Transcriber’s Notes

Obvious typographical errors have been silently corrected. Variations
in hyphenation have been standardised but all other spelling and
punctuation remains unchanged.

Italics are represented thus _italic_, bold thus =bold= and
superscripts thus y^{en}.




                                WORLDS
                             IN THE MAKING

                     THE EVOLUTION OF THE UNIVERSE


                                  BY
                           SVANTE ARRHENIUS

                DIRECTOR OF THE PHYSICO-CHEMICAL NOBEL
                         INSTITUTE, STOCKHOLM


                             TRANSLATED BY
                             DR. H. BORNS


                              ILLUSTRATED

                               [colophon]

                          NEW YORK AND LONDON
                    HARPER & BROTHERS PUBLISHERS
                               MCMVIII


                Copyright, 1908, by HARPER & BROTHERS.

                        _All rights reserved._

                        Published March, 1908.




                           TABLE OF CONTENTS


    I. VOLCANIC PHENOMENA AND EARTHQUAKES                               1

    Destruction caused by volcanism and by earthquakes.—Different
    kinds of volcanoes.—Vesuvius.—Products of eruption.—Volcanic
    activity diminishing.—Structure of volcanoes.—Geographical
    distribution of volcanoes.—Temperature in the interior of
    the earth.—Significance of water for volcanism.—Composition
    of the earth’s interior.—Geographical distribution of
    earthquakes.—Fissures in the earth’s crust.—Groups of
    earthquakes.—Waves in the sea and in the air accompanying
    earthquakes.—Their connection with volcanism.—Systems of
    fissures.—Seismograms.

    II. THE CELESTIAL BODIES, IN PARTICULAR THE EARTH, AS ABODES OF
    LIVING BEINGS                                                     39

    Manifold character of the worlds.—The earth probably at
    first a ball of gases.—Formation of the earth crust and
    its rapid cooling.—Balance between heat received and heat
    lost by radiation.—Life already existing on the earth for
    a milliard of years.—The waste of solar heat.—Temperature
    and habitability of the planets.—Heat-preserving influence
    of the atmosphere.—Significance of carbon dioxide in the
    atmosphere.—Warm and cold geological ages.—Fluctuations in
    the percentage of carbon dioxide of the air.—Combustion,
    decay, and growth.—Atmospheric oxygen.—Vegetable life more
    ancient than animal life.—The atmospheres of planets.—Chances
    of an improvement in the climate.

    III. RADIATION AND CONSTITUTION OF THE SUN                        64

    Stability of the solar system.—Losses and possible
    gains of heat by the sun.—Theses of Mayer and of
    Helmholtz.—Temperatures of the white, yellow, and
    reddish stars, and of the sun.—Sun-spots and sun
    faculæ.—Prominences.—Spectra of the parts of the
    sun.—Temperature of the sun.—The interior of the sun.—Its
    composition according to the mechanical theory of heat.—The
    losses of heat by the sun probably covered by the enormous
    solar energy.

    IV. THE RADIATION PRESSURE                                        94

    Newton’s law of gravitation.—Kepler’s observation of comets’
    tails.—The thesis of Euler.—Proof of Maxwell.—The radiation
    pressure.—Electric charges and condensation.—Comets’ tails
    and radiation pressure.—Constituents and properties of comets’
    tails.—Weight of the solar corona.—Loss and gain of matter
    by the sun.—Nature of meteorites.—Electric charge of the
    sun.—Electrons drawn into the sun.—Magnetic properties of
    the sun and appearance of the corona.—Constituents of the
    meteors.—Nebulæ and their heat and light.

    V. THE SOLAR DUST IN THE ATMOSPHERE. POLAR LIGHTS AND THE
    VARIATIONS OF TERRESTRIAL MAGNETISM                              118

    The supply of dust from the sun rather
    insignificant.—Polarization of the light of the sky.—The
    upper clouds.—Different kinds of auroræ.—Their
    connection with the corona of the sun.—Polar lights and
    sun-spots.—Periodicity of polar lights.—Polar lights and
    magnetic disturbances.—Velocity of solar dust.—Fixation of
    atmospheric nitrogen.—The Zodiacal Light.

    VI. END OF THE SUN.—ORIGIN OF NEBULÆ                             148

    The extinction of the sun.—Collision between two celestial
    bodies.—The new star in Perseus.—Formation of nebulæ.—The
    appearance of nebulæ.—The nebulæ catch wandering meteors and
    comets.—The ring nebula in Lyra.—Variable stars.—Eta in
    Argus.—Mira Ceti.—Lyra and Algol stars.—Evolution of the
    stars.

    VII. THE NEBULAR AND THE SOLAR STATES                            191

    The energy of the universe.—The entropy of the
    universe.—The entropy increases in the suns, but
    decreases in the nebulæ.—Temperature and constitution
    of the nebulæ.—Schuster’s calculations of the condition
    of a celestial body consisting of gases.—Action of the
    loss of heat on nebulæ and on suns.—Development of a
    rotating nebula into a planetary system.—The hypothesis of
    Kant-Laplace.—Objections to it.—The views of Chamberlin
    and Moulton.—The radiation pressure balances the effect of
    Newtonian gravitation.—The emission of gases from the nebulæ
    balances the waste of heat characteristic to the solar systems.

    VIII. THE SPREADING OF LIFE THROUGH THE UNIVERSE                 212

    Stability of the species.—Theory of mutation.—Spontaneous
    generation.—Bathybius.—Panspermia.—The stand-points of
    Richter, Ferdinand Cohn, and Lord Kelvin.—The radiation
    pressure enables spores to escape.—The effect of strong
    sunlight and of cold on the germinating power.—Transport of
    spores through the atmosphere into universal space and through
    it to other planets.—General conclusions.


                  EXPLANATION OF ABBREVIATIONS, ETC.

The temperatures are stated in degrees centigrade (° C.), either on the
Celsius scale, on which the freezing-point of water is 0°, or on the
absolute scale, whose zero lies 273 degrees below the freezing-point of
water, at -273° C. The equivalent temperatures on the Fahrenheit scale
(freezing-point of water 32° F.) are added in brackets (° F.).

1 metre (m.) = 10 decimetres (dm.) = 100 centimetres (cm.) = 1000
millimetres (mm.) = 3.28 ft.; 1 kilometre (km.) = 1000 metres (m.) =
0.62 miles; 1 mile = 1.6 kilometres (km.).

Light travels in vacuo at the rate of 300,000 km. (nearly 200,000
miles) per second.




                             ILLUSTRATIONS


    FIG.                                                           PAGE

    1. VESUVIUS, AS SEEN FROM THE ISLAND OF NISIDA, IN
    MODERATE ACTIVITY                                                  2

    2. ERUPTION OF VESUVIUS IN 1882                                    4

    3. ERUPTION OF VESUVIUS IN 1872                                    6

    4. PHOTOGRAPH OF VESUVIUS, 1906. CHIEFLY CLOUDS OF
    ASHES                                                              8

    5. BLOCK LAVA ON MAUNA LOA                                        10

    6. THE EXCELSIOR GEYSER IN YELLOWSTONE PARK, U. S. A.
    REMNANT OF THE POWERFUL VOLCANIC ACTIVITY IN THE
    TERTIARY AGE                                                      11

    7. MATO TEPEE IN WYOMING, U. S. A. TYPICAL VOLCANIC
    “NECK”                                                            12

    8. CLEFTS FILLED WITH LAVA AND VOLCANIC CONE OF ASHES,
    TOROWHEAP CAÑON, PLATEAU OF COLORADO                              13

    9. THE KILAUEA CRATER ON HAWAII                                   15

    10. CHIEF EARTHQUAKE CENTRES, ACCORDING TO THE BRITISH
    ASSOCIATION COMMITTEE                                             22

    11. CLEFTS IN VALENTIA STREET, SAN FRANCISCO, AFTER THE
    EARTHQUAKE OF 1906                                                25

    12. SAND CRATERS AND FISSURES, PRODUCED BY THE CORINTH
    EARTHQUAKE OF 1861. IN THE WATER, BRANCHES OF
    FLOODED TREES                                                     27

    13. EARTHQUAKE LINES IN LOWER AUSTRIA                             30

    14. LIBERTY BUILDING OF LELAND STANFORD JUNIOR UNIVERSITY,
    IN CALIFORNIA, AFTER THE EARTHQUAKE OF 1906                       32

    15. EARTHQUAKE LINES IN THE TYRRHENIAN DEPRESSION                 34

    16. SEISMOGRAM RECORDED AT SHIDE, ISLE OF WIGHT, ON
    AUGUST 31, 1898                                                   35

    17. PHOTOGRAPH OF THE SURFACE OF THE MOON, IN THE VICINITY
    OF THE CRATER OF COPERNICUS                                       62

    18. SUN-SPOT GROUP AND GRANULATION OF THE SUN                     74

    19. PART OF THE SOLAR SPECTRUM OF JANUARY 3, 1872                 75

    20. METALLIC PROMINENCES IN VORTEX MOTION                         76

    21. FOUNTAIN-LIKE METALLIC PROMINENCES                            76

    22. QUIET PROMINENCES OF SMOKE-COLUMN TYPE                        77

    23. QUIET PROMINENCES, SHAPE OF A TREE                            77

    24. DIAGRAM ILLUSTRATING THE DIFFERENCES IN THE SPECTRA
    OF SUN-SPOTS AND OF THE PHOTOSPHERE                               78

    25. SPECTRUM OF A SUN-SPOT, THE CENTRAL BAND BETWEEN
    THE TWO PORTIONS OF THE PHOTOSPHERE SPECTRUM                      78

    26. THE GREAT SUN-SPOT OF OCTOBER 9, 1903                         79

    27. THE GREAT SUN-SPOT OF OCTOBER 9, 1903                         80

    28. THE GREAT SUN-SPOT OF OCTOBER 9, 1903                         81

    29. THE GREAT SUN-SPOT OF OCTOBER 9, 1903                         82

    30. PHOTOGRAPH OF THE SOLAR CORONA OF 1900                        83

    31. PHOTOGRAPH OF THE SOLAR CORONA OF 1870                        84

    32. PHOTOGRAPH OF THE SOLAR CORONA OF 1898                        85

    33. PHOTOGRAPH OF ROERDAM’S COMET (1893 II.), SUGGESTING
    SEVERAL STRONG NUCLEI IN THE TAIL                                100

    34. PHOTOGRAPH OF SWIFT’S COMET (1892 I.)                        101

    35. DONATI’S COMET AT ITS GREATEST BRILLIANCY IN 1858            102

    36. IMITATION OF COMETS’ TAILS                                   104

    37. GRANULAR CHONDRUM FROM THE METEORITE OF SEXES.
    ENLARGEMENT 1:70                                                 109

    38. ARCH-SHAPED AURORÆ BOREALIS, OBSERVED BY NORDENSKIOLD
    DURING THE WINTERING OF THE VEGA IN
    BERING STRAIT 1879                                               124

    39. AURORA BOREALIS, WITH RADIAL STREAMERS                       125

    40. AURORA WITH CORONA, OBSERVED BY GYLLENSKIÖLD ON
    SPITZBERGEN, 1883                                                126

    41. POLAR-LIGHT DRAPERIES, OBSERVED IN FINNMARKEN, NORTHERN
    NORWAY                                                           127

    42. CURVE OF MAGNETIC DECLINATION AT KEW, NEAR LONDON,
    ON NOVEMBER 15 AND 16, 1905                                      138

    43. CURVE OF HORIZONTAL INTENSITY AT KEW ON NOVEMBER
    15 AND 16, 1905                                                  139

    44. ZODIACAL LIGHT IN THE TROPICS                                146

    45. SPECTRUM OF NOVA AURIGÆ, 1892                                154

    46. DIAGRAM INDICATING THE CONSEQUENCES OF A COLLISION
    BETWEEN TWO EXTINCT SUNS                                         157

    47. SPIRAL NEBULA IN THE CANES VENATICI                          159

    48. SPIRAL NEBULA IN THE TRIANGLE                                161

    49. THE GREAT NEBULA IN ANDROMEDA                                163

    50. RING-SHAPED NEBULA IN LYRA                                   164

    51. CENTRAL PORTION OF THE GREAT NEBULA IN ORION                 165

    52. NEBULAR STRIÆ IN THE STARS OF THE PLEIADES                   167

    53. NEBULAR STRIÆ IN THE SWAN                                    169

    54. NEBULA AND STAR RIFT IN THE SWAN, IN THE MILKY WAY           171

    55. GREAT NEBULA NEAR RHO, IN OPHIUCHUS                          172

    56. STAR CLUSTER IN HERCULES                                     173

    57. STAR CLUSTER IN PEGASUS                                      175

    58. CONE-SHAPED STAR CLUSTER IN GEMINI                           176

    59. COMPARISON OF SPECTRA OF STARS OF CLASSES 2, 3, 4            185

    60. COMPARISON OF SPECTRA OF STARS OF CLASSES 2, 3, 4            186




                                PREFACE


When, more than six years ago, I was writing my _Treatise of Cosmic
Physics_, I found myself confronted with great difficulties. The
views then held would not explain many phenomena, and they failed in
particular in cosmogonic problems. The radiation pressure of light,
which had not, so far, been heeded, seemed to give me the key to the
elucidation of many obscure problems, and I made a large use of this
force in dealing with those phenomena in my treatise.

The explanations which I tentatively offered could, of course, not
claim to stand in all their detail; yet the scientific world received
them with unusual interest and benevolence. Thus encouraged, I tried
to solve more of the numerous important problems, and in the present
volume I have added some further sections to the complex of explanatory
arguments concerning the evolution of the Universe. The foundation
to these explanations was laid in a memoir which I presented to
the Academy of Sciences at Stockholm in 1900. The memoir was soon
afterwards printed in the _Physikalische Zeitschrift_, and the subject
was further developed in my _Treatise of Cosmic Physics_.

It will be objected, and not without justification, that scientific
theses should first be discussed and approved of in competent circles
before they are placed before the public. It cannot be denied that,
if this condition were to be fulfilled, most of the suggestions on
cosmogony that have been published would never have been sent to the
compositors; nor do I deny that the labor spent upon their publication
might have been employed for some better purpose. But several
years have elapsed since my first attempts in this direction were
communicated to scientists. My suggestions have met with a favorable
reception, and I have, during these years, had ample opportunity
carefully to re-examine and to amend my explanations. I therefore feel
justified in submitting my views to a larger circle of readers.

The problem of the evolution of the Universe has always excited
the profound interest of thinking men. And it will, without doubt,
remain the most eminent among all the questions which do not have any
direct, practical bearing. Different ages have arrived at different
solutions to this great problem. Each of these solutions reflected the
stand-point of the natural philosophers of its time. Let me hope that
the considerations which I offer will be worthy of the grand progress
in physics and chemistry that has marked the close of the nineteenth
and the opening of the twentieth century.

Before the indestructibility of energy was understood, cosmogony
merely dealt with the question how matter could have been arranged
in such a manner as to give rise to the actual worlds. The most
remarkable conception of this kind we find in Herschel’s suggestion
of the evolution of stellar nebulæ, and in the thesis of Laplace
concerning the formation of the solar system out of the universal
nebula. Observations more and more tend to confirm Herschel’s view.
The thesis of Laplace, for a long time eulogized as the flower of
cosmogonic speculations, has more and more had to be modified. If we
attempt, with Kant, to conceive how wonderfully organized stellar
systems could originate from absolute chaos, we shall have to admit
that we are attacking a problem which is insoluble in that shape. There
is a contradiction in those very attempts to explain the origin of the
Universe in its totality, as Stallo[1] emphasizes: “The only question
to which a series of phenomena gives legitimate rise relates to their
filiation and interdependence.” I have, therefore, only endeavored to
show how nebulæ may originate from suns and suns from nebulæ; and I
assume that this change has always been proceeding as it is now.

    [Footnote 1: Stallo: _Concepts and Theories of Modern Physics_.
    London, 1900, p. 276.]

The recognition of the indestructibility of energy seemed to accentuate
the difficulties of the cosmogonic problems. The theses of Mayer and
of Helmholtz, on the manner in which the Sun replenishes its losses of
heat, have had to be abandoned. My explanation is based upon chemical
reactions in the interior of the Sun in accordance with the second law
of thermodynamics. The theory of the “degradation” of energy appeared
to introduce a still greater difficulty. That theory seems to lead to
the inevitable conclusion that the Universe is tending towards the
state which Clausius has designated as “_Wärme Tod_” (heat death),
when all the energy of the Universe will uniformly be distributed
through space in the shape of movements of the smallest particles. That
would imply an absolutely inconceivable end of the development of the
Universe. The way out of this difficulty which I propose comes to this:
the energy is “degraded” in bodies which are in the solar state, and
the energy is “elevated,” raised to a higher level, in bodies which are
in the nebular state.

Finally, I wish to touch upon one cosmogonical question which
has recently become more actual than it used to be. Some kind of
“spontaneous generation,” origination of life from inorganic matter,
had been acquiesced in. But just as the dreams of a spontaneous
generation of energy—_i.e._, of a _perpetuum mobile_—have been
dispelled by the negative results of all experiments in that direction,
just in the same way we shall have to give up the idea of a spontaneous
generation of life after all the repeated disappointments in this field
of investigation. As Helmholtz[2] says, in his popular lecture on the
growth of the planetary system (1871): “It seems to me a perfectly just
scientific procedure, if we, after the failure of all our attempts to
produce organisms from lifeless matter, put the question, whether life
has had a beginning at all, or whether it is not as old as matter, and
whether seeds have not been carried from one planet to another and have
developed everywhere where they have fallen on a fertile soil.”

    [Footnote 2: Helmholtz, _Populäré Wissenschaftliche Vorträge_.
    Braunschweig, 1876, vol. iii., p. 101.]

This hypothesis is called the hypothesis of panspermia, which I have
modified by combining it with the thesis of the radiation pressure.

My guiding principle in this exposition of cosmogonic problems has been
the conviction that the Universe in its essence has always been what
it is now. Matter, energy, and life have only varied as to shape and
position in space.

                                                          THE AUTHOR.

  STOCKHOLM, December, 1907.




                         WORLDS IN THE MAKING




                                   I

                  VOLCANIC PHENOMENA AND EARTHQUAKES


                       The Interior of the Earth

The disasters which have recently befallen the flourishing settlements
near Vesuvius and in California have once more directed the attention
of mankind to the terrific forces which manifest themselves by volcanic
eruptions and earthquakes.

The losses of life which have been caused in these two last instances
are, however, insignificant by comparison with those which various
previous catastrophes of this kind have produced. The most violent
volcanic eruption of modern times is no doubt that of August 26 and
27, 1883, by which two-thirds of the island of Krakatoa, 33 square
kilometres (13 square miles) in area, situated in the East Indian
Archipelago, were blown into the air. Although this island was itself
uninhabited, some 40,000 people perished on that occasion, chiefly by
the ocean wave which followed the eruption and which caused disastrous
inundations in the district. Still more terrible was the destruction
wrought by the Calabrian earthquake of February and March, 1783, which
consisted of several earthquake waves. The large town of Messina
was destroyed on February 5th, and the number of people killed by
this event has been estimated at 100,000. The same region, especially
Calabria, has, moreover, frequently been visited by disastrous
earthquakes—again in 1905 and 1907. Another catastrophe upon which
history dwells, owing to the loss of life (not less than 90,000),
was the destruction of the capital of Portugal on November 1, 1755.
Two-thirds of the human lives which this earthquake claimed were
destroyed by a wave 5 m. in height rushing in from the sea.

    [Illustration: Fig. 1.—Vesuvius, as seen from the Island of
    Nisida, in moderate activity]

Vesuvius is undoubtedly the best studied of all volcanoes. During
the splendor of Rome this mountain was quite peaceful—known as an
extinct volcanic cone so far as history could be traced back. On the
extraordinarily fertile soil about it had arisen big colonies of such
wealth that the district was called Great Greece (Græcia Magna). Then
came, in the year 79 A.D., the devastating eruption which destroyed,
among others, the towns of Herculaneum and Pompeii. The volumes of gas,
rushing forth with extreme violence from the interior of the earth,
pushed aside a large part of the volcanic cone whose remnant is now
called Monte Somma, and the falling masses of ashes, mixed with streams
of lava, built up the new Vesuvius. This mountain has repeatedly
changed its appearance during later eruptions, and was provided with
a new cone of ashes in the year 1906. The outbreak of the year 79
was succeeded by new eruptions in the years 203, 472, 512, 685, 993,
1036, 1139, 1500, 1631, and 1660, at quite irregular intervals. Since
that time Vesuvius has been in almost uninterrupted activity, mostly,
however, of a harmless kind, so that only the cloud of smoke over its
crater indicated that the internal glow was not yet extinguished. Very
violent eruptions took place in the years 1794, 1822, 1872, and 1906.

Other volcanoes behave quite differently from these violent volcanoes,
and do hardly any noteworthy damage. Among these is the crater-island
of Stromboli, situated between Sicily and Calabria. This volcano has
been in continuous activity for thousands of years. Its eruptions
succeed one another at intervals ranging from one minute to twenty
minutes, and its fire serves the sailors as a natural light-house. The
force of this volcano is, of course, unequal at different periods.
In the summer of 1906 it is said to have been in unusually violent
activity. Very quiet, as a rule, are the eruptions of the great
volcanoes on Hawaii.

Foremost among the substances which are ejected from volcanoes is water
vapor. The cloud floating above the crater is, for this reason, the
surest criterion of the activity of the volcano. Violent eruptions
drive the masses of steam up into the air to heights of 8 km. (5
miles), as the illustrations (Figs. 1 to 4) will show.

The height of the cloud may be judged from the height of Vesuvius, 1300
metres (nearly 4300 ft.) above sea-level. The illustration on page 4
(Fig. 2) is a reproduction of a drawing by Poulett Scrope, representing
the Vesuvius eruption of the year 1822. There seems to have been no
wind on this day; the masses of steam formed a cloud of a regular
shape which reminds us of a pine-tree. According to the description of
Plinius, the cloud noticed at the eruption of Vesuvius in the year 79
must have been of the same kind. When the air is not so calm the cloud
assumes a more irregular shape (Fig. 3). Clouds which rise to such
elevations as we have spoken of are distinguished by strong electric
charges. The vivid flashes of lightning which shoot out of the black
clouds add to the terror of the awful spectacle.

    [Illustration: Fig. 2.—Eruption of Vesuvius in 1882. (After a
    contemporaneous drawing by Poulett Scrope)]

The rain which pours down from this cloud is often mixed with ashes
and is as black as ink. The ashes have a color which varies between
light-gray, yellow-gray, brown, and almost black, and they consist
of minute spherules of lava ejected by the force of the gases and
rapidly congealed by contact with the air. Larger drops of lava harden
to volcanic sand—the so-called “lapilli” (that is, little stones),
or to “bombs,” which are often furrowed by the resistance offered by
the air, and turn pear-shaped. These solid products, as a rule, cause
the greatest damage due to volcanic eruptions. In the year 1906 the
weight of these falling masses (Fig. 4) crushed in the roofs of houses.
A layer of ashes 7 m. (23 ft.) in thickness buried Pompeii under a
protective crust which had covered it up to days of modern excavations.
The fine ashes and the muddy rain clung like a mould of plaster to the
dead bodies. The mud hardened afterwards into a kind of cement, and as
the decomposition products of the dead bodies were washed away, the
moulds have provided us with faithful casts of the objects that had
once been embedded in them. When the ashes fall into the sea, a layer
of volcanic tuffa is formed in a similar manner, which enshrines the
animals of the sea and algæ. Of this kind is the soil of the Campagna
Felice, near Naples. Larger lumps of solid stones with innumerable
bubbles of gases float as pumice-stone on the sea, and are gradually
ground down into volcanic sand by the action of the waves. The floating
pumice-stone has sometimes become dangerous or, at any rate, an
obstacle to shipping, through its large masses; that was, at least, the
case with the Krakatoa eruption of 1883.

    [Illustration: Fig. 3.—Eruption of Vesuvius in 1872. (After a
    photograph.)]

Among the gases which are ejected in addition to water vapor, carbonic
acid should be mentioned in the first instance; also vapors of
sulphur and sulphuretted hydrogen, hydrochloric acid, and chloride of
ammonium, as well as the chlorides of iron and copper, boric acid, and
other substances. A large portion of these bodies is precipitated on
the edges of the volcano, owing to the sudden cooling of the gases.
The more volatile constituents, such as carbonic acid, sulphuretted
hydrogen, and hydrochloric acid, may spread over large areas, and
destroy all living beings by their heat and poison. It was these gases,
for example, which caused the awful devastation at St. Pierre, where
30,000 human lives were destroyed on May 8, 1902, by the eruption of
Mont Pelée. The ejection of hydrogen gas, which, on emerging from the
lava, is burned to water by the oxygen of the air, has been observed in
the crater of Kilauea.

The ashes of the volcanoes are sometimes carried to vast distances
by the air currents—_e.g._, from the western coast of South America
to the Antilles; from Iceland to Norway and Sweden; from Vesuvius
(1906) to Holstein. Best known in this respect is the eruption of the
Krakatoa, which drove the fine ashes up to an elevation of 30 km. (18
miles). The finest particles of these ashes were slowly carried by
the winds to all parts of the earth, where they caused, during the
following two years, the magnificent sunrises and sunsets which were
spoken of as “the red glows.” This glow was also observed in Europe
after the eruption of Mont Pelée. The dust of Krakatoa further supplied
the material for the so-called “luminous clouds of the night,” which
were seen in the years 1883 to 1892 floating at an elevation of about
80 km. (50 miles), and hence illuminated by the light of the sun long
after sunset.

The crater of Kilauea, on the high volcano of Mauna Loa, in
Hawaii—this volcano is about of the same height as Mont Blanc—has
excited special interest. The crater forms a large lake of lava having
an area of about 12 sq. km. (nearly 5 sq. miles), which, however,
varies considerably with time. The lava boiling at red glow is
constantly emitting masses of gas under slight explosions, spurting
out fiery fountains to a height of 20 m. (65 ft.) into the air. Here
and there lava flows out from crevices in the wall of the crater down
the slope of the mountain, until the surface of the lake of lava
has descended below these cracks. As a rule, this lava is of a thin
fluid consistency, and it spreads, therefore, rather uniformly over
large areas. Of a similar kind are also the floods of lava which are
sometimes poured over thousands of square kilometres on Iceland. The
so-called Laki eruption of the year 1783 was of a specially grand
nature. Though occurring in an uninhabited district, it did a great
amount of damage. In the more ancient geological periods, especially in
the Tertiary age, similar sheets of lava of vast extensions have been
spread over England and Scotland (more than 100,000 sq. km., roughly,
40,000 sq. miles); over Deccan, in India, 400,000 sq. kms. (150,000
sq. miles), up to heights of 2000 m. (6500 ft.); and over Wyoming,
Yellowstone Park, Nevada, Utah, Oregon, and other districts of the
United States, as well as over British Columbia.

    [Illustration: Fig. 4.—Photograph of Vesuvius, 1906. Chiefly
    clouds of ashes]

In other cases the slowly ejected lava is charged with large volumes of
gases, which escape when the lava congeals and burst it up into rough,
unequal blocks, forming the so-called block lava (Fig. 5). The streams
of lava can likewise produce terrible devastation when they descend
into inhabited districts; on account of their slow motion, they rarely
cause loss of life, however.

Where the volcanic activity gradually lessens or ceases, we can still
trace it by the exhalations of gas and the springs of warm water which
we find in many districts where, during the Tertiary age, powerful
volcanoes were ejecting their streams of lava. To this class belong
the famous geysers of Iceland, of Yellowstone Park (Fig. 6), and
of New Zealand; also the hot springs of Bohemia, so highly valued
therapeutically (_e.g._, the Karlsbad Sprudel); the Fumaroli of Italy,
Greece, and other countries, exhaling water vapor; the Mofettæ, with
their exhalations of carbonic acid (of frequent occurrence in the
district of the Eifel and on both sides of the middle Rhine, in the
Dogs Grotto near Naples, and in the Valley of Death in Java); the
Solfatara, exhaling vapors of sulphur—sulphuretted hydrogen and
sulphur dioxide (they are found near Naples on the Phlegræan Fields
and in Greece); as well as many of the so-called mud volcanoes,
which eject mud, salt water, and gases (as a rule, carbonic acid and
hydrocarbons)—for example, the mud volcanoes near Parma and Modena, in
Italy, and those near Kronstadt, in Transylvania.

    [Illustration: Fig. 5.—Block lava on Mauna Loa]

The extinct volcanoes, of which some, like the Aconcagua, 6970 m.
(22,870 ft.), in South America, and the Kilimanjaro, in Africa, 6010
m. (19,750 ft.), rank among the highest mountains, are exposed to a
rapid destruction by the rain, because they consist largely of loose
materials—volcanic ashes with interposed layers of lava. Where these
lava streams expand gradually, they protect the ground underneath from
erosion by water, and in this way proper cuts are formed on the edges
of the lava streams, passing through the old volcano and through the
sedimentary strata at deeper levels.

    [Illustration: Fig. 6.—The Excelsior Geyser in Yellowstone
    Park, U. S. A. Remnant of powerful volcanic activity in the
    Tertiary age]

The old volcano of Monte Venda, near Padua, affords an interesting
example of this type. We can observe there how the sedimentary
limestone has been changed by the lava, which was flowing over it, into
marble to a depth of about 1 m. (3 ft.) Sometimes the limestone which
is lying over the lava has also undergone the same transformation,
which would indicate that lava has not only been flowing above the edge
of the crater, but has also forced itself out on the sides through
the fissures between two layers of limestone. Massive subterranean
lava streams of this kind are found in the so-called lakkolithes of
Utah and in the Caucasus. There the superior layers have been forced
upward by the lava pressing from below; the lava froze, however,
before it reached the surface of the earth, where it might have formed
a volcano. Quite a number of granites, the so-called batholithes,
chiefly occurring in Norway, Scotland, and Java, are of similar origin.
Occasionally it is only the core of congealed lava that has remained of
the whole volcano. These cores, which originally filled the pipe of the
crater, are frequent in Scotland and in North America, where they are
designated “necks” (Fig. 7).

    [Illustration: Fig. 7.—Mato Tepee in Wyoming, U. S. A. Typical
    volcanic “Neck”]

The so-called cañons of the Colorado Plateau, with their almost
vertical walls, are the results of the erosive action of rivers. A
drawing by Dutton shows a wall of this kind more than 800 m. (2600 ft.)
in height, through four fissures of which lava streams have forced
their way up to the surface (Fig. 8). Over one of these fissures
a small cone of volcanic ashes is still visible, while the cones
which probably overtopped the three other fissures have been washed
away, so that the veins end in small “necks.” Evidently a very fluid
lava—strong percentages of magnesia and of oxide of iron render the
lava more fluid than an admixture of silicic acid, and the fluidity
is further increased by the presence of water—has been forced into
the fissures which were already present, and has reached the surface
of the earth before it froze. The driving force behind them must have
been pretty strong; else the lava streams could not have attained the
necessary velocity of flow.

    [Illustration: Fig. 8.—Clefts filled with lava and volcanic
    cone of ashes, Torowheap Cañon, Plateau of Colorado. Diagram.]

When the Krakatoa was blown into the air in 1883 half of the
volcano remained behind. This half clearly shows the section of the
cone of ashes, which has been but very slightly affected by the
destructive action of the water. We find there in the central part the
light-colored stopper of lava in the volcano pipe, and issuing from it
more light-colored beds of lava, between which darker strata of ashes
can be seen.

The distribution of volcanoes over the surface of the earth is marked
by striking regularities. Almost all the volcanoes are situated near
the shores of the sea. A few are found in the interior of East Africa;
but they are, at any rate, near the Great Lakes of the equatorial
regions. The few volcanoes which are supposed to be situated in Central
Asia must be regarded as doubtful. We miss, however, volcanoes on
some sea-coasts, as in Australia and along the long coast-lines of
the Northern Arctic Ocean to the north of Asia, Europe, and America.
Volcanoes occur only where great cracks occur in the crust of the earth
along the sea-coast. Where such fissures are found, but where the sea
or large inland lake basins are not near—as, for instance, in the
Austrian Alps—we do not meet with any volcanoes; such districts are,
however, renowned for their earthquakes.

Since ancient ages the belief has been entertained that the molten
masses of the interior of the earth find an outlet through the
volcanoes. Attempts have been made to estimate the depth of the hearths
of volcanoes, but very different values have been deduced. Thus, the
hearth under the volcano of Monte Nuovo, which was thrown up in the
year 1538 on the Phlegræan Fields, near Naples, has been credited with
depths varying from 1.3 km. to 60 km. (1 mile to 40 miles); for the
Krakatoa, estimates of more than 50 km. (30 miles) have been made. All
these calculations are rather aimless; for the volcanoes are probably
situated on folds of the earth-crust, through which the fluid mass (the
magma) rushes forth in wedges from the interior of the earth, and it
will presumably be very difficult to say where the hearth of magma ends
and where the volcanic pipe commences. The Kilauea gives the visitor
the impression that he is standing over an opening in the crust of the
earth, through which the molten mass rushes forth directly from the
interior of the earth. (Fig. 9.)

    [Illustration: Fig. 9.—The Kilauea Crater on Hawaii]

As regards the earth-crust, we know from observations in bore-holes
made in different parts of the world that the temperature increases
rather rapidly with the depth, on an average by about thirty degrees
Cent. per kilometre (about 1.6° F. per 100 feet). It must be remarked,
however, that the depth of our deepest bore-holes hardly exceeds 2
km. (Paruchowitz, in Silesia, 2003 m., or 6570 ft.; Schladebach,
near Merseburg, Prussian Saxony, 1720 m.). If the temperature should
go on increasing at the rate of 30 degrees Cent. for each further
kilometre, the temperature at a depth of 40 kilometres should attain
degrees at which all the common rocks would melt. But the melting-point
certainly rises at the same time as the pressure. The importance of
this circumstance was, however, much exaggerated when it was believed
that for this reason the interior of the earth might possibly be solid.
Tammann has shown by direct experiments that the temperature of fusion
only rises up to a certain pressure, and that it begins to decrease
again on a further increase of pressure. The depths indicated above are
therefore not quite correct. If we assume, however, that other kinds of
rock behave like diabase—the melting-point of which, according to the
determinations of Barus, rises by 1° Cent. for each 40 atmospheres of
pressure corresponding to a depth of 155 m.—we should conclude that
the solid crust of the earth could not have a greater thickness than 50
or 60 km. (40 miles). At greater depths we should therefore penetrate
into the fused mass. On account of its smaller density the silicic acid
will be concentrated in the upper strata of the molten mass, while
the basic portions of the magma, which are richer in iron oxide, will
collect in the lower strata, owing to their greater density.

This magma we have to picture to ourselves as an extremely viscid
liquid resembling asphalt. The experiments of Day and Allen show that
rods, supported at their ends, of 30 × 2 × 1 mm. of different minerals,
like the feldspars microcline and albite, could retain their shape for
three hours without curving noticeably, although their temperature was
about a hundred degrees above their melting-point, and although they
appeared completely fused, or, more correctly, completely vitrified
when taken out of the furnace. These molten silicates behave very
differently from other liquids like water and mercury, with which we
are more accustomed to deal.

The motion and diffusion in the magma, and especially in the very
viscous and sluggish acid portions of the upper strata, will therefore
be exceedingly small, and the magma will behave almost like a solid
body, like the minerals of the experiments of Day and Allen. The magmas
of volcanoes like Etna, Vesuvius, and Pantellaria may, therefore, have
quite different compositions, as we should conclude from their lavas
without our being forced to believe, with Stübel, that these three
hearths of volcanoes are completely separated, though not far removed
from one another. In the lava of Vesuvius a temperature of 1000 or 1100
degrees has been found at the lower extremity of the stream. From the
occurrence in the lava of certain crystals like leucite and olivin,
which we have reason to assume must have been formed before the lava
left the crater, it has been concluded that the lava temperature cannot
have been higher than 1400 degrees before it left the volcanic pipe.

It would, however, be erroneous to deduce from the temperature of the
lava of Vesuvius that the hearth of the volcano must be situated at a
depth of approximately 50 kilometres. Most likely its depth is much
smaller, perhaps not even 10 kilometres. For there, as everywhere where
volcanoes occur, the crust of the earth is strongly furrowed, and the
magma will just at the spots where we find volcanoes come much nearer
to the surface of the earth than elsewhere.

The importance of water for the formation of volcanoes probably lies
in the fact that, in the neighborhood of cracks under the bottom of
the sea, the water penetrates down to considerable depths. When the
water reaches a stratum of a temperature of 365 degrees—the so-called
critical temperature of water—it can no longer remain in the liquid
state. That would not prevent, however, its penetrating still farther
into the depths, in spite of its gaseous condition. As soon as the
vapor comes in contact with magma, it will eagerly be absorbed by the
magma. The reason is that water of a temperature of more than 300
degrees is a stronger acid than silicic acid; the latter is therefore
expelled by it from its compounds, the silicates, which form the main
constituents of the magma. The higher the temperature, the greater the
power of the magma to absorb water. Owing to this absorption the magma
swells and becomes at the same time more fluid. The magma is therefore
pressed out by the action of a pressure which is analogous to the
osmotic pressure by virtue of which water penetrates through a membrane
into a solution of sugar or salt. This pressure may become equivalent
to thousands of atmospheres, and this very pressure would raise the
magma up the volcanic pipe even to a height of 6000 m. (20,000 feet)
above the sea-level. As the magma is ascending in the volcanic pipe
it is slowly cooled, and its capacity for binding water diminishes
with falling temperature. The water will hence escape under violent
ebullition, tearing drops and larger lumps of lava with it, which fall
down again as ashes or pumice-stone. After the lava has flown out of
the crater and is slowly cooling, it continues to give off water,
breaking up under the formation of block lava (see Fig. 5). If, on the
other hand, the lava in the crater of the volcano is comparatively
at rest, as in Kilauea, the water will escape more slowly; owing to
the long-continued contact of the surface layer of lava with the air,
little water will remain in it, the water being, so to say, removed by
aeration, and the lava streams will therefore, when congealing, form
more smooth surfaces.

In some cases volcanoes have been proved (Stübel and Branco) not to
be in connection with any fractures in the crust of the earth. That
holds, for instance, for several volcanoes of the early Tertiary age
in Swabia. We may imagine that the pressure produced by the swelling
of the magma became so powerful as to be able to break through the
earth-crust at thinner spots, even in the absence of previous fissures.

If, in our consideration, we follow the magma farther into the depths,
we shall not find any reason for assuming that the temperature will not
rise farther towards the interior of the earth. At depths of 300 or
400 km. (250 miles) the temperature must finally attain degrees such
that no substance will be able to exist in any other state than the
gaseous. Within this layer the interior of the earth must, therefore,
be gaseous. From our knowledge of the behavior of gases at high
temperatures and pressures, we may safely conclude that the gases in
the central portions of the earth will behave almost like an extremely
viscid magma. In certain respects they may probably be compared to
solid bodies; their compressibility, in particular, will be very small.

We might think that we could not possibly learn anything concerning
the condition of those strata. Earthquakes have, however, supplied us
with a little information. Such gaseous masses must fill by far the
greatest part of the earth, and they must have a very high specific
gravity; for the average density of the earth is 5.52, and the outer
strata, the ocean and the masses of the surface which are known to us,
have smaller densities. The ordinary rocks possess a density ranging
from 2.5 to 3. It must, therefore, be assumed that the materials of
the innermost portions of the earth must be metallic, and Wiechert,
in particular, has advocated this view. Iron will presumably form the
chief constituent of this gas of the central earth. Spectrum analysis
teaches us that iron is a very important constituent of the sun. We
know, further, that the metallic portions of the meteorites consist
essentially of iron; and finally terrestrial magnetism indicates that
there must be large masses of iron in the interior of the earth.
We have also reason to believe that the native iron occurring in
nature—_e.g._, the well-known iron of Ovifak, in Greenland—is of
volcanic origin. The materials in the gaseous interior of the earth
will, owing to their high density, behave in chemical and physical
respects like liquids. As substances like iron will, also at very high
temperatures, have a far higher specific gravity than their oxides,
and these again have a higher gravity than their silicates, we have to
assume that the gases in the core of the earth will almost exclusively
be metallic, that the outer portions of the core will contain
essentially oxides, and those farther out again mostly silicates.

The fused magma will, on penetrating in the shape of batholithes
into the upper layers, probably be divided into two portions, of
which one, the lighter and gaseous, will contain water and substances
soluble in it; while the other, heavier portion, will essentially
consist of silicates with a lower percentage of water. The more fluid
portion, richer in water, will be secreted in the higher layers, will
penetrate into the surrounding sedimentary strata, especially into
their fissures, and will fill them with large crystals, often of
metallurgical value—_e.g._, of the ores of tin, copper, and other
metals, while the water will slowly evaporate through the superposed
strata. The more viscid and sluggish mass of silicates, on the other
hand, will congeal, thanks to its great viscosity, to glass, or, when
the cooling is very slow, to small crystals.

We now turn to earthquakes. No country has been absolutely spared by
earthquakes. In the districts bounding upon the Baltic, and especially
in northern Russia, they have, however, been of a quite harmless type.
The reason is that the earth-crust there has been lying undisturbed for
long geological epochs and has never been fractured. The comparatively
severe earthquake which shook the west coast of Sweden on October 23,
1904, to an unusually heavy degree, without, however, causing any
noteworthy damage (a few chimneys were knocked over), was caused by a
fault of relatively pronounced character for those districts in the
Skager-Rack—a continuation of the deepest fold in the bottom of the
North Sea, the so-called Norwegian Trough, which runs parallel to the
Norwegian coast. In Germany, the Vogtland and the districts on both
sides of the middle Rhine have frequently been visited by earthquakes.
Of other European countries, Switzerland, Spain, Italy, and the Balkan
Peninsula, as well as the Karst districts of Austria, have often
suffered from earthquakes.

    [Illustration: Fig. 10.—Chief earthquake centres, according to
    the British Association Committee]

According to the committee appointed by the British Association for the
investigation of earthquakes—a committee which has contributed a great
deal to our knowledge of these great natural phenomena—earthquakes of
some importance emanate from certain centres which have been indicated
on the subjoined map (Fig. 10). The most important among these regions
comprises Farther India, the Sunda Isles, New Guinea, and Northern
Australia; it is marked on the map by the letter F. From this district
have emanated in the six-year period 1899-1904 no fewer than 249
earthquakes, which have been recorded in many observatories far removed
from one another. This earthquake centre F is closely related to the
one marked E, in Japan, from which 189 earthquakes have proceeded. Next
to this comes the extensive district K with 174 earthquakes, comprising
the most important folds in the crust of the Old World, including
the mountain chains from the Alps to the Himalaya. This district is
interesting, because it has been disturbed by a great many earthquakes,
although it is almost entirely situated on the Continent. After that
we have the districts A, B, C, with 125, 98, and 95 earthquakes. They
are situated near lines of fracture in the earth-crust along the
American coast of the Pacific Ocean and the Caribbean Sea. District D,
with 78 earthquakes, is similarly situated. The three last-mentioned
districts, B, C, D, as well as G, between Madagascar and India, with
85 earthquakes, all seem to be surpassed by the district H in the
eastern Atlantic, with its 107 earthquakes. These latter are, however,
relatively feeble, and we owe their accurate records probably to the
circumstances that a great many earthquake observatories are situated
within the immediate surroundings of this district. The same may be
said of the district I, or Newfoundland, which is not characterized
by many earthquakes, and of the district J, between Iceland and
Spitzbergen, with 31 and 19 earthquakes respectively. The last on
the list used to be the district L, situated about the South Pole,
with only eight earthquakes. This small number is probably merely
due to the want of observatories in those parts of the earth. Another
district, M, has finally been added, which extends to the southwest
from New Zealand. No fewer than 75 intense earthquakes were recorded
between March 14 and November 23, 1903, by the Discovery Expedition, in
70° southern latitude and 178° eastern longitude.

Earthquakes commonly occur in swarms or groups. Thus, more than 2000
shocks were counted on Hawaii in March, 1868. During the earthquakes
which devastated the district of Phokis, in Greece, in 1870-73, shocks
succeeded one another for a long time at intervals of three seconds.
During the whole period of three and a half years about half a million
shocks were counted, and, further, a quarter of a million subterranean
reports which were not accompanied by noticeable concussions. Yet of
all these shocks only about 300 did noteworthy damage, and only 35
were considered worth being reported in the newspapers. The concussion
of October 23, 1904, belonged to a group which lasted from October
10 to October 28, and in which numerous small tremors were noticed,
especially on October 24 and 25. The earthquake of San Francisco
commenced on April 18, 1906, at 5 hrs. 12 min. 6 sec. A.M. (Pacific
Ocean time), and ended at 5 hrs. 13 min. 11 sec, lasting therefore 1
minute and 5 seconds. Twelve smaller shocks succeeded in the following
hour. Before 6 hrs. 52 min. P.M., nineteen further concussions were
counted, and various smaller shocks succeeded in the following days.

With such groups of earthquakes weaker tremors usually precede the
violent destructive shocks and give a warning. Unfortunately this
is not always so, and no warning was given by the earthquakes which
destroyed Lisbon in 1755 and Caracas in 1812, nor by those which
devastated Agram in 1880, nor, finally, in the case of the San
Francisco disaster. A not very severe earthquake without feebler
precursors befell Ischia in 1881, while the violent catastrophe
which devastated this magnificent island in 1883 was heralded by
several warnings. As in San Francisco and Chili in 1906, less violent
concussions generally succeed the destructive shocks. Earthquakes like
that of Lisbon in 1755, consisting of a single shock, are very rare.

The violent concussions often produce large fissures in the ground.
Such were noticed in several places at San Francisco. One of the
largest fissures known, that of Midori, in Japan, was caused by the
earthquake of October 20, 1891. It left a displacement of the ground
ranging up to 6 m. (20 ft.) in the vertical and 4 m. (13 ft.) in the
horizontal direction. This crack had a length of not less than 65 km.
(40 miles). Extensive fissures were also formed by the earthquakes of
Calabria, in 1783, at Monte San Angelo, and in the sandstones of the
Bálpakrám Plateau in India, in 1897. In mountainous districts falls
of rock are a frequent consequence of the formation of fissures and
earthquakes. A large number of rocks fell in the neighborhood of Delphi
during the Phokian earthquake. On January 25, 1348, an earthquake
sent down a large portion of Mount Dobratsch (in the Alps of Villach,
in Carinthia, which is now much frequented by tourists) and buried
two towns and seventeen villages. The earthquake of April 18, 1906,
in California started from a crack which extends from the mouth of
Alder Creek, near Point Arena, running parallel with the coast-line
mostly inland, then entering the sea near San Francisco, and turning
again inland between Santa Cruz and San José, finally proceeding _via_
Chittenden up to Mount Pinos, a distance of about 600 km. (400 miles),
in the direction of N. 35° W. to S. 35° E. Along this crack the two
masses of the earth have been displaced so that the ground situated
to the southwest of the fissure has been moved by about 3 m. (10
ft.), and in some spots even by 6 m. (20 ft.) towards the northwest.
In some localities in Sonoma and Mendocino counties the southwestern
part has been raised, but nowhere by more than 1.2 m. (4 ft.). This is
the longest crack which has ever been noticed in connection with an
earthquake.

    [Illustration: Fig. 11.—Clefts in Valentia Street, San
    Francisco, after the earthquake of 1906]

The earthquake over, the ground does not always return to its
original position, but remains in a more or less wavy condition. This
can most easily be observed in districts where streets or railways
cross the ground. It is reported, for instance, that the track of
the tramway-lines in Market Street, the chief thoroughfare of San
Francisco, formed large wavelike curves after the earthquake.

As a consequence of the displacements in the interior of the earth
and of the formation of fissures, river courses are changed, springs
become exhausted, and new springs arise. That was the case, for
instance, in California in 1906. The ground water often rushes out
with considerable violence, tearing with it sand and mud and stones,
and piling them up, occasionally forming little craters (Fig. 12).
Extensive floods may also be caused on such occasions. By such a flood
the ancient Olympia was submerged under a layer of river sand which for
some time preserved from destruction the ancient Greek masterpieces
of art—among them the famous statue of Hermes. The floods afterwards
receded, and the treasures of ancient Olympia could be excavated.

Like the natural water channels and arteries in the interior of the
earth, water mains are displaced by the concussions. The direct damage
caused by the floods is often less important than the damage due to the
impossibility of extinguishing the fires which follow the destruction
of the buildings. It was the fires that did most of the enormous
material damage in the destruction of San Francisco.

Still greater devastation is wrought by the ocean waves thrown up by
earthquakes. We have already referred to the flood of Lisbon in 1755,
which was felt on the western coast of Norway and Sweden. Another
wave, in 1510, devoured 109 mosques and 1070 houses in Constantinople.
Another wave, again, invaded Kamaïshi, in Japan, on June 15, 1896,
swept away 7600 houses and killed 27,000 people.

We have repeatedly alluded to the disastrous flood-wave of Krakatoa of
1883. This wave traversed the whole of the Indian Ocean, passing to the
Cape of Good Hope and Cape Horn, and travelled round half the globe
afterwards. Even more remarkable was the aerial wave, which spread like
an explosion wave.

    [Illustration: Fig. 12.—Sand craters and fissures, produced
    by the Corinth earthquake of 1861. In the water, branches of
    flooded trees]

While the most violent cannonades are rarely heard for more than
150 km. (95 miles)—in a single case at a distance of 270 km. (170
miles)—the eruption of Krakatoa was heard at Alice Springs, at a
distance of 3600 kilometres, and on the island of Rodriguez, at almost
4800 km. (3000 miles). The barographs of the meteorological stations
first marked a sudden rise and then a decided sinking of the air
pressure, succeeded by a few smaller fluctuations. These air pulses
were repeated in some places as many as seven times. We may therefore
assume that the aerial wave passed these places three times in the one
direction, and three times in the other, travelling round the earth.
The velocity of propagation of this wave was 314.2 m. (1030 ft.) per
second, corresponding to a temperature of -27° Cent. (17° F.) which
prevails at an altitude of about 8 km. (5 miles) above the earth’s
surface, at which altitude this wave may have travelled.

Within the last decade a peculiar phenomenon (leading to what is
designated variation of latitudes) has been studied. The poles of
the axis of the earth appear to move in a very irregular curve about
their mean axis. The movement is exceedingly small. The deviation of
the North Pole from its mean position does not amount to more than
10 m. (about 33 ft.). It has been believed that these motions of
the North Pole are subject to sudden fluctuations after unusually
violent earthquakes, especially when such concussions follow at rapid
intervals. That would give us, perhaps more than any other observation,
an idea of the force of earthquakes, since they would appear to be able
to disturb the equilibrium of the whole mass of our globe.

A severely felt effect of earthquakes, though most people perhaps
pay little attention to it, is the destruction of submarine cables.
The gutta-percha sheaths of cables are frequently found in a fused
condition, suggesting volcanic eruptions under the bottom of the sea.
We take care now to avoid earthquake centres in laying telegraphic
cables. Their positions have been ascertained by the most modern
investigations (see Fig. 10).

People have always been inclined to look for a connection between
earthquakes and volcanic eruptions. The connection is unquestionable in
a large number of violent earthquakes. In order to establish it, the
above-mentioned committee of the British Association has compiled the
following table of the history of the earthquakes of the Antilles:

    1692.—Port Royal, Jamaica, destroyed by an earthquake; land
    sinking into the sea. Eruption on St. Kitts.

    1718.—Terrible earthquake on St. Vincent, followed by an
    eruption.

    1766-67.—Great shocks in northeastern South America, in Cuba,
    Jamaica, and the Antilles. Eruption on Santa Lucia.

    1797.—Earthquake in Quito, loss of 40,000 lives. Concussions
    in the Antilles, eruption on Guadeloupe.

    1802.—Violent shocks in Antigua. Eruption on Guadeloupe.

    1812.—Caracas, capital of Venezuela, totally destroyed by
    earthquake. Violent shocks in the Southern States of North
    America, commencing on November 11, 1811. Eruptions on St.
    Vincent and Guadeloupe.

    1835-36.—Violent concussions in Chili and Central America.
    Eruption on Guadeloupe.

    1902.—April 19. Violent shocks, destroying many towns of
    Central America. Mont Pelée, on Martinique, in activity.
    Eruption on May 3. Submarine cables break, sea recedes. Renewed
    violent movements of the sea on May 8, 19, 20. Eruption on St.
    Vincent, cable destroyed on May 7. Violent eruption of Mont
    Pelée on May 8. Destruction of St. Pierre. Numerous smaller
    earthquakes.

This table distinctly marks the restless state of affairs in that part
of the earth, and how quiet and safe matters are comparatively in old
Europe, especially in the north. Some parts of Central America are so
persistently visited by earthquakes that one of them, Salvador, has
been christened “Schaukelmatte.” It is not saying too much to assert
that the earth is there incessantly trembling. Other districts which
are very frequently visited are the Kuriles and Japan, as well as the
East Indian islands. In all these countries the crust of the earth has
been broken and folded within comparatively recent epochs (chiefly in
the Tertiary age) by numerous fissures, and their compression is still
going on.

    [Illustration: Fig. 13.—Earthquake lines in lower Austria]

The smaller earthquakes, of which not less than 30,000 are counted in
the course of a year, do not stand in any closer relation to volcanic
eruptions. This is also the case for a number of large earthquakes,
among which we have to count the San Francisco earthquake.

It is assured with good reason that earthquakes are often produced
at the bottom of the sea, where there is a strong slope, by slips of
sedimentary strata which have been washed down from the land into the
sea in the course of centuries. Milne believes that the seaquake of
Kamaïshi of June 15, 1896, was of this character. Concussions may even
be promoted by the different loading of the earth resulting from the
fluctuations in the pressure of the air above it.

Smaller, though occasionally rather violent, earthquakes are not
infrequent in the neighborhood of Vienna. On the map (Fig. 13) we see
three lines. The line A B is called the thermal line, because along it
a number of hot springs, the thermæ of Meidling, Baden, Vöslau, etc.,
are located, which are highly valued; the other line B C is called the
Kamp line, because it is traversed by the river Kamp; and the third B F
is called the Mürz line, after the river Mürz. The main railway-track
between Vienna and Bruck follows the valleys of A B and E F.

These lines, which probably correspond to large fissures in the
earth-crust, are known as sources of numerous earthquakes. The district
about Wiener Neustadt, where the three lines intersect, is often shaken
by violent earthquakes; some of their dates have been marked on the map.

The curve which is indicated by the letters X X on the map marks the
outlines of an earthquake which started on January 3, 1873, from both
sides of the Kamp line. It is striking to see how the earthquake spread
in the loose ground of the plain between St. Pölten and Tulln, while
the masses of rock situated to the northwest and southeast formed
obstacles to the propagation of the earthquake waves.

    [Illustration: Fig. 14.—Library building of Leland Stanford
    Junior University, in California, after the earthquake of 1906.
    The photograph shows the great strength of iron structures in
    comparison to the strength of brickwork. The effect of the
    earthquake on wooden structures can be seen in Fig. 11]

Similar conclusions have been deduced from the study of the spreading
of the waves which destroyed Charleston, South Carolina, in 1886.
Twenty-seven lives were destroyed by this shock. It was the most
terrible earthquake that ever visited the United States before the year
1906. In the Charleston concussion the Alleghany Mountains proved a
powerful bar against the further propagation of the shocks, which all
the more easily travelled in the loose soil of the Mississippi Valley.
In San Francisco, likewise, the worst devastation fell upon those parts
of the town which had been built upon the loose, partly made ground
in the neighborhood of the harbor, while the buildings erected on the
famous mountain ridges of San Francisco suffered comparatively little
damage, in so far as they were not reached by the destructive fires. As
regards the destructive effects of the earthquake in San Francisco, the
building-ground of that city has been divided into four classes (the
first is the safest, the last the most unsafe)—namely: 1. Rocky soil.
2. Valleys situated between rocks and filled up by nature in the course
of time. 3. Sand-dunes. 4. Soil created by artificial filling up. This
latter soil “behaved like a semiliquid jelly in a dish,” according to
the report of the Earthquake Commission.

For similar reasons the sky-scrapers, constructed of steel on deep
foundations, stood firmest. After them came brick houses, with
well-joined and cemented walls on deep foundations. The weakness of
wooden houses proved mainly due to the poor connection of the beams,
a defect which might easily be remedied. The superiority of the steel
structure will be apparent from the illustrations (Figs. 11 and 14).

The spots situated just over the crack, of which we spoke on page 25,
suffered the most serious damage. Next to them, devastation befell
especially localities which, like Santa Rosa, San José, and Palo Alto
with Leland Stanford Junior University, are situated on the loose soil
of the valley, whose deepest portions are covered by the bay of San
Francisco. The splendidly endowed California University, in Berkeley,
and the famous Lick Observatory, both erected on rocky ground,
fortunately escaped without any notable damage.

The map sketch (Fig. 15) by Suess represents the earthquake lines of
Sicily and Calabria. These districts have, as mentioned before, been
devastated by severe earthquakes, of which the most terrible occurred
in the year 1783, and again in 1905 and 1907. They have also been the
scene of many smaller concussions.

[Illustration: Fig. 15.—Earthquake lines in the Tyrrhenian depression]

The bottom of the Tyrrhenian Sea—between Italy, Sicily, and
Sardinia—has been lowered in rather recent ages and is still sinking.
We notice on the map five dotted lines, corresponding to cracks in
the crust of the earth. These lines would intersect in the volcanic
district of the Lipari Islands. We further see a dotted circular arc
corresponding to a fissure which is regarded as the source of the
Calabrian earthquakes of 1783, 1905, and 1907. The earth-crust behaved
somewhat after the manner of a windowpane which was burst by a heavy
impact from a point corresponding to the Island of Lipari. From this
point radiate lies of fracture, and fragments have been broken off from
the earth-crust by arc-shaped cracks. The volcano Etna is situated on
the intersection of the radial and circular fissures.

[Illustration: Fig. 16.—Seismogram recorded at Shide, Isle of Wight,
on August 31, 1898]

In recognition of the high practical importance of earthquake
observations, seismological stations have in recent days been erected
in many localities. At these observatories the earthquakes are recorded
by pendulums whose styles draw lines on tapes of paper moved by
clock-work. As long as the earth is quiet the drawn line is straight.
When earthquakes set in, the line passes into a wavy curve. As long
as the movement of the paper is slow, the curve merely looks like a
widened straight line. The subjoined illustration (Fig. 16) represents
a seismogram taken at the station of Shide, on the Isle of Wight, on
August 31, 1898. The earthquake recorded originated in the Centre G,
in the Indian Ocean. The origin has been deduced from the moments of
arrival of the different waves at different stations. We notice on the
seismogram a faint widening of the straight line at 20 hrs. 5 min. 2
sec. (8 hrs. 5 min. 2 sec. P.M.). The amplitude of the oscillations
then began to widen, and the heaviest concussions were noticed at 20
hrs. 36 min. 25 sec., and 20 hrs. 42 min. 49 sec., after which the
amplitudes slowly decreased with smaller shocks. The first shock of
20 hrs. 5 min. 2 sec. is called the preliminary tremor. This tremor
passes through the interior of the earth at a velocity of propagation
of 9.2 km. (5-3/4 miles) per second. It would require twenty-three
minutes to pass through the earth along a diameter. The tremor is
very feeble, which is ascribed to the extraordinarily great friction
characteristic of the strongly heated gases which are confined in the
interior of the earth. The principal violent shock at 20 hrs. 36 min.
25 sec. was caused by a wave travelling through the solid crust of the
earth. The intensity of this shock is much less impaired than that of
the just-mentioned tremor, and it travels with the smaller velocity of
about 3.4 km. (2.1 miles) along the earth’s surface.

The velocity of propagation of concussion pulses has been calculated
for a mountain of quartz, in which it would be 3.6 km. (2.2 miles) per
second, very nearly the same as the last-mentioned figure. We should
expect this, since the firm crust of the earth consists essentially
of solid silicates—_i.e._, compounds of quartz endowed with similar
properties.

Measured at small distances from the origin, the velocity of
propagation of the wave appears smaller, and the first preliminary
tremor is frequently not observed. The velocity may be diminished to
2 km. (1-1/4 miles) per second. The reason is that the pulse partly
describes a curve in the more solid portions of the crust, and partly
passes through looser strata, through which the wave travels at a much
slower rate than in firm ground; for instance, at 1.2 km. through loose
sandstones, at 1.4 km. through the water of the ocean, and at 0.3
km. through loose sand. We recognize that it should be possible to
calculate the distance between the point of observation and the origin
of the earthquake from the data relating to the arrivals of the first
preliminary tremor and of the principal shock of maximum amplitude. The
violent shock is sometimes repeated after a certain time, though with
decreased intensity. It has often been observed that this secondary,
less violent, shock seems to have travelled all round the earth _via_
the longest road between the origin and the point of observation,
just like one portion of the aerial waves in the eruption of Krakatoa
(compare page 27). The velocity of propagation of this secondary shock
is the same as that of the principal shock.

Milne has deduced from his observations that, when the line joining the
origin of the earthquake and the point of observation does not at its
lowest level descend deeper than 50 km. below the surface of the earth,
the pulse will travel undivided through the solid crust of the earth.
For this reason we estimate the thickness of the solid crust at 50 km.
The value is in almost perfect agreement with the one which we had (on
page 16) derived from the increase of temperature with greater depths.
It should further be mentioned, perhaps, that the density of the earth
in the vicinity has been determined from pendulum observation, and that
this density seems to be rather variable down to the depths of 50 or 60
km., but to become more uniform at greater depths. These 50 or 60 km.
(31 or 37 miles) would belong to the solid crust of the earth.

The movement of earthquake shocks through the earth thus teaches us
that the solid earth-crust cannot be very thick, and that the core of
the earth is probably gaseous. The similar conclusions, to which these
various considerations had led us, may therefore come very near the
truth. A careful study of seismograms may, we hope, help us to learn
more about the central portions of the earth, which at first sight
appear to be absolutely inaccessible to scientific research.




                                  II

 THE CELESTIAL BODIES, IN PARTICULAR THE EARTH, AS ABODES OF ORGANISMS


There is no more elevating spectacle than to contemplate the sky with
its thousands of stars on a clear night. When we send our thoughts
to those lights glittering in infinite distance, the question forces
itself upon us, whether there are not out there planets like our own
that will sustain organic life. How little interest do we take in a
barren island of the Arctic Circle, on which not a single plant will
grow, compared to an island in the tropics which is teeming with life
in its most wonderful variety! The unknown worlds occupy our minds much
more when we may fancy them inhabited than when we have to regard them
as dead masses floating about in space.

We have to ask ourselves similar questions with regard to our own
little planet, the earth. Was it always covered with verdure, or was
it once sterile and barren? And if that be so, what are the conditions
under which the earth can fulfil its actual part of harboring
organic life? That “the earth was without form” in the beginning is
unquestionable. It does not matter whether we assume that it was once
all through an incandescent liquid, which may be the most probable
assumption, or that it was, as Lockyer and Moulton think, formed by the
accumulation of meteoric stones which became incandescent when arrested
in their motion.

We have seen that the earth probably consists of a mass of gas encased
within a shell which is solid on the outside and remains a viscid
liquid on the inner side. We presume with good reason that the earth
was originally a mass of gas separated from the sun, which is still in
the same state. By radiation into cold space the sphere of gas which,
on the whole, would behave as our sun does now, would gradually lose
its high temperature, and finally a solid crust could form on its
surface. Lord Kelvin has calculated that it would not require more than
one hundred years before the temperature of this crust would sink to
100°. Supposing, even, that Kelvin’s calculations should not quite be
confirmed, we may yet maintain that not many thousands of years would
have elapsed from the time when the earth assumed its first crust at
about 1000° till the age when this temperature had fallen below 100°
(212° F.). Living beings certainly could not exist so long, since
the albumen of the cells would at once coagulate at the temperature
of boiling water, like the white of an egg. Yet it has been reported
that some of the hot springs of New Zealand contain algæ, although at
a temperature of over 80°. When I went to Yellowstone Park to inquire
into the correctness of this statement, I found that the algæ existed
only at the edge of the hot springs, where the temperature did not
exceed 60° (140° F.). The famous American physiologist Loeb states that
we do not meet with algæ in hot springs at temperatures above 55°.

Since, now, the temperature of the earth-crust would much more quickly
sink from 100° to 55° than it had fallen from 1000° to 100°, we may
imagine that only a few thousands of years may have intervened between
the formation of the first crust of the earth and the cooling down
to a temperature such as would sustain life. Since that time the
temperature has probably never been so low that the larger portion
of the earth’s surface would not have been able to support organisms,
although there have been several glacial ages in which the arctic
districts inaccessible to life must have extended much farther than
at present. The ocean will also have been free of ice over much the
greatest portion of its surface at all times, and may therefore have
been inhabited by organisms in all ages. The interior of the earth
cools continually, though slowly, because heat passes from the inner,
warmer portions to the other, cooler portions through the crust of the
earth.

The earth is able to serve as the abode of living beings because its
outer portions are cooled to a suitable temperature (below 55°) by
radiation, and because the cooling does not proceed so far that the
open sea would continually be frozen over, and that the temperature on
the Continent would always remain below freezing-point. We owe this
favorable intermediate stage to the fact that the radiation from the
sun balances the loss of heat by radiation into space, and that it is
capable of maintaining the greater portion of the surface of the earth
at a temperature above the freezing-point of water. The temperature
conditioning life on a planet is therefore maintained only because,
on the one side, light and heat are received by radiation from the
sun in sufficient quantities, while on the other side an equivalent
radiation of heat takes place into space. If the heat gain and the heat
loss were not to balance each other, the term of suitable conditions
would not last long. The temperature of the earth-crust could sink
in a few hundreds or thousands of years from 1000° to 100°, because
when the earth was at this high temperature its radiation into space
predominated over the radiation received from the sun. On the other
hand, about a hundred million years have passed, according to Joly,
since the age when the ocean originated. The temperature of the earth,
therefore, required this long space of time in order to cool down from
365° (at which temperature water vapor can first be condensed to liquid
water) to its present temperature. The cooling afterwards proceeded at
a slower rate, because the difference between the radiations inward and
outward was lessened with the diminishing temperature of the earth.
Various methods have been applied in estimating these periods. Joly
based his estimate on the percentage of salt in the sea and in the
rivers. If we calculate how much salt there is in the sea, and how much
salt the rivers can supply to it in the course of a year, we arrive at
the result that the quantity of salt now stored in the ocean might have
been supplied in about a hundred million years.

We arrive at still higher numbers when we calculate the time which
must have elapsed during the deposition of all the stratified and
sedimentary layers. Sir Archibald Geikie estimates the total thickness
of those strata, supposing them to have been undisturbed, at 30,000
m. (nearly 20 miles). He concludes, further, from the examination of
more recent strata, that every stratum one metre in thickness must
have required from 3000 to 20,000 years for its formation. We should,
therefore, have to allow a space of from ninety to six hundred million
years for the deposition of all the sedimentary strata. The Finnish
geologist Sederholm even fixes the time at a thousand million years.

Another method again starts from the consideration that, while the
temperature of the surface of the earth remains fairly steady owing to
the heat exchange between solar radiation and terrestrial radiation
into space, the interior of the earth must have shrunk with the
cooling. How far this shrinkage extends we may estimate from the
formation of the mountain chains which, according to Rudzki, cover
1.6 per cent. of the earth’s surface. The earth’s radius should
consequently have contracted by about 0.8 per cent., corresponding to
a cooling through about 300°, which would require two thousand million
years.

Quite recently the renowned physical chemist Rutherford has expounded
a most original method of estimating the age of minerals. Uranium and
thorium are supposed to produce helium by their slow dissociation, and
we know how much helium is produced from a certain quantity of uranium
or thorium in a year. Now Ramsay has determined the percentage of
helium in the uranium mineral fergusonite and in thorianite. Rutherford
then calculates the time which would have passed since the formation of
these minerals. He demands at least four hundred million years, “for
very probably some helium has escaped from the minerals during that
time.” Although this estimate is very uncertain, it is interesting to
find that it leads to an age for the solid earth-crust of the same
order of magnitude as the other methods.

During this whole epoch of almost inconceivable length of between one
hundred million and two thousand million years, organisms have existed
on the surface of the earth and in the sea which do not differ so very
much from those now alive. The temperature of the surface may have
been higher than it is at present; but the difference cannot be very
great, and will amount to 20° Cent. (36° F.) at the highest. The actual
mean temperature of the surface of the earth is 16° Cent. (61° F.). It
varies from about -20° Cent. (-4° F.) at the North Pole, and -10° Cent.
(+14° F.) at the South Pole to 26° Cent. (79° F.) in the tropical zone.
The main difference between the temperatures of the earth’s surface in
the most remote period from which fossils are extant and the actual
state rather seems to be that the different zones of the earth are now
characterized by unequal temperatures, while in the remote epochs the
heat was almost uniformly distributed over the whole earth.

The condition for this prolonged, almost stationary state was that the
gain of heat of the earth’s surface by radiation from the sun and the
loss of heat by radiation into space nearly balanced each other. That
the replenishing supply by radiation from an intensely hot body—in
our case the sun—is indispensable for the existence of life will be
evident to everybody. Not everybody may, however, have considered that
the loss of heat into cold space or into colder surroundings is just as
indispensable. To some people, indeed, the assumption that the earth as
well as the sun should waste the largest portions of their vital heat
as radiation into cold space appears so unsatisfactory that they prefer
to believe radiation to be confined to radiation between celestial
bodies; there is no radiation into space, in their opinion. All the
solar heat would thus benefit the planets and the moons in the solar
system, and only a vanishing portion of it would fall upon the fixed
stars, because their visual angles are so small. If that were really
correct, the temperature of the planets would rise at a rapid rate
until it became almost equal to that of the sun, and all life would
become impossible. We are therefore constrained to admit that “things
are best as they are,” although the great waste of solar heat certainly
weakens the solar energy.

The opinion that all the solar heat radiated into infinite space is
wasted, starts moreover from a hypothesis which is not proved, and
which is highly improbable—namely, that only an extremely small
portion of the sky is covered with celestial bodies. That might
certainly be correct if we assumed, as has formerly been done, that
the majority of the celestial bodies must be luminous. We do not
possess, however, any reliable knowledge of the number and size of the
dark celestial bodies. In order to account for the observed movements
of different stars, it has been thought that there must be in the
neighborhood of some of them dark stars of enormous size whose masses
would surpass the mass of our sun, or, at least, be equal to it. But
the largest number of the dark celestial bodies which hide the rays
from the stars behind them probably consist of smaller particles,
such as we observe in meteors and in comets, and to a large extent of
so-called cosmical dust. The observations of later years, by the aid
of most powerful instruments, have shown that so-called nebulæ and
nebulous stars abound throughout the heavens. In their interior we
should probably find accumulations of dark masses.

The light intensity of most of the nebulæ is, moreover, far too weak to
permit of their being perceived. We have, therefore, to imagine that
there are bodies all through infinite space, and about as numerous as
they are in the immediate neighborhood of our solar system. Thus every
ray from the sun, of whatever direction, would finally hit upon some
celestial body, and nothing would be lost of the solar radiation, nor
of the stellar radiation.

As regards the radiation-heat exchange, the earth might be likened to
a steam-engine. In order that the steam-engine shall perform useful
work, it is necessary not only that the engine be supplied with heat
of high temperature from a furnace and a boiler, but also that the
engine be able to give its heat up again to a heat reservoir of lower
temperature—a condenser or cooler. It is only by transferring heat
from a body of higher temperature to another body of lower temperature
that the engine can do work. In a similar way no work can be done on
the earth, and no life can exist, unless heat be conferred by the
intermediation of the earth from a hot body, the sun, to the colder
surroundings of universal space—_i.e._, to the cold celestial bodies
in it.

To a certain extent the temperature of the earth’s surface, as we shall
presently see, is conditional by the properties of the atmosphere
surrounding it, and particularly by the permeability of the latter for
the rays of heat.

If the earth did not possess an atmosphere, or if this atmosphere were
perfectly diathermal—_i.e._, pervious to heat radiations—we should
be able to calculate the mean temperature of the earth’s surface,
given the intensity of the solar radiation, from Stefan’s law of the
dependence of heat radiation on its temperature. Starting from the not
improbable assumption that, at a mean distance of the earth from the
sun, the solar rays would send 2.5 gramme-calories per minute to a body
of cross section of 1 sq. centimetre at right angles to the rays of the
sun, Christiansen has calculated the mean temperatures of the surfaces
of the various planets. The following table gives his figures, and
also the mean distances of the planets from the sun, in units of the
mean distance of the earth from the sun, 149.5 million km. (nearly 93
million miles):

   ─────────┬─────────┬───────────┬──────────┬───────────────┬──────────
            │ Radius  │    Mass   │  Mean    │      Mean     │  Density
    Planet  ├─────────┴───────────┤ distance │   temperature │ according
            │ According to See    │          │               │  to See
   ─────────┼─────────┬───────────┼──────────┼───────────────┼──────────
   Mercury  │   0.341 │    0.0224 │   0.39   │ + 178°(332°)  │   0.564
   Venus    │   0.955 │    0.815  │   0.72   │ +  65°        │   0.936
   Earth    │   1     │    1      │   1      │ +   6.5°      │   1
   Moon     │   0.273 │    0.01228│   1      │ +   6.5°(105°)│   0.604
   Mars     │   0.53  │    0.1077 │   1.52   │ -  37°        │   0.729
   Jupiter  │  11.13  │  317.7    │   5.2    │ - 147°        │   0.230
   Saturn   │   9.35  │   95.1    │   9.55   │ - 180°        │   0.116
   Uranus   │   3.35  │   14.6    │  19.22   │ - 207°        │   0.388
   Neptune  │   3.43  │   17.2    │  30.12   │ - 221°        │   0.429
   Sun      │ 109.1   │ 332,750   │     0    │ +6200°        │   0.256
   ─────────┴─────────┴───────────┴──────────┴───────────────┴──────────

In the case of Mercury, I have added another figure, 332°. Mercury
always turns the same side to the sun, and the hottest point of this
side would reach a temperature of 397°; its mean temperature, according
to my calculation, is 332°, while the other side, turned away from the
sun, cannot be at a temperature much above absolute zero, -273°. I have
made a similar calculation for the moon, which turns so slowly about
its axis (once in twenty-seven days) that the temperature on the side
illuminated by the sun remains almost as high (106°) as if the moon
were always turning the same face to the sun. The hottest point of
this surface would attain a temperature of 150°, while the poles of
the moon and that part of the other side which remains longest without
illumination can, again, not be much above absolute zero temperature.
This estimate is in fair agreement with the measurements made of the
lunar radiation and the temperature estimate based upon it. The first
measurement of this kind was made by the Earl of Rosse. He ascertained
that the moon disk as illuminated by the sun—that is to say, the full
moon—would radiate as much heat as a black body of the temperature
110° Cent. (230° F.). A later measurement by the American Very seems
to indicate that the hottest point of the moon is at about 180°, which
would be 30° higher than my estimate. In the cases of the moon and
of Mercury, which do not possess any atmosphere to speak of, this
calculation may very fairly agree with the actual state of affairs.

The temperature of the planet Venus would be about 65° Cent. (149° F.)
if its atmosphere were perfectly transparent. We know, however, that
dense clouds, probably of water drops, are floating in the atmosphere
of this planet, preventing us from seeing its land and water surfaces.
According to the determinations made by Zöllner and others, Venus
would reflect not less than 76 per cent. of the incident light of the
sun, and the planet would thus be as white as a snow-ball. The rays of
heat are not reflected to the same extent. We may estimate that the
portion of heat absorbed by the planet is about half the incident heat.
The temperature of Venus will therefore be reduced considerably, but it
is partly augmented again by the protective action of this atmosphere.
The mean temperature of Venus may, hence, not differ much from the
calculated temperature, and may amount to about 40° (104° F.). Under
these circumstances the assumption would appear plausible that a very
considerable portion of the surface of Venus, and particularly the
districts about the poles, would be favorable to organic life.

Passing to the earth, we find that the temperature-reducing influence
of the clouds must be strong. They protect about half of the earth’s
surface (52 per cent.) from solar radiation. But even with a perfectly
clear sky, not all the light from the sun really reaches the earth’s
surface; for finely distributed dust is floating even in the purest
air. I have estimated that this dust would probably absorb 17 per cent.
of the solar heat. Clouds and dust would therefore together deprive the
earth of 34 per cent. of the heat sent to it, which would lead to a
reduction of the temperature by about 28°. Dust and the water-bubbles
in the clouds also prevent the radiation of heat from the earth, so
that the total loss of heat to be charged to clouds and dust will
amount to about 20° (36° F.).

It has now been ascertained that the mean temperature of the earth is
16° (61° F.), instead of the calculated 6.5° (43.7° F.). Deducting the
20° due to the influence of dust and clouds, we obtain -14° (7° F.), and
the observed temperature would therefore be higher than the calculated
by no less than 30° (54° F.). The discrepancy is explained by the
heat-protecting action of the gases contained in the atmosphere, to
which we shall presently refer (page 51).

There are but few clouds on Mars. This planet is endowed with an
atmosphere of extreme transparency, and should therefore have a high
temperature. Instead of the temperature of -37° (35° F.), calculated,
the mean temperature seems to be +10° (+50° F.). During the winter
large white masses, evidently snow, collect on the poles of Mars, which
rapidly melt away in spring and change into water that appears dark to
us. Sometimes the snow-caps on the poles of Mars disappear entirely
during the Mars summer; this never happens on our terrestrial poles.
The mean temperature of Mars must therefore be above zero, probably
about +10°. Organic life may very probably thrive, therefore, on Mars.
It is, however, rather sanguine to jump at the conclusion that the
so-called canals of Mars prove its being inhabited by intelligent
beings. Many people regard the “canals” as optical illusions; Lowell’s
photographs, however, do not justify this opinion.

As regards the other large planets, the temperatures which we have
calculated for them are very low. This calculation is, however, rather
illusory, because these planets probably do not possess any solid or
liquid surface, but consist altogether of gases. Their densities, at
least, point in this direction. In the case of the inner planets,
Mars and our moon included, the density is rather less than that of
the earth. Mercury stands last among them, with its specific gravity
of 0.564. There follows a great drop in the specific gravities of
the outer large planets. Saturn, with a density of 0.116, is last in
this order; the densities of the two outermost planets lie somewhat
higher—by 0.3 or 0.4 about—but these last data are very uncertain.
Yet these figures are of the same order of magnitude as that assumed
for the sun—0.25—and we believe that the sun, apart from the small
clouds, is wholly a gaseous body. It is therefore probable that
the outer planets, including Jupiter, will also be gaseous and be
surrounded by dense veils of clouds which prevent our looking down
into their interior. That view would contend against the idea that
these planets can harbor any living beings. We could rather imagine
their moons to be inhabited. If these moons received no heat from their
planets, they would assume the above-stated temperatures of their
central bodies. Looked at from our moon, the earth appears under a
visual angle, 3.7 times as large as that of the sun. As the temperature
of the sun has, from its radiation, been estimated at 6200° Cent., or
6500° absolute, the moon would receive as much heat from the earth as
from the sun, if the earth had a temperature of about 3100° Cent., or
3380° absolute. When the first clouds of water vapor were being formed
in the terrestrial atmosphere, the earth’s temperature was about 360°,
and the radiation from the earth to the moon only about 1.25-thousandth
of that of the sun. The present radiation from the earth does not
even attain one-twentieth of this value. It is thus manifest that
the radiation from the earth does not play any part in the thermal
household of the moon.

The relations would be quite different if the earth had the 11.6 times
greater diameter of Jupiter, or the diameter of Saturn, which is
9.3 times greater than its own. The radiation from the earth to the
moon would then make up about a sixth or a ninth of the actual solar
radiation, taking the temperature of the earth’s surface at 360°. We
can easily calculate, further, that Jupiter and Saturn would radiate
as much heat against a moon at a distance of 240,000 or 191,000 km.
respectively (since the distance of the moon from the earth amounts
to 384,000 km.) as the sun sends to Mars—taking the temperature of
those planets at 360° Cent. Now we find, near Jupiter as well as near
Saturn, moons at the distances of 126,000 and 186,000 km. respectively,
which are smaller than those mentioned, and it is not inconceivable
that these moons receive from their central bodies sufficient heat
to render life possible, provided that they be enveloped by a
heat-absorbing atmosphere. The conditions appear to be less favorable
for the innermost satellites of Jupiter and Saturn. When their planets
are shining at the maximum brilliancy, their light intensity is only
a sixth or a ninth of the solar light intensity, which upon these
satellites is itself only one-twenty-seventh or one-ninetieth of the
intensity on the earth. During the incandescence epoch of these planets
their moons will certainly for some time have been suitable for the
development of life.

That the atmospheric envelopes limit the heat losses from the planets
had been suggested about 1800 by the great French physicist Fourier.
His ideas were further developed afterwards by Pouillet and Tyndall.
Their theory has been styled the hot-house theory, because they thought
that the atmosphere acted after the manner of the glass panes of
hot-houses. Glass possesses the property of being transparent to heat
rays of small wave lengths belonging to the visible spectrum; but it is
not transparent to dark heat rays, such, for instance, as are sent out
by a heated furnace or by a hot lump of earth. The heat rays of the sun
now are to a large extent of the visible, bright kind. They penetrate
through the glass of the hot-house and heat the earth under the glass.
The radiation from the earth, on the other hand, is dark and cannot
pass back through the glass, which thus stops any losses of heat, just
as an overcoat protects the body against too strong a loss of heat by
radiation. Langley made an experiment with a box, which he packed with
cotton-wool to reduce loss by radiation, and which he provided, on the
side turned towards the sun, with a double glass pane. He observed
that the temperature rose to 113° (235° F.), while the thermometer
only marked 14° or 15° (57° or 59° F.) in the shade. This experiment
was conducted on Pike’s Peak, in Colorado, at an altitude of 4200 m.
(13,800 ft.), on September 9, 1881, at 1 hr. 4 min. P.M., and therefore
at a particularly intense solar radiation.

Fourier and Pouillet now thought that the atmosphere of our earth
should be endowed with properties resembling those of glass, as regards
permeability of heat. Tyndall later proved this assumption to be
correct. The chief invisible constituents of the air which participate
in this effect are water vapor, which is always found in a certain
quantity in the air, and carbonic acid, also ozone and hydrocarbons.
These latter occur in such small quantities that no allowance has been
made for them so far in the calculations. Of late, however, we have
been supplied with very careful observations on the permeability to
heat of carbonic acid and of water vapor. With the help of these data
I have calculated that if the atmosphere were deprived of all its
carbonic acid—of which it contains only 0.03 per cent. by volume—the
temperature of the earth’s surface would fall by about 21°. This
lowering of the temperature would diminish the amount of water vapor
in the atmosphere, and would cause a further almost equally strong
fall of temperature. The examples, so far as they go, demonstrate that
comparatively unimportant variations in the composition of the air
have a very great influence. If the quantity of carbonic acid in the
air should sink to one-half its present percentage, the temperature
would fall by about 4°; a diminution to one-quarter would reduce the
temperature by 8°. On the other hand, any doubling of the percentage of
carbon dioxide in the air would raise the temperature of the earth’s
surface by 4°; and if the carbon dioxide were increased fourfold, the
temperature would rise by 8°. Further, a diminution of the carbonic
acid percentage would accentuate the temperature differences between
the different portions of the earth, while an increase in this
percentage would tend to equalize the temperature.

The question, however, is whether any such temperature fluctuations
have really been observed on the surface of the earth. The geologists
would answer: yes. Our historical era was preceded by a period in which
the mean temperature was by 2° (3.6 F.) higher than at present. We
recognize this from the former distribution of the ordinary hazel-nut
and of the water-nut (_Trapa natans_). Fossil nuts of these two species
have been found in localities where the plants could not thrive in
the present climate. This age, again, was preceded by an age which,
we are pretty certain, drove the inhabitants of northern Europe from
their old abodes. The glacial age must have been divided into several
periods, alternating with intervals of milder climates, the so-called
inter-glacial periods. The space of time which is characterized by
these glacial periods, when the temperature—according to measurements
based upon the study of the spreading of glaciers in the Alps—must
have been about 5° (8° F.) lower than now, has been estimated by
geologists at not less than 100,000 years. This epoch was preceded by
a warmer age, in which the temperature, to judge from fossilized plants
of those days, must at times have been by 8° or 9° (14° or 16° F.)
higher than at present, and, moreover, much more uniformly distributed
over the whole earth (Eocene). Pronounced fluctuations of this kind in
the climate have also occurred in former geological periods.

Are we now justified in supposing that the percentage of carbon dioxide
in the air has varied to an extent sufficient to account for the
temperature changes? This question has been answered in the affirmative
by Högbom, and, in later times, by Stevenson. The actual percentage of
carbonic acid in the air is so insignificant that the annual combustion
of coal, which has now (1904) risen to about 900 million tons and is
rapidly increasing,[3] carries about one-seven-hundredth part of its
percentage of carbon dioxide to the atmosphere. Although the sea, by
absorbing carbonic acid, acts as a regulator of huge capacity, which
takes up about five-sixths of the produced carbonic acid, we yet
recognize that the slight percentage of carbonic acid in the atmosphere
may by the advances of industry be changed to a noticeable degree in
the course of a few centuries. That would imply that there is no real
stability in the percentage of carbon dioxide in the air, which is
probably subject to considerable fluctuations in the course of time.

    [Footnote 3: It amounted in 1890 to 510 million tons; in 1894,
    to 550; in 1899, to 690; and in 1904, to 890 million tons.]

Volcanism is the natural process by which the greatest amount of
carbonic acid is supplied to the air. Large quantities of gases
originating in the interior of the earth are ejected through the
craters of the volcanoes. These gases consist mostly of steam and of
carbon dioxide, which have been liberated during the slow cooling of
the silicates in the interior of the earth. The volcanic phenomena
have been of very unequal intensity in the different phases of the
history of the earth, and we have reason to surmise that the percentage
of carbon dioxide in the air was considerably greater during periods
of strong volcanic activity than it is now, and smaller in quieter
periods. Professor Frech, of Breslau, has attempted to demonstrate
that this would be in accordance with geological experience, because
strongly volcanic periods are distinguished by warm climates, and
periods of feeble volcanic intensity by cold climates. The ice age
in particular was characterized by a nearly complete cessation of
volcanism, and the two periods at the commencement and at the middle of
the Tertiary age (Eocene and Miocene) which showed high temperatures
were also marked by an extraordinarily developed volcanic activity.
This parallelism can be traced back into more remote epochs.

It may possibly be a matter of surprise that the percentage of
carbon dioxide in the atmosphere should not constantly be increased,
since volcanism is always pouring out more carbon dioxide into our
atmosphere. There is, however, one factor which always tends to reduce
the carbon dioxide of the air, and that is the weathering of minerals.
The rocks which were first formed by the congelation of the volcanic
masses (the so-called magma) consist of compounds of silicic acid
with alumina, lime, magnesia, some iron and sodium. These rocks were
gradually decomposed by the carbonic acid contained in the air and
in the water, and it was especially the lime, the magnesia, and the
alkalies, and, in some measure also the iron, which formed soluble
carbonates. These carbonates were carried by the rivers down into the
seas. There lime and magnesia were secreted by the animals and by the
algæ, and their carbonic acid became stored up in the sedimentary
strata. Högbom estimates that the limestones and dolomites contain at
least 25,000 times more carbonic acid than our atmosphere. Chamberlin
has arrived at nearly the same figure—from 20,000 to 30,000; he
does not allow for the precambrian limestones. These estimates are
most likely far too low. All the carbonic acid that is stored up in
sedimentary strata must have passed through the atmosphere. Another
process which withdraws carbonic acid from the air is the assimilation
of plants. Plants absorb carbonic acid under secretion of carbon
compounds and under exhalation of oxygen. Like the weathering, the
assimilation increases with the percentage of carbonic acid. The
Polish botanist E. Godlewski showed as early as 1872 that various
plants (he studied _Typha latifolia_ and _Glyceria spectabilus_ with
particular care) absorb from the air an amount of carbonic acid which
increases proportionally with the percentage of carbonic acid in the
atmosphere up to 1 per cent., and that the assimilation then attains,
in the former plant, a maximum at 6 per cent., and in the latter
plant at 9 per cent. The assimilation afterwards diminishes if the
carbonic acid percentage is further augmented. If, therefore, the
percentage of carbon dioxide be doubled, the absorption by the plants
would also be doubled. If, at the same time, the temperature rises by
4°, the vitality will increase in the ratio of 1: 1.5, so that the
doubling of the carbon dioxide percentage will lead to an increase
in the absorption of carbonic acid by the plant approximately in the
ratio of 1: 3. The same may be assumed to hold for the dependence
of the weathering upon the atmospheric percentage of carbonic acid.
An increase of the carbon dioxide percentage to double its amount
may hence be able to raise the intensity of vegetable life and the
intensity of the inorganic chemical reactions threefold.

According to the estimate of the famous chemist Liebig, the quantity of
organic matter (freed of water) which is produced by one hectare (2.5
acres) of soil, meadowland, or forest is nearly the same, approximately
2.5 tons per year in central Europe. In many parts of the tropics the
growth is much more rapid; in other places, in the deserts and arctic
regions, much more feeble. We may be justified in accepting Liebig’s
figure as an average for the firm land on our earth. Of the organic
substances to which we have referred, and which mainly consist of
cellulose, carbon makes up 40 per cent. Thus the actual annual carbon
production by plants would amount to 13,000 million tons—_i.e._,
not quite fifteen times more than the consumption of coal, and about
one-fiftieth of the quantity of the carbon dioxide in the air. If,
therefore, all plants were to deposit their carbon in peat-bogs, the
air would soon be depleted of its carbon dioxide. But it is only a
fraction of one per cent. of the coal which is produced by plants that
is stored up for the future in this way. The rest is sent back into the
atmosphere by combustion or by decay.

Chamberlin relates that, together with five other American geologists,
he attempted to estimate how long a time would be required before
the carbon dioxide of the air would be consumed by the weathering of
rocks. Their various estimates yielded figures ranging from 5000 to
18,000 years, with a probable average of 10,000 years. The loss of
carbonic acid by the formation of peat may be estimated at the same
figure. The production of carbonic acid by the combustion of coal would
therefore suffice to cover the loss of carbonic acid by weathering and
by peat formation seven times over. Those are the two chief factors
deciding the consumption of carbonic acid, and we thus recognize that
the percentage of carbonic acid in the air must be increasing at a
constant rate as long as the consumption of coal, petroleum, etc., is
maintained at its present figure, and at a still more rapid rate if
this consumption should continue to increase as it does now.

This consideration enables us to picture to ourselves the possibility
of the enormous plant-growth which must have characterized certain
geological periods of our earth—for instance, the carboniferous period.

This period is known to us from the extraordinarily large number of
plants which we find embedded in the clay of the swamps of those days.
Those plants were slowly carbonized afterwards, and their carbon is in
our age returned to its original place in the household of nature in
the shape of carbonic acid. A great portion of the carbonic acid has
disappeared from the atmosphere of the earth, and has been stored up as
coal, lignite, peat, petroleum, or asphalt in the sedimentary strata.
Oxygen was liberated at the same time, and passed into our atmospheric
sea. It has been calculated that the amount of oxygen in the air—1216
billion tons—approximately corresponds to the mass of fossil coal
which is stored up in the sedimentary strata. The supposition appears
natural, therefore, that all the oxygen of the air may have been
formed at the expense of the carbonic acid in the air. This view was
first advanced by Kœhne, of Brussels, in 1856, and later discussions
have strengthened its probability. Part of the oxygen is certainly
consumed by weathering processes, and absorbed—_e.g._, by sulphides
and by ferro-salts; without this oxidation the actual quantity of
oxygen in the air would be greater. On the other hand, there are in
the sedimentary strata many oxidizable compounds—_e. g._, especially
iron sulphides—which have probably been reduced by the interaction
of carbon (by organic compounds). A large number of the substances
which consume oxygen during their decomposition and decay have also
been produced by the intermediation of the coal which had previously
been deposited under liberation of oxygen, so that these substances
are, by their oxidation, restored to their original state. We may
hence take it as established that the masses of free oxygen in the air
and of free carbon in the sedimentary strata approximately correspond
to each other, and that probably all the oxygen of the atmosphere
owes its existence to plant life. This appears plausible also for
another reason. We know for certain that there is some free oxygen in
the atmosphere of the sun, and that hydrogen abounds in the sun. The
earth’s atmosphere may originally have been in the same condition.
When the earth cooled gradually, hydrogen and oxygen combined to
water, but an excess of hydrogen must have remained. The primeval
atmosphere of the earth may also have contained hydrocarbons, as they
play an important part in the gases of comets. To these gases there
were added carbonic acid and water vapor, coming from the interior of
the earth. Thanks to its chemical inertia, the nitrogen of the air may
not have undergone much change in the course of the ages. An English
chemist, Phipson, claims to have shown that both higher plants (the
corn-bind) and lower organisms (various bacteria) can live and develop
in an atmosphere devoid of oxygen when it contains carbonic acid and
hydrogen. It is also possible that simple forms of vegetable life
existed before the air contained any oxygen, and that these plants
liberated the oxygen from the carbonic acid exhaled by the craters.
This oxygen gradually (possibly under the influence of electric
discharges) converted the hydrogen and the hydrocarbons of the air into
water and carbonic acid until those elements were consumed. The oxygen
remained in the air, whose composition gradually approached more the
actual state.[4]

    [Footnote 4: According to the opinion of a colleague of mine,
    a botanist, the results of the experiments of Phipson must be
    regarded as very doubtful, and some oxygen would appear to be
    indispensable for the growth of plants. We have to imagine the
    development somewhat as follows: As the earth separated from
    the solar nebula, its temperature was very high at first in its
    outer portions. At this temperature it was not able to retain
    the lighter gases, like hydrogen and helium, for a long period;
    the heavy gases, like nitrogen and oxygen, remained. The
    original excess of hydrogen and helium disappeared, therefore,
    before the crust of the earth had been formed, and the
    atmosphere of the earth immediately after the formation of the
    crust contained some oxygen, besides much nitrogen, carbonic
    acid, and water vapor. The main bulk of the actual atmospheric
    oxygen would therefore have been reduced from carbon dioxide by
    the intermediation of plants. The view that celestial bodies
    may lose part of their atmosphere is due to Johnstone Stoney.
    The atmospheric gases escape the more rapidly the lighter their
    molecules and the smaller the mass of the celestial bodies. On
    these lines we explain that the smaller celestial bodies like
    the moon and Mercury, have lost almost all their atmosphere,
    while the earth has only lost hydrogen and helium, which again
    have been retained by the sun.]

This oxygen is an essential element for the production of animal life.
As animal life stands above vegetable life, so animal life could only
originate at a later stage than plant life. Plants require, in addition
to suitable temperature, only carbonic acid and water, and these
gases will probably be found in the atmospheres of all the planets
as exhalations of their inner incandescent masses which are slowly
cooling. The presence of water vapor has directly been established, by
means of the spectroscope, in the atmospheres of other planets—Venus,
Jupiter, and Saturn—and indirectly by the observation of a snow-cap on
Mars. The spectroscope further gives us indication of the presence of
other gases. There is an intense band in the red part of the spectra of
Jupiter and Saturn, of wave-length 0.000618 mm. Other new constituents
of unknown nature have been discerned in the spectra of Uranus and
Neptune. On the other hand, there is hardly any, or at any rate only
a quite insignificant, atmosphere on the moon and on Mercury. This
is easily understood. The temperature on that side of Mercury which
is turned away from the sun is near absolute zero. All the gases of
the planetary atmosphere would collect and condense there. If, then,
Mercury had originally an atmosphere, it must have lost it as it lost
its own rotation, compelling it to turn always the same face towards
the sun. Similar reasons may account for the absence of a lunar
atmosphere. If Venus should likewise always turn the same side towards
the sun, as many astronomers assert, Venus should not have any notable
atmosphere, nor clouds either. We know, however, that this planet is
surrounded by a very marked developed atmosphere.[5]

    [Footnote 5: That results from the very strong refraction which
    light undergoes in the atmosphere of Venus when this planet
    is seen in front of the sun’s edge during the so-called Venus
    transits.]

And that is the strongest objection to the assumption that Venus
follows the example of Mercury as regards the rotation about its own
axis.

Since, now, warm ages have alternated with glacial periods, even after
man appeared on the earth, we have to ask ourselves: Is it probable
that we shall in the coming geological ages be visited by a new ice
period that will drive us from our temperate countries into the hotter
climates of Africa? There does not appear to be much ground for such
an apprehension. The enormous combustion of coal by our industrial
establishments suffices to increase the percentage of carbon dioxide
in the air to a perceptible degree. Volcanism, whose devastations—
on Krakatoa (1883) and Martinique (1902)—have been terrible in late
years, appears to be growing more intense. It is probable, therefore,
that the percentage of carbonic acid increases at a rapid rate. Another
circumstance points in the same direction; that is, that the sea
seems to withdraw carbonic acid from the air. For the carbonic acid
percentage above the sea and on islands is on an average 10 per cent.
less than the above continents.

    [Illustration: Fig. 17.—Photograph of the surface of the moon,
    in the vicinity of the crater of Copernicus. Taken at the
    Yerkes Observatory, Chicago, U. S. A. Scale: Diameter of moon,
    0.55 m. = 21.7 in. Owing to the absence of an atmosphere and of
    atmospheric precipitations, the precipitous walls of the crater
    and other elevations do not indicate any signs of decay.]

If the carbonic acid percentage of the air had kept constant for
ages, the percentage of the water would have found time to get into
equilibrium with it; but the sea actually absorbs carbonic acid
from the air. Thus the sea-water must have been in equilibrium with
an atmosphere which contained less carbonic acid than the present
atmosphere. Hence the carbonic acid percentage has been increasing of
late.

We often hear lamentations that the coal stored up in the earth is
wasted by the present generation without any thought of the future, and
we are terrified by the awful destruction of life and property which
has followed the volcanic eruptions of our days. We may find a kind of
consolation in the consideration that here, as in every other case,
there is good mixed with the evil. By the influence of the increasing
percentage of carbonic acid in the atmosphere, we may hope to enjoy
ages with more equable and better climates, especially as regards the
colder regions of the earth, ages when the earth will bring forth
much more abundant crops than at present, for the benefit of rapidly
propagating mankind.




                                  III

                 RADIATION AND CONSTITUTION OF THE SUN


The question has often been discussed in past ages, and again in
the last century, in how far the position of our earth within the
solar system may be regarded as secure. One might apprehend two
things. Either the distance of the earth from the sun might increase
or decrease, or the rotation of the earth about its axis might be
arrested; and either of these possibilities would threaten the
continuance of life on the earth. The problem of the stability of
the solar system has been investigated by the astronomers, and their
patrons have offered high prizes for a solution of the problem. If the
solar system consisted merely of the sun and the earth, the earth’s
existence would be secure for ages; but the other planets exercise a
certain, though small, influence upon the movements of the earth. That
this influence can only be of slight importance is due to the fact
that the total mass of all the planets does not aggregate more than
one-seven-hundred-and-fiftieth of the mass of the sun, and, further,
to the fact that the planets all move in nearly circular orbits around
the centre, the sun, so that they never approach one another closely.
The calculations of the astronomers demonstrate that the disturbances
of the earth’s orbit are merely periodical, representing long cycles
of from 50,000 to 2,000,000 years. Thus the whole effect is limited to
a slight vacillation of the orbits of the planets about their mean
positions.

So far everything is well and good. But our solar system is traversed
by other celestial bodies, mostly of unknown, but certainly not of
circular orbits—namely, the comets. The fear of a collision with a
comet still alarmed the thinkers of the past century. Experience has,
however, taught us that collisions between the earth and comets do not
lead to any serious consequence. The earth has several times passed
through the tails of comets—for instance, in 1819 and 1861—and it
was only the calculating astronomer who became aware of the fact. Once
on such an occasion we have thought that we observed a glow like that
of an aurora in the sky. When the earth was drawing near the denser
parts of the comet, particles fell on the earth in the shape of showers
of shooting-stars, without doing any appreciable damage. The mass of
comets is too small perceptibly to disturb the paths of the planets.

The rotation of the earth about its axis should slowly be diminished by
the effects of the tides, since they act like a brake applied to the
surface of the earth. This retardation is, however, so unimportant that
the astronomers have not been able to establish it in historical times.
The slow shrinkage of the earth somewhat counteracts this effect.
Laplace believed that we were able to deduce, from an analysis of the
observations of solar eclipses in ancient centuries, that the length of
the day had not altered by more than 0.01 second since the year 729 B.C.

We know that the sun, unaccompanied by its planets, is moving in space
towards the constellation of Hercules with a velocity of 20 km. (13
miles) per second, which is amazing to our terrestrial conceptions.
Possibly the constituents of our solar system might collide with some
other unknown celestial body on this journey. But as the celestial
bodies are sparsely distributed, we may hope that many billions of
years will elapse before such a catastrophe will take place.

In mechanical respects the stability of our system appeared to be well
established. Since the modern theory of heat has made its triumphant
entry into natural science, however, the aspect of matters has changed.
We are convinced that all life and all motion on the earth can be
traced back to solar radiation. The tidal motions alone make a rather
unimportant exception. We have to ask ourselves: Will not the store of
energy in the sun, which goes out, not only to the planets, but to a
far greater extent into unknown domains of cold space, come to an end,
and will not that be the end of all the joys and sorrows of earthly
existence? The position appears desperate when we consider that only
one part in 2300 millions of the solar radiation benefits the earth,
and perhaps ten times as much the whole system, with all its moons.
The solar radiation is so powerful that every gramme of the mass of
the sun loses two calories in the course of a year. If, therefore, the
specific heat of the sun were the same as that of water, which in this
respect surpasses most other substances, the solar temperature would
fall by 2° Cent. (3.6° F.) every year. As, now, the temperature of the
sun in its outer portion has been estimated at from 6000° to 7000°, the
sun should have cooled completely within historical times. And though
the interior of the sun most probably has a vastly higher temperature
than the outer portions which we can observe, we should, all the
same, have to expect that the solar temperature and radiation would
noticeably have diminished in historical times. But all the documents
from ancient Babylon and Egypt seem to point out that the climate at
the dawn of historical times was in those countries nearly the same as
at present, and that, therefore, the sun shone over the most ancient
representatives of culture in the same way as it shines on their
descendants now.

The thesis has frequently been advanced, therefore, that the sun has
in its heat balance not only an expenditure side, but also an almost
equally substantial income side. The German physician R. Mayer,
who has the immortal merit of first having given expression to the
conception of a relation between heat and mechanical work, directed his
attention also to the household of the sun. He suggested that swarms
of meteorites, rushing into the sun with an amazing velocity (of over
600 km. per second), would, when stopped in their motion, generate
heat at the rate of 45 million calories per gramme of meteorites. In
future ages it would be the turn of the planets to sustain for some
time longer the spark of life in the sun, by the sacrifice of their
own existences. The sun would therefore, like the god Saturn, have to
devour its own children in order to continue its existence. Of how
little avail that would be we learn from the consideration that the
fall of the earth into the sun would not be able to prolong the heat
expenditure of the sun by as many as a hundred years. By their rush
into the sun, almost uniformly from all sides, the meteorites would,
moreover, long since have put a stop to the rotation of the sun about
its axis. Further, by virtue of the increasing mass and the hence
augmenting attraction of the sun, the length of our year would have
had to diminish by about 2.8 seconds per year, which is in absolute
contradiction to the observations of the astronomers. According to
Mayer’s thesis, a corresponding number of meteorites would, finally,
also have to tumble upon the surface of the earth, and (according to
data which will be furnished in Chapter IV.) they should raise the
surface temperature to about 800°. The thesis is therefore misleading.

We must look for another explanation. It occurred to Helmholtz, one of
the most eminent investigators in the domain of the mechanical theory
of heat, that, instead of the meteorites, parts of the sun itself
might fall towards its centre, or, in other words, that the sun was
shrinking. Owing to the high gravitation of the sun (27.4 times greater
than on the surface of the earth), the shrinkage would liberate a great
amount of heat. Helmholtz calculated that, in order to cover the heat
expenditure of the sun, a shrinkage of its diameter by 60 m. annually
would be required. If the sun’s diameter should only be diminished by
one-hundredth of one per cent.—a change which we should not be able
to establish—the heat loss would be covered for more than 2000 years.
That seems at first satisfactory. But if we proceed with our estimate,
we find that if the sun went on losing as much heat as at present for
seventeen million years it would have to contract within this period to
a quarter of its present volume, and would therefore acquire a density
like that of the earth. Long before that, however, the radiation from
the sun would have been decreased so powerfully that the temperature
on the earth’s surface would no longer rise above freezing-point.
Helmholtz, on this argument, limited the further existence of the
earth to about six million years. That is less satisfactory. But we
know nothing of the future and must be content with possibilities. Not
so, however, if we calculate back with the aid of Helmholtz’s theory.
According to this theory, and according to Helmholtz’s own data, a
state like the present cannot have existed for more than ten million
years. Since, now, geologists have come to the conclusion that the
petrefactions which we find in the fossil-bearing strata of the earth
have needed at least a hundred million years for their formation, and
more probably a thousand million years, and since, moreover, the still
more ancient formations—the so-called precambrian strata—have been
deposited in equally long or still longer periods, we see that the
theory of Helmholtz is unsatisfactory.

A somewhat peculiar way out of the dilemma has been suggested by a few
scientists. We know that one gramme of the wonderful element radium
emits about 120 calories per hour, or in the course of a year, in
round numbers, a million calories. This radiation seems to continue
unimpaired for years. If we now assume that each kilogramme of the mass
of the sun contains only two milligrammes of radium, that amount would
be sufficient to balance the heat expenditure of the sun for all future
ages. Without some further auxiliary hypothesis, we can, however, not
listen to this suggestion. It presupposes that heat is created out
of nothing. Some scientists, indeed, believe that radium may absorb
a radiation, coming from space, in some unknown manner and convert
it into heat. Before we enter seriously into a discussion of this
explanation we shall have to answer the questions where that radiation
comes from and where it takes its store of energy.

We must, therefore, again search for another source of heat energy for
the sun. Before we can hope to find it, we had better study the sun
itself a little.

All scientists are agreed that the sun is of the same constitution as
the thousands of luminous stars which we see in the sky. According to
the color of the light which they emit, stars are classified as white,
yellow, and red stars. The differences in their light become much
more distinct when we examine them spectroscopically. In the white
stars the helium and hydrogen lines predominate decidedly; the helium
stars contain, in addition, oxygen. Metals are comparatively little
represented; but they play a main part in the spectra of the yellow
stars, in which, further, some bands become visible. In the spectra
of the red stars we notice many bands which indicate that chemical
compounds are present in the outer portions. Everybody knows that the
platinum wire or the filament of an incandescent lamp which has been
heated to incandescence by the electric current first shines reddish,
then yellow when the current is increased, and finally more and more
white. At the same time the temperature rises. We can estimate the
temperature from the brightness of the glow. If we know the wave-length
of the radiations of that color which emits the greatest amount of
heat in the spectrum (it should be a normal spectrum), it is easy to
calculate the temperature of the star from Wien’s law of displacements.
We need only divide 2.89 by the respective wave-length expressed in
mm. to find the absolute temperature of the star; by deducting 273
from the result, we obtain the temperature in degrees Cent. on the
ordinary scale. For the sun the maximum of heat radiation lies near
wave-length 0.00055 (in the greenish-yellow light), and therefore the
absolute temperature of the radiating disk of the sun, the so-called
photosphere, should be 5255° absolute, or nearly 5000° Cent. But our
atmosphere weakens the sunlight, and it also causes a displacement of
the maximum radiation in the spectrum. The same applies to the sun’s
own atmosphere, so that we have to adopt a higher estimate than 5000°
Cent. By means of Stefan’s law of radiation, the solar temperature has
been estimated at about 6200°, which would correspond to a wave-length
of about 0.00045 mm. This correction is therefore significant. About
half of it has to be ascribed to the influence of the solar atmosphere,
the other half to the terrestrial atmosphere. A Hungarian astronomer,
Harkányi, has determined in the same way the temperature of several
white stars (Vega and Sirius), and found it to be about 1000° higher
than that of the sun, while the red star Betelgeuse, the most prominent
star in Orion, would have a temperature by 2500° lower than that of the
sun.

It must expressly be stated that in making these estimates we
understand by the temperature of the star in this case the temperature
of a radiating body which emits the same light as that which reaches
us from the star. But the stellar light undergoes important changes
on its way to us. We learn from observing new stars that a star may
be surrounded by a cloud of cosmical dust which sifts the blue rays
out and permits the red ones to pass. The star then shines with a
less brilliantly white light than in the absence of the cloud. The
consequence is that we estimate the temperature lower than it really
is. In the red stars bands have been noticed, indicating, as we have
already said, the presence of chemical compounds. The most interesting
of these are the compounds of cyanogen and of carbon, probably with
hydrogen, which appear to resemble those observed by Swan in the
spectrum of gas flames and which were named after him. It was formerly
thought that the presence of these compounds implied lower temperature.
But we shall see that this conclusion is not firmly established. Hale
has found during eclipses of the sun that exactly the same compounds
occur immediately above the luminous clouds of the sun. They are
probably more numerous below the clouds, where the temperature is no
doubt higher, than above them.

However that may be, we have reason to assume that the now yellow sun
was once a white star like the brilliant Sirius, that it has slowly
cooled down to its present appearance, and that it will some day shine
with the reddish light of Betelgeuse. The sun will then only radiate a
seventh of the heat which it emits now, and it is very likely that the
earth will have been transformed into a glacial desert long before that
time.

It has already been pointed out that the atmospheres of both the sun
and of the earth produce a strong absorption of the solar rays, and
especially of the blue and white rays. It is for this reason that the
light of the sun appears more red in the evening than at noon, because
in the former case it has to pass through a thicker layer of air, which
absorbs the blue rays. For the same reason the limb of the sun appears
more red in spectroscopic examinations than the centre of the sun.
This weakening of the sun’s light is due to the fine dust pervading
the atmospheres of the earth and the sun. When the products of strong
volcanic eruptions, like the eruptions of Krakatoa in 1883 and of Mont
Pelée in 1902, filled the atmosphere with a fine volcanic dust, the sun
appeared distinctly red when standing low in the horizon. It was this
dust that caused the red glow.

When we examine an image of the sun which has been thrown on a screen
by the aid of a lens or a system of lenses, we notice on the sun’s
disk a mottling of characteristic darker spots. These spots struck the
attention of Galileo, and they were discovered almost simultaneously by
him, by Fabricius, and by Scheiner (1610-1611). These spots have since
been the most diligently studied features of the sun. We carefully
determine their number and sizes, and combine these two data to make
the so-called sun-spot numbers. These numbers change from year to
year in a rather irregular way, the period amounting on an average to
11.1 years. The spots appear in two belts on the sun, and they glide
over the disk in the course of thirteen or fourteen days. Sometimes
they reappear after another thirteen or fourteen days. It is therefore
believed that they lie comparatively quiet on the surface of the sun,
and that the sun rotates about its own axis in about twenty-seven
days, so that after that period the same points are again opposite
the earth. This is the so-called synodical period. The great interest
which attaches to the study of these features lies in the fact that
simultaneously with these spots several other phenomena seem to vary
which attain their maxima at the same time. Such are, in the first
instance, the polar lights and the magnetic variations, and, to a
lesser degree, the cirrus clouds and temperature changes, as well as
several other meteorological phenomena (compare Chapter V.).

About the sun-spots we notice the so-called faculæ—portions which are
much brighter than their surroundings. When we carefully examine a
strongly magnified image of the sun, we find that it has a granulated
appearance (Fig. 18). Langley compares the disk to a grayish-white
cloth almost hidden by flakes of snow. The less bright portions are
designated “pores,” the brighter portions “granules.” It is generally
assumed that the granules correspond to clouds which rise like the
clouds of our atmosphere on the top of ascending convection currents.
But while the terrestrial clouds are formed of drops of rain or of
crystals of ice, the granules consist probably of soot—that is to
say, condensed carbon—and of drops of metals, iron, and others. The
smallest granule which we are able to discern has a diameter of about
200 km. (130 miles).

    [Illustration: Fig. 18.—Sun-spot group and granulation of the
    sun. (Photographed at the Meudon Observatory, near Paris, April
    1, 1884)]

The faculæ are formed by very large accumulations of clouds which are
carried up by strong ascending currents and spread over large areas,
as in our cyclones. The spots correspond to descending masses of gas
with rising temperatures, which are therefore “dry” and do not carry
any clouds, as in terrestrial anticyclones. Through these holes in the
walls of solar clouds we peep a little farther into the gigantic masses
of gas, and we obtain an idea of the state of affairs in the deeper
strata of the sun. The depth of the wall of cloud is, of course, not
large compared to the radius of the sun.

    [Illustration: Fig. 19.—Part of the solar spectrum of January
    3, 1872. After Langley. The bright horizontal bands are due
    to prominences. In the middle (at 208) the hydrogen line F,
    strongly distorted by violent agitation]

The study of the spectra affords us the best insight into the nature
of the different parts of the sun. The spectra teach us not only the
constituents of these parts, but also the velocities with which they
move. We have learned in this way that, lying above the luminous
clouds of the sun which are radiating to us, there are great masses
of gas containing most of our terrestrial elements. We distinguish
particularly in them iron, magnesium, calcium, sodium, helium, and
hydrogen. The two last-mentioned constituents, being the least dense,
are found particularly in the outermost strata of the atmosphere. The
solar atmosphere becomes visible when, during an eclipse of the sun,
the disk of the moon has proceeded so far as to cover the intensely
luminous clouds in the so-called photosphere. Owing to its strong
percentage of hydrogen, the gaseous atmosphere generally shines in the
purple hue which is characteristic of this element. This stratum of gas
is also called the chromosphere (from the Greek word χρῶμα, meaning
color). Its thickness is estimated at from 7000 to 9000 km. (5000 to
6000 miles). From it rise rays of fire over the surrounding surface
like blades of grass on meadows, to which their appearance has been
likened.

    [Illustration: Fig. 20.—Metallic prominences in vortex motion.
    The white spot marks the size of the earth]

    [Illustration: Fig. 21.—Fountain-like metallic prominences]

When these flames rise still higher, to about 15,000 km. (9300 miles)
or more, they are called protuberances or prominences. Their number
as well as their altitude grow with the number of sun-spots. They are
distinguished as metallic and as quiet prominences. The former are
characterized by particularly violent motion, as will become apparent
from Figs. 20 and 21, and they contain large amounts of metallic
vapor. They appear only within the belt of sun-spots which are most
pronounced at a distance of about 20° from the solar equator. Their
movements are so violent that they often traverse several hundreds of
kilometres in a second. The Hungarian Fényi observed, indeed, on July
15, 1895, a prominence whose greatest velocity in the line of sight,
measured spectroscopically, amounted to 862 km. (536 miles), and whose
maximum velocity at right angles to this direction was 840 km. per
second. These colossal velocities distinguish the highest parts, while
the lower portions, which are the most dense and which contain most
metallic vapor, are less mobile, as might be expected. Their altitude
above the sun’s surface may reach exceedingly high figures, and this
applies also to the quiet prominences. The above-mentioned prominence
of July 15, 1895, reached a height of 500,000 km., and Langley
observed, on October 7, 1880, one at an altitude of 560,000 km., whose
tip, therefore, nearly attained an elevation equal to that of a radius
of the sun, 690,000 km. above the limb of the sun’s photosphere. The
mean altitude of these prominences is 40,000 km. After their discovery
by Lector Vassenius, of Götheborg, in 1733, they could only be studied
during total solar eclipses, until Lockyer and Janssen taught us, in
the year 1868, how to observe them in full sunlight by means of the
spectroscope.

    [Illustration: Fig. 22.—Quiet prominences of smoke-column type]

    [Illustration: Fig. 23.—Quiet prominences, shape of a tree. The
    white spot indicates the size of the earth]

    [Illustration: Fig. 24.—Diagram illustrating the differences
    in the spectra of sun-spots and of the photosphere. Some lines
    in the spot spectrum are stronger, others fainter, than in the
    photosphere spectrum. In the central portion, two reversals; to
    the right, two bands. After Mitchell]

    [Illustration: Fig. 25.—Spectrum of a sun-spot, the central
    band between the two portions of the photosphere spectrum. The
    spot spectrum is bordered with the half-shadows of the edge of
    the spot. After Mitchell]

The quiet prominences consist almost exclusively of hydrogen and
helium; sometimes they contain also traces of metallic gases. They
resemble clouds floating quietly in the solar atmosphere, or masses
of smoke coming from a chimney. They may appear anywhere on the sun,
and their stability is so great that they have sometimes been watched
during a complete solar rotation (for about forty days); this is
possible only when they occur in the neighborhood of the poles, where
they always remain visible outside the sun’s limb. Figs. 22 and 23 show
several such prominences according to Young.

    [Illustration: Fig. 26.—The great sun-spot of October 9,
    1903. Taken with the photo-heliograph of Greenwich in the
    usual manner. The spot is shown at mean level of the calcium
    faculæ. The two following photographs show a lower-level and a
    higher-level section through the calcium faculæ]

Sometimes the matter of the prominences seems to fall back upon the
surface of the sun between the smaller flames of fire which we have
likened to blades of grass (Fig. 21). In most cases, however, the
prominences appear slowly to dissolve. When their brilliant glow fades
owing to their intense radiation, they can no longer be observed. The
quiet prominences, which seem to float at heights of about 50,000 km.
and at still greater heights, must there be almost in a vacuum. Their
particles cannot be supported by any surrounding gases, after the
manner of the drops of water in terrestrial clouds. In order that they
may remain floating they must be pushed away from the sun by a peculiar
force—the radiation pressure (see Chapter IV.).

    [Illustration: Fig. 27.—The great sun-spot of October 9, 1903.
    Photograph of the low-level calcium faculæ with the aid of the
    light of the calcium line H. The spot is not obscured by the
    faculæ—at least, not so much as in the following illustrations]

The faculæ can be studied in the same way as the prominences, and
of late Deslandres and Hale have used for this purpose a special
instrument, the heliograph (compare Figs. 26 to 29). When the faculæ
approach the limb of the sun they appear particularly brilliant by
comparison with their surroundings. That seems to indicate that they
are lying at a great altitude, and that their light is hence not
weakened by the superposed hazy stratum. When they reach the sun’s
limb they appear to us like raised portions of the photosphere. The
clouds which form these faculæ are carried upward by powerful ascending
streams of gas whose expansion is due to the diminution of the gaseous
pressure.

    [Illustration: Fig. 28.—The great sun-spot of October 9, 1903.
    Photograph of the higher-level calcium faculæ, taken with the
    light of the central portion of the line H (calcium). The
    higher-level faculæ hide the spot, indicating that the faculæ
    spread considerably during their ascent]

    [Illustration: Fig. 29.—The great sun-spot of October 9, 1903.
    Photograph of the hydrogen faculæ, taken with the light of the
    spectral line F (hydrogen). Only the darkest portions of the
    spot are visible. The other portions are obscured by masses of
    the hydrogen, which were evidently in a restless state]

Sun-spots display many peculiarities in their spectra (Figs. 24 and
25). Very prominent is always the helium line; prominent likewise the
dark sodium lines, which are markedly widened and which show in their
middle portions a bright line—the so-called reversal of lines (Fig.
24). This occurrence indicates that the metal is lying in a deeper
stratum. In the red portion of the spectrum we find bands, just as in
the spectra of the red stars. These bands, which appear to be resolved
into crowds of lines by the aid of powerful instruments, indicate the
presence of chemical compounds. Since the spot is comparatively of
feeble intensity, its spectrum appears superposed like a less bright
ribbon upon the background of the spectrum of the more luminous
photosphere. The violet end of the sun-spot spectrum is particularly
weakened. Although the spot has the appearance of a pit in the
photosphere, and when on the sun’s limb makes it look as if a piece had
been cut out of the edge, it yet does not appear darker than the sun’s
edge. That points to the conclusion that the light emitted by the spot
emanates chiefly from its upper, cold portions.

    [Illustration: Fig. 30.—Photograph of the solar corona of 1900.
    (After Langley and Abbot.) Illustrating the appearance of the
    corona in years of minimum sun-spot frequency]

The light coming from the deeper portions is distinctly absorbed to
a large degree by the higher-lying strata. The sun-spots also appear
to become narrower in their lower parts, owing to the compression of
the gases at greater depths, and one may regard their funnel-shaped
cloud-walls as “half-shadows,” which appear darker than the
surroundings, but brighter than the so-called core of the spot. The
weakening of the violet end of the spectrum is probably due to the
presence of fine particles of dust in the solar gases, just as they
cause the corresponding weakening of the violet end of the spectrum of
the sun’s limb. The bands in the red parts of the sun-spot spectrum may
originate from the deeper portions of the spot, because all the higher
parts of the solar atmosphere yield simple, sharp lines. The bands
suggest that chemical compounds can exist at the higher pressure of the
inner portions of the sun, and that these compounds are decomposed
in the outer parts of the sun, to give the line spectra of chemical
elements.

    [Illustration: Fig. 31.—Photograph of the solar corona of 1870.
    (After Davis.) The year 1870 was one of maximum sun-spot frequency]

The enigmatical corona lies farther out in the atmosphere of the sun.
It consists of streamers which may extend beyond the disk of the sun to
the length of several solar diameters. The corona can only be observed
at total eclipses of the sun. Figs. 30 to 32 illustrate the appearance
of this very peculiar phenomenon.

    [Illustration: Fig. 32.—Photograph of the solar corona of 1898.
    (After Maunder.) 1898 was a year of average solar activity]

When the number of sun-spots is small, the corona streamers extend like
huge brooms from the equatorial parts, and the feebler rays of the
corona near the solar poles are then bent downward to the equator, just
like the lines of force about the poles of a magnet (Fig. 30).

We suppose, for this reason, that the sun acts like a strong magnet,
whose poles are situated near the geographical poles of the sun. In
years which are richer in sun-spots the distribution of the streamers
of the corona is more uniform. At moderate sun-spot frequency, large
numbers of rays seem to emanate from the neighborhood of the maximum
belt of sun-spots, so that the corona often assumes a quadrangular
shape (compare Fig. 32).

These remarks hold for the “outer corona,” while the inner portion,
the so-called “inner corona,” shines in a more uniform light. The
spectroscopic examination demonstrates that the light consists
mainly of hydrogen gas and of an unknown gas designated coronium,
which particularly seems to occur in the higher parts of the inner
corona. The outer streamers of the corona, on the contrary, yield a
continuous spectrum which shows that the light is radiated by solid or
liquid particles. In the spectrum of the coronal rays at an extreme
distance from the disk, astronomers have sometimes fancied that they
discerned dark lines on a bright ground, just as in the spectrum of the
photosphere. It has been assumed that this light is reflected sunlight,
originating from small solid or liquid particles of the outer corona.
It must be reflected, because it is partly polarized. The radiating
disposition of the outer corona indicates the action of a force, the
radiation pressure, which drives the smaller particles away from the
centre of the sun.

As regards the temperature of the sun, we have already seen that
the two methods applied for its determination have yielded somewhat
unequal results. From the intensity of the radiation, Christiansen,
and afterwards Warburg, calculated a temperature of about 6000° Cent.
Wilson and Gray found for the centre of the sun 6200°, which they
afterwards corrected into 8000°. Owing to the absorption of light by
the terrestrial and the solar atmospheres, we always find too low
values. That applies, to a still greater extent, to any estimate based
upon the determination of that wave-length for which the heat emission
from the solar spectrum is maximum. Le Chatelier compared the intensity
of sunlight filtered through red glass with the intensities of light
from several terrestrial sources of fairly well-known temperatures
treated in the same way. These estimates yielded to him a solar
temperature of 7600° Cent. Most scientists reckon with an absolute
temperature of 6500°, corresponding to about 6200° Celsius. That is
what is known as the “effective temperature” of the sun. If the solar
rays were not partially absorbed, this temperature would correspond
to that of the clouds of the photosphere. Since red light is little
absorbed comparatively, Le Chatelier’s value of 7600°, and the almost
equal value of Wilson and Gray of 8000°, should approximately represent
the average temperature of the outer portions of the clouds of the
photosphere. The higher temperature of the faculæ is evident from
their greater light intensity, which, however, may partly be due to
their greater height. Carrington and Hodgson saw, on September 1,
1859, two faculæ break out from the edge of a sun-spot. Their splendor
was five or six times greater than that of the surrounding parts of
the photosphere. That would correspond to a temperature of about
10,000 or 12,000° Cent. The deeper parts of the sun which broke out on
these occasions evidently have a higher temperature, and this is not
unnatural, since the sun is losing heat by radiation from its outer
portions.

We know that the temperature of our atmosphere decreases with greater
heights. The movements of the air are concerned in this change. A
sinking mass of air is compressed by the increased pressure to which
it is being exposed, and its temperature rises, therefore, just as
the temperature rises in a pneumatic gas-lighter when the piston is
pressed down. If the air were dry and in strong vertical motion, its
temperature would change by 10° Cent. (18° F.) per km. If it stood
still, it would assume an almost uniform temperature; that is to say,
there would be no lowering of the temperature as we proceed upward. The
actual value lies between the two extremes. As the gravitation in the
photosphere of the sun is 27.4 times greater than on the surface of the
earth, we can deduce that, if the air on the sun were as dense as on
the earth, the temperature on the sun would vary 27.4 times as much as
on the earth with the increasing height—that is to say, by 270 degrees
per kilometre, provided its atmosphere were in violent agitation. Now,
the outer portions of the solar atmosphere are, indeed, in violent
motion, so that this latter assumption seems to be justified. But
this part consists essentially of hydrogen, which is 29 times lighter
than the air. We must, therefore, reduce the value at which we arrive
to one-twenty-ninth. As a result, the final temperature gradient per
kilometre would only be 9° Cent. (16.2° F.). But the radiation is
extremely powerful on the sun, and it tends to equalize the conditions.
Nine degrees per kilometre is therefore, without doubt, too high a
value. Further, in the interior of the sun the gases are much heavier.
At a small depth, however, they will be so strongly compressed by the
upper strata that their further compressibility will be limited, and
the calculation which we have just made loses its validity. Yet, in any
case, the temperature of the sun must increase as we penetrate nearer
to its centre. If we accept a temperature gradient per kilometre of
the value above indicated, 9°—it is three times greater in the solid
earth-crust—we should obtain for the centre of the sun a temperature
of more than six million degrees.

All substances melt and evaporate as their temperature is raised. If
the temperature exceeds a certain limit, the “critical temperature,”
the substance can no longer be condensed to a liquid, however high the
pressure may be pushed, and the substance will only exist as a gas.
If we start from -273° as absolute zero, this critical temperature is
nearly one and a half times as high as the ebullition temperature of
the substance under atmospheric pressure. So far as our experience
goes, it does not appear probable that the critical temperature of any
substance could be higher than 10,000° or 12,000° Cent., the highest
values which we have calculated for the temperature of the faculæ. The
inner portions of the sun must hence be gaseous, and the whole sun
be a strongly compressed mass of gas of extremely high temperature,
which, owing to the high pressure, is at a density 1.4 times as great
as that of water, and which in many respects, therefore, will resemble
a liquid. It must, for instance, be extremely viscid, and that accounts
for the relatively great stability of the sun-spots (one sun-spot held
out for a year and a half in 1840 and 1841). The sun would thus have
to be regarded as a sphere of gas, in the outer portions of which a
certain amount of condensations of cloud character have taken place,
owing to radiation and to the outward movements of the gaseous masses.
The pressure in the photosphere—that is, in those parts in which these
clouds are floating—has been averaged at five or six atmospheres, a
figure which, considering the very high gravitation, would suggest
a layer of superposed gas above it corresponding to not more than a
fifth of our terrestrial atmosphere. At an approximately corresponding
height, 11,500 m. (38,000 ft.), there are floating in the terrestrial
atmosphere the highest cirrus clouds, to which the clouds of the
photosphere may in many respects be compared.

We turn back to the unanswered question whence the sun takes the
compensation for the heat which it constantly radiates into space. The
most powerful source of heat known to us is that of chemical reactions.
The most familiar reaction of daily life is the combustion of coal.
By burning one gramme of carbon we obtain 8000 calories. If the sun
consisted of pure carbon, its energy would not hold out more than 4000
years. It is not to be wondered at, therefore, that most scientists
soon abandoned the hope of solving the problem in this way. The French
astronomer Faye attempted to explain the replenishment of the losses of
heat by radiation from the sun by arguments in which he resorted to the
heat of a combination of the constituents of the sun. He said: “So high
a temperature must prevail in the interior of the sun that everything
there will be decomposed into its elementary constituents. When the
atoms afterwards penetrate into the outer layers, they are again
united, and they liberate heat.” Faye thus imagined that new masses of
elements would constantly rise from the interior of the sun and would
be reunited in chemical combination on the surface. But if new masses
are to penetrate upward to the surface, those which were at first above
must go back to the centre of the sun, in order to be re-decomposed
by the great heat there; and this re-decomposition would consume just
as much heat as was gained by the rising of the same masses to the
surface. This convection can therefore only help to transport the store
of heat from the interior to the surface. The total amount of heat
stored in the sun would in this way, supposing the mean temperature to
be six million degrees, be able to cover the heat expenditure for about
three million years.

We have, moreover, seen that the highest strata of the sun are
distinguished by line spectra, suggestive of simple chemical compounds,
while at greater depth in the sun-spots chemical combinations occur
which are characterized by band spectra. It is quite incorrect to
assert that high temperatures must necessarily decompose all chemical
compounds into their elements. The mechanical theory of heat teaches
us only that at rising temperatures products are formed whose
formation goes hand in hand with an absorption of heat. Thus, at a
high temperature, ozone is formed from oxygen, although ozone is more
complex in composition than oxygen, and by this reaction 750 calories
are consumed when one gramme of oxygen is transformed into one gramme
of ozone. We likewise know that in the electric arc, at a temperature
of about 3000°, a compound is formed under consumption of heat by the
oxygen and nitrogen of the atmosphere. A new method for the technical
preparation of nitric acid from the nitrogen of the air is based upon
this reaction. Again, the well-known compounds benzene and acetylene
are formed from their elements, carbon and hydrogen, under absorption
of heat. All these bodies can only be synthesized from their elementary
constituents at high temperatures. We further know from experience
that the higher the temperature at which a reaction takes place, the
greater, in general, the amount of heat which it absorbs.

A similar law applies to the influence of pressure. When the pressure
is increased, such processes will be favored as will yield products of
a smaller volume. If we imagine that a mass of gas rushes down from a
higher stratum of the sun into the depths of the sun’s interior, as
gases do in sun-spots, complex compounds will be produced by virtue
of the increased pressure. This pressure must increase at an immense
rate towards the interior of the sun, by about 3500 atmospheres
per kilometre. The gases which dissociate into atoms at the lower
pressures and the higher temperatures of the extreme solar strata above
the photosphere clouds enter into chemical combination in the depths
of the spots, as we learn from spectroscopic examination. Owing to
their high temperatures, these compounds absorb enormous quantities of
heat in their building up, and these quantities of heat are to those
which are concerned in the chemical processes of the earth in the same
ratio as the temperature of the sun is to that at which the chemical
reactions are proceeding on the earth. As these gases penetrate
farther into the sun, temperature and pressure are still more and more
increased, and there will result products more and more abounding in
energy and concentration. We may, therefore, imagine the interior
of the sun charged with compounds which, brought to the surface of
the sun, would dissociate under an enormous evolution of heat and an
enormous increase of volume. These compounds have to be regarded as
the most powerful blasting agents, by comparison with which dynamite
and gun-cotton would appear like toys. In confirmation of this view,
we observe that gases when penetrating into the photosphere clouds are
able to eject prominences at a stupendous velocity, obtaining several
hundred kilometres per second. This velocity surpasses that of the
swiftest rifle-bullet about a thousandfold. We may hence ascribe to
the explosives which are confined in the interior of the sun energies
which must be a million times greater than the energy of our blasting
agents. (For the energy increases with the square of the velocity.)
And yet these solar blasting agents have already given up a large part
of their energy during their passage from the sun’s interior. It thus
becomes conceivable that the solar energy—instead of holding out for
4000 years, as it would if it depended upon the combustion of a solar
sphere made out of carbon—will last for something like four thousand
million years. Perhaps we may further extend this period to several
billions.

That there are such energetic compounds we have learned from the
discovery of the heat evolution of radium. According to Rutherford,
radium is decomposed by one-half in the space of about 1300 years. In
this decomposition a quantity of about a million calories is evolved
per gramme and per year, and we thus find that the decomposition of
radium into its final products is accompanied by a heat evolution of
about two thousand millions of calories per gramme—about a quarter of
a million times more heat than the combustion of one gramme of carbon
would yield.

In chemical respects as well, then, the earth is a dwarf compared to
the sun, and we have every reason to presume that the chemical energy
of the sun will be sufficient to sustain the solar heat during many
thousand millions and possibly billions of years to come.




                                  IV

                        THE RADIATION PRESSURE


Next to simple measuring and simple calculations, astronomy appears
to be the most ancient science. Yet, though man has worshipped the
sun from the most remote ages, it was not fully comprehended before
the middle of the past century that the sun is the source of all life
and of all motion. Part of the veneration for the sun was transferred
to the moon, with its mild light, and to the smaller celestial
lights. It did not escape notice that their positions in the sky
were always changing simultaneously with the annual variations in
the weather, and all human undertakings depended upon the weather
and the seasons. The moon and the stars were worshipped—we know
now, without any justification whatever—as ruling over the weather,
and consequently over man’s fate.[6] Before anything was undertaken
people attempted first to assure themselves of the favorable aspect
of the constellations, and since the most remote ages astrologers
have exercised a vast influence over the ignorant and superstitious
multitude.

    [Footnote 6: The moon strongly, and more than any other agent,
    influences the tides. Apart from this effect the position of
    the moon has only a feeble influence upon the air pressure and
    upon atmospheric electricity and terrestrial magnetism. The
    influence of the stars is imperceptible.]

In spite of the vehement enunciation of Giordano Bruno (1548-1600),
this superstition was still deeply rooted when Newton succeeded in
proving, in 1686, that the movements of the so-called wandering stars,
or planets, and of their moons could be calculated with the aid of
one very simple law: that all these celestial bodies are attracted
by the sun or by their respective central bodies with a force which
is proportional to their own mass and to the mass of the central
body and inversely proportional to the square of their distance from
that central body. Newton’s contemporary, Halley, applied the law of
gravitation also to the mysterious comets, and calculating astronomy
has since been based upon this, its firmest law, to which there has
not been found any exception. The world was thus at once rid of
the paralyzing superstition which exacted belief in a mysterious
ruling of the stars. The contemporaries of Newton, as well as their
descendants, have rightly valued this discovery more highly than any
other scientific triumph of this hero’s. According to Newton’s law, all
material bodies would tend to become more and more concentrated and
united, and the development of the universe would result in the sucking
up of the smaller celestial bodies—the meteorites, for instance—by
the larger bodies.

It must, however, be remarked that Newton’s great precursor, Kepler,
observed in 1618 that the matter of the comets is repelled by the sun.
Like Newton, he believed in the corpuscular theory of light. The sun
and all other luminous bodies radiated light, they thought, because
they ejected minute corpuscles of light matter in all directions.
If, now, these small corpuscles hit against the dust particles in
the comets’ tails, the dust particles would be carried away with
them, and their repulsion by the sun would become intelligible. It
is characteristic that Newton would not admit this explanation of
Kepler’s, although he shared Kepler’s opinion on the nature of light.
According to Newton, the deviation of the tails of comets from his
law of general attraction was only apparent. The tails of comets, he
argued, behaved like the columns of smoke rising from a chimney, which,
although the gases of combustion are attracted by the earth, yet ascend
because they are lighter than the surrounding air. This view, which has
been characterized by Newcomb as no longer to be seriously taken into
consideration, demonstrates the strong tendency of Newton to explain
everything with the aid of his law.

The astronomers followed faithfully in the footsteps of their
inimitable master, Newton, and they brushed aside every phenomenon
which would not fit into his system. An exception was made by the
famous Euler, who, in 1746, expressed the opinion that the waves of
light exerted a pressure upon the body upon which they fell. This
opinion, however, could not prevail against the criticisms with which
others, and especially De Mairan, assailed it. That Euler was right,
however, was proved by Maxwell’s great theoretical treatise on the
nature of electricity (1873). He showed that rays of heat—and the
same applies, as Bartoli established in 1876, to radiations of any
kind—must exercise a pressure just as great as the amount of energy
contained in a unit volume, by virtue of their radiation. Maxwell
calculated the magnitude of this pressure, and he found it so small
that it could hardly have been demonstrated with the experimental means
then at our disposal. But this demonstration has since been furnished,
with the aid of measurements obtained in a vacuum, by the Russian
Lebedeff and by the Americans Nichols and Hull (1900, 1901). They have
found that this pressure, the so-called radiation pressure, is exactly
as great as Maxwell predicted.

In spite of Maxwell’s great authority, astronomers quite overlooked
this important law of his. Lebedeff, indeed, tried in 1892 to apply it
to the tails of comets, which he regarded as gaseous; but the law is
not applicable in this case. As late as the year 1900, shortly before
Lebedeff was able to publish his experimental verification of this
law, I attempted to prove its vast importance for the explanation of
several celestial phenomena. The magnitude of the radiation pressure
of the solar atmosphere must be equivalent to 2.75 milligrammes if the
rays strike vertically against a black body one square centimetre in
area. I also calculated the size of a spherule of the same specific
gravity as water, such that the radiation pressure to which it would be
exposed in the vicinity of the sun would balance the attraction by the
sun. It resulted that equilibrium would be established if the diameter
of the sphere were 0.0015 mm. A correction supplied by Schwarzschild
showed that the calculation was only valid when the sphere completely
reflects all the rays which fall upon it. If the diameter of the
spherule be still smaller, the radiation pressure will prevail over
the attraction, and such a sphere would be repelled by the sun. Owing
to the refraction of light, this will, according to Schwarzschild,
further necessitate that the circumference of the spherule should be
greater than 0.3 times the wave-length of the incident rays. When the
sphere becomes still smaller, gravitation will once more predominate.
But spherules whose sizes are intermediate between these two limits
will be repelled. It results, therefore, that molecules, which have far
smaller dimensions than those mentioned, will not be repelled by the
radiation pressure, and that therefore Maxwell’s law does not hold for
gases. When the circumference of the spherule becomes exactly equal to
the wave-length of the radiation, the radiation pressure will act at
its maximum, and it will then surpass gravity not less than nineteen
times. These calculations apply to all spheres, totally reflecting
the light, of a specific gravity like water, and to a radiation and
attraction corresponding to that of the sun. Since the sunlight is not
homogeneous, the maximum effect will somewhat be diminished, and it
is nearly equal to ten times the gravity for spheres of a diameter of
about 0.00016 mm.[7]

    [Footnote 7: One centimetre of water contains 470 billions of
    these spheres. Such a little drop of water, again, contains 96
    millions of molecules, and there are probably organisms which
    are smaller than these drops. Compare the experiments with
    ultra-microscopic organisms by E. Raehlmann, N. Gaidukow, and
    others.]

Before we had recourse to the radiation pressure for the explanation
of the repulsion phenomena such as have been observed in the tails of
comets, it was generally believed with Zöllner that the repulsion was
due to electrical forces. Electricity undoubtedly plays an important
part in these phenomena, as we shall see. The way in which it acts
in these instances was explained by a discovery of C. T. R. Wilson
in 1899. Gases can in various ways be transformed into conductors of
electricity which as a rule they do not conduct. The conducting gases
are said to be ionized—that is to say, they contain free ions, minute
particles charged with positive or negative electricity. Gases can be
ionized, among other ways, by being radiated upon with Röntgen rays,
kathode rays, or ultra-violet light, as well as by strong heat. Since
the light of the sun contains a great many ultra-violet radiations, it
is indisputable that the masses of gases in the neighborhood of the sun
(_e.g._, probably in comets when they come near the sun) will partly
be ionized, and will contain both positive and negative ions. Ionized
gases are endowed with the remarkable capability of condensing vapors
upon themselves. Wilson showed that this property is possessed to a
higher degree by the negative ions than by the positive ions (in the
condensation of water vapor). If there are, therefore, water vapors in
the neighborhood of the sun which can be condensed by cooling, drops of
water will, in the first instance, be condensed upon the negative ions.
When these drops are afterwards repelled by the radiation pressure,
or when they sink, owing to gravity, as drops of rain sink in the
terrestrial atmosphere, they will carry with them the charge of the
negative ions, while the corresponding positive charge will remain
behind in the gas or in the air. In this way the negative and positive
charges will become separated from each other, and electric discharges
may ensue if sufficiently large quantities of opposite electricity have
been accumulated. By reason of these discharges the gases will become
luminescent, although their temperature may be very low. Stark has even
shown that low temperatures are favorable for the display of a strong
luminosity in electric discharges.

We have stated that Kepler, as early as the beginning of the
seventeenth century, came to the conclusion that the tails of comets
were repelled by the sun. Newton indicated how we might, from the shape
of the comets’ tails, calculate their velocity. The best way, however,
is to determine this velocity by direct observation. The comets’ tails
are not so uniform in appearance as they are generally represented in
illustrations, but they often contain several luminous nuclei (Fig.
33), whose motions can be directly ascertained.

    [Illustration: Fig. 33.—Photograph of Roerdam’s comet (1893
    II.), suggesting several strong nuclei in the tail]

From a study of the movements of comets’ tails, Olbers concluded,
about the beginning of the last century, that the repulsion of the
comets’ tails by the sun is inversely proportional to the square of
their distance—that is to say, that the force of the repulsion is
subject to the same law as the force of gravitation. We can, therefore,
express the repulsion effect in units of solar gravitation, and this
has generally been done. That the radiation pressure will in the same
manner change with the distance is only natural. For the radiation
against the same surface is also inversely proportional to the square
of the distance from the radiating body, the sun.

    [Illustration: Fig. 34.—Photograph of Swift’s comet (1892 I.)]

In the latter part of the past century the Russian astronomer Bredichin
conducted a great many measurements on the magnitude of the forces with
which comets’ tails are repelled by the sun. He considered himself,
on the strength of these measurements, justified in dividing comets’
tails into three classes. In the first class the repulsion was 19 times
stronger than gravitation; in the second class, from 3.2 to 1.5 times
stronger; and in the third class, from 1.3 to 1 times stronger. Still
higher values have, however, been deduced for several comets. Thus
Hussey found for the comet of 1893 (Roerdam’s comet, 1893 II., Fig. 33)
a repulsion 37 times as strong as gravitation; and Swift’s comet (1892
I.) yields the still higher value of 40.5 (Fig. 34). Some comets show
several tails of different kinds, as the famous comet of Donati (Fig.
35). Its two almost straight tails would belong to the first class, and
the more strongly developed and curved third tail to the second class.

    [Illustration: Fig. 35.—Donati’s comet at its greatest
    brilliancy in 1858]

Schwarzschild, as already stated, calculated that small spherules
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.

As regards the other objection, we have to remark that any light
emission must exceed a certain minimum intensity to become visible.
There may be nebulæ, and they probably constitute the majority, which
we cannot observe because the number of electrically charged particles
rushing into them is far too insignificant. A confirmation of this
view was furnished by the flashing-up of the new star in Perseus on
February 21 and 22, 1901. This star ejected two different kinds of
particles, of which the one kind travelled with nearly double the
velocity of the other. The accumulations of dust formed two spherical
shells around the new star, corresponding in every respect to the two
kinds of comets’ tails of Bredichin’s first and second classes, which
we have sometimes observed together in the same comet (Fig. 35). When
these dust particles, on their road, hit against nebular masses, the
latter became luminescent, and we thereby obtained knowledge of the
presence of large stellar nebulæ of whose existence we previously
had not the faintest suspicion. Conditions, no doubt, are similar
in other parts of the heavens where “we have not so far discovered
any nebulæ—we believe, because of the small number of these charged
particles straying about in those parts. On the same grounds we may
explain the variability of certain nebulæ which formerly appeared quite
enigmatical.”




                                   V

  THE SOLAR DUST IN THE ATMOSPHERE—POLAR LIGHTS AND THE VARIATIONS OF
                         TERRESTRIAL MAGNETISM


We have so far dwelt on the effects which the particles expelled
from the sun and the stars exert on distant celestial bodies. It may
be asked whether this dust does not act upon our own earth. We have
already recognized the peculiar luminescence which on clear nights is
diffused over the sky as a consequence of electrical discharges of this
straying dust. This leads to the question whether the magnificent polar
lights, which according to modern views are also caused by electric
discharges in the higher strata of the atmosphere, are not produced
by dust which the sun sends to us. It will, indeed, be seen that we
can in this way explain quite a number of the peculiarities of these
mysterious phenomena which have always excited man’s imagination.

We know that meteorites and shooting-stars are rendered incandescent
by the resistance which they encounter in the air at an average height
of 120 km. (75 miles), sometimes of 150 and 200 km. In isolated cases
meteorites are supposed to have become visible even at still greater
altitudes. It would result that there must be appreciable quantities of
air still at relatively high elevation, and that the atmosphere cannot
be imperceptible at an altitude of less than 100 km., as was formerly
assumed. Bodies smaller than the meteorites as well as the solar dust
we have spoken of—which, owing to their minuteness and to the strong
cooling by heat radiation and conduction that they undergo in passing
through the atmosphere, could never attain incandescence—would be
stopped at greater heights. We will assume that they are arrested at a
mean height of about 400 km. (250 miles).

The masses of dust which are expelled by the sun are partly uncharged,
partly charged with positive or negative electricity. Only the latter
can be connected with the polar lights; the former would remain dark
and slowly sink through our atmosphere to the surface of the earth.
They form the so-called cosmical dust, of whose great importance
Nordenskiöld was so firmly convinced. He estimated that the yearly
increase in the weight of the earth by the addition of the meteorites
was at least ten million tons, or five hundred times more than we
stated above (page 108). Like Lockyer and, in more recent days,
Chamberlin, he believed that the planets were largely built up of
meteorites.

The dust reaching the earth from the sun would not, were it not
electrically charged, amount to more than 200 tons in a year. Although
this figure may be far too low, yet the supply of matter by these means
is certainly very small in comparison with the 20,000 tons which the
earth receives in the shape of meteorites and shooting-stars. But owing
to its extremely minute distribution, the effect of this dust is very
important, and it may constitute a much greater portion of the finely
distributed cosmical dust in the highest strata of the atmosphere than
the dust introduced by falling meteorites and shooting-stars.

That these particles exert a noticeable influence upon terrestrial
conditions, in spite of their relatively insignificant mass, is due to
two causes. They are extremely minute and therefore remain suspended
in our atmosphere for long periods (for more than a year in the case of
the Krakatoa dust), and they are electrically charged.

In order to understand their action upon the earth, we will examine how
the terrestrial conditions depend upon the position of the earth with
regard to the various active portions of the sun, and upon the change
of the sun itself in regard to its emission of dust particles. For this
examination we have to avail ourselves of extensive statistical data;
for only a long series of observations can give us a clear conception
of the action of solar dust.

These particles withdraw from the sun gases which they were able
to condense on their surface, and which had originally been in the
chromosphere and in the corona of the sun. The most important among
these gases is hydrogen; next to it come helium and the other noble
gases which Ramsay has discovered in the atmosphere, in which they
occur in very small quantities. As regards hydrogen, Liveing and
(after him) Mitchell have maintained that it is not produced in the
terrestrial atmosphere. Occasionally it is certainly found in volcanic
gases. Thus hydrogen escapes, for instance, from the crater of Kilauea,
on Hawaii, but it is burned at once in the atmosphere. If hydrogen were
present in the atmosphere, it would gradually combine with the oxygen
to water vapor; and we have to assume, therefore, that the hydrogen
must be introduced into our atmosphere from another source—namely,
from the sun. Mitchell finds in this view a strong support for the
opinion that solar dust is always trickling down through our atmosphere.

The quantity of solar dust which reaches our atmosphere will naturally
vary in proportion with the eruptive activity of the sun. The quantity
of dust in the higher strata influences the color of the light of the
sun. After the eruption of the volcano Rakata on Krakatoa, in 1883,
and again, though to a lesser degree, after the eruption of Mont Pelée
on Martinique, red sunsets and sunrises were observed all over the
globe. At the same time, another phenomenon was noticed which could be
estimated quantitatively. The light of the sky is polarized with the
exception of the light coming from a few particular spots. Of these
spots, one called Arago’s Point is situated a little above the antipode
of the sun, and another, Babinet’s Point, is situated above the sun.
If we determine the elevation of these points above the horizon at
sunset, we find in accordance with the theoretical deduction that this
elevation is greater when the higher strata of the atmosphere are
charged with dust (as after the eruption of Rakata) than under normal
conditions. Busch, a German scientist, analyzed the mean elevation
of these points (stated in degrees of arc) at sunset, and found the
following peculiar numbers:

                │1886 │ ’87 │ ’88 │ ’89 │ ’90  │
 Arago’s Point  │20.1 │19.7 │18.4 │17.8 │=17.7=│
 Babinet’s Point│23.9 │21.9 │17.9 │56.8 │=15.4=│
 Sun-spot Number│21.1 │19.1 │ 6.7 │=6.1=│ 6.5  │

                │ ’91 │ ’92 │ ’93  │  ’94 │ ’95 │Mean
 Arago’s Point  │20.6 │19.6 │ 20.2 │=20.7=│18.8 │19.4
 Babinet’s Point│23.3 │21.5 │=24.2=│ 23.3 │19.0 │20.7
 Sun-spot Number│35.6 │73.8 │=84.9=│ 78.0 │63.9 │40.0

There is a distinct parallelism in these series of figures. Almost
simultaneously with the sun-spot maximum the height of the two
so-called neutral points above the horizon attains its maximum at
sunset, and the same applies to the minimum. That the phenomena in the
atmosphere take place a little later than the phenomena on the sun
which caused them is perhaps only natural.

When the air is rich in dust, or when it is strongly ionized by kathode
rays, conditions are favorable for the formation of clouds. This can
be observed, for instance, with auroral lights. They regularly give
rise to a characteristic cloud formation, so much so that Adam Paulsen
was able to recognize polar lights by the aid of these clouds in
full daylight. Klein has compiled a table on the connection between
the frequency of the higher clouds, the so-called cirrus clouds, at
Cologne, and the number of sun-spots during the period 1850-1900. He
demonstrates that during this half-century, which comprises more than
four sun-spot periods, the sun-spot maxima fell in the years in which
the greatest number of cirrus clouds had been observed. The minima of
the two phenomena are likewise in agreement.

A similarly intensified formation of clouds seems also to occur on
Jupiter when sun-spots are frequent. Vogel states that Jupiter at such
times shines with a whiter light, while at sun-spot minima it appears
of a deeper red. The deeper we are able to peep into the atmosphere of
Jupiter, the more reddish it appears. During periods of strong solar
activity the higher portions of Jupiter’s atmosphere therefore appear
to be crowded with clouds.

The discharge of the charged solar dust in our atmosphere calls forth
the polar lights.

The polar lights occur, as the name indicates, most frequently in
the districts about the poles of the earth. They are, however, not
actually more frequent the nearer we come to the poles; but they
attain a maximum of frequency in circles which enclose the magnetic
and the geographical poles. The northern maximum belt passes, via
Cape Tscheljuskin, north of Novaja Semlja, along the northwestern
coast of Norway, a few degrees to the south of Iceland and Greenland,
right across Hudson Bay and over the northwestern extension of Alaska.
When we go to the south of this belt, the auroras, or boreal lights,
diminish markedly. They are four times less frequent in Edinburgh, and
fifteen times less frequent in London or New York, than in the Orkney
Islands or Labrador.

Paulsen divides the auroras into two classes, which behave quite
differently in several respects. The great difficulties which the
solution of the problems of polar lights has so far offered seem to a
large extent to be due to the fact that all polar lights were treated
as being of the same kind.

The polar lights of the first class do not display any streamers. They
cover a large portion of the sky in a horizontal direction. They are
very quiet, and their light is strikingly constant. As a rule, they
drift slowly towards the zenith, and they do not give rise to any
magnetic disturbances.

These polar lights generally have the shape of an arch whose apex is
situated in the direction of the magnetic meridian (Fig. 38). Sometimes
several arches are grouped one above another.

Nordenskiöld observed these arches quite regularly during the polar
night when he was wintering near Pitlekaj, in the neighborhood
of Bering Sound. Adam Paulsen has often seen them on Iceland and
Greenland, which are situated within the maximum belt spoken of, where
northern lights are very common. Occasionally auroras are also seen
farther from the poles, as circular arches of a milky white, which may
be quite high in the heavens.

Sometimes we perceive in the arctic regions that large areas of the
heavens are covered by a diffused light which might best be compared
to a luminous, transparent cloud; the darker portions in it probably
appear dark by contrast. This phenomenon was frequently observed
during the Swedish expedition of 1882-1883, near Cape Thordsen.

    [Illustration: Fig. 38.—Arch-shaped auroræ borealis, observed
    by Nordenskiöld during the wintering of the _Vega_ in Bering
    Strait, 1879]

Masses of light at so low a level that the rocks behind them are
obscured have frequently been observed to float in the air, especially
in the arctic districts. Thus Lemström saw an aurora on the island
of Spitzbergen in front of a wall of rock only 300 m. (1000 ft.) in
height. In northern Finland he observed the auroral line in the light
of the air in front of a black cloth only a few metres distant. Adam
Paulsen counts these phenomena also as polar lights of the first class,
and he regards them as phosphorescent clouds which have been carried
down by convection currents to an unusually low level of our atmosphere.

Polar lights of the second class are distinguished by the
characteristic auroral rays or streamers. Sometimes these streamers are
quite separated from one another (see Fig. 39); as a rule they melt
into one another, especially below, so as to form draperies which are
so easily moved and unsteady that they appear to flutter in the wind
(Fig. 41.) The streamers run very approximately in the direction of the
inclination (magnetic dip) needle, and when they are fully developed
around the celestial dome their point of convergence is distinctly
discernible in the so-called corona (Fig. 40). When the light is at its
greatest intensity the aurora is traversed by numerous waves of light.

    [Illustration: Fig. 39.—Aurora borealis, with radial streamers]

The draperies are very thin. Paulsen watched them sometimes drifting
over his head in Greenland. The draperies then appeared foreshortened,
in the shape of striæ or ribbons of light in convolutions. These polar
lights influence the magnetic needle. When they pass the zenith their
influence changes sign, so that the deviation of the magnetic needle
changes from east to west when the ribbon is moving from north to
south. Paulsen therefore concluded that negative electricity (kathode
rays) was moving downward in these rays. These polar lights correspond
to violent displacements of negative electricity, while polar lights
of the first class appear to consist of a phosphorescent matter which
is not in strong agitation. The streamers may penetrate down into
rather low atmospheric strata, at least in districts which are near the
maximum belt of the northern lights. Thus Parry observed at Port Bowen
an auroral streamer in front of a cliff only 214 m. (700 ft.) in height.

    [Illustration: Fig. 40.—Aurora with corona, observed by
    Gyllenskiöld on Spitzbergen, 1883]

Polar lights of the first order may pass into those of the second
order, and _vice versa_. We frequently see rays suddenly flash out from
the arch of the aurora, mostly downward, but, when the display is very
intense, also upward. On the other hand, the violent agitation of a
“drapery light” may cease, and may give way to a diffused, steady glow
in the sky. The polar light of the first class is chiefly observed in
the arctic regions. To it corresponds, in districts farther removed
from the pole, the diffused light which appears to be spread uniformly
over the heavens and which gives the auroral line.

    [Illustration: Fig. 41.—Polar-light draperies, observed in
    Finnmarken, northern Norway]

The usually observed polar lights (speaking not only of those seen on
arctic expeditions) belong to the second class, which comprises also
all those included in the subjoined statistics, with the exception
of the auroral displays reported from Iceland and Greenland. While
the streamer lights distinctly conform to the 11.1 years’ period, and
become more frequent at times of sun-spot maxima, this is not the case,
according to Tromholt, with the auroras of Iceland and Greenland. Their
frequency, on the contrary, seems to be rather independent of the
sun-spot frequency. Not rarely auroral maxima corresponding to sun-spot
maxima are subdivided into two by a secondary minimum. This phenomenon
is most evident in the polar regions, but it can also be traced in the
statistics from Scandinavia and from other countries.

Better to understand the nature of auroras, we will consider the sun’s
corona during the time of a minimum year, taking as an example the year
1900 (compare Fig. 30). The rays of the corona in the neighborhood
of the poles of the sun are laterally deflected by the action of the
magnetic lines of force of the sun. The small, negatively charged
particles have evidently only a low velocity, so that they move quite
close to the lines of force in the neighborhood of the solar poles
and are concentrated near the equator. There the lines of force are
less crowded—that is to say, the magnetic forces are weaker—and the
solar dust can therefore be ejected by the radiation pressure and will
accumulate to a large disk expanding in the equatorial plane. To us
this disk appears like two large streams of rays which project in the
direction of the solar equator. Part of this solar dust will come near
the earth and be deflected by the magnetic lines of force of the earth;
it will hence be divided into two streams which are directed towards
the two terrestrial magnetic poles. These poles are situated below the
earth’s crust, and therefore not all the rays will be concentrated
towards the apparent position of the magnetic poles upon the surface of
the earth. It is to be expected that the negatively charged particles
coming from the sun will chiefly drift towards that district which is
situated somewhat to the south of the magnetic north pole, when it is
noon at this pole. When it is midnight at the magnetic pole, most
of the negatively charged particles will be caught by the lines of
force before they pass the geographical north pole, and the maximum
belt of the auroras will for this reason surround the magnetic and
the geographical poles, as has already been pointed out (compare page
122). The negatively charged solar dust will thus be concentrated in
two rings above the maximum belts of the polar lights. Where the dust
collides with molecules of the air, it will produce a phosphorescent
glow, as if these molecules were hit by the electrically charged
particles of radium. This phosphorescent glow rises in the shape of a
luminous arch to a height of about 400 km. (250 miles)—according to
Paulsen—and the apex of this arch will in every part seem to lie in
the direction where the maximum belt is nearest to the station of the
observer. That will fairly coincide with the direction of the magnetic
needle.

The solar corona of a sun-spot maximum year is of a very different
appearance (Fig. 31). The streamers radiate straight from the sun in
almost all directions; and if there be some privileged directions,
it will be those above the sun-spot belts. The velocity of the solar
dust is evidently so great that the streamers are no longer visibly
deflected by the magnetic lines of force of the sun. Nor is this
charged dust influenced to any noticeable degree by the magnetic lines
of force of the earth. It will in the main fall straight down in that
part of the atmosphere in which the radiation is most intense: As
these “hard” rays of the sun[8] seem to issue from the faculæ of the
sun which are most frequent in maximum sun-spot years, some polar
lights will also be seen in districts which are far removed from the
maximum belt of the auroras, especially when the number of sun-spots
is large. The opposite relation holds for the “soft” streams of solar
dust which fall near the maximum belt of the polar lights. These
streams occur most frequently with low sun-spot frequency, as we know
from observations of the solar corona. Possibly they are carried
along by the stream of harder rays in maximum years. The polar lights
corresponding to these rays therefore attain their maximum with few
sun-spots. Hard and soft dust streams occur, of course, simultaneously;
but the former predominate in maximum sun-spot years, the latter in
minimum years.

    [Footnote 8: The designations “hard” and “soft” streams of
    solar dust correspond to the terms used with regard to kathode
    rays. The soft rays have a smaller velocity, and are therefore
    more strongly deflected by external forces, as, for instance,
    magnetic forces.]

That the periodicity of the polar lights in regions without the maximum
belt follows very closely the periodicity of the sun-spots was shown by
Fritz as early as 1863. The length of the period varies between 7 and
16 years, the average being 11.1 years. The years of maxima and minima
for sun-spots and for northern auroras are the following:


                             MAXIMUM YEARS

    Sun-spots       1728  ’39  ’50  ’62  ’70  ’78  ’88  1804  ’16  ’30
                    1837  ’48  ’60  ’71  ’83  ’93  1905
    Northern lights 1730  ’41  ’49  ’61  ’73  ’78  ’88  1805  ’19  ’30
                    1840  ’50  ’62  ’71  ’82  ’93  1905


                             MINIMUM YEARS

    Sun-spots       1734  ’45  ’55  ’67  ’76  ’85  ’98  1811  ’23  ’34
                    1844  ’56  ’67  ’78  ’89  1900
    Northern lights 1735  ’44  ’55  ’66  ’75  ’83  ’99  1811  ’22  ’34
                    1844  ’56  ’66  ’78  ’89  1900

There are, in addition, as De Mairan proved in his classical memoir of
the year 1746, longer periods common to both the number of sun-spots
and the number of auroras. According to Hansky, the length of this
period is 72 years; according to Schuster, 33 years. Very pronounced
maxima occurred at the beginning and the end of the eighteenth century,
the last in the year 1788; afterwards auroras became very rare in the
years 1800-1830, just as in the middle of the eighteenth century. In
1850, and particularly in 1871, there were strong maxima; they have
been absent since then.

The estimates of the heights of the polar lights vary very
considerably. The height seems to be the greater, on the whole, the
nearer the point of observation is to the equator, which would well
agree with the slight deflection of the kathode rays towards the
surface of the earth in regions which are farther removed from the
pole. Gyllenskiöld found on Spitzbergen a mean height of 55 km.;
Bravais, in northern Norway, 100 to 200 km.; De Mairan, in central
Europe, 900 km.; Galle, again, 300 km. In Greenland, Paulsen observed
northern lights at very low levels. In Iceland he fixed the apex of
the northern arch which may be considered as a point where the charged
particles from the sun are discharged into the air at about 400 km. Not
much reliance can be placed upon the earlier determinations; but the
heights given conform approximately to the order of magnitude which we
may deduce from the height at which the solar dust will be stopped by
the terrestrial atmosphere.

The polar lights possess, further, a pronounced yearly periodicity
which is easily explicable by the aid of the solar dust theory. We have
seen that sun-spots are rarely observed near the solar equator, and the
same applies to solar faculæ. They rapidly increase in frequency with
higher latitudes of the sun, and their maximum occurs at latitudes of
about fifteen degrees. The equatorial plane of the sun is inclined by
about seven degrees towards the plane of the earth’s orbit. The earth
is in the equatorial plane of the sun on December 6th and June 4th,
and most distant from it three months later. We may, therefore, expect
that the smallest number of solar-dust particles will fall on the earth
when the earth is in the equator of the sun—that is, in December and
June—and the greatest number in March and September. These relations
are somewhat disturbed by the twilight, which interferes with the
observation of auroras in the bright summer nights of the arctic
region, while the dark nights of the winter favor the observation
of these phenomena. The distribution of the polar lights over the
different seasons of the year will become clear from the subjoined
table compiled by Ekholm and myself:

                                    Iceland and  United   Southern
                  Sweden    Norway   Greenland   States    auroræ
                 (1883-96) (1861-95) (1872-92)  (1871-93) (1856-94)

    January         1056      =251=     804       1005      =56=
    February        1173       331      734       1455      126
    March          =1312=     =335=     613       1396     =183=
    April            568        90      128      =1724=     148
    May              170         6        1       1270       54
    June             =10=       =0=      =0=     =1061=      40
    July              54         0        0       1233      =35=
    August           191        18       40       1210       75
    September       1055       209      455      =1735=     120
    October        =1114=     =353=     716       1630     =192=
    November        1077       326      811       1240      112
    December        =940=      260     =863=      912        81
     Average number  727       181      430       1322      102

In zones where the difference between the lengths of day and night of
the different seasons is not very great, as in the United States, and
in districts in which the southern light is observed (about latitude
40° S.), the chief minimum falls in winter: on the northern hemisphere,
in December; on the southern hemisphere, in June or July. A less
pronounced minimum occurring in the summer. Twice in the course of the
year the earth passes through the plane of the solar equator. During
these periods a minimum of solar dust trickles down upon the earth, and
that period is characterized by a larger number of polar lights which
is distinguished by a higher elevation of the sun above the horizon.
We may expect this; for most solar dust will fall upon that portion
of the earth over which the sun is highest at noon. The two maxima of
March or April and of September or October, when the earth is at its
greatest distance from the plane of the solar equator, are strongly
marked in all the series, except in those for the polar districts
Iceland and Greenland. There the auroral frequency is solely dependent
upon the intensity of the twilight, so that we find a single maximum in
December and the corresponding minimum in June. More recent statistics
(1891-1903) indicate, however, a minimum in December. For the same
reason the summer minimum in countries of high latitudes, like Sweden
and Norway, is very much accentuated.

Similar reasons render it difficult for most localities to indicate the
daily periodicity of the polar lights. Most of the solar dust falls
about noon; and most polar lights should therefore be counted a few
hours after noon, just as the highest temperature of the day is reached
a little after noon. On account of the intense sunlight, however,
this maximum can only be established in the wintry night of the polar
regions, and even there only when a correction has been made for the
disturbing effect of the twilight. In this way Gyllenskiöld found a
northern-light maximum at 2.40 P.M. for Cape Thordsen, on Spitzbergen,
the corresponding minimum being at 7.40 A.M. In other localities we
can only ascertain that the polar lights are more intense and more
frequent before than after midnight. In central Europe the maximum
occurs at about 9 P.M.; in Sweden and Norway (in latitude 60° N.), half
an hour or an hour later.

A few other periods, approximately of the length of a month, have
been suggested with regard to polar lights. A period lasting 25.93
days predominates in the southern lights, where the maximum exceeds
the average by 44 per cent. For the northern lights in Norway the
corresponding excess percentage is 23; for Sweden, only 11.[9]

    [Footnote 9: The reason is that in the southern district only
    very few, and chiefly the most intense, auroras are recorded.
    If we observe very assiduously in a large country, and conduct
    the observations at different spots, we shall find polar light
    almost every night. This consideration partly wipes out the
    just-mentioned differences.]

The same period of nearly twenty-six days had already been pointed out
for a long series of other especially magnetic phenomena which, as we
shall see, are very closely connected with auroras, and it had also
been found in the frequency of thunder-storms and in the variations of
the barometer. This periodicity has often been thought to be connected
with the axial rotation of the sun. The Austrian scientist Hornstein
has even gone so far as to propose that the length of this period
should be carefully determined, “because it would give a more accurate
value for the rotation of the sun than the direct determinations.”
We know now that the length of the solar revolution is different for
different solar altitudes, a circumstance with which observations
of sun-spot movements at different latitudes had already made
Carrington and Spörer familiar, but which was not safely established
before Dunér’s spectroscopical determination of the movement of the
solar photosphere. Dunér found the following sidereal revolutions
for different latitudes of the sun to which the subjoined synodical
revolution would correspond. (By sidereal revolution of a point on the
sun we understand the time which elapses between the two moments when a
certain star passes, on two consecutive occasions, through the meridian
plane of the point—that is to say, through a plane laid through the
poles of the sun and the point in question. The synodical revolution
is determined by the passage of the earth through this meridian. On
account of the proper motion of the earth the synodical period is
longer than the sidereal period.)

    Latitude on the sun (degrees)   0    15    30    45    60    75
    Sidereal revolution (days)    25.4  26.4  27.6  30.0  33.9  38.5
    Synodical revolution (days)   27.3  28.5  29.9  32.7  37.4  43.0

That the periods of rotation of the solar photosphere, and, in a
similar way, the periods of the spots, the faculæ, and the prominences,
should become so considerably longer with increasing latitudes is one
of the most mysterious problems of the physics of the sun. Something
similar applies to the clouds of Jupiter, but the difference in that
case is much smaller—only about one per cent. The clouds of our
atmosphere behave quite differently, a fact which is explained by our
atmospheric circulation.[10]

    [Footnote 10: The very highest strata of our atmosphere (at
    levels of from 20 to 80 km., 15 to 50 miles) may perhaps form
    an exception. The luminous clouds which were observed in the
    years 1883-1892 at Berlin (after the eruption of Krakatoa), and
    which were floating at a very high level, showed a drift with
    regard to the surface of the earth opposite to the drift of the
    cirrus clouds, which are directed eastward.]

In our case, of course, the position of the sun with regard to
the earth—that is to say, the synodical period—can alone be of
importance. We recognize that the period of 25.93 days does not at all
agree with any period of the solar photosphere. The solar equatorial
zone differs least, and it would be appropriate to reckon with this
period, since the earth never moves very far from the plane of the
solar equator, and returns to that plane, at any rate, twice in the
course of a year.

But there is another peculiarity. The higher a point is situated
in the atmosphere of the sun, the shorter is its period. Thus the
synodical period of the faculæ near the equator is on an average 26.06,
the period of the spots 26.82, of the photosphere 27.3 days. Faculæ
situated at higher levels revolve still more rapidly, and we are thus
driven to the conclusion that the period to which we have alluded
agrees with the period of the faculæ situated at higher levels in the
equatorial zone of the sun, and is probably dependent upon them. That
would conform to our ideas concerning the physics of the sun. For the
faculæ are produced in the ascending currents of gas and at rather
lower levels than the spherules which are expelled by the radiation
pressure. This radiation pressure is strongest just in the neighborhood
of the faculæ.

For the same reason the repulsion of the solar dust becomes
particularly powerful when the faculæ are strongly developed—that is
to say, just in the time of pronounced eruptive activity of the sun
which is characterized by many sun-spots.

We must thus imagine that the radiation of the sun will be stronger
in times of strong eruptive activity than during the days of low
sun-spot frequencies. Direct observations of the intensity of the solar
radiation which have been made by Saveljeff in Kieff confirm this
assumption. It must be pointed out, however, that another phenomenon
investigated by Köppen seems to contradict this conclusion. Köppen
ascertained that in our tropics the temperature was by 0.32° Cent.
(nearly 0.6° F.) lower during sun-spot maxima than the average, and
that five years later, a year before the sun-spot minimum, it reached
its maximum value of 0.41° Cent. (0.7° F.) above the average. A similar
peculiarity can be traced to other zones, but on account of the less
steady climates it is much less marked there than in the tropics. A
French physicist, Nordmann, has fully confirmed the observations of
Köppen. On the other hand, Very, an American astronomer, has found that
the temperature in very dry (desert) districts of the tropics (near
Port Darwin, 12° 28´ S., and near Alice Springs, 23° 38´ S., both in
Australia) is higher at sun-spot maxima than at minima; but Very was
in this research guided merely by the records of maximum and minimum
thermometers. From Very’s investigation it would appear that the solar
radiation is really more intense with larger sun-spot numbers.[11]
This, it must be remarked, is only noticeable in exceedingly dry
districts in which there is no cloud formation worth mentioning. In
other districts the cloud formation which accompanies sun-spot maxima
interferes with the simplicity of the phenomena. The cooling effect of
the clouds seems in these cases by far to surpass the direct heating
effect of the solar rays, and in this manner Köppen’s conclusion
would become explicable. If we could observe the temperatures of the
atmospheric strata above the clouds, their variation would no doubt be
in the same degree as that in the desert.

    [Footnote 11: According to Memery (_Bull. Soc. Astr._, March 7,
    1906, p. 168) an instantaneous rise of temperature is observed
    immediately when a sun-spot is first seen, and the temperature
    sinks again when the sun-spot disappears.]

Finally, we have to note another period in the phenomena of the polar
lights—the so-called tropical month, whose length is 27.3 days. The
nature of this period is little understood. It is possibly connected
with the electric charge of the moon. The peculiarity of this period
is that it acts in an opposite way in the northern and southern
hemispheres. When the moon is above the horizon, it seems to prevent
the formation of polar lights; but for this case the difficulties of
observation caused by the moonlight must, of course, be taken into
consideration.

    [Illustration: Fig. 42.—Curve of magnetic declination at
    Kew, near London, on November 15 and 16, 1905. The violent
    disturbance of November 15, 9 P.M., corresponds to the maximum
    intensity of the aurora. Compare the following figure]

Celsius and Hiorter observed in 1741 that the polar lights exercise an
influence on the magnetic needle. From this circumstance we have drawn
the conclusion that the polar lights are in some way due to electric
discharges which act upon the magnetic needle. These magnetic effects,
the disturbances of the otherwise steady position of the magnetic
needle, are not influenced by the light of the sun and moon, and can
therefore be studied to greater advantage than auroras. We have
already pointed out that it is only the aurora of the radial, streamer
type which exerts this magnetic influence (compare Figs. 42 and 43).

    [Illustration: Fig. 43.—Curve of horizontal intensity at Kew on
    November 15 and 16, 1905. On November 15 a magnificent aurora
    was observed in Galicia, Germany, France, Norway, England,
    Ireland, and Nova Scotia, with a maximum at 9 P.M. The polar
    light was unusually brilliant as early as 6 P.M.]

These magnetic variations show exactly the same periods as the northern
lights and the sun-spots. As regards, first, the long period of 11.1
years, our observations prove that the so-called magnetic disturbances
of the position of the magnetic needle faithfully reflect the
variations in the sun-spots. This connection was discovered in 1852 by
Sabine in England, by Wolf in Switzerland, and by Gautier in France.
Even the more irregular diurnal variations in the magnetic elements
are subjected to a solar period. The magnetic needle points in our
districts with its north end towards the north—not exactly, though,
being deflected towards the west. This western deviation or declination
is greatest soon after noon, about one o’clock, and this diurnal change
is greater in summer than in winter, and the fluctuation of the
position of the magnetic needle greater in daytime than at night-time.
It is, therefore, manifest that we have to deal with some solar
effect. This becomes still more distinct when we study the change with
reference to the daily variation in the number of sun-spots. In the
subjoined table the variation in the declination has been compiled for
Prague for the years 1856 to 1889. Only years with maxima and minima of
sun-spots and of magnetic variations have been noticed in this table:

                      1856  1860  1867   1871  1879  1884  1889

    Sun-spot number    4.3  95.7   7.3  139.1   3.4  63.7   6.3


                    DAILY VARIATIONS IN DECLINATION

                      1856   1859  1867  1871   1878  1883  1889

    Observed          5.98  10.36  6.95  11.43  5.65  8.34  5.99
    Calculated        6.08  10.20  6.22  12.15  6.04  8.76  6.17

We see that the maxima and minima years of the two phenomena very
nearly coincide. The accord is so evident that we may calculate the
diurnal variation as proportional to the increase in the number of
sun-spots. This is shown by the two last lines of the table.

The yearly variation is again exactly the same as that of polar lights,
as the following table indicates, in which the disturbances of magnetic
declination, horizontal intensity, and vertical intensity are compiled
for Toronto, Canada; for comparison the means of these three magnitudes
are added for Greenwich. The unit of this table is the average annual
variation:


                                TORONTO

                  Jan.   Feb.  Mar.  April    May    June   July

    Declination  =0.57=  0.84  1.11  =1.42=   0.98  =0.53=  0.94
    Horizontal   =0.56=  0.94  0.94  =1.50=   0.90  =0.36=  0.61
    Vertical     =0.57=  0.74  1.08  =1.49=   1.12  =0.50=  0.71

                  Aug.  Sept.   Oct.  Nov.  Dec.

    Declination   1.16  =1.62=  1.31  0.78  0.76
    Horizontal    0.75  =1.71=  1.48  0.98  0.58
    Vertical      1.08  =1.61=  1.29  0.75  0.61


                               GREENWICH

             Jan.   Feb.   Mar.  April  May    June   July

    Means    0.93  =1.23=  1.22  1.09   0.81  =0.71=  0.81

             Aug.   Sept.   Oct.   Nov.   Dec.

    Means    0.90   1.15  =1.18=  1.02   =0.83=


The daily variation of the disturbances has been analyzed by Van
Bemmelen for the period 1882-1893 and for the observatory of Batavia,
on Java. The maximum occurs there about 1 P.M., and is about 1.86 times
as great as the average value for the day. The minimum of 0.48 occurs
at 11 P.M. Between 8 P.M. and 3 A.M. the disturbances are almost as
rare as about 11 o’clock at night.

The variation is greatest with that declination which has its maximum
of 3.26 at 12 M., and its minimum of 0.14 at 11 P.M.

The period of almost 26 days first investigated by Hornstein has also
been refound in the magnetic variations and disturbances by Broun,
Liznár, and C. A. Müller. It must be added, however, that Schuster does
not consider these data as in any way conclusive.

The moon has also a slight influence upon the magnetic needle, as
Kreil proved as early as 1841. The effect is in a different sign in
the northern and southern hemispheres, and may be likened to a tidal
phenomenon.

The ultra-violet rays of the sun are strongly absorbed by the
atmosphere, and they cause an ionization of the molecules of the air.
This ionization is, on the whole, more marked at higher altitudes. The
ascending air currents carry with them water vapor which is condensed
on the ions, particularly on the negative ions. In this way most
clouds become negatively charged; this interesting fact—_i.e._, that
they are more frequently charged with negative than with positive
electricity—was first proved by Franklin in his kite experiments.
When the rain-drops have fallen, the air above remains positively
charged; this has been observed during balloon ascensions. The clouds
which are formed at high levels are most strongly charged; for this
reason thunder-storms over land occur mostly in the summer-time. The
thunder-storms also show the 26-day period, as Bezold has proved for
southern Germany, and Ekholm and myself have shown for Sweden.

A vast amount of material concerning these questions and magnetic
phenomena in particular has been collected by the various
meteorological observatories and is awaiting analysis.

Although some observers like Sidgreaves question the correlation of
sun-spots and polar lights or magnetic disturbances, because strong
spots have been seen on the disk of the sun without any magnetic
disturbances having been noticed, yet the view predominates that the
magnetic disturbances are caused by sun-spots when the sun-spots cross
the central meridian of the sun which is opposite the earth. Thus
Maunder observed a magnetic storm and a northern light succeeding the
passage of a large sun-spot through the central solar meridian on the
8th to the 10th of September, 1898. The magnetic effect attained its
maximum about twenty-one hours after the passage through the meridian.

Similarly Riccò established in ten instances, in which exact
determination was possible, a time interval of 45.5 hours on an
average between the meridian passage of a spot and the maximum
magnetic effect. Riccò also submitted to an analysis the data which
Ellis had collected and which Maunder had investigated. He found for
these instances, on an average, almost exactly the same numbers,
the time interval being 42.5 hours. That would correspond to a mean
velocity of the solar dust of from 910 to 980 km. per second. On the
other hand, we have no difficulty at all in calculating the time
which a spherule of a diameter of 0.00016 mm. (those particles travel
fastest) and of the specific gravity of water would need in order to
reach the earth, under the influence of solar gravitation and of a
mechanical radiation pressure 2.5 times as large from the outside of
the sun. The time found, 56.1 hours, corresponds to a mean velocity
of 740 km. per second. In order that the solar dust may move with the
velocity calculated by Riccò, its specific gravity should be less than
1—_viz._, 0.66 and 0.57. This value looks by no means improbable,
when we assume that the spherules consist of hydrocarbons saturated
with hydrogen, helium, and other noble gases. We should also arrive
at larger velocities for the solar dust, as has already been pointed
out with regard to the tails of comets, when we presume that the
particles consist of felted marguerites of carbon or silicates, or of
iron—materials which we regard as the main constituents of meteorites.

It should, perhaps, be mentioned that the most intense spectrum line
of the polar lights has been found to be characteristic of the noble
gas krypton. As this gas is found only in very small quantities in our
atmosphere, it does not appear improbable that it has been carried
to us together with the solar dust, and that its spectrum becomes
perceptible during the discharge phenomena. The other spectrum lines
of the polar lights belong to the spectra of nitrogen, argon, and of
the other noble gases. The volumes of noble gases which are brought
into our terrestrial atmosphere in this manner would in any case be
exceedingly small.

The electrical phenomena of our terrestrial atmosphere indirectly
possess considerable importance for organic life and, consequently,
for human beings. By the electrical discharges part of the nitrogen is
made to combine with the oxygen and hydrogen (liberated by the electric
decomposition of water vapor) of our air, and it thus forms the
ammonia compounds, as well as the nitrates and nitrites, which are so
essential to vegetable growth. The ammonia compounds which play a most
important part in the temperate zones appear especially to be formed
by the so-called silent discharges which we connect with auroras. The
oxygen compounds of nitrogen, on the other hand, seem to be chiefly the
products of the violent thunder-storms of the tropics. The rains carry
these compounds down into the soil, where they fertilize the plants.

The supply of nitrogen thus fixed amounts in the course of a year to
about 1.25 gramme per square metre in Europe, and to almost fourfold
that figure in the tropics. If we accept three grammes as the average
number for the whole firm land of the earth, that would mean 3 tons per
square kilometre, and about 400 million tons per year for the whole
firm land of 136 million square kilometres. A very small portion of
this fixed nitrogen, possibly one-twentieth, falls on cultivated soil;
a larger portion will help to stimulate plant growth in the forests
and on the steppes. We may mention, for comparison, that the nitrogen
contained in the saltpetre which the mines of Chili yield to us has
risen from 50,000 tons in 1880 to 120,000 tons in 1890, to 210,000 tons
in 1900, and to 260,000 tons in 1905. The nitrogen produced in the
shape of ammonium salts (sulphate) by the gas-works of Europe amounts
to about one-quarter of the last-mentioned figure. To this figure we
have, of course, to add the production of coal-gas ammonia in the
United States and elsewhere. Yet even allowing for this item, we shall
find that the artificial supply of combined nitrogen on the earth does
not represent more than about one-thousandth of the natural supply.

As the nitrogen contents of the air may be estimated at 3980 billion
tons, we recognize that only one part in three millions of the nitrogen
of the atmosphere is every year fixed by electric discharges, presuming
that the nitrogen supply to the sea is as great per square kilometre
as to the land. The nitrogen thus fixed benefits the plants of the sea
and of the land, and passes back into the atmosphere or into the water
during the life of the plants or during their decay. Water absorbs
some nitrogen, and equilibrium between the nitrogen contents of the
atmosphere and of the sea is thus maintained. Hence we need not fear
any noteworthy depletion of the nitrogen contents of the air. This
conclusion is in accord with the fact that no notable accumulation of
fixed nitrogen appears to have taken place in the solid and liquid
constituents of the earth.

The reader may remember (compare page 57) that during the annual cycle
of vegetation not less than one-fiftieth of the atmospheric contents
in the carbon dioxide is absorbed. Since oxygen is formed from this
carbon dioxide, and since the air contains about seven hundred times as
many parts per volume of oxygen as of carbon dioxide, the exchange of
atmospheric oxygen is about one part in thirty-five thousand. In other
words, the oxygen of the air participates about one hundred times more
energetically in the processes of vegetation than the nitrogen, and
this is in accordance with the general high chemical activity of oxygen.

Before we close this chapter we will briefly refer to the peculiar
phenomenon known as the Zodiacal Light, which can be seen in the
tropics almost any clear night for a few hours after or before sunset
or sunrise. In our latitudes the light is rarely visible, and is best
seen about the periods of the vernal and autumnal equinoxes. The
phenomenon is generally described as a luminous cone whose basis lies
on the horizon, and whose middle line coincides with the zodiac. Hence
the name. According to Wright and Liais, its spectrum is continuous. It
is stated that the Zodiacal Light is as strong in the tropics as that
of the Milky Way.

    [Illustration: Fig. 44.—Zodiacal Light in the tropics]

There can be no doubt that this glow is due to dust particles
illuminated by the sun. It has therefore been assumed that this dust is
floating about the sun in a ring, and that it represents the rest of
that primeval nebula out of which the solar system has been condensed,
according to the theory of Kant and Laplace (compare Chapter VII.).
Sometimes a fairly luminous band seems to shoot out from the apex of
the cone of the Zodiacal Light and to cross the sky in the plane of
the ecliptic. In that part of the sky which is just opposite the sun
this band expands to a larger, diffused, not well-defined spot of
light covering about 12° of arc in latitude and 90° in longitude. This
luminescence is called the counter-glow (Gegenschein), and was first
described by Pezenas in 1780.

The most probable view concerning the nature of this counter-glow is
that it is caused by small particles of meteorites or dust which fall
towards the sun. Like the position of the corona of the aurora, the
position of this counter-glow seems to be an effect of perspective; the
orbits of the little particles are directed towards the sun, and they
therefore appear to radiate from a point opposite to it.

We know very little about this phenomenon. Even the position of the
Zodiacal Light along the zodiac which has given rise to its name
has been questioned, and it would appear from recent investigations
that the glow is situated in the plane of the solar equator. However
that may be, the view is very generally held that the glow is due to
particles which come from the sun or enter into it. We have already
adduced arguments to prove that the mass of solar dust cannot be
unimportant; this dust may therefore be the cause of the phenomenon
which we have just been discussing.




                                  VI

                    END OF THE SUN—ORIGIN OF NEBULÆ


We have seen that the sun is dissipating and wasting almost
inconceivable amounts of heat every year: 3.8. 10^{33} gramme-calories,
corresponding to 2 gramme-calories for each gramme of its mass. We have
also obtained an idea as to how the enormous storage of heat energy in
the sun may endure this loss for ages. Finally, however, the time must
come when the sun will cool down and when it will cover itself with a
solid crust, as the earth and the other planets—so far, probably, in a
gaseous state—have done long since or will do before long. No living
being will be able to watch this extinction of the sun despairingly
from one of the wandering planets; for, in spite of all our inventions,
all life will long before have ceased on the satellites of the sun for
want of heat and light.

The further development of the cold sun will recall the actual progress
of our earth, except in so far as the sun will have no life-spending,
central source of light and heat near it. In the beginning the thin,
solid crust will again and again be burst by gases, and streams of
lava will rush out from the interior of the sun. After a while these
powerful discharges will stop, the lava will freeze, and the fragments
will close up more firmly than before. Only on some of the old
fissures volcanoes will rise and allow the gases to escape from the
interior—water vapor and, to a less extent, carbonic acid, liberated
by the cooling.

Then water will be condensed. Oceans will flood the sun, and for a
short period it will resemble the earth in its present condition,
though with the one important difference. The extinct sun, unlike
our earth, will not receive life-giving heat from the outside,
excepting the small amount of radiation from universal space and
the heat generated by the fall of meteorites. The temperature fall
will therefore be rapid, and the vanishing clouds of the attenuated
atmosphere will not long check radiation. The ocean will become
covered with a crust of ice. Then the carbonic acid will commence
to condense, and will be precipitated as a light snow in the solar
atmosphere. Finally, at a temperature of about -200° Cent., the gases
of the atmosphere will be condensed, and new oceans, now principally
of nitrogen, will be produced. Let the temperature sink another 20°,
and the energy of the inrushing meteorites will just suffice to balance
a further loss of heat by radiation. The solar atmosphere will then
consist essentially of helium and hydrogen—the two gases which are
most difficult to condense—and of some nitrogen.

In this stage the heat loss of the sun will be almost imperceptible.
Owing to the low thermal conductive power of the earth’s crust,
there escapes through each square mile of this crust scarcely
one-thousand-millionth part of the heat which the sun is radiating
from an equal area of its surface. In future days, when the solar
crust will have attained a thickness of 60 km. (40 miles), its loss
of heat will be diminished to the same degree. The temperature on the
surface of the sun may then still be some 50° or 60° above absolute
zero, and volcanic eruptions will raise the temperature only for short
periods and over small areas. Yet in the interior the temperature will
still be at nearly the same actual intensity, something like several
million degrees, and the compounds of infinite explosive energy will
be stored up there as today. Like an immense dynamite magazine, the
dark sun will float about in universal space without wasting much
of its energy in the course of billions of years. Immutable, like a
spore, it will retain its immense store of force until it is awakened
by external forces into a new span of life similar to the old life. A
slow shrinkage of the surface, due to the progressive loss of heat of
the core and to the consequent contraction, will in the meanwhile have
covered the sun with the wrinkles of old age.

Let us suppose that the crusts of the sun and the earth have the same
thermal conductivity—namely, that of granite. According to Homén, a
slab of granite one centimetre in thickness, whose two surfaces are
at a temperature difference of 1° Cent., will permit 0.582 calorie
to pass per minute per square centimetre of surface. By analogy, the
earth’s crust, with an increase of temperature of 30° per kilometre,
as we penetrate inward, would allow 1.75 .10^{-4} calorie to pass per
minute and per square centimetre (this is 1/3580 of the mean heat
supply of the earth, 0.625 calorie per minute per square centimetre);
while the sun, with a crust of the same thickness as the earth, but
with a diameter 108.6 times larger, would lose 3.3 times more heat per
minute than the earth receives from it at the present time. At present
the sun loses 2260 million times more heat than the earth receives;
consequently, the loss of heat would be reduced to 1/686,000,000 of
the present amount. If the thickness of the solar crust amounted to
1/140 of the solar radius—that is to say, to the same fraction that
the thickness of the earth’s crust represents of the terrestrial
radius—the sun would in 74,500 million years not lose any more heat
than it does now in a single year. This number has to be diminished, on
account of the colder surface which the sun would have by that time,
to about 60,000 million years. Considering that the mean temperature
of the sun may be as high as 5 million degrees Celsius, the cooling
down to the freezing-point of water might occupy 150,000 billion years,
assuming that its mean specific heat is as great as that of water.
During this time the crust of the sun would increase in thickness and
the cooling would, of course, proceed at a decreasing rate. In any
case, the total loss of energy during a period of a thousand billion
years could, under these circumstances, only constitute a very small
fraction of the total stored energy.

When an extinct star moves forward through infinite spaces of time, it
will ultimately meet another luminous or likewise extinct star. The
probability of such a collision is proportional to the angle under
which the star appears—which, though very small, is not of zero
magnitude—and to the velocity of the sun. The probability is increased
by the deflection which these celestial bodies will undergo in their
orbits on approaching each other. Our nearest neighbors in the stellar
universe are so far removed from us that light, the light of our sun,
requires, on an average, perhaps ten years to reach them. In order
that the sun, with its actual dimensions and its actual velocity in
space—20 km. (13 miles) per second—should collide with another star
of similar kind, we should require something like a hundred thousand
billion years. Suppose that there are a hundred times more extinct than
luminous stars—an assumption which is not unjustifiable—the probable
interval up to the next collision may be something like a thousand
billion years. The time during which the sun would be luminous would
represent perhaps one-hundredth of this—that is to say, ten billion
years. This conclusion does not look unreasonable. For life has only
been existing on the earth for about a thousand million years, and this
age represents only a small fraction of the time during which the sun
has emitted light and will continue to emit light. The probability of
a collision between the sun and a nebula is, of course, much greater;
for the nebulæ extend over very large spaces. In such a case, however,
we need not apprehend any more serious consequences than result when a
comet is passing through the corona of the sun. Owing to the very small
amount of matter in the corona, we have not perceived any noteworthy
effects in these instances. Nevertheless, the entrance of the sun into
a nebula would increase the chance of a collision with another sun; for
we shall see below that dark and luminous celestial bodies appear to be
aggregated in the nebulæ.

From time to time we see new stars suddenly flash up in the sky,
rapidly decrease in splendor again, to become extinguished or, at
any rate, to dwindle down to faint visibility once more. The most
remarkable of these exceedingly interesting events occurred in
February, 1901, when a star of the first magnitude appeared in the
constellation of Perseus. This star was discovered by Anderson, a
Scotchman, on the morning of February 22, 1901. It was then a star of
the third magnitude.[12] On a photograph which had been taken only
twenty-eight hours previous to the discovery of this star, the star
was not visible at all, although the plate marked stars down to the
twelfth magnitude. The light intensity of this new star would hence
appear to have increased more than five-thousand-fold within that
short space of time. On February 23d the new star surpassed all other
stars except Sirius in intensity. By February 25th it was of the first
magnitude, by February 27th of the second, by March 6th of the third,
and by March 18th of the fourth magnitude. Then its brightness began to
fluctuate periodically up to June 22d, with a period first of three,
then of five days, while the average light intensity decreased. By
June 23d it was of the sixth magnitude. The light intensity diminished
then more uniformly. By October, 1901, it was a star of the seventh
magnitude; by February, 1902, of the eighth magnitude; by July, 1902,
of the ninth magnitude; by December, 1902, of the tenth magnitude; and
since then it has gradually dwindled to the twelfth magnitude. When
this star was at its highest intensity it shone with a bluish-white
light. The shade then changed into yellow, and by the beginning of
March, 1901, into reddish. During its periodical fluctuations the hue
was whitish yellow at its maximum and reddish at its minimum intensity.
Since then the color has gradually passed into pure white.

    [Footnote 12: Stars are classified in magnitude, the order
    being such that the most luminous stars have the lowest
    numbers. A star of the first magnitude is 2.52 times brighter
    than a star of the second magnitude; this, again, 2.52 times
    brighter than a star of the third magnitude, and so on. All
    this from the point of view of an observer on the earth.]

The spectrum of this star shows the greatest similarity to that of the
new star in the constellation Auriga (Nova Aurigæ) of the year 1892
(Fig. 45).

    [Illustration: Fig. 45.—Spectrum of Nova Aurigæ, 1892]

On the whole, it is characteristic of new stars that the spectrum lines
appear double—dark on the violet and bright on the red side. In the
spectrum of Nova Aurigæ this peculiarity is, among others, striking
in the three hydrogen lines C, F, and H, in the sodium line, in the
nebula lines, and also in the magnesium line. In the spectrum of
Nova Persei the displacement of the hydrogen lines towards the violet
is so great that, according to Doppler’s principle,[13] the hydrogen
gas which absorbed the light must have been moving towards us with
a velocity of 700 or more kilometres (450 miles) per second. Some
calcium lines show a similar displacement, which is less noticeable
in the case of the other metals. This would appear to indicate that
relatively cold masses of gas are issuing from the stars and streaming
with enormous velocities towards the earth. The luminous parts of the
stars were either at a stand-still or they were moving away from us.
The simplest explanation of these phenomena would be that a star when
flashing up by virtue of its high temperature and high pressure shows
enhanced (widened) spectral lines, whose violet portion is absorbed by
the strongly cooled masses of gas which are moving towards us and are
cooled by their own strong expansion. These gases stream, of course, in
all directions from the star, but we only become aware of those gases
which absorb the light of the stars—that is to say, those situated
between the star and the earth, and streaming in our direction.

    [Footnote 13: When, standing on a station platform, we watch
    an express train rushing through the station, the pitch of the
    engine whistle seems to become higher as long as the train
    is approaching us, and deeper again when the train is moving
    away from us. The pitch of a note depends upon the number of
    oscillations which our ear receives per second. Now, when the
    train is fast approaching us, more vibrations are sent into our
    ear than when the train is at a stand-still, and the pitch,
    therefore, appears to become higher. The same reasoning holds
    for light waves, of which Doppler, of Prague (Bohemia), was in
    fact thinking when first announcing his principle in 1842. The
    wave-length of a particular color of the spectrum is fixed with
    the aid of some Fraunhofer line characteristic of a certain
    metal. If we compare the spectrum of a star and the spectrum of
    a glowing metal, photographed on the same plate, the stellar
    lines will appear shifted towards the violet end (violet light
    is produced by nearly twice as many vibrations of the ether per
    second as red light) when the star is moving towards us in the
    line of sight. This principle has successfully been applied by
    Huggins, H. C. Vogel, and others, for determining the motion
    of a star in our line of sight. When a star is revolving about
    its own axis, the equatorial belt will seem to come nearer to
    us (or to recede from us), while the polar regions will seem to
    be at a stand-still; the lines will then appear oblique (not
    vertical). In this way Keeler proved that the rings of Saturn
    consists of swarms of meteorites moving at different velocities
    in the different rings.—H. B.]

Gradually the light of the metallic lines and of the continuous
spectrum on which they were superposed began to fade, first in the
violet, while the hydrogen lines and nebular lines still remained
distinct; like other new stars, this star showed, after a while, the
nebular spectrum. This interesting fact was first noticed by H. C.
Vogel in the new star in the Swan (Nova Cygni, 1876). The star P in the
Swan, which flashed up in the year 1600, still offers us a spectrum
which indicates the emission of hydrogen gas. It is not impossible
that this “new” star has not yet reached its equilibrium, and is still
continuing to emit cold streams of gases. Insignificant quantities
of gas suffice for the formation of an absorption spectrum; thus the
emission of gas might continue for long periods without exhausting the
supply.

We have already mentioned (page 116) the peculiar clouds of light which
were observed around Nova Persei. Two annular clouds moved away from
this star with velocities of 1.4 and 2.8 seconds of arc per day between
March 29, 1901, and February, 1902. If we calculate backward from
these dates the time which must have elapsed since those gases left the
star, we arrive at the date of the week—February 8 to 16, 1901—in
satisfactory agreement with the period of greatest luminosity of the
star of February 23d. It would, therefore, appear that these emanations
came from the star and were ejected by the radiation pressure. Their
light did not mark any noticeable polarization, and could not be
reflected light for this reason. We may suppose that the dust particles
discharged their electric charges, and that the gases became thereby
luminous.

In this case we were evidently witnesses of the grand _finale_ of
the independent existence of a celestial body by collision with some
other body of equal kind. The two colliding bodies were both dark, or
they emitted so little light that their combined light intensities
did not equal that of a star of the twelfth magnitude. As, now, their
splendor after the collision was greater than that of a star of the
first magnitude, although their distance has been estimated to be at
least 120 light years,[14] their radiation intensity must have exceeded
that of the sun several thousand times. Under these circumstances the
mechanical radiation pressure must also have been many times more
powerful than on the surface of the sun, and the masses of dust which
were ejected by the new star must have possessed a velocity very much
greater than that of solar dust. Yet this velocity must have been
smaller than that of light, which, indeed, the effect of the radiation
pressure can never equal.

    [Footnote 14: One light year corresponds to 9.5 billion
    kilometres, and it is the distance which the light traverses in
    the course of a year.]

    [Illustration: Fig. 46.—Diagram indicating the consequences
    of a collision between two extinct suns, A and B ‘moving’ in
    the direction of the straight arrows. A rapid rotation in
    the direction of the curved arrows results, and two powerful
    streamers are ejected by A B, the explosive substances from the
    deeper strata of A and B being brought up to the surface by the
    collision]

It is not difficult to picture to ourselves the enormous violence
with which this “collision” must have taken place. A strange body—for
instance, a meteorite—which rushes from the infinite universe into
the sun has at its collision a velocity of 600 km. (400 miles) per
second, and the velocity of the two colliding suns must have been
of approximately that order. The impact will in general be oblique,
and, although part of the energy will of course be transformed into
heat, the rest of the kinetic energy must have produced a rotational
velocity of hundreds of kilometres per second. By comparison with
this number the actual circumferential speed of the sun, about 2 km.
(1-1/4 miles) per second on the equator, would vanish altogether; and
the difference is still more striking for the earth, with its 0.465
km. per second at the equator. We shall, therefore, not commit an
error of any consequence if we presume the two bodies to have been
practically devoid of circumferential speeds before their collision. At
the collision, matter will have been ejected from both these celestial
bodies at right angles to the relative directions of their motion in
two powerful torrents, which would be situated in the plane in which
the two bodies were approaching each other (compare Fig. 46). The
rotational speed of the double star, which will be diminished by this
ejection of matter, will have contributed to increase the energy of
ejection. We remember, now, that when matter is brought up from the
interior to the surface of the sun it will behave like an explosive of
enormous power. The ejected gases will be hurled in terrific flight
about the rapidly revolving central portions. We obtain an idea (though
a very imperfect one) of these features when we look at a revolving
pinwheel in a fireworks display. Two pinwheels have been attached
to the ends of a diameter and belch forth fire in radial lines. The
farther removed from the wheel, the smaller will be the actual velocity
and also the angular velocity of these torrents of fire. Similarly with
the star. The streams are rapidly cooled, owing to the rapid expansion
of the gas. They will also contain fine dust, largely consisting of
carbon, probably, which had been bound by the explosive materials.
The clouds of fine dust will obscure the new star more and more, and
will gradually change its white brilliancy into yellow and reddish,
because the fine dust weakens blue-and-green rays more than it does
yellow-and-red rays. At first the clouds were so near to the star that
they possessed a high angular velocity of their own; they then appeared
to surround the star completely. But after March 22, 1901, the outer
particles of the streams attained greater distances and assumed longer
periods of revolution (six days); the star then became more obscured
when the extreme dust clouds of the streams covering it happened to
get between us and the star. As the streams of particles were moving
farther away, their rotational periods increased gradually to ten days.
The star, therefore, became periodical with a slowly growing length
of period, and its glow turned more reddish at its minimum than at
its maximum of intensity. At the same time, the absorptive power of
the marginal particles decreased, partly owing to their increasing
expansion, partly because the dust was slowly aggregating to coarser
particles; possibly, also, because the finest particles were being
driven away by the radiation pressure. The sifting influence which the
dust exercised upon the light, and owing to which the red-and-yellow
rays were more readily transmitted than the blue-and-green, gradually
became impaired; hence the color of the light turned more gray, and
after a certain time the star appeared once more of a whitish hue.
This white color indicates that the star must still have a very high
temperature. By the continued ejection of dust-charged masses of gas,
probably with gradually decreasing violence, the light intensity of the
star must slowly diminish (as seen from the earth) and the distribution
of the layers of dust around the luminous core will more and more
become uniform. How violent the explosion must have been, we recognize
from the observation that the first ejected masses of hydrogen
rushed out with an apparent velocity of at least 700 km. per second.
This velocity is of the same order as that of the most remarkable
prominences of the sun.

    [Illustration: Fig. 47.—Spiral nebula in the Canes Venatici.
    Messier 51. Taken at the Yerkes Observatory on June 3, 1902.
    Scale, 1 mm. = 13.2 sec. of arc]

It will be admitted that these arguments present us with a faithful
simile even of the details of the observed course of events, and it is
therefore highly probable that our view is in the main correct. But
what has meanwhile become of the new star? Spectrum analysis tells
us that it has been converted into a stellar nebula like other new
stars. The continuous light of the central body has more and more been
weakened by the surrounding masses of dust. By the radiation pressure
these masses are driven towards the outer particles of the surrounding
gaseous envelope consisting principally of hydrogen, helium, and
“nebular matter.” There the dust discharges its negative electricity,
and thus calls forth a luminescence which equals that of the nebulæ.

    [Illustration: Fig. 48.—Spiral nebula in the Triangle. Messier
    33. Taken at the Yerkes Observatory on September 4 and 6, 1902.
    Scale, 1 mm. = 30.7 sec. of arc]

We have to consider next that owing to the incredibly rapid rotation,
the central main mass of the two stars will, in its outer portions,
be exposed to centrifugal forces of extraordinary intensity, and will
therefore become flattened out to a large revolving disk.[15] As the
pressure in the outer portions will be relatively small, the density of
the gases will likewise be diminished there. The energetic expansion
and, more still, the great heat radiation will lower the temperature
at a rapid rate. We have thus to deal with a central body whose inner
portion will possess a high density, and which will resemble the mass
of the sun, while the outer portion will be attenuated and nebular.
Distributed about the central body we shall find the rest of the two
streams of gases which were ejected immediately after the violent
collision between the two celestial bodies. A not inconsiderable
portion of the matter of these spirally arranged outer parts will
probably travel farther away into infinite space, finally to join some
other celestial body or to form parts of the great irregular spots of
nebular matter which are collected around the star clusters. Another
portion, not able to leave the central body, will remain in circular
movement about it. In consequence of this circular movement, which
will be extremely slow, the outlines of the two spirals will gradually
become obliterated, and the spirals will themselves more and more
assume the shape of nebular rings about the central mass.

    [Footnote 15: A. Ritter has calculated that when two suns of
    equal size collide with one another from an infinite distance,
    the energy of the collision is not more than sufficient to
    enlarge the volume of the suns to four times the previous
    amount. The largest portion of the mass will therefore probably
    remain in the centre, and it will only be masses of light gases
    which will be ejected.]

    [Illustration: Fig. 49.—The great nebula in Andromeda. Taken at
    the Yerkes Observatory on September 18, 1901. Scale, 1 mm. =
    54.6 sec. of arc]

    [Illustration: Fig. 50.—Ring-shaped nebula in Lyra. Taken at
    the Yerkes Observatory]

This spiral form (Figs. 47 and 48) of the outer portions of the nebulæ
has for a long time excited the greatest attention. In almost all the
investigated instances it has been observed that two spiral branches
are coiling about the central body. This would indicate that the matter
is in a revolving movement about the central axis of the spiral, and
that it has streamed away from the axis in two opposite directions.
Sometimes the matter appears arranged as in a coil; of this type the
great nebula of Andromeda is the best-known example (Fig. 49). A closer
inspection of this nebula with more powerful instruments indicates,
however, that it is also spiral and that it appears coiled, because we
are looking at it from the side. The late famous American astronomer
Keeler, who has studied these nebulæ with greater success than any one
else, has catalogued a great many of them in all the divisions of the
heavens which were accessible to his instruments, and he has found
that these formations are predominatingly of a spiral nature.

    [Illustration: Fig. 51.—Central portion of the great nebula in
    Orion. Taken at the Yerkes Observatory. Scale, 1 mm. = 12 sec.
    of arc]

Some nebulæ, like the so-called planetary nebulæ, offer rather the
appearance of luminous spheres. We may assume in these cases that the
explosions were less violent, and that the spirals, therefore, are
situated so closely together that they seem to merge into one another.
Possibly the inequalities in their development have become equalized
in the course of time. A few nebulæ are ring-shaped, as the well-known
nebula of Lyra (see Fig. 50). These rings may, again, have been
formed out of spiral nebulæ, and the spirals may have gradually been
obliterated by rotation, while the central nebulous matter may have
been concentrated on the planets travelling round the central star.
Schaeberle, an eminent American astronomer, has discovered traces of
spiral shape also in the Lyra nebula.

Another kind of nebula is the ordinary nebula of vast extension and
irregular shape, evidently formed out of most extremely attenuated
matter; well-known characteristic examples are found in Orion, about
the Pleiades, and in the Swan (Figs. 51, 52, and 53). In these nebulæ
portions of a spiral structure have likewise often been discerned.

    [Illustration: Fig. 52.—Nebular striæ in the stars of the
    Pleiades. Taken at the Yerkes Observatory on October 19, 1901.
    Scale, 1 mm. = 42.2 sec. of arc]

We have said that the collision between two celestial bodies would
result in the formation of a spiral with two wings. If the impact is
such that the two centres of the celestial bodies move straight towards
each other, a disk will arise, and not a spiral; or if one star is
much smaller than the other, possibly a cone, because the gases will
uniformly be spread in all directions about the line of impact. A
perfectly central impact is obviously very rare; but there may be cases
which approach this limiting condition more or less, especially when
the relative velocity of the two bodies is small. By slow diffusion a
feebly developed spiral may also be converted into a rotating disklike
structure. The extension of these nebular structures will depend upon
the ratio between the mass of the system and the velocity of ejection
of the gases. If, for example, two extinct suns of nearly equal
dimensions and mass, like our sun, should collide, some gas masses
would travel into infinite space, being hurled out with a velocity
of more than 900 km. (550 miles) per second; while other particles,
moving at a slower rate, would remain in the neighborhood of the
central body. The nearer to that body, the smaller was their velocity.
From their position they might fall back into the central body, to be
reincorporated in it, if two circumstances did not prevent this. The
one circumstance is the enormous radiation pressure of the glowing
central mass. That pressure keeps masses of dust particles floating,
which by friction will carry the surrounding masses of gas with them.
Owing to the absorption of the radiation by the dust particles, only
the finer particles will be supported farther outside, and at the
extreme margin of the nebula even the very finest dust will no longer
be maintained in suspension by the greatly weakened radiation pressure.
Thus we arrive at an outer limit for the nebula. The other circumstance
is the violent rotation which is set up by the impact of the central
bodies. The rotation and the centrifugal forces will produce a
disk-shaped expansion of the whole central mass. Owing to molecular
collisions and to tidal effects, the angular velocity will in the
denser portions tend to become uniform, so that the whole will rotate
like a flattened-out ball filled with gas, and the spiral structure
will gradually disappear in those parts. In the more remote particles
the velocity will only increase to such an extent as to equal that of
a planet moving at the same distance—that is to say, the gravitation
towards the central body will be balanced by the centrifugal force, and
at the very greatest distances the molecular bombardments, as well as
gravitation towards the centre, will become so insignificant that any
masses collected there will retain their shape for an almost unlimited
space of time.

In the centre of this system the main bulk of the matter would be
concentrated in a sun of extreme brightness, whose light intensity
would, however, owing to strong radiation, diminish with comparative
rapidity.

    [Illustration: Fig. 53.—Nebular striæ in the Swan. New General
    Catalogue, 6992. Taken at the Yerkes Observatory on October 5,
    1901. Scale, 1 mm. = 41 sec. of arc]

Such an extensive nebular system, in which gravitation, on account
of the enormous distances, would act feebly and very slowly, would
yet, in spite of the extraordinary attenuation of matter in its outer
portions, and just on account of its vast extension, be able to stop
the movement of the particles of dust penetrating into it. If the
gases of the nebula are not to escape into space, notwithstanding the
infinitesimal gravitation, their molecules must be assumed to be almost
at a stand-still, and their temperature must not rise by more than
50° or 60° Cent. above absolute zero. At such low temperatures the
so-called adsorption plays an enormously important part (Dewar). The
small dust particles form centres about which the gases are condensed
to a remarkable degree. The extremely low density of these gases does
not prevent their condensation; for the adsorption phenomenon follows a
law according to which the mass of condensed gas will only be reduced
by about one-tenth when the density of the surrounding gas has been
decreased by one-ten-thousandth. The mass of dust particles or dust
grains will thus be augmented, and when they collide they will be
cemented together by the semiliquid films condensed upon them. There
must, hence, be a relatively energetic formation of meteorites in
the nebulæ, and especially in their interiors. Then stars and their
satellites, migrating through space, will stray into these swarms of
gases and meteorites within the nebulæ. The larger and more rapidly
moving celestial bodies will crush through this relatively less dense
matter; but thousands of years may yet be occupied in their passing
through nebulæ of vast dimensions.

    [Illustration: Fig. 54.—Nebula and star rift in the Swan, in
    the Milky Way. Taken by M. Wolf, Königstuhl, near Heidelberg]

    [Illustration: Fig. 55.—Great nebula near Rho, in Ophiuchus.
    Photograph by E. E. Barnard, Lick Observatory. There are
    several empty spots and rifts near the larger stars of the
    nebula]

An extraordinarily interesting photograph obtained by the celebrated
Professor Max Wolf, of Heidelberg, shows us a part of the nebula in
the Swan into which a star has penetrated from outside. The intruder
has collected about it the nebulous matter it met on its way, and
has thus left an empty channel behind it marking its track. Similar
spots of vast extent, relatively devoid of nebulous matter, occur
very frequently in the irregular nebulæ; they are frequently called
“fissures,” or by the specifically English term “rifts,” because they
have generally a long-drawn-out appearance. The presumption that these
rifts represent the tracks of large celestial bodies which have cut
their way through widely expanded nebular masses (Fig. 54) has been
entertained for a long time.

    [Illustration: Fig. 56.—Star cluster in Hercules. Messier 13.
    Taken at the Yerkes Observatory. Scale, 1 mm. = 9.22 sec. of
    arc]

The smaller and more slowly moving immigrants, on the other hand,
are stopped by the particles of the nebulæ. We therefore see the
stars more sparsely distributed in the immediate neighborhood of
the nebulæ, while in the nebulæ themselves they appear more densely
crowded. This fact had struck Herschel in his observations of nebulæ;
in recent days it has been investigated by Courvoisier and M. Wolf. In
this way several centres of attraction are created in a nebula; they
condense the gases surrounding the nebula, and catch, so to say, any
stray meteorites and collect them especially in the inner portions of
the nebula. We frequently observe, further, how the nebular matter
appears attenuated at a certain distance from the luminous bright
stars (compare Figs. 52 and 55). Finally, the nebulæ change into star
clusters which still retain the characteristic shapes of the nebulæ;
of these the spiral is the most usual, while we also meet with conical
shapes, originating from conical nebulæ, and spherical shapes (compare
Figs. 56, 57, and 58).

This is, broadly, the type of evolution through which Herschel,
relying upon his observations, presumed a nebula to pass. He was,
however, under the impression that the nebulous matter would directly
be condensed into star clusters without the aid of strange celestial
immigrants.

    [Illustration: Fig. 57.—Star cluster in Pegasus. Messier 15.
    Taken at the Yerkes Observatory. Scale, 1 mm. = 6.4 sec. of arc]

It has been known since the most ancient times, and has been confirmed
by the observations of Herschel and others in a most convincing manner,
that the stars are strongly concentrated about the middle line of the
Milky Way. It is not improbable that there was originally a nebula of
enormous dimensions in the plane of the Milky Way, produced possibly by
the collision of two such giant suns as Arcturus. This gigantic nebula
has gathered up the smaller migrating celestial bodies which, in their
turn, have condensed upon themselves nebular matter, and have thereby
become incandescent, if they were not so before. The rotational
movement in those parts which were far removed from the centre of the
Milky Way may be neglected. At a later period collisions succeeded
between the single stars which had been gathered up, and it is for this
reason that gaseous nebulæ, as well as new stars, are comparatively
frequent phenomena in the plane of the Milky Way. This view may some
day receive confirmation, when we succeed in proving the existence of
a central body in the Milky Way, evidence of which might possibly be
deduced from the curvature of the orbits of the sun or of other stars.

    [Illustration: Fig. 58.—Cone-shaped star cluster in Gemini.]

As regards the ring-shaped nebula in the Lyre (Fig. 50), the most
recent measurements made by Newkirk point to the result that the star
visible in its centre is distant from us about thirty-two light-years.
As it appears probable that this star really forms the central core
of the nebula, the distance of the nebula itself must be thirty-two
light-years. From the diameter of the ring-shaped nebula which Newkirk
estimates at one minute of arc, this astronomer has calculated that
the distance of the ring from its central body is equal to about three
hundred times the radius of the earth’s orbit—that is to say, the ring
is about ten times as far from its sun as Neptune is from our sun.
There is a faint luminescence within this ring. The nebular matter may
originally have been more concentrated at this spot than in the outer
portions of the ring itself. But this mass was probably condensed on
meteors which immigrated from outside, and when these meteors coalesced
dark planets were produced which move about the central body, and which
have gathered about them most of the gases. If that central body were
as heavy as our sun, the matter in the ring should revolve about it
in five thousand years. That rotation would suffice to wipe out the
original spiral shape, enough of which has yet been left to permit of
our distinctly discerning the two wings of the spiral. The central
body of this ring-shaped nebula gives a continuous spectrum of bright
lines which is particularly developed on the violet side. The star
would therefore appear to be much younger and much hotter than our
sun, and its radiation pressure would therefore be much more intense.
The period of rotation of the nebula may, for this reason, have to be
estimated at a considerably higher figure.

The eminent Dutch astronomer Kapteyn has deduced from the proper
motions of 168 nebulæ that their average distance from the earth is
about seven hundred light-years and equal to that of stars of the tenth
magnitude. The old idea, that the nebulæ must be infinitely farther
removed from us than the fainter stars, would therefore appear to be
erroneous. According to the measurements of Professor Bohlin, the
nebula in Andromeda may indeed be at a distance of not more than forty
light-years.

The “new stars” form a group among the peculiar celestial bodies which
on account of their variable light intensity have been designated as
“variable stars,” and of which a few typical cases should be mentioned,
because a great scientific interest attaches to these problems. The
star Eta, in Argus, may be said to illustrate the strange fate that
a star has to pass through when it has drifted into a nebula filled
with immigrated celestial bodies. It is one of the most peculiar
variable stars. The star shines through one of the largest nebular
clouds in the heavens. Whether it stands in any physical connection
with its surroundings cannot be stated without further examination.
The star might, for instance, be at a considerable distance in front
of the nebula, between the latter and ourselves. Its frequent change
in light intensity suggests, however, a series of collisions, which do
not appear unnatural to us when we suppose that the star is within a
nebula into which many celestial bodies have drifted.

As this star belongs to the southern hemisphere, it was not observed
before our astronomers commenced to visit that hemisphere. In 1677
it was classed as a star of the fourth magnitude; ten years later it
was of the second magnitude; the same in 1751. In 1827 it was of the
first magnitude, and it was found to be variable—that is to say, it
shone with variable brightness. Herschel observed that it fluctuated
between the first and second magnitudes, and that it increased in
brightness after 1837, so that it was by 1838 of magnitude 0.2. After
that it began to decrease in intensity up to April, 1839, when it had
the magnitude 1.1. It remained for four years approximately at this
intensity; then it increased rapidly again in 1843, and surpassed all
stars except Sirius (magnitude -1.7).[16] Afterwards its intensity
slowly diminished once more, so that it remained just visible to the
naked eye (sixth magnitude); by 1869 it had become invisible. Since
then it has been fluctuating between the sixth and seventh magnitudes.

    [Footnote 16: This figure, -1.7, signifies that the brightness
    of Sirius is 2.52^{2.7} = 12 times greater than that of a
    star of magnitude 1. Next to Sirius comes Canopus, with
    magnitude -1.0, being 6.3 times brighter than a star of
    magnitude 1.]

The last changes in the intensity of this star strongly recall the
behavior of the new star in Perseus, only that the latter has been
passing through its phases at a much more rapid rate. It appears to
be certain, however, that Eta, in Argus, was from the very beginning
far brighter than Nova Persei, and that at least once before the
great collision in 1843 (after which it was surrounded by obscuring
clouds of increasing opacity)—namely, in January, 1838, it had been
exposed to a slight collision of quickly vanishing effect. This lesser
collision was probably of the kind which Mayer imagined for the earth
and sun. It would give rise to heat development corresponding to the
heat expenditure of the sun in about a hundred years. As it had been
observed that the star was variable in an irregular manner before
that, we may, perhaps, presume that it had already undergone another
collision.

According to the observations of Borisiak, a student in Kief, the new
star in Perseus would have been, on the evening of February 21, 1901,
of 1.5 magnitude, while a few hours previously it had been of magnitude
12, and the following evening of magnitude 2.7; afterwards its
intensity increased up to the following evening, when it outshone all
the other stars in the northern sky. If this statement is not based on
erroneous observations, the new star must have been in collision with
another celestial body two days before its great collision, perhaps
with a small planet in the neighborhood of the sun, with which it later
collided. That would account for its temporary brilliancy.

New stars are by no means so rare as one might perhaps assume. Almost
every year some new star is discovered. By far most of these are seen
in the neighborhood of the Milky Way, where the visible stars are
unusually crowded, so that a collision which would become visible to us
may easily occur.

For similar reasons we find there also most of the gaseous nebulæ.

Most of the star clusters are also in the neighborhood of the Milky
Way. This is in consequence of the facts just alluded to. The nebulæ
which are produced by collisions between two suns are soon crossed by
migrating celestial bodies such as meteorites or comets, which there
occur in large numbers; by the condensing action of these intruders
they are then transformed into star clusters. In parts of the heavens
where stars are relatively sparse (as at a great distance from the
Milky Way), most of the nebulæ observed exhibit stellar spectra. They
are nothing but star clusters, so far removed from us that the separate
stars can no longer be distinguished. That single stars and gaseous
nebulæ are so rarely perceived in these regions is, no doubt, due to
their great distance.

Among the variable stars we find quite a number which display
considerable irregularity in their fluctuations of brightness,
and which remind us of the new stars. To this class belongs the
just-mentioned star Eta, in Argus. Another example (the first one
which was recognized as “variable”) is Mira Ceti, which may be
translated, “The Wonderful Star in the Constellation of the Whale.”
This mysterious body was discovered by the Frisian priest Fabricius,
on August 12, 1596, as a star of the second magnitude. The priest, an
experienced astronomer, had not previously noticed this star, and he
looked for it in vain in October, 1597. In the years 1638 and 1639 the
variability of the star was recognized, and it was soon ascertained
to be irregular. The period has a length of about eleven months, but
it fluctuates irregularly about this figure as a mean value. At its
greatest intensity the star ranks with those of the first or second
order. Sometimes it is weaker, but it is always of more than the fifth
magnitude. Ten weeks after a maximum the star is no longer visible,
and its brightness may diminish to magnitude 9.5. In other words, its
intensity varies about in the ratio of 1: 1000 (or possibly more).
After a minimum the brightness once more increases, the star becomes
visible again—that is to say, it attains the sixth magnitude—and
after another six weeks it will once more be at its maximum. We have
evidently to deal with several superposed periods.

The spectrum of this star is rather peculiar. It belongs to the red
stars with a band spectrum which is crossed by bright hydrogen lines.
The star is receding from us with a velocity of not less than 63 km.
(39 miles) per second. The bright hydrogen lines which correspond
to the spectrum of the nebula may sometimes be resolved into three
components, of which the middle one corresponds to a mean velocity of
60 km., and the two others have variable receding velocities of 35 and
82 km.—that is to say, velocities of 25 or 22 km. less or more than
the mean velocity. Evidently the star is surrounded by three nebulæ;
one is concentrated about its centre; the two others lie on a ring
the matter of which has been concentrated on two opposite sides. The
ring, which recalls the ring nebula in the Lyre, seems to move about
the star with a velocity of 23.5 km. per second. As this revolution
is accomplished within eleven—or, more correctly, within twenty-two
months, since there must be two maxima and two minima during one
revolution—the total circumference of the ring will be 23.5 × 86,400 ×
670—1361 millions, and the radius of its orbit 217 million km., which
is 1.45 times greater than the radius of the earth’s orbit. Now the
velocity of the earth in its orbit is 29.5 km. (18.3 miles) per second.
A planet at 1.45 times that distance from the sun would have the (1.203
times smaller) velocity of 24.5 km. per second, which is approximately
that of the hypothetical ring of Mira Ceti. We conclude, therefore,
that the mass of the central sun in Mira Ceti will nearly equal the
mass of our sun. The calculation really suggests that Mira would be
about eight per cent. smaller; but the difference lies within the range
of the probable error.

Chandler has directed attention to a striking regularity in these
stars. The longer the period of their variation, the redder in general
their color. This is easily comprehended. The denser the original
atmosphere, the more widely the gases will have extended outward from
the star, and the more dust will have been caught or secreted by it. We
have seen that the limb of the sun has a reddish light because of the
quantities of dust in the solar atmosphere. The effect is chiefly to
be ascribed to the absorption of the blue rays by the dust; but it may
partly be explained on the assumption that the solar radiations render
the dust incandescent, though its temperature may be lower than that
of the photosphere, because the dust lies outside the sun, and that it
will therefore emit a relatively reddish light. The more dust there is
in a nebula, the redder will be its luminescence; and as the quantity
of dust increases in general with the extension of the nebula, that
star which is surrounded by wider rings of nebulæ will in general be
more red; but the greater the radius of the ring, the longer also will
in general be its period.

The so-called red stars show, in addition to the bright hydrogen lines,
banded spectra which indicate the presence of chemical compounds.
On this account such stars were formerly credited with a lower
temperature. But the same peculiarity is also observed in sun-spots,
although the latter, on account of their position, must have a higher
temperature than the surrounding photosphere. The presence of bands in
the spectrum certainly suggests high pressure, however. The red stars
are evidently surrounded by a very extensive atmosphere of gases, in
the inner portions of which the pressure is so high that the atoms
enter into combination. The spectra of the red stars display, on the
whole, a striking resemblance to those of the sun-spots. The violet
portion of the spectrum is weakened, because the masses of dust have
extinguished this light. Owing to the large masses of dust which lie
in our line of sight, the spectrum lines are in both cases markedly
widened and sometimes accompanied by bright lines.

Another class of stars, distinguished by bright lines, comprises those
studied by Wolf and Rayet, and named after them. These stars are
characterized by a hydrogen atmosphere of enormous extension, large
enough in some cases, it has been calculated, to fill up the orbit of
Neptune. These stars are evidently either hotter and more strongly
radiating than the red stars, or there is not so much dust in their
neighborhood—the dust may possibly have been expelled by the strong
radiating pressure. They are, therefore, classed with the yellow, and
not with the red stars. Although there is every reason to suppose that
their central bodies are at least as hot as those of the white stars,
the dust is yet able to reduce the color to yellow, owing to the vast
extensions of their atmospheres.

The unequal periods in stars like Mira may be explained by the
supposition that there are several rings of dust moving about them, as
in the case of the planet Saturn. In the case of the inner rings which
have a short period, there has probably been sufficient time during
the uncounted number of revolutions to effect a uniform distribution
of the dust. Hence we do not discern any noteworthy nuclei in them,
such as we observe in the tails of comets; the dust rings only help
to impart to the star a uniform reddish hue. In the outer rings the
distribution of dust will, however, not be uniform. One of the rings
may be responsible for the chief proper period. By the co-operation
of other less important dust rings, the maximum or minimum, we shall
easily understand, may be displaced, and thus the time interval
between the maxima and minima be altered. This alteration of the
period is so strong for some stars that we have not yet succeeded in
establishing any simple periodicity. The best-known star of this type
is the bright-red star Betelgeuse in the constellation of Orion. The
brightness of this star fluctuates irregularly between the magnitudes
1.0 and 1.4.

By far the largest number of variable stars belong to the type of Mira.
Others resemble the variable star Beta in the constellation of the
Lyre, and thus belong to the Lyre type. The variability of the spectra
of a great many of these stars indicates that they must be moving
about a dark star as companion, or rather that they both move about a
common centre of gravity. The change in the light intensity is, as a
rule, explained by the supposition that the bright star is partially
obscured at times by its dark companion. Many irregularities, however,
in their periods and other circumstances prove that this explanation is
not sufficient. The assumption of rings of dust circulating about the
star and of larger condensation centres affords a better elucidation
of the variability of these stars. They are grouped with the white or
yellow stars, in whose surroundings the dust does not play so large a
part as in that of Mira Ceti. The period of their variability is, as
a rule, very short, moreover—generally only a few days (the shortest
known, only four hours)—while the period of the Mira stars amounts to
at least sixty-five days, and may attain two years. There may be still
longer periods so far not investigated.

Nearly related to the Lyre stars are the Algol stars, whose variability
can be explained by the assumption that another bright or dark star is
moving within their vicinity, partially cutting off their light. There
is no dust in these cases, and the spectrum characterizes these stars
as stars of the first class—that is, as white stars—so far as they
have been studied up to the present.

We must presume for all the variable stars that the line of sight from
the observer to the star falls in the plane of their dust rings or of
their companions. If this were not so, they would appear to us like a
nebula with a central condensation nucleus, or, so far as Algol stars
are concerned, like the so-called spectroscopic doubles whose motion
about each other is recognized from the displacement of their spectral
lines.

The evolution of stars from the nebulous state has been depicted by the
famous chief of the Lick Observatory, in California, W. W. Campbell, as
follows (compare the spectra of the stars of the 2d, 3d, and 4th class,
Figs. 59 and 60):

[Illustration:
         74         My, in     Sun        280
    Schjellerup’s   Gemini   Class 2  Schjellerup’s
      Catalogue     Class 3            Catalogue
       Class 4                          Class 4

    Fig. 59.—Comparison of spectra of stars of classes 2,
    3, 4. After photographs taken at the Yerkes Observatory.
    Blue portions of spectrum. Wave-lengths in millionths of a
    millimetre]

[Illustration:
         280         Sun    My, in       74
    Schjellerup’s  Class 2  Gemini   Schjellerup’s
      Catalogue             Class 3   Catalogue
       Class 4                         Class 4

    Fig. 60.—Comparison of spectra of stars of classes 2, 3, 4.
    After photographs taken at the Yerkes Observatory. Green and
    yellow portions of spectrum. Wave-lengths in millionths of a
    millimetre]

“It is not difficult to select a long list of well-known stars which
cannot be far removed from nebular conditions. These are the stars
containing both the Huggins and the Pickering series of bright hydrogen
lines, the bright lines of helium, and a few others not yet identified.
Gamma Argus and Zeta Puppis are of this class. Another is DM +30.3639°,
which is actually surrounded with a spherical atmosphere of hydrogen
some five seconds of arc in diameter. A little further removed from the
nebular state are the stars containing both bright and dark hydrogen
lines—caught, so to speak, in the act of changing from bright-line
to dark-line stars. Gamma Cassiopeiæ, Pleione, and My Centauri are
examples. Closely related to the foregoing are the helium stars. Their
absorption lines include the Huggins hydrogen series complete, a score
or more of the conspicuous helium lines, frequently a few of the
Pickering series, and usually some inconspicuous metallic lines. The
white stars in Orion and in the Pleiades are typical of this age.

“The assignment of the foregoing types to an early place in stellar
life was first made upon the evidence of the spectroscope. The
photographic discovery of nebulous masses in the regions of a large
proportion of the bright-line and helium stars affords extremely
strong confirmation of their youth. Who that has seen the nebulous
background of Orion (Fig. 51) or the remnants of nebulosity in which
the individual stars of the Pleiades (Fig. 52) are immersed can doubt
that the stars in these groups are of recent formation?

“With the lapse of time, stellar heat radiates into space, and, so
far as the individual star is concerned, is lost. On the other hand,
the force of gravity on the surface strata increases. The inevitable
contraction is accompanied by increasing average temperature. Changes
in the spectrum are the necessary consequence. The second hydrogen
series vanishes, the ordinary hydrogen absorption is intensified, the
helium lines become indistinct, and calcium and iron absorptions begin
to assert themselves. Vega and Sirius are conspicuous examples of this
period. Increasing age gradually robs the hydrogen lines of their
importance, the H and K lines broaden, the metallic lines develop, the
bluish-white color fades in the direction of the yellow, and, after
passing through types exemplified by many well-known stars, the solar
stage is reached. The reversing layer in solar stars represents but
four or five hydrogen lines of moderate intensity; the calcium lines
are commandingly permanent, and some twenty thousand metallic lines are
visible. The solar type seems to be near the summit of stellar life.
The average temperature of the mass must be nearly a maximum; for the
low density indicates a constitution that is still gaseous [compare
Chapter VII.].

“Passing time brings a lowering of the average temperature. The
color passes from yellow to red, in consequence of lower radiation,
temperature, and increasing general absorption by the atmosphere.
The hydrogen lines become indistinct, metallic absorption remains
permanent, and broad absorption bands are introduced. In one type
(Secchi’s Type III.), of which Alpha Herculis is an example, these
bands are of unknown origin. In another class (Secchi’s Type IV.),
illustrated by the star 19 Piscium, they have been definitely
identified as of carbon origin.

“There is scarcely room for doubt that these types of stars (Type
IV.) are approaching the last stages of stellar development. Surface
temperatures have been lowered to the point of permitting more complex
chemical combinations than those in the sun.

“Secchi’s Type III. includes the several numbered long-period variable
stars of the Mira Ceti class, whose spectra at maximum brilliancy show
several bright lines of hydrogen and other chemical elements.[17] It
is significant that the dull-red stars are all very faint; there are
none brighter than magnitude 5.5. Their effective radiatory power is
undoubtedly very low.”

    [Footnote 17: This circumstance indicates that the red color of
    these stars, as we have already remarked with regard to Mira
    Ceti, is not to be traced back to a low temperature, but rather
    to the dust surrounding them. The most extraordinary brightness
    of some stars, like Arcturus and Betelgeuse, which are redder
    than the sun, and whose spectra, according to Hale, resemble
    those of the sun-spots, presuppose a very high temperature.
    The characteristic lines of their spectra are produced by the
    relatively cool vapors of their outer portions.]

The state of evolution, which succeeds that characterized as the
Secchi Type IV., may be elucidated with the aid of the examples of
Jupiter and the earth, with which we are more familiar. These planets
would be invisible if they were not shining in borrowed light.

Jupiter has not advanced so far as the earth. The specific gravity
of Jupiter is somewhat lower than that of the sun (1.27 against
1.38), and, apart from the clouds in its atmosphere, this planet is
probably altogether in a gaseous condition, while the earth, with its
mean density of 5.52, possesses a solid cold crust, enclosing its
incandescent interior. This state of the earth corresponds to the last
stage in the evolution of the stars.

Of the streams of gaseous matter which are ejected when stars collide
with one another, the metallic vapors are rapidly condensed by cooling;
only helium and hydrogen will remain in the gaseous condition and
form nebular masses about the central body. These nebulæ yield bright
lights. Their luminosity is due to the negative particles which are
sent to them by the radiation pressure of near stars, and especially by
the central bodies of the nebula.

With the new stars which have so far been observed, this pressure of
radiation soon diminishes, and the nebular light likewise decreases
in such cases. In other instances, as with the stars characterized by
bright hydrogen and helium lines, the radiation of the central body or
stars in their vicinity seems to be maintained at full force for long
periods.

The nebulous accumulations of helium and hydrogen will gradually escape
and be condensed in near-by stars under the formation of “explosive”
compounds. The tendency to enter into combination seems to be strongest
in the case of helium; it disappears first from the stellar atmosphere.
That helium enters into compounds at high temperatures seems to follow
from the researches of Ramsay, Cooke, and Kohlschütter.

Hydrogen will afterwards be absorbed, and the light of the central body
will then show the predominating occurrence of the vapors of calcium
and of other metals in its atmosphere. Simultaneously with these,
chemical compounds will be noticed, among which the carbon compounds
will play an important part—in the outer portions of the sun-spots, in
the stars of the Secchi Type IV., as well as in the gaseous envelopes
of the comets.[18]

    [Footnote 18: The presence of carbon bands in the spectrum
    need not be taken as a mark of low temperature. Crew and Hale
    have observed that these bands gradually vanished from an arc
    spectrum as the temperature was lowered by decreasing the
    current intensity.]

Finally a crust will form. The star is extinct.




                                  VII

                   THE NEBULAR AND THE SOLAR STATES


We will now proceed to a more intimate consideration of the chemical
and physical conditions which probably characterize the nebulæ in
distinction from the suns. These properties differ in many respects
essentially from those which we are accustomed to associate with matter
as investigated by us, which may, from this point of view, be styled
relatively concentrated.

The differences must be fundamental. For the motto of Clausius, which
comprises the sum of our knowledge of the nature of heat, cannot apply
to nebulæ. This motto reads:

    “The energy of the universe is constant. The entropy of the
    universe tends to a maximum.”

Everybody understands what is meant by energy. We know energy in
many forms. The most important are: energy of position (a heavy body
has larger energy by virtue of its having been raised to a certain
height above the surface of the earth than when it is lying on the
surface); energy of motion (a discharged rifle-bullet has an energy
which is proportional to the mass of the bullet and to the square of
its velocity); energy of heat, which is regarded as the energy of the
motion of the smallest particles of a body; electrical energy, such as
can, for instance, be stored in an accumulator battery, and which,
like all other modifications of energy, may be converted into energy of
heat; and chemical energy, such as is displayed by a mixture of eight
grammes of oxygen with one gramme of hydrogen, which can be transformed
into water under a strong evolution of heat. When we say that the
energy of a system to which energy is not imparted from outside is
constant, we merely mean that the different forms of energy of the
separate parts of this system may be transformed into other forms of
energy, but that the sum total of all the energies must always remain
unchanged. According to Clausius this law is valid throughout the
infinite space of the universe.

By entropy we understand the quantity of heat of a body divided by its
absolute temperature. If a quantity of heat, of Q calories, of a body
at a temperature of 100° (absolute temperature, 373°) passes over to
another body of 0° (absolute temperature, 273°), the total entropy of
the two will have been decreased by Q/373, and increased by Q/273. As
the latter quantity is the greater, the entropy of the whole will have
increased. By itself, we know, heat always passes, either by radiation
or by conduction, from bodies of higher temperature to bodies of lower
temperature. That evidently implies an increase in entropy, and it is
in agreement with the law of Clausius that entropy tends to increase.

The most simple case of heat equilibrium is that in which we place a
number of bodies of unequal temperatures in an enclosure which neither
receives heat from outside nor communicates heat to the outside. In
some way or other, usually by conduction or radiation, the heat will
pass from the warmer to the colder bodies, until at last equilibrium
ensues and all the bodies have the same temperature. According to
Clausius, the universe tends to that thermal equilibrium. If it be
ever attained, all sources of motion, and hence of light, will have
been exhausted. The so-called “heat-death” (Wärmetod) will have come.

If Clausius were right, however, this heat-death, we may object, should
already have occurred in the infinitely long space of time that the
universe has been in existence. Or we might argue that the world has
not yet been in existence sufficiently long, but that, anyhow, it had a
beginning. That would contradict the first part of the law of Clausius,
that the energy of the universe is constant; for in that case all the
energy would have originated in the moment of creation. That is quite
inconceivable, and we must hence look for conditions for which the
entropy law of Clausius does not hold.

The famous Scotch physicist Clerk-Maxwell has conceived of such a case.
Imagine a vessel which is divided by a partition into two halves, both
charged with a gas of perfectly uniform temperature. Let the partition
be provided with a number of small holes which would not allow more
than one gas molecule to pass at a time. In each hole Maxwell places a
small, intelligent being (one of his “demons”), which directs all the
molecules which enter into the hole, and which have a greater velocity
than the mean velocity of all the molecules, to the one side, and which
sends to the other side all the molecules of a smaller velocity than
the average.[19] All the undesirable molecules the demon bars by means
of a little flap. In this way all the molecules of a velocity greater
than the average may be collected in the one compartment, and all the
molecules of a lesser velocity in the other compartment. In other
words, heat—for heat consists in the movements of molecules—will pass
from the one constantly cooling side to the other, which is constantly
raising its temperature, and which must therefore become warmer than
the former.

    [Footnote 19: The kinetic theory of gases imagines all the
    molecules of a gas to be in constant motion. The internal
    pressure of the gas depends upon the mean velocity of the
    particles; but some particles will move at a greater, and some
    at a smaller velocity than the average.—H.B.]

In this instance heat would therefore pass from a colder to a warmer
body, and the entropy would diminish.

Nature, of course, does not know any such intelligent beings.
Nevertheless, similar conditions may occur in celestial bodies in
the gaseous state. When the molecules of gas in the atmosphere of a
celestial body have a sufficient velocity—which in the case of the
earth would be 11 km. (7 miles) per second—and when they travel
outward into the most extreme strata, they may pass from the range of
attraction out into infinite space, after the manner of a comet, which,
if endowed with sufficient velocity when near the sun, must escape
from the solar system. According to Stoney, it is in this way that the
moon has lost its original atmosphere. This loss of gas is certainly
imperceptible in the case of our sun and of large planets like the
earth. But it may play an important part in the household of the
nebulæ, where all the radiation from the hot celestial bodies is stored
up, and where, owing to the enormous distances, the restraining force
of gravity is exceedingly feeble. Thus the nebulæ will lose their most
rapid molecules from their outer portions, and they will therefore be
cooling in these outer strata. This loss of heat is compensated by the
radiation from the stars. If, now, there were only nebulæ of one kind
in the whole universe, those escaped molecules would finally land on
some other nebula, heat equilibrium would thus be established between
the different nebulæ, and the “heat-death” be realized. But we have
already remarked that the nebulæ enclose many immigrated celestial
bodies, which are able to condense the gases from their neighborhood,
and which thereby assume a higher temperature.

The lost molecules of gases may also stray into the vast atmosphere of
these growing stars, and the condensation will then be hastened under a
continuous lowering of the entropy. By such processes the clock-work of
the universe may be maintained in motion without running down.

About the bodies which have drifted into nebulæ, and about the remnants
of new stars which lie inside the nebulæ, the gases will thus collect
which had formerly been scattered through the outer portions of the
nebula. These gases originate from the explosive compounds which had
been stored in the interior of the new stars. Hydrogen and helium
are, most likely, the most important of these; for they are the most
difficult to be condensed, and can exist in notable quantities at
extremely low temperatures, such as must prevail in the outermost
portions of the nebulæ, in which gases of other substances would be
liquefied. Even if the nebulæ had an absolute temperature of 50° (-223°
C.), the vapor of the most volatile of all the metals, mercury, would
even in the saturated state be present in such a small quantity that
a single gramme would occupy the space of a cube whose side would
correspond to about two thousand light-years—that is to say, to 450
times the distance of the earth from the nearest fixed star. One gramme
of sodium, likewise a very volatile metal, and of a comparatively high
importance in the constitution of the fixed stars, would fill the side
of a cube that would be a thousand million times as large. Still more
inconceivable numbers result for magnesium and iron, which are very
frequent constituents of fixed stars, and which are less volatile than
the just-mentioned metals. We thus recognize the strongly selective
action of the low temperatures upon all the substances which are less
difficult to condense than helium and hydrogen. As we now know that
there is another substance in the nebulæ, which has been designated
nebulium, and which is characterized by two spectral lines not found in
any terrestrial substance, we must conclude that this otherwise unknown
element nebulium must be almost as difficult to condense as hydrogen
and helium. Its boiling-point will probably lie below 50° absolute,
like that of those gases.

That hydrogen and helium, together with nebulium, alone seem to occur
in the vastly extended nebulæ is probably to be ascribed to their
low boiling-points. We need not look for any other explanation. The
supposition of Lockyer that all the other elements would be transformed
into hydrogen and helium at extreme rarefaction is quite unsupported.

In somewhat lower strata of the nebula, where its shape resembles
a disk, other not easily condensable substances, such as nitrogen,
hydrocarbons of simple composition, carbon monoxide, further, at deeper
levels, cyanogen and carbon dioxide, and, near the centre, sodium,
magnesium, and even iron may occur in the gaseous state. These less
volatile constituents may exist as dust in the outermost strata. This
dust would not be revealed to us by the spectroscope. In the strongly
developed spiral nebulæ, however, the extreme layers, which seem
to hide the central body, appear to be so attenuated that the dust
floating in them is not able to obscure the spectrum of the metallic
gases. The spectrum of the nebula then resembles a star spectrum,
because the deepest strata contain incandescent layers of dust clouds,
whose light is sifted by the surrounding masses of gases.

It has been observed that the lines of the different elements are
not uniformly distributed in the nebulæ. Thus Campbell observed,
for instance, when investigating a small planetary nebula in the
neighborhood of the great Orion nebula, that the nebulium had not the
same distribution as the hydrogen. The nebulium, which was concentrated
in the centre of the nebula, probably has a higher boiling-point than
hydrogen, therefore, and occurs in noticeable quantities in the inner,
hotter parts of the nebula. Systematic investigations of this kind may
help us to a more perfect knowledge of the temperature relations in
these peculiar celestial objects.

Ritter and Lane have made some interesting calculations on the
equilibrium in a gaseous celestial body of so low a density that the
law of gases may be applied to it. That is only permissive for gases
or for mixtures of gases whose density does not exceed one-tenth of
that of water or one-fourteenth of the actual density of the sun.
The pressure in the central portions of such a mass of gas would,
of course, be greater than the pressure in the outer portions, just
as the pressure rises as we penetrate from above downward into our
terrestrial atmosphere. If we imagine a mass of the air of our
atmosphere transferred one thousand metres higher up, its volume will
increase and its temperature will fall by 9.8° C. (18° F.). If there
were extremely violent vertical convection currents in the air, its
temperature would diminish in this manner with increasing altitude; but
internal radiation tends to equalize these temperature differences.
The following calculation by Schuster concerning the conditions of a
mass of gas of the size of the sun is based on Ritter’s investigation.
It has been made under the hypothesis that the thermal properties of
this mass of gas are influenced only by the movements in it, and not by
radiation. The calculation is applied to a star which has the same mass
as the sun (1.9 × 10^{33} grammes, or 324,000 times the mass of the
earth), and a radius of about ten times that of the sun (10 × 690,000
km.), whose mean density would thus be 1000 times smaller than that of
the sun, or 0.0014 times the density of water at 4° C. In the following
table the first column gives the distance of a point from the centre
of the star as a fraction of its radius; the density (second column)
is expressed in the usual scale, water being the unit; pressures
are stated in thousands of atmospheres, temperatures in thousands
of degrees Centigrade. The temperature will vary proportionately
to the molecular weight of the gas of which the star consists; the
temperatures, in the fourth column of the table, concern a gas of
molecular weight 1—that is to say, hydrogen gas dissociated into
atoms, as it will be undoubtedly on the sun and on the star. If the
star should consist of iron, we should have to multiply these latter
numbers by 56, the molecular weight of iron; the corresponding figures
will be found in the fifth column.

                                                      Temperature in
    Distance from    Density    Pressure in 10^3        10^3° Cent.
       centre                     atmospheres       Hydrogen     Iron

         0           0.00844         852              2460      137,500
         0.1         0.00817         807              2406      134,600
         0.2         0.00739         683              2251      126,100
         0.3         0.00623         513              2007      112,400
         0.4         0.00488         342              1707       95,600
         0.5         0.00354         200              1377       77,100
         0.6         0.00233         100              1043       58,400
         0.7         0.00136          40               728       48,800
         0.8         O.00065          12               445       24,900
         0.9         0.00020           1.7             202       11,300
         1.0         0.00000           0                 0            0

Schuster’s calculation was really made for the sun—that is to say,
for a celestial body whose diameter is ten times smaller, and
whose specific gravity is therefore a thousand times greater than
the above-assumed values. According to the laws of gravitation and
of gases, the pressure must there be 10,000 times greater, and the
temperature ten times higher, than those in our table. The density of
the interior portions would, however, become far too large to admit
of the application of the gas laws. I have therefore modified the
calculations so as to render them applicable to a celestial body of ten
times the radius of the sun or of 1080 times the radius of the earth;
the radius would then represent one-twenty-second of the distance
from the centre of the sun to the earth’s orbit, and the respective
celestial body would have very small dimensions indeed if compared to a
nebula.

The extraordinarily high pressure in the interior portions of the
celestial body is striking; this is due to the great mass and to the
small distances. In the centre of the sun the pressure would amount to
8520 million atmospheres, since the pressure increases inversely as the
fourth power of the radius. The pressure near the centre of the sun is,
indeed, almost of that order. If the sun were to expand to a spherical
planetary nebula of a thousand times its actual linear dimensions (when
it would almost fill the orbit of Jupiter), the specific gravity at
its centre would be diminished to one-millionth of the above-mentioned
value—that is to say, matter in this nebula would not, even at the
point of greatest concentration, be any denser than in the highly
rarefied vacuum tubes which we can prepare at ordinary temperatures.
The pressure would likewise be greatly diminished—namely, to about six
millimetres only, near the centre of the gaseous mass. The temperature,
however, would be rather high near the centre—namely, 24,600° C., if
the nebula should consist of atomatic hydrogen, and fifty-six times
as high again if consisting of iron gas. Such a nebula would restrain
gases with 1.63 times the force which the earth exerts. Molecules of
gases moving outward with a velocity of about 18 km. (11 miles) per
second would forever depart from this atmosphere.

The estimation of the temperature in such masses of gases is certainly
somewhat unreliable. We have to presume that neither radiation nor
conduction exert any considerable influence. That might be permitted
for conduction; but we are hardly justified in neglecting radiation.
The temperatures within the interior of the nebula will, therefore, be
lower than our calculated values. It is, however, difficult to make any
definite allowance for this factor.

If the mass of the celestial body should not be as presumed—for
instance, twice as large—we should only have to alter the pressure and
the density of each layer in the same proportion, and thus to double
the above values. The temperature would remain unchanged. We are hence
in a position to picture to ourselves the state of a nebula of whatever
dimensions and mass.

Lane has proved, what the above calculations also indicate, that the
temperature of such nebula will rise when it contracts in consequence
of its losing heat. If heat were introduced from outside, the nebula
would expand under cooling. A nebula of this kind presumably loses
heat and gradually raises its own temperature until it has changed
into a star, which will at first have an atmosphere of helium and of
hydrogen like that of the youngest stars (with white light). By-and-by,
under a further rise of temperature, the extremely energetic chemical
compounds will be formed which characterize the interior of the sun,
because helium and hydrogen—which were liberated when the nebula was
re-formed and which dashed out into space—will diffuse back into the
interior of the star, where they will be bound under the formation of
the compounds mentioned. The atmosphere of hydrogen and of helium will
disappear (helium first), the star will contract more and more, and the
pressure and the convection currents in the gases will become enormous.
There will be a strong formation of clouds in the atmosphere of the
star, which will gradually become endowed with the properties which
characterize our sun. The sun behaves very differently from the gaseous
nebulæ for which the calculations of Lane, Ritter, and Schuster hold.
For when the contraction of a gas shall have proceeded to a certain
limit, the pressure will increase in the ratio 1: 16, while the volume
will decrease in the ratio 8: 1, provided there be no change in the
temperature. When the gas has reached this point and is still further
compressed, the temperature will remain in steady equilibrium. At still
higher pressures, however, the temperature must fall if equilibrium
is to be maintained. According to Amagat, this will occur at 17° C.
(290° absolute) in gases like hydrogen and nitrogen, which at this
temperature are far above their critical points, and at a pressure of
300 or 250 atmospheres. When the temperature is twice as high on the
absolute scale, or at 307° C., twice the pressure will be required.

We can now calculate when our nebula will pass through this critical
stage, to which a lowering of the temperature must succeed. Accepting
the above figures, we find that half the mass of the nebula will fill
a sphere of a radius 0.53 of that of the nebula. If the mass were
everywhere of the same density, half of it would fill a sphere of 0.84
of this radius. When will the interior mass cross the boundary of the
above stage, while the exterior portions still remain below this stage?
That will be at about the time when the nebula in its totality will
pass through its maximum temperature. We will now base our calculations
on the temperatures which apply to iron in the gaseous state; for in
the interior of the nebula the mean molecular weight will at least be
56 (that of iron). We shall find that the pressure at the distance 0.53
will be about 177,000 atmospheres, and the temperature approximately
71 million degrees—_i.e._, 245,000 times higher than the absolute
temperature in the experiments of Amagat. The specified stage will
then be reached when the pressure will be 245,000 times as large as
250 atmospheres—viz., 61 million atmospheres. As, now, the pressure
is only 177,000 atmospheres, our nebula will yet be far removed from
that stage at which cooling will set in. We can easily calculate that
this will take place when the nebula has contracted to a volume about
three times that of our sun. The assertion which is so often made
that the sun might possibly attain higher temperatures in the future
is unwarranted. This celestial body has long since passed through the
culminating-point of its thermal evolution, and is now cooling. As the
temperatures which Schuster deduced were no doubt much too high, the
cooling must, indeed, have set in already in an earlier stage. But
stars like Sirius, whose density is probably not more than one per
cent, of the solar density, are probably still in a rising-temperature
stage. Their condition approximates that of the mass of gas of our
example.

The planetary nebulæ are vastly more voluminous. The immense space
which these celestial bodies may occupy will be understood from the
fact that the largest among them, No. 5 in Herschel’s catalogue,
situated near the star B in the Great Bear, has a diameter of 2.67
seconds of arc. If it were as near to us as our nearest star neighbor,
its diameter would yet be more than three times that of the orbit
of Neptune; doubtless it is many hundreds of times larger. This
consideration furnishes us with an idea of the infinite attenuation
in such structures. In their very densest portions the density cannot
be more than one-billionth of the density of the air. In the outer
portions of such nebulæ the temperature must also be exceedingly low;
else the particles of the nebula could not be kept together, and only
hydrogen and helium can occur in them in the gaseous state.

Yet we may regard the density and temperature of such celestial bodies
as gigantic by comparison with those of the gases in the spirals of the
nebulæ. There never is equilibrium in these spirals, and it is only
because the forces in action are so extraordinarily small that these
structures can retain their shapes for long periods without noticeable
changes. It is, probably, chiefly in those parts in which the cosmical
dust is stopped in its motion that meteorites and comets are produced.
The dust particles wander into the more central portions of the
nebulæ, into which they penetrate deeply, owing to their relatively
large mass, to form the nuclei for the growth of planets and moons. By
their collisions with the masses of gases which they encounter, they
gradually assume a circular movement about the axis of rotation of the
nebula. In this rotation they condense portions of the gases on their
surface, and hence acquire a high temperature—which they soon lose
again, however, owing to the comparatively rapid radiation.

So far as we know, spiral nebulæ are characterized by continuous
spectra. The splendor of the stars within them completely outshines the
feeble luminosity of the nebula. The stars in them are condensation
products and undoubtedly in an early stage of their existence;
they may therefore be likened to the white stars, like the new star
in Perseus and the central star in the ring nebula of the Lyre.
Nevertheless, it has been ascertained that the spectrum of the
Andromeda nebula has about the same length as that of the yellow stars.
That may be due to the fact that the light of the stars in this nebula,
which we only seem to see from the side, is partly extinguished by dust
particles in its outer portion, as was the case with the light of the
new star in Perseus during the period of its variability.

Our considerations lead to the conclusion that there is rotating about
the central body of the nebula an immense mass of gas, and that outside
this mass there are other centres of condensation moving about the
central body together with the masses of gas concentrated about them.
Owing to the friction between the immigrated masses and the original
mass of gas which circulated in the equatorial plane of the central
body, all these masses will keep near the equatorial plane, which will
therefore deviate little from the ecliptic. We thus obtain a proper
planetary system, in which the planets are surrounded by colossal
spheres of gas like the stars in the Pleiades (Fig. 52). If, now, the
planets have very small mass by comparison with the central body—as
in our solar system—they will be cooled at an infinitely faster rate
than the sun. The gaseous masses will soon shrink, and the periods of
rotation will be shortened; but for those planets, at least, which are
situated near the centre, these periods will originally differ little
from the rotation of the central body. The dimensions of the central
body will always be very large, and the planets circulating about it
will produce very strong tidal effects in its mass. Its period of
rotation will be shortened, while the orbital rotation of the planets
will tend to become lengthened. Thus the equilibrium is disturbed;
it is re-established again, because the planet is, so to say, lifted
away from the sun, as G. H. Darwin has so ingeniously shown with
regard to the moon and the earth. Similar relations will prevail in
the neighborhood of those planets which will thus become provided with
moons. Hence we understand the peculiar fact that all the planets move
almost in the same plane, the so-called ecliptic, and in approximately
circular orbits; that they all move in the same direction, and that
they have the same direction of rotation in common with their moons and
with the central body, the sun. It is only the outermost planets, like
Uranus and Neptune, in whose cases the tidal effects were not of much
consequence, that form exceptions to this rule.

In explanation of these phenomena various philosophers and astronomers
have advanced a theory which is known as the Kant-Laplace theory,
after its most eminent advocates. Suggestions pointing in the same
direction we find in Swedenborg (1734). Swedenborg assumed that our
planetary system had been evolved under the formation of vortices from
a kind of “chaos solare,” which had acquired a more and more energetic
circulating motion about the sun under the influence of internal
forces, possibly akin to magnetic forces. Finally a ring had been
thrown off from the equator, and had separated into fragments, out of
which the planets had been formed.

Buffon introduced gravitation as the conservational principle. In an
ingenious essay, “Formation des Planètes” (1745), he suggests that
the planets may have been formed from a “stream” of matter which was
ejected by the sun when a comet rushed into it.

Kant started from an original chaos of stationary dust, which under
the influence of gravitation arranged itself as a central body, with
rings of dust turning around it; the rings, later on, formed themselves
into planets. The laws of mechanics teach, however, that no rotation
can be set up in a central body, which is originally stationary, by
the influence of a central force like gravitation. Laplace, therefore,
assumed with Swedenborg that the primeval nebula from which our
solar system was evolved had been rotating about the central axis.
According to Laplace, rings like those of Saturn would split off, as
such a system contracted, and planets and their moons and rings would
afterwards be formed out of those rings. It is generally believed at
present, however, that only meteorites and small planets, but not the
larger planets, could have originated in this way. We have, indeed,
such rings of dust rotating about Saturn, the innermost more rapidly,
the outer rings more slowly, just as they would if they were crowds of
little moons.

Many further objections have later been raised against the hypothesis
of Laplace, first by Babinet, later especially by Moulton and
Chamberlin. In its original shape this hypothesis would certainly not
appear to be tenable. I have therefore replaced it by the evolution
thesis outlined above. It is rather striking that the moons of the
outermost planets, Neptune and Uranus, do not move in the plane of
the ecliptic, and that their moons further describe a “retrograde”
movement—that is to say, they move in the direction opposite to that
conforming to the theory of Laplace. The same seems to hold for the
moon of Saturn, which was discovered in 1898 by Pickering. All these
facts were, of course, unknown to Laplace in 1776; and if he had known
them he would scarcely have advanced his thesis in the garb in which
he offered it. The explanation of these facts does not cause any
difficulty. We may assume that the matter in the outer portions of
the primeval nebula was so strongly attenuated that the immigrating
planet did not attain a sufficient volume to have the large common
rotation in the equatorial plane of the sun impressed upon it by the
tidal effects. Charged only with the small mass of matter which they
met on their road, the planet and its moon, on the contrary, remained
victorious in the limited districts in which they were rotating. Only
the slow orbital movement about the central body was influenced, and
that adapted itself to the common direction and the circular orbit. It
is not inconceivable that there may be, farther out in space, planets
of our solar system, unknown to us, moving in irregular paths like the
comets. The comets, Laplace assumed, probably immigrated at a later
period into our solar system when the condensation had already advanced
so far that the chief mass of the nebular matter had disappeared from
interplanetary space.

Chamberlin and Moulton have attempted to show that the difficulties
of the hypothesis of Laplace may be obviated by the assumption that
the solar system has evolved from a spiral nebula, into which strange
bodies intruded which condensed the nebular mass of their surroundings
upon themselves. We have pointed out examples of how the nebula seems
to vanish in the vicinity of the stars, which would correspond to
growing planets, located in nebulæ.

In concluding this consideration, we may draw a comparison between the
views which were still entertained a short time ago and the views and
prospects which the discoveries of modern days open to our eyes.

Up to the beginning of this century the gravitation of Newton seemed
to rule supreme over the motions and over the development of the
material universe. By virtue of this gravitation the celestial bodies
should tend to draw together, to unite in ever-growing masses. In the
infinite space of past time the evolution should have proceeded so far
that some large suns, bright or extinct, could alone persist. All life
would be impossible under such conditions.

And yet we discern in the neighborhood of the sun quite a number
of dark bodies, our planets, and we may surmise that similar dark
companions or satellites exist in the vicinity of other suns and stars;
for we could not understand the peculiar to-and-fro motions of those
stars on any other view. We further observe that quite a number of
small celestial bodies rush through space in the shapes of meteorites
or shooting-stars which must have come to us from the most remote
portions of the universe.

The explanation of these apparent deviations from what we may regard
as a necessary consequence of the exclusive action of gravity will
be found under two heads—in the action of the mechanical radiation
pressure of light, and in the collisions between celestial bodies. The
latter produce enormous vortices of gases about nebular structures in
the gaseous condition; the radiation pressure carries cosmical dust
into the vortices, and the dust collects into meteorites and comets and
forms, together with the condensation products of the gaseous envelope,
the planets and the moons accompanying them.

The scattering influence of the radiation pressure therefore balances
the tendency of gravitation to concentrate matter. The vortices of
gases in the nebulæ only serve to fix the position of the dust, which
is ejected from the suns through the action of the radiation pressure.

The masses of gas within the nebulæ form the most important centres of
concentration of the dust which is ejected from the sun and stars.
If the world were limited, as people used to fancy—that is to say,
if the stars were crowded together in a huge heap, and only infinite,
empty space outside of this heap, the dust particles ejected from the
suns during past ages by the action of the radiating pressure would
have been lost in infinite space, just as we imagined that the radiated
energy of the sun was lost.

If that were so, the development of the universe would long since
have come to an end, to an annihilation of all matter and of all
energy. Herbert Spencer, among others, has explained how thoroughly
unsatisfactory this view is. There must be cycles in the evolution of
the universe, he has emphasized. That is manifestly indispensable if
the system is to last. In the more rarefied, gaseous, cold portions of
the nebulæ we find that part of the machinery of the universe which
checks the waste of matter and, still more, the waste of force from
the suns. The immigrating dust particles have absorbed the radiation
of the sun and impart their heat to the separate particles of the
gases with which they collide. The total mass of gas expands, owing to
this absorption of heat, and cools in consequence. The most energetic
molecules travel away, and are replaced by new particles coming from
the inner portions of the nebulæ, which are in their turn cooled by
expansion. Thus every ray emitted by a sun is absorbed, and its energy
is transferred, through the gaseous particles of the nebulæ, to suns
that are being formed and which are in the neighborhood of the nebula
or in its interior portions. The heat is hence concentrated about
centres of attraction that have drifted into the nebula or about the
remnants of the celestial bodies which once collided there. Thanks to
the low temperature of the nebula, the matter can again accumulate,
while the radiation pressure, as Poynting has shown, will suffice
to keep bodies apart if their temperature is 15° C., their diameter
3.4 cm., and their specific gravity as large as that of the earth,
5.5. At the distance of the orbit of Neptune, where the temperature
is about 50° absolute and approximates, therefore, that of a nebula,
this limit of size is reduced to nearly one millimetre. It has already
been suggested (compare page 153) that capillary forces, which would
prevail under the co-operation of the gases condensed upon the dust
grains, rather than gravity, play a chief part in the accumulation
and coalescence of the small particles. In the same manner as matter
is concentrated about centres of attraction energy may be accumulated
there in contradiction to the law of the constant increase of entropy.

During this conservational activity the layers of gas are rapidly
rarefied, to be replaced by new masses from the inner parts of the
nebula, until this centre is depleted, and the nebula has been
converted into a star cluster or a planetary system which circulates
about one or several suns. When the suns collide once more new nebulæ
are created.

The explosive substances, consisting probably of hydrogen and helium
(and possibly of nebulium), in combination with carbon and metals, play
a chief part in the evolution from the nebular to the stellar state,
and in the formation of new nebulæ after collisions between two dark
or bright celestial bodies. The chief laws of thermodynamics lead to
the assumption that these explosive substances are formed during the
evolution of the suns and are destroyed during their collisions. The
enormous stores of energy concentrated in these bodies perform, in a
certain sense, the duty of powerfully acting fly-wheels interposed in
the machinery of the universe in order to regulate its movements and
to make certain that the cyclic transition from the nebular to the
star stage, and vice versa, will occur in a regular rhythm during the
immeasurable epochs which we must concede for the evolution of the
universe.

By virtue of this compensating co-operation of gravity and of the
radiation pressure of light, as well as of temperature equalization
and heat concentration, the evolution of the world can continue in an
eternal cycle, in which there is neither beginning nor end, and in
which life may exist and continue forever and undiminished.




                                 VIII

              THE SPREADING OF LIFE THROUGH THE UNIVERSE


We have just recognized the probability of the assumption that solar
systems have been evolved from nebulæ, and that nebulæ are produced
by the collision of suns. We likewise consider it probable that there
circulate about the newly formed suns smaller celestial bodies which
cool more rapidly than the central sun. When these satellites have
provided themselves with a solid crust, which will partly be covered
by water, they may, under favorable conditions, harbor organic life,
as the earth and probably also Venus and Mars do. The satellites would
thereby gain a greater interest for us than if we had to imagine them
as consisting entirely of lifeless matter.

The question naturally arises whether we may believe that life can
really originate on a celestial body as soon as circumstances are
favorable for its evolution and propagation. This question will occupy
us in this last chapter.

Men have been pondering over these problems since the remotest ages.
All living beings, past ages recognized, must have been generated and
they had to die after a certain shorter or longer life. Somewhat later,
and yet still in a very early epoch, experience must have taught men
that organisms of one kind can only generate other organisms of the
same kind; that the species are invariable, as we now express it. The
idea was that all species originally came from the hands of the Creator
endowed with their present qualities. This view may still be said to
represent the general or “orthodox” doctrine.

This view has also been called the Linnæan thesis, because Linné, in
the fifth edition of his _Genera Plantarum_, adheres to it strictly:
“Species tot sunt, quot diversas formas ab initio produxit Infinitum
Ens, quae deinde formae secundum generationis inditas leges produxere
plures, at sibi semper similes, ut species nunc nobis non sint plures
quam fuerunt ab initio.” Which we may render: “There are as many
different kind of species as the Infinite Being has created different
forms in the beginning. These forms have later engendered other beings
according to the laws of inheritance, always resembling them, so that
we have at the present time not any more species than there were from
the beginning.” Time was ripe, however, even then for a less rigid
conception of nature, more in accordance with our present views. The
first foundations of the theory of evolution in the biological sciences
were laid by Lamarck (in 1794), Treviranus (in 1809), Goethe and Oken
(in 1820). But a reaction set in. Cuvier and his authority forced
public opinion back to the ancient stand-point. In his view the now
extinct species of past geological epochs had been destroyed by natural
revolutions, and new species had again been generated by a new act of
the Creator.

Within the last few decades, however, the general belief has rapidly
been revolutionized, and the theory of evolution, especially since the
immortal Charles Darwin came forth with his epoch-making researches,
now meets with universal acceptance.

According to this theory the species adapt themselves in the course of
time to their surroundings, and the changes may become so great that a
new species may be considered to have originated from an old species.
The researches of De Vries have, within quite recent times, further
accentuated this view, so that we now concede cases to be extant where
new species spring forth from old ones under our very eyes. This thesis
has become known as the theory of mutation.

At the present time we accordingly imagine that living organisms,
such as we see around us, have all descended from older organisms,
rather unlike them, of which we still find traces and remnants in the
geological strata which have been deposited during past ages. From this
stand-point all living organisms might possibly have originated from
one single, most simple organism. How that was generated still remains
to be explained.

The common view, to which the ancients inclined, is that the lower
organisms need not necessarily have originated from seeds. It was
noticed that some low-type organisms, larvæ, etc., took rise in putrid
meat; Vergil describes this in his _Georgicas_. It was not until the
seventeenth century that this belief was disproved by many experiments,
among others by those of Swammerdam and Leuwenhoek. The thesis of the
so-called “Generatio spontanea” once more blossomed into new life upon
the discovery of the so-called infusoria, the small animal organisms
which seem to arise spontaneously in infusions and concoctions.
Spallanzani, however, demonstrated in 1777 that when the infusions, and
the vessel containing them, as well as the air above them, were heated
to a sufficiently high temperature to kill all the germs present, the
infusions would remain sterile, and no living organisms could develop
in them. To this fact we owe our ordinary methods of making preserves.
It is true that objections were raised against this demonstration.
The air, it was objected, is so changed by heating that subsequent
development of minute organisms is rendered impossible. But this last
objection was refuted by the chemists Chevreul and Pasteur, as well
as by the physicist Tyndall in the sixties and seventies of the past
century. These scientists demonstrated that no organisms are produced
in air which is freed from the smallest germs by some other means than
heating—_i.e._, by filtration through cotton-wool. The researches of
Pasteur, in particular, and the methods of sterilization which are
based upon them and which are applied every day in bacteriological
laboratories, have more and more forced the conviction upon us that a
germ is indispensable for the origination of life.

And yet eminent scientists take up the pen again and again in order to
demonstrate the possibility of the “Generatio spontanea.” In this they
do not rely upon the safe methods of natural science, but they proceed
on philosophical lines of argument. Life, they suggest, must once have
had a beginning, and we are hence forced to believe that spontaneous
generation, even if not realizable under actual conditions, must have
once occurred. Considerable interest was excited when the great English
physiologist Huxley believed he had discovered in the mud brought
up from the very bottom of the sea an albuminoid substance which he
called “Bathybius Haeckelii,” in honor of the zealous German Darwinist
Haeckel. In this bathybius (deep-sea organism) one fancied for a time
that the primordial ooze, which had originated from inorganic matter
and from which all organisms might have been evolved, and of which Oken
had been dreaming, had been discovered. But the more exact researches
of the chemist Buchanan demonstrated that the albuminoid substance in
this primordial ooze consisted of flocks of gypsum precipitated by
alcohol.

People then had recourse to the most fantastic speculations. Life, it
was argued, might possibly have had its origin in the incandescent mass
of the interior of the earth. At high temperatures organic compounds
of cyanogen and its derivatives might be formed which would be the
carriers of life (Pflüger). There is, however, little need of our
entering into any of these speculations until they have been provided
with an experimental basis.

Almost every year the statement is repeated in biological literature
that we have at last succeeded in producing life from dead matter.
Among the most recent assertions of this kind, the discovery claimed
by Butler-Burke has provoked much comment. He asserted that he had
succeeded, with the aid of the marvellous substance radium, in
instilling life into lifeless matter—namely, a solution of gelatine.
Criticism has, however, relegated this statement, like all similar
ones, to the realm of fairy tales.

We fully share the opinion which the great natural philosopher
Lord Kelvin has expressed in the following words: “A very ancient
speculation, still clung to by many naturalists (so much so that I have
a choice of modern terms to quote in expressing it), supposes that,
under meteorological conditions very different from the present, dead
matter may have run together or crystallized or fermented into ‘germs
of life,’ or ‘organic cells,’ or ‘protoplasm.’ But science brings a
vast mass of inductive evidence against this hypothesis of spontaneous
generation. Dead matter cannot become living without coming under the
influence of matter previously alive. This seems to me as sure a
teaching of science as the law of gravitation.”

Although the latter verdict may be a little dogmatic, it yet
demonstrates how strongly many scientists feel the necessity of
finding another way of solving the problem. The so-called theory of
panspermia really shows a way. According to this theory life-giving
seeds are drifting about in space. They encounter the planets, and fill
their surfaces with life as soon as the necessary conditions for the
existence of organic beings are established.

This view was probably foreshadowed long ago. Definite suggestions in
this direction we find in the writings of the Frenchman Sales-Guyon de
Montlivault (1821), who assumed that seeds from the moon had awakened
the first life on the surface of the earth. The German physician H. E.
Richter attempted to supplement the doctrine of Darwin by combining the
conception of panspermia with it. Flammarion’s book on the plurality
of inhabited worlds suggested to Richter the idea that seeds had come
from some other inhabited world to our earth. He emphasizes the fact
that carbon has been found in meteorites which move in orbits similar
to those of the comets which wander about in space; and in this carbon
he sees the rests of organic life. There is no proof at all for this
latter opinion. The carbon found in meteorites has never exhibited any
trace of organic structure, and we may well imagine the carbon—_e.g._,
that which appears to occur in the sun—to be of inorganic origin.
Still more fantastic is his idea that organisms floating high in our
atmosphere are caught by the attraction of meteorites flying past
our planet, and are in this way carried out into universal space and
deposited upon other celestial bodies. As the surface of meteorites
becomes incandescent in their flight through the atmosphere, any germs
which they might possibly have caught would be destroyed; and if, in
spite of that, a meteorite should become the conveyor of live germs,
those germs would be burned in the atmosphere of the planet on which
they descended.

In one point, however, we must agree with Richter. There is logic
in his statement that “The infinite space is filled with, or (more
correctly) contains, growing, mature, and dying celestial bodies. By
mature worlds we understand those which are capable of sustaining
organic life. We regard the existence of organic life in the universe
as eternal. Life has always been there; it has always propagated itself
in the shape of living organisms, from cells and from individuals
composed of cells.” Man used to speculate on the origin of matter, but
gave that up when experience taught him that matter is indestructible
and can only be transformed. For similar reasons we never inquire into
the origin of the energy of motion. And we may become accustomed to the
idea that life is eternal, and hence that it is useless to inquire into
its origin.

The ideas of Richter were taken up again in a popular lecture delivered
in 1872 by the famous botanist Ferdinand Cohn. The best-known
expression of opinion on the subject, however, is that of Sir William
Thomson (later Lord Kelvin) in his presidential address to the British
Association at Edinburgh in 1871:

“When two great masses come into collision in space, it is certain
that a large part of each is melted; but it seems also quite certain
that in many cases a large quantity of débris must be shot forth in
all directions, much of which may have experienced no greater violence
than individual pieces of rock experience in a landslip or in blasting
by gunpowder. Should the time when this earth comes into collision
with another body, comparable in dimensions to itself, be when it is
still clothed as at present with vegetation, many great and small
fragments carrying seed and living plants and animals would undoubtedly
be scattered through space. Hence, and because we all confidently
believe that there are at present, and have been from time immemorial,
many worlds of life besides our own, we must regard it as probable
in the highest degree that there are countless seed-bearing meteoric
stones moving about through space. If at the present instant no life
existed upon this earth, one such stone falling upon it might, by what
we blindly call _natural_ causes, lead to its becoming covered with
vegetation. I am fully conscious of the many objections which may be
urged against this hypothesis. I will not tax your patience further by
discussing any of them on the present occasion. All I maintain is that
I believe them to be all answerable.”

Unfortunately we cannot share Lord Kelvin’s optimism regarding this
point. It is, in the first instance, questionable whether living beings
would be able to survive the violent impact of the collision of two
worlds. We know, further, that the meteorite in its fall towards the
earth becomes incandescent all over its surface, and any seeds on it
would therefore be deprived of their germinating power. Meteorites,
moreover, show quite a different composition from that of the fragments
from the surface of the earth or a similar planet. Plants develop
almost exclusively in loose soil, and a lump of earth falling through
our atmosphere would, no doubt, be disintegrated into a shower of small
particles by the resistance of the atmosphere. Each of these particles
would by itself flash up like a shooting-star, and could not reach the
earth in any other shape than that of burned dust. Another difficulty
is that such collisions, which, as we presume, are responsible for the
flashing-up of so-called new stars, are rather rare phenomena, so that
little likelihood remains of small seeds being transported to our earth
in this manner.

The question has, however, entered into a far more favorable stage
since the effects of radiation have become understood.

Bodies which, according to the deductions of Schwarzschild, would
undergo the strongest influence of solar radiation must have a diameter
of 0.00016 mm., supposing them to be spherical. The first question is,
therefore: are there any living seeds of such extraordinary minuteness?
The reply of the botanist is that the so-called permanent spores of
many bacteria have a size of 0.0003 or 0.0002 mm., and there are, no
doubt, much smaller germs which our microscopes fail to disclose. Thus,
yellow-fever in man, rabies in dogs, the foot-and-mouth disease in
cattle, and the so-called mosaic disease—common to the tobacco plant
in Netherlandish India, and also observed in other countries—are, no
doubt, parasitical diseases; but the respective parasites have not yet
been discovered, presumably because they are too minute to be visible
under the microscope.[20]

    [Footnote 20: Meanwhile a large number of organisms which are
    invisible under the ordinary microscope have been rendered
    visible by the aid of the ultra-microscope, among others the
    presumable microbe of the foot-and-mouth disease.]

It is, therefore, very probable that there are organisms so small
that the radiation pressure of a sun would push them out into space,
where they might give rise to life on planets, provided they met with
favorable conditions for their development.

We will, in the first instance, make a rough calculation of what would
happen if such an organism were detached from the earth and pushed out
into space by the radiation pressure of our sun. The organism would,
first of all, have to cross the orbit of Mars; then the orbits of the
smaller and of the outer planets; and, having passed the last station
of our solar system, the orbit of Neptune, it would drift farther into
infinite space towards other solar systems. It is not so difficult to
estimate the time which the smallest particles would require for this
journey. Let their specific gravity be that of water, which will very
fairly correspond to the facts. The organisms would cross the orbit of
Mars after twenty days, the Jupiter orbit after eighty days, and the
orbit of Neptune after fourteen months. Our nearest solar system, Alpha
Centauri, would be reached in nine thousand years. These calculations
have been made under the supposition that the radiation pressure is
four times as strong as gravitation, which would be nearly correct
according to the figures of Schwarzschild.[21]

    [Footnote 21: The radiation pressure has here been assumed to
    be somewhat greater than on page 103, because the spores are
    here regarded as opaque, while the drops of hydrocarbons have
    been regarded as partially translucid to luminous rays.]

These time intervals required for the organisms to reach the different
planets of our solar system are not too long for the germs in question
to preserve their germinating power. The estimate is more unfavorable
in the case of their transference from one planetary system to another,
which will require thousands of years. But we shall see further on that
the very low temperature of those parts of space (about -220° C.) would
suspend the extinction of the germinating power, as it arrests all
chemical reactions.

As regards the period during which the germinating power can be
preserved at ordinary temperature, we have been told that the so-called
“mummy wheat” which had been found in ancient Egyptian tombs was still
capable of germination. Critics, however, have established that the
respective statements of the Arabs concerning the sources of that wheat
are very doubtful. The French scientist Baudoin asserts that bacteria
capable of germination were found in a Roman tomb which had certainly
remained untouched for eighteen hundred years; but this statement is to
be received with caution. It is certain, however, that both seeds of
some higher plants and spores of certain bacteria—_e.g._, anthrax—do
maintain their germinating power for several years (say, twenty), and
thus for periods which are much longer than those we have estimated as
necessary for their transference to our planet.

On the road from the earth the germs would for about a month be exposed
to the powerful light of the sun, and it has been demonstrated that
the most highly refrangible rays of the sun can kill bacteria and
their spores in relatively short periods. As a rule, however, these
experiments have been conducted in such a manner that the spores could
germinate on the moist surface on which they were deposited (for
instance, in Marshall Ward’s experiments). That, however, does not at
all conform to the conditions prevailing in planetary space. For Roux
has shown that anthrax spores, which are readily killed by light when
the air has access, remain alive when the air is excluded. Some spores
do not suffer from insulation at all. That applies, for instance,
according to Duclaux, to _Thyrothrix scaber_, which occurs in milk and
which may live for a full month under the intense light of the sun. All
the botanists that I have been able to consult are of the opinion that
we can by no means assert with certainty that spores would be killed
by the light rays in wandering through infinite space.

It may further be argued that the spores, in their journey through
universal space, would be exposed during most of that period to an
extreme cold which possibly they might not be able to endure. When the
spores have passed the orbit of Neptune, their temperature will have
sunk to -220°, and farther out it will sink still lower. In recent years
experiments have been made in the Jenner Institute, in London, with
spores of bacteria which were kept for twenty hours at a temperature
of -252° in liquid hydrogen. Their germinating power was not destroyed
thereby.

Professor Macfadyen has, indeed, gone still further. He has
demonstrated that micro-organisms may be kept in liquid air (at -200°)
for six months without being deprived of their germinating power.
According to what I was told on the occasion of my last visit to
London, further experiments, continued for still longer periods, have
only confirmed this observation.

There is nothing improbable in the idea that the germinating power
should be preserved at lower temperatures for longer periods than at
our ordinary temperatures. The loss of germinating power is no doubt
due to some chemical process, and all chemical processes proceed at
slower rates at lower temperatures than they do at higher. The vital
functions are intensified in the ratio of 1: 2.5 when the temperature
is raised by 10° C. (18° F.). By the time that the spores reached the
orbit of Neptune and their temperature had been lowered to -220°, their
vital energy would, according to this ratio, react with one thousand
millions less intensity than at 10°. The germinating power of the
spores would hence, at -220°, during the period of three million years,
not be diminished to any greater degree than during one day at 10°.
It is, therefore, not at all unreasonable to assert that the intense
cold of space will act like a most effective preservative upon the
seeds, and that they will in consequence be able to endure much longer
journeys than we could assume if we judged from their behavior at
ordinary temperatures.

It is similar with the drying effect which may be so injurious to plant
life. In interplanetary space, which is devoid of atmosphere, absolute
dryness prevails. An investigation by B. Schröber demonstrates that the
green alga _Pleurococcus vulgaris_, which is so common on the trunks
of trees, can be kept in absolute dryness (over concentrated sulphuric
acid in a desiccator) for twenty weeks without being killed. Seeds and
spores may last still longer in a dry atmosphere.

Now, the tension of water vapor decreases in nearly the same ratio as
the speed of the reaction with lower temperatures. The evaporation of
water—_i.e._, the drying effect—may hence, at a temperature of -220°,
not proceed further in three million years than it will in one day at
10°. We have thus several plausible reasons for concluding that spores
which oppose an effective resistance to drying may well be carried from
one planet to another and from one planetary system to another without
sacrificing their vital energy.

The destructive effect of light is, according to the experiments of
Roux, no doubt due to the fact that the rays of light call forth
an oxidation by the intermediation of the surrounding air. This
possibility is excluded in interplanetary space. Moreover, the
radiation of the sun is nine hundred times fainter in the orbit
of Neptune than in the orbit of the earth, and half-way to the
nearest fixed star, Alpha Centauri, twenty million times feebler.
Light, therefore, will not do much harm to the spores during their
transference.

If, therefore, spores of the most minute organisms could escape from
the earth, they might travel in all directions, and the whole universe
might, so to say, be sown with them. But now comes the question: how
can they escape from the earth against the effect of gravitation?
Corpuscles of such small weight would naturally be carried away by any
aerial current. A small rain-drop, 1/50 mm. in diameter, falls, at
ordinary air pressure, about 4 cm. per second. We can calculate from
this observation that a bacteria spore 0.00016 mm. in diameter would
only fall 83 m. in the course of a year. It is obvious that particles
of this minuteness would be swept away by every air current they met
until they reached the most diluted air of the highest strata. An air
current of a velocity of 2 m. per second would take them to a height
where the air pressure is only 0.001 mm.—_i.e._, to a height of about
100 km. (60 miles). But the air currents can never push the particle
outside of our atmosphere.

In order to raise the spores to still higher levels we must have
recourse to other forces, and we know that electrical forces can help
us out of almost any difficulty. At heights of 100 km. the phenomena
of the radiating aurora take place. We believe that the auroræ are
produced by the discharge of large quantities of negatively charged
dust coming from the sun. If, therefore, the spore in question should
take up negative electricity from the solar dust during an electric
discharge, it may be driven out into the sea of ether by the repulsive
charges of the other particles.

We suppose, now, that the electrical charges—like matter—cannot be
subdivided without limit. We must finally come to a minimum charge,
and this charge has been calculated at about 3.5.10^{-10} electrostatic
units.

We can, without difficulty, calculate the intensity of an electric
field capable of urging the charged spore of 0.00016 mm. upward against
the force of gravity. The required field-strength is only 200 volts per
metre. Such fields are often observed on the surface of the earth with
a clear sky, and they are, indeed, almost normal. The electric field of
a region in which an auroral display takes place is probably much more
intense, and would, without doubt, be of sufficient intensity to urge
the small electrically charged spores which convection currents had
carried up to these strata, farther out into space against the force of
gravity.

It is thus probable that germs of the lowest organisms known to us are
continually being carried away from the earth and the other planets
upon which they exist. As seeds in general, so most of these spores,
thus carried away, will no doubt meet death in the cold infinite space
of the universe. Yet a small number of spores will fall on some other
world, and may there be able to spread life if the conditions be
suitable. In many cases conditions will not be suitable. Occasionally,
however, the spores will fall on favorable soil. It may take one
million or several millions of years from the age at which a planet
could possibly begin to sustain life to the time when the first seed
falls upon it and germinates, and when organic life is thus originated.
This period is of little significance in comparison with the time
during which life will afterwards flourish on the planet.

The germs which in this way escape from the planets on which their
ancestors had found abode, may either wander unobstructed through
space, or they may, as we have indicated, reach outer planets, or
planets moving about other suns, or they may meet with larger
particles of dust rushing towards the sun.

We have spoken of the Zodiacal Light and that part of it which has been
designated the counter-glow. This latter glow is regularly seen in the
tropics and occasionally in that portion of our heavens which is just
opposite the sun. Astronomers ascribe the counter-glow to streams of
fine dust which are drawn towards the sun (compare page 147). Let us
assume that a seed of the diameter of 0.00016 mm. strikes against a
grain of dust which is a thousand times as large (0.0016 mm. diameter),
and attaches itself to its surface. This spore will be carried by the
grain of dust towards the sun; it will cross the orbits of the inner
planets, and it may descend in their atmospheres. Those grains of dust
do not, by any means, require very long spaces of time to pass from one
planetary orbit to another. If we assume that the spore starts with
zero velocity near Neptune (in which case the seed might originate
from the moon of Neptune; for Neptune itself, like Uranus, Saturn, and
Jupiter, is not yet sufficiently cooled to sustain life), the spore
would reach the orbit of Uranus in twenty-one years, and of Mercury in
twenty-nine years. With the same initial velocity such particles would
be twelve years in passing between the orbits of Uranus and Saturn,
four years between Saturn and Jupiter, two years between Jupiter and
Mars, eighty-four days between Mars and the earth, forty days between
the earth and Venus, and twenty-eight days between Venus and Mercury.

We see from these time estimates that the germs, together with the
grains of dust to which they have attached themselves, might move
towards the sun with much smaller velocity (from ten to twenty times
smaller) without our having to fear any loss of their germinating
powers during the transit. In other words, if these seeds adhere to the
particles, ninety or ninety-five per cent. of whose weight is balanced
by the radiation pressure, they may soon fall into the atmosphere of
some inner planet with the moderate velocity of a few kilometres per
second. It is easy to calculate that if such a particle should, in
falling, be arrested in its motion after the first second, it would
yet, thanks to the strong heat radiation from it, not be heated by more
than 100° Cent. (212° F.) above the temperature of its surroundings.
Such a temperature can be borne by the spores of bacteria without fatal
effects for much more than one second. After the particles, together
with the seed adhering to them, have once been stopped, they will
slowly descend, or will be carried down to the surface of the nearest
planet by descending convection currents.

In this way life would be transferred from one point of a planetary
system, on which it had taken root, to other locations in the same
planetary system which favor the development of life.

The seeds not caught by such particles of dust may be taken over to
other solar systems, and finally be stopped by the radiation pressure
of their suns. They cannot penetrate any farther than to spots at
which the radiation pressure is as strong as at their starting-points.
Consequently, germs from the earth, which is five times as near the sun
as Jupiter, could approach another sun within a fifth of the distance
at which germs from Jupiter would be stopped.

Somewhere near the suns, where the seeds are arrested by the radiation
pressure to be turned back into space, there will evidently be
accumulations of these seeds. The planets which circulate around their
suns have therefore more chance of meeting them than if they were not
in the vicinity of a sun. The germs will have lost the great velocity
with which they wandered from one solar system to another, and they
will not be heated so greatly in falling through the atmospheres of the
planets which they meet.

The seeds which are turned back into space when coming near a sun will
there perhaps meet with particles whose weight is somewhat greater than
the repelling power of the radiation pressure. They would, therefore,
turn back to the suns. Like the germs, and for similar reasons, these
particles would consequently be concentrated about the sun. The small
seeds have, therefore, a comparatively better chance of being arrested
before their return to space by contact with such particles, and of
being carried to the planets near that sun.

In this manner life may have been transplanted for eternal ages from
solar system to solar system and from planet to planet of the same
system. But as among the billions of grains of pollen which the wind
carries away from a large tree—a fir-tree, for instance—only one
may on an average give birth to a new tree, thus of the billions, or
perhaps trillions, of germs which the radiation pressure drives out
into space, only one may really bring life to a foreign planet on which
life had not yet arisen, and become the originator of living beings on
that planet.

Finally, we perceive that, according to this version of the theory
of panspermia, all organic beings in the whole universe should be
related to one another, and should consist of cells which are built up
of carbon, hydrogen, oxygen, and nitrogen. The imagined existence of
living beings in other worlds in whose constitution carbon is supposed
to be replaced by silicon or titanium must be relegated to the realm of
improbability. Life on other inhabited planets has probably developed
along lines which are closely related to those of our earth, and this
implies the conclusion that life must always recommence from its very
lowest type, just as every individual, however highly developed it may
be, has by itself passed through all the stages of evolution from the
single cell upward.

All these conclusions are in beautiful harmony with the general
properties characteristic of life on our earth. It cannot be denied
that this interpretation of the theory of panspermia is distinguished
by perfect consistency, which is the most important criterion of the
probability of a cosmogonical theory.

There is little probability, though, of our ever being able to
demonstrate the correctness of this view by an examination of seeds
falling down upon our earth. For the number of germs which reach us
from other worlds will be extremely limited—not more, perhaps, than a
few within a year all over the earth’s surface; and those, moreover,
will presumably strongly resemble the single-cell spores with which the
winds play in our atmosphere. It would be difficult, if not impossible,
to prove the celestial origin of any such germs if they should be found
contrary to our assumption.


                                THE END