Evolution of the Chemical Elements, and its Relations to Stellar Evolution.
We come now to the consideration of a subject which has a most important bearing on the question of stellar evolution, viz. the genesis and dissociation of the chemical elements. The evolution of one element from another is, it is true, as yet but a mere hypothesis, but it is an hypothesis well supported by a host of facts and considerations, and held by a large number of our leading chemists and physicists. “The demonstrated unity of force,” says Professor F. W. Clarke,[[65]] “leads us by analogy to expect a similar unity of matter; and the many strange and hitherto unexplained relations between the different elements tend to encourage our expectations.” The hypothesis throws much light on some obscure points in stellar evolution. In regard to this, Professor Clarke justly remarks that “it is plain that the nebular hypothesis would be doubled in importance, and our views of the universe greatly expanded, if it could be shown that an evolution of complex from simple forms of matter accompanied the development of planets from the nebulæ. Evolution could look for no grander triumph.” In fact, it is difficult to understand how our sun and the stars could have been evolved from nebulæ without assuming an evolution of the chemical elements. The true nebulæ show the presence of only two elements, nitrogen and hydrogen, but our sun contains more than a dozen of distinct elements, and the planets more than three times that number. How, then, could all these have arisen out of nebulæ composed simply of nitrogen and hydrogen? The matter is plain if we assume an evolution of the elements.
The stars have been classed into four groups, which, as Professor Clarke has remarked, indicate different stages in the process of evolution. The first class, containing white stars like Sirius, show the predominance of hydrogen and a scarcity of the metallic elements. In the second class the metallic elements become more numerous and the hydrogen less distinct; while in the third class hydrogen is difficult to detect.[[66]] This seems to show a gradual development of the chemical elements as the star cools and grows older. I shall now give a brief account of the views expressed on the subject by some of our leading physicists and chemists.
It will be observed, in reference to the theories we have just considered, that the process of evolution is supposed to take place from the smaller to the larger aggregates of matter. Beginning with an extreme condition of tenuity, by aggregation, the materials become successively larger and more complex. In passing backwards in the process we find the aggregates becoming less and less till they reach the “cosmical dust,” or “fire-mist,” out of which the primitive nebulæ were supposed to be formed. Receding still farther back, we have the universal atmosphere from which the fire-mist is assumed to have been derived.
This universal atmosphere, though in a state of extreme tenuity, is, as we shall see, supposed by some to be in a more elemental form than anything revealed to us in the laboratory. The suggestion of the dissociation of the chemical elements and its application to stellar physics was, I think, first advanced by Sir Benjamin Brodie in 1866, and more fully in 1867. In the latter year views similar were considered more fully by Dr. T. Sterry Hunt. The question of the dissociation of elements has been ably discussed by Mr. Lockyer in his various writings. It has been suggested by Mr. Lockyer that the coincidence of rays emitted by different chemical elements when subjected to very high temperatures affords evidence of a common element in the composition of the metals producing the coincident rays. Mr. Lockyer states that many trains of thought suggested by solar and stellar physics point to the hypothesis that the elements themselves, or at all events some of them, are compound bodies.[[67]] This view was also put forward by Professor Graham, who says “that it is conceivable that the various kinds of matter now recognised in different elementary substances may possess one and the same element or atomic molecule existing in different conditions of mobility. The essential unity of matter,” he adds, “is an hypothesis in harmony with the equal action of gravity upon all bodies.” Similar views have been advocated by M. Dumas, who based the suggestion of the composite nature of the elementary atoms on certain relations of atomic weights. The composite nature of the chemical elements has also been maintained by Henri Sainte-Claire Deville, and also by Berthelot, who held that the atoms of the elements are the same, and distinguished only by their modes of motion. Professor Schuster, in a paper read before the British Association in 1880, supports the view of the dissociation of the chemical elements.
That all the purely physical sciences will one day be brought under a few general laws and principles, and the whole of the recognised chemical elements will be resolved into one or two material elements, is a conclusion towards which physical science seems at present slowly tending. There is certainly something fascinating in this view of the unity of nature. There is in this idea more than a purely physical interest attached to it. It has, as I hope to show in a future work, an important bearing on questions relating to the foundations of the true theory of evolution.
The question of the unity of the chemical elements is one, however, yet in a hypothetical condition. Professors Liveing and Dewar, who have given attention to this subject, say: “The supposition that the different elements may be resolved into simpler constituents, or into a single one, has long been a favourite speculation with chemists; but, however probable this hypothesis may appear à priori, it must be acknowledged that the facts derived from the most powerful method of analytical investigation yet devised give it scant support.”[[68]]
Sir Benjamin Brodie on the pre-nebular condition of matter.—There are, considers Sir Benjamin Brodie, very forcible reasons which lead us to suspect that chemical substances are really composed of a primitive system of elemental bodies, analogous in their general nature to our present elements: that some of those bodies which we speak of as elements may be compounds. These ideal elements assumed by him, he says, “though now revealed to us by the numerical properties of chemical equations only as implicit and dependent existences, we cannot but surmise may sometimes become, or may in the past have been, isolated and independent existences”—as, for instance, in the case of the sun, where the temperature is excessive. “We may,” he further adds, “consider that in remote ages the temperature of matter was much higher than it is now, and that these other things [ideal elements] existed then in the state of perfect gases—separate existences—uncombined.”[[69]] He then refers to certain observations of Mr. Huggins and Dr. Miller on the spectra of nebulæ where one of the lines of nitrogen was found alone; and that this suggested to them that the line might have been produced by one of the elements of nitrogen; and that nitrogen may therefore be compound. He mentions as a significant fact that a large proportion of the class of elements which he has termed “composite elements” has not been found in the sun, they having probably been decomposed by the intense heat.
Dr. T. Sterry Hunt on the pre-nebular condition of matter.—A year after the foregoing views regarding chemical dissociation had been advanced by Sir Benjamin Brodie, Dr. T. Sterry Hunt, in a lecture on “The Chemistry of the Primeval Earth,” delivered at the Royal Institution (May 31, 1867), put forward, apparently quite independently, opinions on dissociation similar to those of Brodie. In this lecture he says: “I considered the chemistry of nebulæ, sun, and stars in the combined light of spectroscopic analysis and Deville’s researches on dissociation, and concluded with the generalisation that the breaking-up of compounds, or dissociation of elements, by intense heat is a principle of universal application, so that we may suppose that all the elements which make up the sun, or our planet, would, when so intensely heated as to be in the gaseous condition which all matter is capable of assuming, remain uncombined, that is to say, would exist together in the state of chemical elements, whose further dissociation in stellar or nebulous masses may even give us evidence of matter still more elemental than that revealed in the experiments of the laboratory, where we can only conjecture the compound nature of many of the so-called elementary substances.”[[70]] And in his address at the grave of Priestley he referred to the suggestion of Lavoisier that hydrogen, nitrogen, and oxygen, with heat and light, might be regarded as simpler forms of matter from which all others are derived. This suggestion was considered in connection with the fact that the nebulæ, which we conceive to be condensing into suns and planets, have hitherto shown evidences only of the presence of the first two of these elements, which, as is well known, make up a large part of the gaseous envelope of our planet, in the forms of air and aqueous vapour. With this he connected the hypothesis advanced by Grove, “that our atmosphere and ocean are but portions of the universal medium which, in an attenuated form, fills the interstellary spaces;[[71]] and further suggested as a legitimate and plausible speculation that these same nebulæ and their resulting worlds may be evolved by a process of chemical condensation from this universal atmosphere, to which they would sustain a relation somewhat analogous to that of clouds and rain to the aqueous vapour around us.”
Professor Oliver Lodge on the pre-nebular condition of matter.—Some have gone still farther back and supposed that the material universe may have arisen out of the luminiferous ether—the hypothetical medium which is assumed to pervade all space. The universal world-stuff scattered through boundless space may in an extreme state of attenuation be, says Professor Winchell, the ethereal medium, and out of this semi-spiritual substance may have germinated the molecules of common matter. “It is certainly possible,” he says, “to conceive these cosmical atoms as a rising-out of some transformation of the ethereal medium; but we know too little of the nature of ether to ground a scientific inference of this kind.”[[72]]