CHAPTER IV

THE DISTRIBUTION OF THE ELEMENTS IN SPACE

It is now an old story, but still possessing its interest, how Fraunhofer analysed the light of the Sun by making it pass through a narrow slit and a prism, and found that the broad rainbow-tinted band of light so obtained was interrupted by hundreds of narrow dark lines, images in negative of the slit; and how Kirchhoff succeeded in proving that two of these dark lines were caused by the white light of the solar photosphere having suffered absorption at the Sun by passing through a stratum of glowing sodium vapour. From that time forward it has been known that the Sun is surrounded by an atmosphere of intensely heated gases, among which figure many of those elements familiar to us in the solid form on the Earth, such as iron, cobalt, nickel, copper, manganese, and the like. These metals, here the very types of solid bodies, are permanent gases on the Sun.

The Sun, then, is in an essentially gaseous condition, enclosed by the luminous shell which we term the photosphere. This shell Prof. C. A. Young and the majority of astronomers regard as consisting of a relatively thin layer of glowing clouds, justifying the quaint conceit of R. A. Proctor, who spoke of the Sun as a "Bubble"; that is, a globe of gas surrounded by an envelope so thin in comparison as to be a mere film. There has been much difference of opinion as to the substance forming these clouds, but the theory is still widely held which was first put forward by Dr. Johnstone Stoney in 1867, that they are due to the condensation of carbon, the most refractory of all known elements. Prof. Abbot, however, refuses to believe in a surface of this nature, holding that the temperature of the Sun is too high even at the surface to permit any such condensation.

The application of the spectroscope to astronomy is not confined to the Sun, but reaches much further. The stars also yield their spectra, and we are compelled to recognize that they also are suns; intensely heated globes of glowing gas, rich in the same elements as those familiar to us on the Earth and known by their spectral lines to be present on the Sun. The stars, therefore, cannot themselves be inhabited worlds any more than the Sun, and at a stroke the whole of the celestial luminaries within the furthest range of our most powerful telescopes are removed from our present search. Only those members of our solar system that shine by reflecting the light of the Sun can be cool enough for habitation; the true stars cannot be inhabited, for, whatever their quality and order, they are all suns, and must necessarily be in far too highly heated a condition to be the abode of life. Many of them may, perhaps, be a source of light and heat to attendant planets, but there is no single instance in which such a planet has been directly observed; no dark, non-luminous body has ever been actually seen in attendance on a star. Many double or multiple stars are known, but these are all instances in which one sun-like body is revolving round another of the same order.[9] We see no body shining by reflected light outside the limits of the solar system. Planets to the various stars may exist in countless numbers, but they are invisible to us, and we cannot discuss conditions where everything is unknown. Enquiry in such a case is useless, and speculation vain.

The stars, as revealed to us by the spectroscope are all of the same order as the Sun, but they are not all of the same species. Quite a large number of stars, of which Arcturus is one of the best-known examples, show spectra that are essentially the same as that of the Sun, but there are other stars of which the spectra bear little or no semblance to it. Nevertheless, it remains true that, on the whole, stellar spectra bear witness to the presence of just the same elements as we recognize in the Sun, though not always in the same proportions or in the same conditions--hydrogen, calcium, sodium, magnesium, iron, titanium, and many more are recognized in nearly all. It is true that not all the known terrestrial elements have yet been identified in either Sun or stars; but, in general, those missing are either "negative" elements like the halogens, or elements of great atomic weight like mercury and platinum. That elements of one class should, as a rule, reveal their presence in Sun and stars wherever these are placed, and, correspondingly, that other classes should as generally fail to show themselves, indicate that such absence is more likely to be due to the general structure of the stellar photospheres and reversing layers than to any irregularity in the distribution of matter in the universe. It is easy, for example, to conceive that the heavy metals may lie somewhat deeper down within the Sun or star than those of low atomic weight. In the case of the Sun, there seems a clear connection between atomic weight and the distinctness with which the element is recognized in the spectrum of the photosphere, the lower atomic weights showing themselves more conspicuously.

It is clear that not all elements present in a Sun or star show themselves in its spectrum. Oxygen is very feebly represented by its elemental lines, but the flutings of titanium oxide are found in sunspots, and with great distinctness in a certain type of stars. Nitrogen, too, though not directly recognized, proves its presence by the lines of cyanogen. The case of helium is one of particular interest; this element was recognized by a very bright yellow line in the solar prominences before it was known to exist on the Earth; indeed, it received the name _helium_ because it then seemed to be a purely solar constituent. Now it is seen as a strong absorption line in the spectrum of many stars; but for some reason it is not in general seen as an absorption line over the Sun's disc, and if our Sun were removed to such distance so as to appear to us only as a star, we should have no evidence that it contained any helium at all. So far, then, as the evidence of the spectroscope goes, the elements present in the Earth are present throughout the whole extent of the universe within our view: the same elements and with the same qualities. For the lines of the spectrum of an element are the revelation of its innermost molecular structure, so that we can confidently affirm that hydrogen and oxygen on Sirius, Arcturus, or the Sun, are essentially the same elements as hydrogen and oxygen on the Earth. On a planet attached to any of these stars, the two gases would combine together to form water under just the same conditions as they do here on the Earth; and at suitable temperatures that water would be a neutral liquid, capable of dissolving just the same chemical substances that it does here. It would freeze as it does here; it would evaporate as it does here; it would be water as completely in all its qualities and conditions as earthly water is. And what applies to one element or compound applies to all. Throughout the whole extent of space, the same building materials have been employed, and throughout they retain the same qualities.

Hydrogen is seen in the spectra of nearly all stars, and also in those of nebulae. The elemental lines of oxygen are not indeed seen in stellar spectra, but that the element is present is shown by the flutings of titanium oxide which distinguish stars like Antares. Nitrogen and carbon again are not recognized by their elemental lines, but the lines of cyanogen are seen in the spectra of comets and of sunspots, and hydrocarbon flutings in the spectra of comets and red stars; while in a few of the hottest stars even sulphur has recently been identified.[10] All the five organo-genetic elements are therefore abundantly diffused through space; the materials for protoplasm, "the albuminous substance with water," are at hand everywhere. This being so, it is reasonable to infer that if organic life exists elsewhere than on this Earth, its essential feature, there as here, is the metabolism of nitrogenous carbon compounds in association with protoplasm.

But it is objected that "we are not yet able to identify all the lines in solar or stellar spectra; may not some of these lines be due to elements of which we know nothing here, and may not such new elements form complex and unstable compounds with each other, or with some of those familiar to us, that would take the place of the five organo-generators, and so give rise to a physical basis of life, different from that we know on this Earth?"

But the development of Mendeleeff's Periodic Law has shown that the elements are not to be regarded as disconnected entities. The Law as given in Mendeleeff's own words, runs: "The properties of the elements as well as the forms and properties of their compounds are in periodic dependence on, or (expressing ourselves algebraically) form a periodic function of the atomic weights of the elements." In other words, they form a series, not only as it regards their atomic weights, but also as it regards their own properties and the forms and properties of their compounds. We are no longer at liberty, as we might have been many years ago, to call into fancied existence new elements having no relation in their properties and compounds to those with which we are acquainted. New elements, no doubt, will be discovered in the future, as in the past; and indeed we may be able to discover them and learn their atomic weights and properties without ever being able to handle them in a terrestrial laboratory.

In a series of remarkable papers communicated to the Royal Astronomical Society during the past year (1911-1912), Dr. J. W. Nicholson has given the result of his computation of the positions of the spectral lines of two elements of simple structure, and has found that the resulting lines correspond, for one dynamical system, to the chief unidentified lines observed in the spectra of nebulae, and for the other, to the chief unidentified lines in the spectrum of the corona. The latter element is probably associated with the halogens, but of much lower atomic weight (namely, 1.3), than fluorine; he therefore gives it the name of _Protofluorine_. The other element, to which he gives the name _Nebulium_, will have an atomic weight of 2.1. Prof. Max Wolf, of Heidelberg, has recently pointed out[11] the evidence of the presence of two other unknown gases in the Ring nebula in Lyra, and there is no reason to suppose that the process of discovery has come to an end. But we cannot imagine that we shall discover any new elements that are more abundant and more universally diffused than the five which give us protoplasm--"the physical basis of life." To take an analogy from the solar system: many hundreds of planetoids have now been discovered between the orbits of Mars and Jupiter, and probably many hundreds more remain to be discovered; but of one thing we are certain, that none of the planetoids yet to be discovered will be of the same rank as either of those two guardians, Mars and Jupiter, who revolve on the confines of the planetoidal zone. Indeed, Ceres, the planetoid first discovered, has a greater mass than the aggregate of all discovered since, and probably of all that exist in the zone.

Water is essential for life here, but the quality in water which restricts the range of terrestrial life is that it freezes at 0 deg. Centigrade, and boils at 100 deg. Centigrade; it is only in the liquid state during the intermediate range of 100 degrees. In order to extend the range for living organisms, we should have, therefore, to discover a new vehicle, that, possessing all the other qualities of water, is not restricted to the liquid state within the same limits. But we are at once met with the difficulty that the first essential for the vehicle is that it should be abundant, and there are no other elements more abundant than hydrogen and oxygen. This new vehicle must, like water, be both neutral and stable, or it would itself interfere with the highly unstable compounds that are a necessity for metabolism. And, if we could find this new vehicle, liquid at temperatures outside the 0 deg. to 100 deg. Centigrade, have we any reason to suppose that protoplasm itself would be able to endure these outlying temperatures? Looking through the range of substances available, we can only say that none other presents itself as approaching water in suitability for its essential office. If we, ourselves, were able to create a vehicle, could we imagine one more perfectly suited?