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.