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]
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]
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.
