Part 44 (1/2)
[Footnote 1145: _Astr. Pop._, p. 661; _La Nature_, January 3, 1880.]
[Footnote 1146: _Proc. Roy. Soc. Edinb._, vols. x., p. 429; xi., p. 89.]
[Footnote 1147: _Vierteljahrsschrift. Astr. Ges._, Jahrg. xxi., p. 206.]
[Footnote 1148: _Proc. Roy. Soc. Edinb._, vol. xxiii., p. 370; _Nature_, vol. lxiv., p. 524.]
[Footnote 1149: _Amer. Jour. of Science_, vol. xx., p. 225.]
CHAPTER IX
_THEORIES OF PLANETARY EVOLUTION_
We cannot doubt that the solar system, as we see it, is the result of some process of growth--that, during innumerable ages, the forces of Nature were at work upon its materials, blindly modelling them into the shape appointed for them from the beginning by Omnipotent Wisdom. To set ourselves to inquire what that process was may be an audacity, but it is a legitimate, nay, an inevitable one. For man's implanted instinct to ”look before and after” does not apply to his own little life alone, but regards the whole history of creation, from the highest to the lowest--from the microscopic germ of an alga or a fungus to the visible frame and furniture of the heavens.
Kant considered that the inquiry into the mode of origin of the world was one of the easiest problems set by Nature; but it cannot be said that his own solution of it was satisfactory. He, however, struck out in 1755 a track which thought still pursues. In his _Allgemeine Naturgeschichte_ the growth of sun and planets was traced from the cradle of a vast and formless ma.s.s of evenly diffused particles, and the uniformity of their movements was sought to be accounted for by the unvarying action of attractive and repulsive forces, under the dominion of which their development was carried forward.
In its modern form, the ”Nebular Hypothesis” made its appearance in 1796.[1150] It was presented by Laplace with diffidence, as a speculation unfortified by numerical b.u.t.tresses of any kind, yet with visible exultation at having, as he thought, penetrated the birth-secret of our system. He demanded, indeed, more in the way of postulates than Kant had done. He started with a sun ready made,[1151] and surrounded with a vast glowing atmosphere, extending into s.p.a.ce out beyond the orbit of the farthest planet, and endowed with a slow rotatory motion.
As this atmosphere or nebula cooled, it contracted; and as it contracted, its rotation, by a well-known mechanical law, became accelerated. At last a point arrived when tangential velocity at the equator increased beyond the power of gravity to control, and equilibrium was restored by the separation of a nebulous ring revolving in the same period as the generating ma.s.s. After a time, the ring broke up into fragments, all eventually reunited in a single revolving and rotating body. This was the first and farthest planet.
Meanwhile the parent nebula continued to shrink and whirl quicker and quicker, pa.s.sing, as it did so, through successive crises of instability, each resulting in, and terminated by, the formation of a planet, at a smaller distance from the centre, and with a shorter period of revolution than its predecessor. In these secondary bodies the same process was repeated on a reduced scale, the birth of satellites ensuing upon their contraction, or not, according to circ.u.mstances. Saturn's ring, it was added, afforded a striking confirmation of the theory of annular separation,[1152] and appeared to have survived in its original form in order to throw light on the genesis of the whole solar system; while the four first discovered asteroids offered an example in which the _debris_ of a shattered ring had failed to coalesce into a single globe.
This scene of cosmical evolution was a characteristic bequest from the eighteenth century to the nineteenth. It possessed the self-sufficing symmetry and entireness appropriate to the ideas of a time of renovation, when the complexity of nature was little accounted of in comparison with the imperious orderliness of the thoughts of man. Since its promulgation, however, knowledge has transgressed many boundaries, and set at naught much ingenious theorising. How has it fared with Laplace's sketch of the origin of the world? It has at least not been discarded as effete. The groundwork of speculation on the subject is still furnished by it. It is, nevertheless, admittedly inadequate. Of much that exists it gives no account, or an erroneous one. The march of events certainly did not everywhere--even if it did anywhere--follow the exact path prescribed for it. Yet modern science attempts to supplement, but scarcely ventures to supersede it.
Thought has, in many directions, been profoundly modified by Mayer's and Joule's discovery, in 1842, of the equivalence between heat and motion.
Its corollary was the grand idea of the ”conservation of energy,” now one of the cardinal principles of science. This means that, under the ordinary circ.u.mstances of observation, the old maxim _ex nihilo nihil fit_ applies to force as well as to matter. The supplies of heat, light, electricity, must be kept up, or the stream will cease to flow. The question of the maintenance of the sun's heat was thus inevitably raised; and with the question of maintenance that of origin is indissolubly connected.
Dr. Julius Robert Mayer, a physician residing at Heilbronn, was the first to apply the new light to the investigation of what Sir John Herschel had termed the ”great secret.” He showed that if the sun were a body either simply cooling or in a state of combustion, it must long since have ”gone out.” Had an equal ma.s.s of coal been set alight four or five centuries after the building of the Pyramid of Cheops, and kept burning at such a rate as to supply solar light and heat during the interim, only a few cinders would now remain in lieu of our undiminished glorious...o...b.. Mayer looked round for an alternative. He found it in the ”meteoric hypothesis” of solar conservation.[1153] The importance in the economy of our system of the bodies known as falling stars was then (in 1848) beginning to be recognised. It was known that they revolved in countless swarms round the sun; that the earth daily encountered millions of them; and it was surmised that the cone of the zodiacal light represented their visible condensation towards the attractive centre. From the zodiacal light, then, Mayer derived the store needed for supporting the sun's radiations. He proved that, by the stoppage of their motion through falling into the sun, bodies would evolve from 4,600 to 9,200 times as much heat (according to their ultimate velocity) as would result from the burning of equal ma.s.ses of coal, their precipitation upon the sun's surface being brought about by the resisting medium observed to affect the revolutions of Encke's comet.
There was, however, a difficulty. The quant.i.ty of matter needed to keep, by the sacrifice of its movement, the hearth of our system warm and bright would be very considerable. Mayer's lowest estimate put it at 94,000 billion kilogrammes per second, or a ma.s.s equal to that of our moon bi-annually. But so large an addition to the gravitating power of the sun would quickly become sensible in the movement of the bodies dependent upon him. Their revolutions would be notably accelerated.
Mayer admitted that each year would be shorter than the previous one by a not insignificant fraction of a second, and postulated an unceasing waste of substance, such as Newton had supposed must accompany emission of the material corpuscles of light, to neutralise continual reinforcement.
Mayer's views obtained a very small share of publicity, and owned Mr.
Waterston as their independent author in this country. The meteoric, or ”dynamical,” theory of solar sustentation was expounded by him before the British a.s.sociation in 1853. It was developed with his usual ability by Lord Kelvin, in the following year. The inflow of meteorites, he remarked, ”is the only one of all conceivable causes of solar heat which we know to exist from independent evidence.”[1154] We know it to exist, but we now also know it to be entirely insufficient. The supplies presumed to be contained in the zodiacal light would be quickly exhausted; a constant inflow from s.p.a.ce would be needed to meet the demand. But if moving bodies were drawn into the sun at anything like the required rate, the air, even out here at ninety-three millions of miles distance, would be thick with them; the earth would be red-hot from their impacts;[1155] geological deposits would be largely meteoric;[1156] to say nothing of the effects on the mechanism of the heavens. Lord Kelvin himself urged the inadmissibility of the ”extra-planetary” theory of meteoric supply on the very tangible ground that, if it were true, the year would be shorter now, actually by six weeks, than at the opening of the Christian era. The ”intra-planetary”
supply, however, is too scanty to be anything more than a temporary makes.h.i.+ft.
The meteoric hypothesis was naturally extended from the maintenance of the sun's heat to the formation of the bodies circling round him. The earth--no less doubtless than the other planets--is still growing.
Cosmical matter in the shape of falling stars and aerolites, to the amount, adopting Professor Newton's estimate, of 100 tons daily, is swept up by it as it pursues its...o...b..tal round. Inevitably the idea suggested itself that this process of appropriation gives the key to the life-history of our globe, and that the momentary streak of fire in the summer sky represents a feeble survival of the glowing hailstorm by which in old times it was fas.h.i.+oned and warmed. Mr. E. W. Brayley supported this view of planetary production in 1864,[1157] and it has recommended itself to Haidinger, Helmholtz, Proctor, and Faye. But the negative evidence of geological deposits appears fatal to it.
The theory of solar energy now generally regarded as the true one was enounced by Helmholtz in a popular lecture in 1854. It depends upon the same principle of the equivalence of heat and motion which had suggested the meteoric hypothesis. But here the movement surrendered and transformed belongs to the particles, not of any foreign bodies, but of the sun itself. Drawn together from a wide ambit by the force of their own gravity, their fall towards the sun's centre must have engendered a vast thermal store, of which 453/454 are computed to be already spent.
Presumably, however, this stream of reinforcement is still flowing. In the very act of parting with heat, the sun develops a fresh stock. His radiations, in short, are the direct result of shrinkage through cooling. A diminution of the solar diameter by 380 feet yearly would just suffice to cover the present rate of emission, and would for ages remain imperceptible with our means of observation, since, after the lapse of 6,000 years, the lessening of angular size would scarcely amount to one second.[1158] But the process, though not terminated, is strictly a terminable one. In less than five million years, the sun will have contracted to half its present bulk. In seven million more, it will be as dense as the earth. It is difficult to believe that it will then be a luminous body.[1159] Nor can an unlimited past duration be admitted. Helmholtz considered that radiation might have gone on with its actual intensity for twenty-two, Langley allows only eighteen million years. The period can scarcely be stretched, by the most generous allowances, to double the latter figure. But this is far from meeting the demands of geologists and biologists.
An attempt was made in 1881 to supply the sun with machinery a.n.a.logous to that of a regenerative furnace, enabling it to consume the same fuel over and over again, and so to prolong indefinitely its beneficent existence. The inordinate ”waste” of energy, which shocks our thrifty ideas, was simultaneously abolished. The earth stops and turns variously to account one 2,250-millionth part of the solar radiations; each of the other planets and satellites takes a proportionate share; the rest, being all but an infinitesmal fraction of the whole, is dissipated through endless s.p.a.ce, to serve what purpose we know not. Now, on the late Sir William Siemens's plan, this reckless expenditure would cease; the solar incomings and outgoings would be regulated on approved economic principles, and the inevitable final bankruptcy would be staved off to remote ages.
But there was a fatal flaw in its construction. He imagined a perpetual circulation of combustible materials, alternately surrendering and regaining chemical energy, the round being kept going by the motive force of the sun's rotation.[1160] This, however, was merely to perch the globe upon a tortoise, while leaving the tortoise in the air. The sun's rotation contains a certain definite amount of mechanical power--enough, according to Lord Kelvin, if directly converted into heat, to keep up the sun's emission during 116 years and six days--a mere moment in cosmical time. More economically applied, it would no doubt go farther. Its exhaustion would, nevertheless, under the most favourable circ.u.mstances, ensue in a comparatively short period.[1161]
Many other objections equally unanswerable have been urged to the ”regenerative” hypothesis, but this one suffices.
Dr. Croll's collision hypothesis[1162] is less demonstrably unsound, but scarcely less unsatisfactory. By the mutual impact of two dark ma.s.ses rus.h.i.+ng together with tremendous speed, he sought to provide the solar nebula with an immense _original_ stock of heat for the reinforcement of that subsequently evolved in the course of its progressive contraction.