The work just mentioned is especially interesting as an attempt to bring the probable destiny of the human soul into connection with the modern theories which explain the past and future career of the physical universe in accordance with the principle of continuity. Its authorship is as yet unknown, but it is believed to be the joint production of two of the most eminent physicists in Great Britain, and certainly the accurate knowledge and the ingenuity and subtlety of thought displayed in it are such as to lend great probability to this conjecture. Some account of the argument it contains may well precede the suggestions presently to be set forth concerning the Unseen World; and we shall find it most convenient to begin, like our authors, with a brief statement of what the principle of continuity teaches as to the proximate beginning and end of the visible universe. I shall in the main set down only results, having elsewhere given a simple exposition of the arguments upon which these results are founded.

The first great cosmological speculation which has been raised quite above the plane of guesswork by making no other assumption than that of the uniformity of nature, is the well-known Nebular Hypothesis. Every astronomer knows that the earth, like all other cosmical bodies which are flattened at the poles, was formerly a mass of fluid, and consequently filled a much larger space than at present. It is further agreed, on all hands, that the sun is a contracting body, since there is no other possible way of accounting for the enormous quantity of heat which he generates. The so-called primeval nebula follows as a necessary inference from these facts.

There was once a time when the earth was distended on all sides away out to the moon and beyond it, so that the matter now contained in the moon was then a part of our equatorial zone. And at a still remoter date in the past, the mass of the sun was diffused in every direction beyond the orbit of Neptune, and no planet had an individual existence, for all were indistinguishable parts of the solar mass. When the great mass of the sun, increased by the relatively small mass of all the planets put together, was spread out in this way, it was a rare vapour or gas.

At the period where the question is taken up in Laplace's treatment of the nebular theory, the shape of this mass is regarded as spheroidal; but at an earlier period its shape may well have been as irregular as that of any of the nebulae which we now see in distant parts of the heavens, for, whatever its primitive shape, the equalization of its rotation would in time make it spheroidal. That the QUANTITY of rotation was the same then as now is unquestionable; for no system of particles, great or small, can acquire or lose rotation by any action going on within itself, any more than a man could pick himself up by his waistband and lift himself over a stone wale So that the primitive rotating spheroidal solar nebula is not a matter of assumption, but is just what must once have existed, provided there has been no breach of continuity in nature's operations.

Now proceeding to reason back from the past to the present, it has been shown that the abandonment of successive equatorial belts by the contracting solar mass must have ensued in accordance with known mechanical laws; and in similar wise, under ordinary circumstances. each belt must have parted into fragments, and the fragments chasing each other around the same orbit, must have at last coalesced into a spheroidal planet.

Not only this, but it has also been shown that as the result of such a process the relative sizes of the planets would be likely to take the order which they now follow; that the ring immediately succeeding that of Jupiter would be likely to abort and produce a great number of tiny planets instead of one good-sized one; that the outer planets would be likely to have many moons, and that Saturn, besides having the greatest number of moons, would be likely to retain some of his inner rings unbroken; that the earth would be likely to have a long day and Jupiter a short one; that the extreme outer planets would be not unlikely to rotate in a retrograde direction; and so on, through a long list of interesting and striking details.

Not only, therefore, are we driven to the inference that our solar system was once a vaporous nebula, but we find that the mere contraction of such a nebula, under the influence of the enormous mutual gravitation of its particles, carries with it the explanation of both the more general and the more particular features of the present system. So that we may fairly regard this stupendous process as veritable matter of history, while we proceed to study it under some further aspects and to consider what consequences are likely to follow.

Our attention should first be directed to the enormous waste of energy which has accompanied this contraction of the solar nebula. The first result of such a contraction is the generation of a great quantity of heat, and when the heat thus generated has been lost by radiation into surrounding space it becomes possible for the contraction to continue. Thus, as concentration goes on, heat is incessantly generated and incessantly dissipated.

How long this process is to endure depends chiefly on the size of the contracting mass, as small bodies radiate heat much faster than large ones. The moon seems to be already thoroughly refrigerated, while Jupiter and Saturn are very much hotter than the earth, as is shown by the tremendous atmospheric phenomena which occur on their surfaces. The sun, again, generates heat so rapidly, owing to his great energy of contraction, and loses it so slowly, owing to his great size, that his surface is always kept in a state of incandescence. His surface-temperature is estimated at some three million degrees of Fahrenheit, and a diminution of his diameter far too small to be detected by the finest existing instruments would suffice to maintain the present supply of heat for more than fifty centuries.

These facts point to a very long future during which the sun will continue to warm the earth and its companion planets, but at the same time they carry on their face the story of inevitable ultimate doom. If things continue to go on as they have all along gone on, the sun must by and by grow black and cold, and all life whatever throughout the solar system must come to an end.

Long before this consummation, however, life will probably have become extinct through the refrigeration of each of the planets into a state like the present state of the moon, in which the atmosphere and oceans have disappeared from the surface. No doubt the sun will continue to give out heat a long time after heat has ceased to be needed for the support of living organisms. For the final refrigeration of the sun will long be postponed by the fate of the planets themselves.

The separation of the planets from their parent solar mass seems to be after all but a temporary separation. So nicely balanced are they now in their orbits that they may well seem capable of rolling on in their present courses forever. But this is not the case. Two sets of circumstances are all the while striving, the one to drive the planets farther away from the sun, the other to draw them all into it. On the one hand, every body in our system which contains fluid matter has tides raised upon its surface by the attraction of neighbouring bodies.

All the planets raise tides upon the surface of the sun and the periodicity of sun-spots (or solar cyclones) depends upon this fact. These tidal waves act as a drag or brake upon the rotation of the sun, somewhat diminishing its rapidity. But, in conformity with a principle of mechanics well known to astronomers, though not familiar to the general reader, all the motion of rotation thus lost by the sun is added to the planets in the shape of annual motion of revolution, and thus their orbits all tend to enlarge, -- they all tend to recede somewhat from the sun.

But this state of things, though long-enduring enough, is after all only temporary, and will at any rate come to an end when the sun and planets have become solid. Meanwhile another set of circumstances is all the time tending to bring the planets nearer to the sun, and in the long run must gain the mastery. The space through which the planets move is filled with a kind of matter which serves as a medium for the transmission of heat and light, and this kind of matter, though different in some respects from ordinary ponderable matter, is yet like it in exerting friction. This friction is almost infinitely little, yet it has a wellnigh infinite length of time to work in, and during all this wellnigh infinite length of time it is slowly eating up the momentum of the planets and diminishing their ability to maintain their distances from the sun. Hence in course of time the planets will all fall into the sun, one after another, so that the solar system will end, as it began, by consisting of a single mass of matter.

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The Unseen World and Other Essays by John Fiske, 1876