We shall later return to the consequences of these peculiarities.
With help of the spectroscope we have ascertained that the gases on the Sun are, in the main, also arranged according to specific weights, so that the lightest reach the greatest heights. Somewhat similar conditions obtain in the gas-appendages of the stars (compare [page 119]).
CHAPTER V
THE CHEMISTRY OF THE ATMOSPHERE
Of a very particular interest is the question of the atmosphere of the planets. The great problem of habitability of the latter is most intimately connected therewith. Primitive fancy, very early, populated the stellar bodies, especially the stars and the Sun, with beings similar to those on Earth. Gradually, however, the insight awoke that these bodies are incandescent and therefore unfit to shelter life of the kind with which we are familiar. Attention then turned to the planets, as they and the Earth belonged to the same order of heavenly bodies. Perhaps they furnished abodes for our kin. And the stars, suns like our Sun, should not they be surrounded each with its throng of planets, gravitating around their central source of light and heat? This beautiful thought vied with the conception of Earth as the centre of the universe and as wholly set apart from the other stellar bodies, whose prime object it were merely to furnish light and to indicate time for the inhabitants of the Earth. Much to be regretted, it was the latter far less attractive theory which gained firm hold on the Church, although a few of its unbiased men, like the renowned Cardinal Nicolaus Cusanus (1401–1464), declared in favour of the contrary opinion, and did so unmolested. But times grew harder, ironclad orthodoxy triumphed, and Giordano Bruno, whose defence was that he simply accepted the theory of the great Cusanus, was burned at the stake to expiate his fearless assertion that other worlds, no less than ours, might be blessed with the presence of living beings.
Undoubtedly, other planets are upbuilt of the same material which enters into the composition of the Earth—as held already by Leonardo da Vinci. Spectral analysis teaches us that the same constituents form all the suns, including our own. If we agree that the suns have furnished the original substance of the planets revolving around them it is a natural conclusion that this matter should combine into similar chemical compounds on planets equally advanced in their evolutionary, that is cooling, process. And we know, indeed, that the same elements compose the Sun and the Earth, and that the samples brought to us from other worlds, i. e. the meteors, are of a composition which strongly reminds us of the rocks in the interior of the Earth. We seek in vain only for indications of the action of water, which substance has left such obvious traces on the surface of our globe and in the immediate strata below. But it should be remembered that the water, in the form of vapour, as previously set forth, has left all the minor stellar bodies, and the meteorites manifestly belong to the small or smallest among the wanderers of the heavens.
There is then no reason to doubt that the material of which the planets are built essentially is the same throughout the universe. Their interior should, like that of our Earth, consist of the heavy metals, principally iron,—strongly prevalent also in the Sun and in the meteors,—and this metallic nucleus should be clothed with the silicates, oxides, carbonates, sulphides, and hydrates of all metals, particularly aluminum, and among the metals we may also count hydrogen. The melting points of these exterior and lighter substances lies above 1000° C (1800° F.). No life could exist in such a molten mass, so that not until a solid crust had been formed through cooling was the possibility of life at hand.
Life is, at least on Earth, tied to certain so-called compounds, in which carbon is the essential common element, while hydrogen, nitrogen, and oxygen together with sulphur, phosphorus, iron, magnesium, and a few other less important elements also enter therein. No substance but carbon possesses this quality of being a prerequisite of life. Silica is a close kin to carbon and a substitute in certain organic compounds, but protoplasm, the main constituent of the living cell, cannot be built without carbon. In the inorganic world, however, silica by virtue of its affinity, which is kindred to that of carbon, plays a rôle somewhat similar to the latter in the almost infinitely variably silicates. Protoplasm cannot endure in a temperature above 60° C. (140° F.) or thereabout—certain algæ, it is sometimes stated, thrive in hot springs up to 80° to 90° C. (176° to 194° F.) but certainly not over 100° C. (212° F.). At these temperatures—strictly speaking at all temperatures between O° C. (32° F.) and 365° C. (689° F.)—water can exist in fluid state and this too is a prerequisite of life. We may say therefore that life is confined to a small temperature range between the freezing and boiling points of water. But wherever water occurs, except in a vessel which it completely fills, there exists also in the adjacent space, if unoccupied by fluids or solids, water vapour of at least 4.6 mm. (.18 inches) pressure. There is, therefore, always an atmosphere of aqueous vapour on any planet whose surface is partly covered by water.
The palæontologists have agreed that all life commenced in the water. The manifold living beings which now inhabit the solid crust of the Earth all descend from ancestors which floated in the waves of the ocean, the cradle of all organisms. It is not absolutely certain that oxygen is necessary to all living beings but many biologists hold that opinion. Certain bacteria are able to draw the oxygen they require for their development from compounds in which oxygen is bound sometimes in a very intimate manner, as in sulphates. But these bacteria are considered degenerate plants, and free oxygen is certainly indispensable to the existence of the animals and probably also to the plants with the exception just mentioned. As we shall see later, free oxygen cannot be present on the planets until a solid crust has been formed. We may therefore state that the conditions for the existence of life on a planet are fulfilled when a true atmosphere containing oxygen and water surround its body.
If we are to understand these conditions, we must study the processes whereby oxygen is supplied to the atmosphere. As the planets are segregations from the Sun, they should originally have the composition of the Sun, particularly that of its outer layers. Here metals occur in greatest abundance, but there are also a few oxides, especially those of titanium and magnesium (according to Fowler), hydrogen in great quantities, oxygen, carbon, cyanogen, and carbon monoxide. It may seem strange that free oxygen exists side by side with a surplus of hydrogen and sodium, strong so-called reducing substances which bind oxygen. But at the high temperatures prevailing on the Sun compounds of oxygen and the reducing substances, for instance hydrogen, i. e. water, are largely dissociated into their constituents. But, if the temperature should drop to about 1200° C. (2200° F.), at which point crust building does not yet take place, the oxygen would be entirely absorbed in the formation of the compounds mentioned. The compound substances of the Earth, like those of the Sun, are also strongly reducing, so that we must infer that free oxygen did not enter into the gas covering of the Earth at the time when its solid crust was formed. We may gain a conception of the gases which then existed in the outmost layers of the Earth by studying the gases on the Sun and on other stellar bodies, particularly the comets, and also by investigating the gases absorbed by the molten interior of the Earth. Previous to the crust formation, the entire mass of the Earth except the gases in its outmost layers were of the same character as possessed by its molten interior now. This molten mass, or magma, when in contact with the surrounding gases partly imbibed them through the process called absorption. An investigation of the gases present in the magma will therefore give us an idea of those existing in the original vapours surrounding the Earth. The magma occasionally comes into view in volcanic eruptions, and the confined gases are then partly given up into the air, but they are also partly retained in the solidifying lava, or volcanic rocks, whence they may be driven off by high temperature and subsequently analyzed. The direct gas emanations from the craters may also be gathered and analyzed. Such investigations have been carried out on a large scale by Albert Brun, Frenchman, Arthur Day, American, and his co-labourers Shepherd and Perret. Brun reached the surprising conclusion that water vapour, hitherto considered the most important of the volcanic gases, in reality was not one of them but originated in the crust of the earth. This theory, however, was completely refuted through the investigations by Day and his associates. As an example, we give the analysis (the mean of several determinations) of volcanic gases from the crater Halemaumau on the volcano Kilauea in Hawaii: