The primitive matter, at first in a chaotic state, we are to suppose slowly reduced to order under the continued agency of these two laws. Its isolated atoms, under gravitation, congregated into groups more or less close, and these again into larger and larger groups, until at length the original diffused matter became resolved into a number of rotating nebular masses of spherical form, of immense volume, and in a state of high heat from the previous shock of their atoms and constituent parts.

According to the nebular hypothesis, these vaporous and rotating spheres slowly cooled by radiation, in cooling contracted, in contracting acquired a more rapid rotatory motion, in consequence of which, through the increased centrifugal force, huge rings of vapour were at length flung off from the equatorial regions. These Saturnian rings successively projected from the spinning spheres as the cooling of the central nebulous mass still continued, in their turn condensed, broke from their annular shape at their points of feeblest cohesion as rings of smoke, united again by the law of gravitation, and settled into the spherical form like their parent masses, with which, however, they continued still connected, as planets, by the powerful invisible chain of gravitation. But the planets, in cooling, had early imitated the process of the parent nebulæ by also expelling rings of vapour which, having gone through the same steps as their generating primaries, appeared as younger families of planets, or planetoids-a third generation, so to speak, and faithful copies, both in form and movements, of their progenitors. Such was the origin of our earth and planets, as well as of their moons-their lawfully-born offspring, and still the followers of their fortunes; while in the rings of Saturn we have an instance of a former

potential moon, whose course of development was checked, the broken fragments not having united—a mishap which, however, had the good result for Science of suggesting the whole hypothesis to its originators.

Such is, in substance, the celebrated speculation of Laplace and Kant, concerning the origin of the earth and the solar system; the first of that startling series of tales issued by Science during the past century, and which has now been seemingly completed in the Darwinian story of the origin of plants and animals, and above all in the crowning one-the metamorphosis of the ape into the man.

The hypothesis is generally allowed to be of a legitimate and scientific character. It postulates only a vera causa, and such laws as we still see in operation. It postulates only a nebulous mass subject to present physical laws; and such nebulæ now exist, while such laws, we may fairly be asked to believe, did govern from eternity the behaviour of matter and energy. Moreover, the hypothesis explains many of the facts requiring explanation, as the fact that the motions of the planets are all nearly in the same plane; that the central mass of our system remains a blazing sun, while the surface of our earth has long since cooled; the fact of the rings of Saturn, the satellites and the various temperatures of the planets, and many other things. Nevertheless, as will presently appear, the hypothesis is not without great and as yet unexplained difficulties.

"The greatest of them," as Professor Newcomb urges, "perhaps, is to show how a ring of vapour surrounding the sun could condense into a single planet encircled by satellites." For the ring of vapour that by supposition

* Newcomb's Astronomy, Part IV. ch. iii., which contains a full consideration of the subject.

condensed into our earth, for instance, must have been at least the diameter of the earth's orbit, that is, some two hundred millions of miles, in breadth. But now, even if we suppose this enormous ring to have been successfully expelled after the manner described by Laplace—a supposition not without its difficulties; if we suppose it to have subsequently condensed as it certainly would, and to have broken up as it probably would;-the further suppositions that we must make constitute a demand on our scientific faith that is scarcely justified by either physical science or analogy. For we are asked to believe that all the sundered parts, probably extremely numerous, some of them separated by the interval of the earth's orbit, and all of them necessarily moving with great velocity in the same direction, at length, after paroxysmal efforts due to the action of gravity, found themselves together again in one symmetrical sphere of vapour, moving orderly round the sun. The exercise of faith required is great; for physical science would rather predict that the broken parts of such a ring, instead of coalescing into a single sphere, would, as the authority just quoted affirms, "condense into a swarm of smaller bodies like the asteroids," or like those still smaller, which compose, according to conjecture, the rings of Saturn. And then we must believe that this precarious process of generating worlds repeated itself without accident again and again; the rings of Saturn showing the only abortive attempt. The earth was also a sphere of vapour which, in shrinking, left behind a ring, which in its turn condensed, broke up, joined again, and finally formed our moon. And Jupiter must have been thus successfully delivered of his numerous progeny of eight. But a much more serious difficulty presents itself in the case of Uranus, whose

moons move in a direction nearly the reverse of that required by the hypothesis. It would seem that either Uranus received a temporary tilt over in his orbit at the moment when each of his rings was disengaged, or else some great perturbation or other exceptional experience occurred to each of his progeny after they were born. And the same must be said of Neptune, whose single moon moves in a more decidedly retrograde order.

Moreover the conditions, if any, under which such a ring could be thrown off from a rotating sphere of cooling vapour have not been investigated mathematically, nor is there any experiment in point,* while under the conditions supposed by Laplace, it has been objected that in the absence of all cohesion between the particles of vapour, the throwing off the ring in the manner alleged was in fact impossible. Finally, the existing nebulæ do not manifest that symmetry of outline which is requisite if they are destined to condense into suns and planets. All which objections, together with others that might be urged, tend to discredit the hypothesis very considerably in the forms propounded by its originators.

The authors of The Unseen Universe have given an improved statement of the nebular hypothesis. They begin with a chaos of atoms compelled to order by the law of gravitation, instead of reaching it slowly by chance after many false starts and failures as in the system of Democritus. We are told that "the original state of the visible universe was a diffused or chaotic state, in which the various particles were widely separated from one another, but

* That of Plateau, sometimes adduced, of a sphere of oil rotating in water, is no confirmation, as it relates to a different thing under different conditions, and on an infinitely smaller scale. See Jevons's Principles of Science.

exerting on one another gravitating force, and therefore possessed of potential energy. As these particles came together, impinged on one another, or gathered into groups, this potential energy was gradually transformed into the energy of heat, and into that of visible motion. We may thus imagine the cooling and (except under very strict conditions of original distribution) necessarily revolving matter, in course of time to have thrown off certain parts of itself, which would thereafter form satellites or planetary attendants, while the central mass would form the sun."

Now, if this be the development hypothesis of Kant and Laplace, as we are expressly told it is, it must be allowed that most of the difficulties which beset the more audacious presentments of it are avoided. But also, it must be said, with this more guarded and general statement of it, nearly all the grandeur of the hypothesis vanishes. With the contracting spheres and the mighty rings of vapour suppressed. which closed together and became worlds, all the charm and attractiveness is gone. That, however, should not signify in a matter that respects only scientific accuracy. True; but also with the removal of the rings and the sphere of vapour, the hypothesis itself is gone. There is nothing. remaining but a mass of matter which threw off parts of itself in its revolution—a different, if a safer, theory. Nor is it wholly satisfactory either. For unless the parts were propelled in a skilful manner that has not been described, they must have either fallen back again or travelled into space, to return no more; and why neither of these accidents, antecedently possible, did actually happen, we are left to surmise.

This last theory has, however, been presented by Sir W. Thomson, its real originator, from a much more striking and *The Unseen Universe, p. 164, Fifth Edit.