reflecting all the incident light and of the specific gravity of water
would be repelled by the sun with a force that might balance ten times
their weight. For a spherule absorbing all the light falling upon
it this figure would be reduced to five times the weight. The small
particles of comets which, according to spectroscopic observations,
probably consist of hydrocarbons are not perfectly absorbing, but they
permit certain rays of the sun to pass. A closer calculation shows that
in this case forces of about 3.3 times the gravity would result.
Larger spherules yield smaller values. Bredichin’s second and third
classes would thus be well adapted to meet the requirements which the
radiation pressure demands.
It is more difficult to explain how such great forces of repulsion
as those of the first group of Bredichin or of the peculiar comets
of Swift and of Roerdam can occur. When a particle or drop of some
hydrocarbon is exposed to powerful radiation, it may finally become
so intensely heated that it will be carbonized. It will yield a
spongy coal, because gases (chiefly hydrogen) will escape during the
carbonization, and the particles of coal will resemble the little
grains of coal-dust which fall from the smokestacks of our steamboats,
and which afterwards float on the surface of the water. It is quite
conceivable that such spherules of coal (consisting probably of
so-called marguerites, felted or pearly structures resembling chains
of bacilli) may have a specific gravity of 0.1, if we make allowance
for the gases they include (compare page 106.) A light-absorbing drop
of this density of 0.1 might, in the most favorable case, experience
a repulsion forty times as strong as the gravitation of the sun. In
this manner we can picture to ourselves the possibility of the greatest
observed forces of repulsion.
The spectra of comets confirm in every respect the conclusions to which
the theory of the radiation pressure leads up. They display a faint,
continuous spectrum which is probably due to sunlight reflected by
the small particles. Besides this, we observe, as already mentioned,
a spectrum of gaseous hydrocarbons and cyanogen. These band spectra
are due to electric discharges; for they are observed in comets whose
distance from the sun is so great that they cannot appear luminous
owing to their own high temperatures. In the tail of Swift’s comet
banded spectra have been observed in portions which were about five
million kilometres from the nucleus. The electric discharges must
chiefly be emitted from the outer parts of the tails, where, according
to the laws of static electricity, the electric forces would be
strongest. For this reason the larger tails of comets look as if they
were enveloped in cloaks of light of a more intense luminosity.
[Illustration: Fig. 36.—Imitation of comets’ tails. Experiment
by Nichols and Hull. The light of an arc-lamp is concentrated
by a lens upon the stream of finely powdered particles]
When a comet comes nearer to the sun, other less volatile bodies also
begin to evaporate. We then find the lines of sodium and, when the
comet comes very close to the sun, also the lines of iron in its
spectrum. These lines are evidently produced by substances which have
been evaporated from the nucleus of the comet. Like the meteorites
falling upon our earth, the nucleus will consist essentially of
silicates, and particularly of the silicate of sodium, and, further, of
iron.
We can easily imagine how the tails of comets change in appearance.
When a comet draws near to the sun, we observe that matter is ejected
from that part of the nucleus which is turned towards the sun. The
case is analogous to the formation of clouds in the terrestrial
atmosphere on a hot summer day. The clouds are provided with a kind of
hood which envelops like a thin, semi-spherical veil that side of the
nucleus which turns to the sun. Sometimes we observe two or more hoods
corresponding to the different layers of clouds in the terrestrial
atmosphere. From the farther side of the hood matter streams away
from the sun. The tails of comets are usually more highly developed
when they approach the sun than when they recede from it. That may
be, as has been assumed for a long time, because a large part of the
hydrocarbons will become exhausted while the comet passes the sun. We
have also noticed that the so-called periodical comets, which return to
the sun at regular intervals, showed at every reappearance a fainter
development of the tail. Comets, further, shine at their greatest
brilliancy in periods of strong solar-spot activity. We may, therefore,
assume that in those periods the surroundings of the sun are charged
to a relatively high degree with the fine dust which can serve as a
condensation nucleus for the matter of the comets’ tails. It is also
probable that in such periods the ionizing radiation of the sun is
more pronounced than usual, owing to the simultaneous predominance of
faculæ.
Nichols and Hull have attempted to imitate tails of comets. They
heated the spores of the fungus _Lycoperdon bovista_, which are almost
spherical and of a diameter of about 0.002 mm., up to a red glow, and
they thus produced little spongy balls of carbon of an average density
of 0.1. These they mixed with emery-powder and introduced them into
a glass vessel resembling an hour-glass (Fig. 36) from which the air
had previously been exhausted as far as possible. They then caused
the powdered mass to fall in a fine stream into the lower part of the
vessel while exposing it at the same time to the concentrated light of
an arc-lamp. The emery particles fell perpendicularly to the bottom,
while the little balls of carbon were driven aside by the radiation
pressure of the light.
We also meet with the effects of the radiation pressure in the
immediate neighborhood of the sun. The rectilinear extension of the
corona streamers to a distance which has been known to exceed six times
the solar diameter (about eight million km.) indicates that repelling
forces from the sun are acting upon the fine dust. Astronomers have
also compared the corona of the sun with the tails of comets, and
Donitsch would class it with Bredichin’s comets’ tails of the second
class. It is possible to calculate the mass of the corona from its
radiation of heat and light. The heat radiated has been measured by
Abbot. At a distance of 30,000 km. from the photosphere, the corona
radiated only as little heat as a body at -55° Cent. The reason is
that the corona in these parts consists of an extremely attenuated
mist whose actual temperature can be estimated by Stefan’s law at
4300° Cent. The corona must, therefore, be so attenuated that it
would only cover a 190,000th part of the sky behind it. We arrive at
the same result when we calculate the amount of light radiated by
the corona; this radiation is of the order of that of the full moon,
being sometimes smaller, sometimes greater, up to twice as great. The
considerations we have been offering apply to the most intense part of
the corona, the so-called inner corona. According to Turner, its light
intensity outward diminishes in the inverse ratio of the sixth power
of its distance from the centre of the sun. At the distance of a solar
radius (690,000 km.) the light intensity would therefore be only 1.6
per cent. of the intensity near the surface of the sun.
Let us assume that the matter of the corona consists of particles of
just such a size that the radiation pressure would balance their weight
(other particles would be expelled from the inner corona); then we find
that the weight of the whole corona of the sun would not exceed twelve
million metric tons. That is not more than the weight of four hundred
of our large ocean steamships (_e.g._, the _Oceanic_), and only about
as much as the quantity of coal burned on the earth within one week.
That the mass of the corona must be extremely rarefied has already
been concluded, from the fact that comets have wandered through the
corona without being visibly arrested in their motion. In 1843 a
comet passed the sun’s surface at a distance of only one-quarter
the sun’s radius without being disturbed in its progress. Moulton
calculated that the great comet of 1881, which approached the sun
within one-half its radius, did not encounter a resistance of more than
one-fifty-thousandth of its mass, and that the nucleus of the comet
was at least five million times denser than the matter of the corona.
Newcomb has possibly expressed the degree of attenuation of the corona
in a somewhat exaggerated way when he said that it contains perhaps one
grain of dust per cubic kilometre (a cube whose side has a length of
three-fifths of a mile).
However small the quantity of matter in the corona may be, and however
unimportant a fraction of this mass may pass into the coronal rays, it
is yet certain that there is a constant loss of finely divided matter
from the sun. The loss, however, is not greater than the supply of
matter (compare below)—namely, about 300 thousand millions of tons in
a year—so that during one billion years not even one-six-thousandth of
the solar mass (2 × 10^{27} tons) will be scattered into space. This
number is very unreliable, however. We know that many meteorites fall
upon the earth, partly as compact stones, partly as the finest dust of
shooting-stars which flash up in the terrestrial atmosphere rapidly to
be extinguished. These masses may be estimated at about 20,000 tons
per year. According to this estimate, the rain of meteorites which
falls upon the sun may amount to 300 thousand millions of tons in a
year. All the suns have emitted matter into space for infinite ages,
and it seems, therefore, a natural inference that many suns would no
longer be in existence if there had not been a supply of matter to
make up for this loss. The cold suns undergo relatively small losses,
but receive just as large inflows of matter as the warm suns. As, now,
our sun belongs to the colder type of stars, it is probable that the
loss of matter from the sun has for this reason been overestimated by
being presumed to be as great as the accession. The presence of dark
celestial bodies may compensate for this overestimation.
[Illustration: Fig. 37.—Granular chondrum from the meteorite of
Sexes.
Enlargement 1: 70. After S. Tschermak]
Whence do the meteorites come? If they were not constantly being
created, their number should have diminished in the course of ages;
for they are gradually being caught up by the larger celestial bodies.
It is not at all improbable that they arise from the accrescence of
small particles which the radiation pressure has been driving out
of the sun. The chondri, which are so characteristic of meteorites,
display a structure as if they had grown together out of a multitude
of extremely fine grains (Fig. 37). Nordenskiöld says: “Most meteoric
iron consists of an extremely delicate texture of various alloys of
metals. This mass of meteoric iron is often so porous that it oxidizes
on exposure to the air like spongy iron. The Pallas iron, when cut
through with a saw, shows this property, which is so distressing for
the collector. The iron of Cranbourne, of Toluca, and others—in fact,
almost all the meteorites with a few exceptions—display the same
texture. It all indicates that these cosmical masses of iron were built
up in the universe by particle being piled upon particle, of iron,
nickel, phosphorus, etc., analogous to the manner in which one atom
of a metal coalesces with another atom when the metal is galvanically
deposited from a solution. Most of the stony meteorites present a
similar appearance. Apart from the crust of slag on the surface, the
stone is often so porous and so loose that it might be used as a
filtering material, and it may easily be crumbled between the fingers.”
When the electrically charged grains of dust coalesce, their small
electrical potential (of about 0.02 volt) may increase considerably.
Under the influence of ultra-violet light these masses of meteorites
are discharged when they approach the sun, as Lenard has shown. Their
negative charge then escapes in the shape of so-called electrons.
Since, now, the sun loses through the rays of the corona large
multitudes of particles, and these particles probably carry, according
to Wilson, negative electricity with them, the positive charge must
remain behind in the stratum from which the coronal rays were emitted,
and also on the sun itself. If this charge were sufficiently powerful,
it would prevent the negatively charged particles in the corona from
escaping from the sun, and all the phenomena which we have ascribed
to the radiation pressure would cease. By the aid of the tenets of
the modern theory of electrons, I have calculated the maximum charge
that the sun could bear, if it is not to stop these phenomena. The
charge would amount to two hundred thousand millions of coulombs—not
by any means too large a quantity of electricity, as it would only be
sufficient to decompose twenty-four tons of water.
By means of this positive charge the sun exerts a vast attractive
power upon all negatively charged particles which come near it. We
have already remarked that the grains of sun-dust which have united to
form meteorites lose under the influence of ultra-violet light their
charge in the shape of negative electrons, extremely minute particles,
of which perhaps one thousand weigh as much as one atom of hydrogen
(1 gramme of hydrogen contains about 10^{24} atoms, corresponding to
10^{27} electrons). These electrons wander about in space. When they
approach a positively charged celestial body they are attracted by it
with great force. If the electrons were moving with a velocity of 300
km. per second, as in Lenard’s experiment, and if the sun were charged
to one-tenth the maximum amount just calculated, it would be able to
draw up all the electrons whose rectilinear path (so far as not curved
by the sun’s attraction) would lie at a distance from the sun 125 times
as great as the distance between the sun and its most remote planet,
Neptune, and 3800 times as great as the distance between the sun and
the earth, which, after all, would only be one-sixtieth of the distance
from our nearest fixed-star neighbor. The sun drains, so to speak, its
surroundings of negative electricity, and this draining effect carries
to the sun, as could easily be proved, a quantity of electricity which
is directly dependent upon the positive charge of the sun. Thus, so
far as electricity is concerned, ample provision has been made for
maintaining equilibrium between the income and expenditure of the sun.
* * * * *
When an electrical particle enters into a magnetic field it describes a
spiral about the so-called magnetic lines of force; when at a greater
distance, the particles appear to move in the direction of the lines
themselves. The rays of the corona emanating from the solar poles show
a distinct curvature like that of the lines of force about a magnet,
and for this reason the sun has been regarded as a big magnet whose
magnetic poles nearly coincide with the geographical poles. The coronal
rays nearer the equator likewise show this curvature (compare Fig.
30). The repelling force of the radiation pressure there is, however,
at right angles to the lines of force and much stronger than the
magnetic force, so that the rays of the corona are compelled to form
two big streams flowing in the equatorial direction. This is especially
noticeable at times of sun-spot minima. During the times of sun-spot
maxima the strength of the radiation pressure of the initial velocity
of the grains of dust seem to predominate so markedly that the magnetic
force is relatively small.
The astronomers tell us that the sun is only a star of small light
intensity compared to the prominent stars which excite our admiration.
The sun further belongs to a group of relatively cold stars. We may
easily imagine, therefore, that the radiation pressure in the vicinity
of these larger stars will be able to move much larger masses of matter
than in our solar system. If the different stars had at any time
consisted of different chemical elements, this difference would have
been equalized in the course of ages. The meteorites may be regarded as
samples of matter collected and despatched from all possible divisions
of space. Now, what bodies do we find in them?
In the comets (compare page 104) iron, sodium, carbon, hydrogen,
and nitrogen (as cyanogen) play the most important part. We know,
especially from the researches of Schiaparelli that meteorites
often represent fragments of comets, and must therefore be related
to them. Thus Biela’s comet, which had a period of 6.6 years, has
disappeared since 1852—it had divided into two parts in 1844-1845.
The comet was rediscovered in a belt of meteorites of the same period
which approaches the orbit of the earth each year on November 27.
Similar relations have been observed with regard to several other
swarms of meteorites. We know also that the just-mentioned elements
which spectrum analysis has proved to exist in comets are the main
constituents of the meteorites, which, in addition, contain the metals
calcium, magnesium, aluminium, nickel, cobalt, and chromium, as well as
the metalloids oxygen, silicon, sulphur, phosphorus, chlorine, arsenic,
argon, and helium. Their composition strongly recalls the volcanic
products of so-called basic nature—that is to say, those which contain
relatively large proportions of metallic oxides, and which have been
thought for good reasons to hail from the deeper strata of the interior
of the earth. Lockyer heated meteoric stones in the electric arc to
incandescence and found their spectra to be very similar to the solar
spectrum.
We therefore draw the conclusion that these messengers from other
solar systems which bring us samples of their chemical elements are
closely related to our sun and to the interior of our earth. That
other stars and comets are essentially composed of the same elements
as our sun and earth, spectrum analysis had already intimated to us.
But various metalloids, like chlorine, bromine, sulphur, phosphorus,
and arsenic, which are of importance for the composition of the earth,
have so far not been traced in the spectra of the celestial bodies,
nor in that of the sun. We find them in meteorites, however, and there
is not the slightest doubt that we must likewise count them among the
essential constituents of the sun and other celestial bodies. It is
with difficulty, however, that the metalloids can be made to exhibit
their spectra, and this is manifestly the reason why spectrum analysis
has not yet succeeded in establishing their presence in the heavens. As
regards the recently discovered so-called noble gases helium, argon,
neon, krypton, and xenon, their presence in the chromosphere has been
discovered on spectrograms taken during eclipses of the sun (Stassano).
According to Mitchell, however, these statements would appear to be
somewhat uncertain as to krypton and xenon.
The small particles of dust which the radiation pressure drives out
into space to all possible distances from the sun and the stars may hit
against one another and may accumulate to larger or smaller aggregates
in the shape of cosmical dust or meteorites. These aggregates
will partly fall upon other stars, planets, comets, or moons, and
partly—and this in very great multitudes—they will float about in
space. There they may, together with the larger dark celestial bodies,
form a kind of haze, which partly hides from us the light of distant
celestial bodies. Hence we do not see the whole sky covered with
luminous stars, which would be the case if, as we may surmise, the
stars were uniformly distributed all through the infinite space of the
universe, and if there were no obstacle to their emission of light. If
there were no other celestial bodies of very low temperature and very
large dimensions which absorbed the heat of the bright suns, the dark
celestial bodies, the meteorites, and the dark cosmical dust would soon
be so strongly heated by solar radiation that they would themselves
turn incandescent, and the whole dome of the sky would appear to us
like one glowing vault whose hot radiation down to the earth would soon
burn every living thing.
These other cold celestial bodies which absorb the solar rays
without themselves becoming hot are known as nebulæ. More recent
researches make us believe that these peculiar celestial bodies occur
nearly everywhere in the sky. The wonderful mechanism which enables
them to absorb heat without raising their own temperature will be
explained later (in Chapter VII.). As these cold nebulæ occupy vast
portions of space, most of the cosmical dust must finally, in its
wanderings through infinite space, stray into them. This dust will
there meet masses of gases which stop the penetration of the small
corpuscles. As the dust is electrically charged (particularly with
negative electricity), these charges will also be accumulated in the
outer layers of the nebulæ. This will proceed until the electrical
tension becomes so strong that discharges are started by the ejection
of electrons. The surrounding gases will therefore be rendered
luminescent, although their temperature may not much (perhaps by 50°)
exceed absolute zero, -273° Cent., and in this way we are enabled to
observe these nebulæ. Most of the particles will be stopped before
they have had time to penetrate very deeply into a nebula, and it
will therefore principally be the outer portions of the nebulæ which
send their light to us. That would conform to Herschel’s description
of planetary nebulæ, which display no greater luminosity in their
centres, but which shine as if they formed hollow spherical shells of
nebulous matter. It is very easy to demonstrate that only substances,
such as helium and hydrogen, which are most difficult to condense,
can at this low temperature exist in gaseous form to any noticeable
degree. The nebulæ, therefore, shine almost exclusively in the light of
these gases. There occurs in the nebulæ, in addition to these gases, a
mysterious substance, nebulium, whose peculiar spectrum has not been
found on the earth nor in the light of stars. Formerly the character
of the nebular spectrum was explained by the assumption either that no
other bodies occurred in nebulæ than the substances mentioned, or that
all the other elements in them were decomposed into hydrogen—helium
was not known then. The simple explanation is that only the gases of
the outer layer of the nebulæ are luminous. How their interiors are
constituted, we do not know.
It has been objected to the view just expressed that the whole sky
should glow in a nebulous light, and that even the outer atmosphere of
the earth should display such a glow. But hydrogen and helium occur
only very sparely in the terrestrial atmosphere. We find, however,
another light, the so-called auroral line, which may possibly be due to
krypton in our atmosphere. Whichever way we turn the spectroscope on a
very clear night, especially in the tropics, we observe this peculiar
green line. It was formerly considered to be characteristic of the
Zodiacal Light, but on a closer examination it has been traced all over
the sky, even where the Zodiacal Light could not be observed. One of
the objections to our view is therefore unjustified.