks of its approach to the sun under
gravity was no greater than that over which the matter of the tail,
seen after the comet had circled around the sun, had been carried in a
few hours. Yet we have no other evidence of any repulsive force at all
being exerted by the sun,--at least no evidence which can be regarded
as demonstrative,--and still less have we any evidence of a repulsive
force exceeding in energy the sun's attracting power." (Proctor.)