PART III.
EVIDENCE IN SUPPORT OF THE THEORY FROM THE PRE-NEBULAR CONDITION OF THE UNIVERSE.
The nebular hypothesis, strictly speaking, is one simply intended to account for the origin of our solar system. “It is,” as remarks Professor A. Winchell, “primarily a genetic explanation of the phenomena of the solar system; and accessorily a co-ordination, in a common conception, of the principal phenomena in the stellar and nebular firmament, as far as human vision has been able to penetrate.”[[62]] The theory starts with the assumption that all the materials composing the solar system once existed in a state of extreme tenuity and diffusion, filling far more than the entire space included within the orbit of the most remote planet. It begins with this diffused nebulous mass tending slowly, under the influence of gravitation, towards a state of aggregation. Beyond this point the received nebular hypothesis does not extend.
It will be observed that the theory here begins in the middle of a process. It begins with the assumption of a mass in the act of condensing under the influence of gravity. It offers no explanation of the origin of the mass, or how it came to be in this attenuated state, or in what condition it existed before the materials began to draw together. These are, however, inquiries which naturally force themselves on our attention. If the nebular theory be a true theory of the origin of the solar system, then this nebulous mass must have had an antecedent history, and we cannot help feeling the instinctive desire of tracing the chain of causation farther back. The mind presses towards an absolute beginning. It is the goal to which it aspires, and no amount of failure will ever deter it from renewing its efforts. Of recent years a considerable amount of attention has been devoted to inquiries in this direction; nearly all of which, it is true, has necessarily been of a speculative and hypothetical character. But hypothesis, as Mr. Locyker remarks, is the life-blood of investigation.
The nebular hypothesis is so highly probable as to have gained almost universal acceptance. In fact, it contains very little of a hypothetical nature. It is, as Mr. Mill says, “an example of legitimate reasoning from a present effect to its past cause, according to the known laws of that cause.” Like the hypothesis of a luminiferous ether, if it is not a true theory, one would almost think that it deserves to be so.
There seems no reason why inquiries should stop at the point where Laplace began. The same line of reasoning may yet carry us back into the pre-nebular region, and perhaps with as great a degree of certainty as it has done in the nebular; though, no doubt, the farther back we proceed, the more difficult probably will the inquiry become. But, be all this as it may, there can be little doubt that the path of investigation is a legitimate one, and also one which is worthy of being traced out.
I shall now briefly refer to some of the leading views which have been expressed in regard to the pre-nebular history of the universe, and shall afterwards consider the additional light which the theory discussed in this volume seems to cast on the subject.
The commonly received opinion is that the nebulæ were formed from ordinary matter existing in a high state of division, and widely diffused through space. The “cosmical dust,” as it is called, was the universal “world-stuff,” out of which all things were supposed to be formed. It is held that in receding backwards in pre-nebular times, the smaller, more simple, and elementary the materials were. Out of this primitive cosmical dust, or world-stuff, by aggregation, the materials became successively larger and more complex. The theory of the origin of nebulæ, on this principle, has been clearly stated by Professor Winchell, and I here give a brief outline of his views on the subject.
Professor A. Winchell on the pre-nebular condition of matter.—This cosmical dust, or world-stuff, he considers to be scattered promiscuously through boundless space. It is cold and non-luminous, and is acted upon by forces of attraction and probably of repulsion. The material particles, either as atoms or less probably as molecules, are drawn by mutual attraction into groups and swarms. Any central attractive force, as of a sun or planet, by causing the particles to move in converging lines, would cause them to become approximated and ultimately aggregated. Thus both mutual attractions and centric movements would tend to produce aggregations dispersed through space. But in the presence of two or more attractive centres, as in the present constitution of the Cosmos, it is impossible that any mass shall fall directly upon its centre of attraction. Hence motions of rotation will be established in the mass, and also orbital motions of masses about each other. In addition to the mutual attraction of the molecules, the convergence of their paths towards centres of attraction must also tend to the formation of masses and swarms of masses and particles. “We have then,” he says, “to picture indefinite space as pervaded by swarms of masses and particles of dark matter. Each mass or particle may, nevertheless, be separated by thousands of miles. It is manifest, therefore, that each mass or particle will eventually dispose itself, under the fixed action of the forces of matter, in some definite order. It is manifest also, from what has been said, that each swarm will have a progressive motion along a path having the essential character of an orbit around some dominant centre of attraction. If, as seems to be the fact, an ethereal medium, or any condition of interplanetary matter, exists in space, it opposes the movements of these swarms by opposing the motion of each constituent mass. But the smaller masses—the particles and molecules—would feel this resistance to the greatest extent. They would therefore fall behind the heavier masses, and would be most deflected toward the attracting centre. The smallest particles would be driven farthest to the rear, and dispersed farthest from the orbit of the train, along the side turned toward the principal attraction. The swarm would present an elongated form, in which the larger and heavier masses would move foremost, and nearest the line of the orbit—that is, near the exterior skirt of the area covered by the general swarm—while the smaller ones would follow, in graduated succession, in a long train which would present a fan-like expansion lying mostly on the inside of the path of the principal masses.”
“This, it may be conceived, is the mode of aggregation of these cosmical matters in the depths of space. Of course the attractions which control them are feeble; their movements are slow, the resistances are relatively inconsiderable, and the elongation of the swarm is correspondingly inconspicuous. What I have described is a tendency which would be present. Sometimes the controlling attraction would be only another cosmical swarm. The two swarms would revolve similarly about their common centre of gravity, while prolonged resistances would cause their slow approximation and final coalescence at the common centre of gravity. Sometimes the controlling attraction would be exerted by a distant sun, around which it would slowly move, continually gathering up additions of matter from the wide fields of space.”