CHAPTER XVI.

THE NEBULAR HYPOTHESIS.

The Arrangement of the Solar System. -The Phenomena for which Gravitation is responsible.-The Phenomena remaining to be accounted for.-Nebulous Matter as found in Comets.-Nebulous Matter possibly in the Heavens.-The Entire Solar System once a Globe of Nebulous Matter.-Motion of Rotation.-Radiation of Heat.-Condensation and its Effects.-Rings disengaged from the Equator of the Revolving Mass. -Formation of Planets and of Satellites.

In our examination of the scheme of worlds which revolve around the sun, we have found that the orbits of the planets are all nearly circular, that their planes are all nearly coincident with the plane of the ecliptic, and that this plane is nearly coincident with the plane of the sun's equator; that the planets all revolve in the same direction around the sun, and that the sun and planets and satellites all rotate on their axes in the same direction; that the periods of revolution grow shorter in the planets and satellites as their distances from their primary grow less; that the sun rotates on his axis in a shorter period than that employed in the revolution of any planet; that every planet accompanied by satellites rotates on its axis in a less time than the period of revolution of any satellite. The law of gravitation is not responsible for any of these facts; and in case we compute the chances of such an organization coming into being by accident, we shall find but one chance in so many millions, that we are compelled to look to some higher cause than mere accident to account for so great a multitude of combined phenomena.

We have said that gravitation is not responsible for the facts above stated. In case a solitary planet be projected with a given force, and in a given direction about the sun, and at a given distance, it will revolve, as we have seen, in

one of four curves, and in any one of these curves it will be held equally by the law of gravitation. The plane in which it revolves may assume any angle with a fixed plane, the direction of the revolution may be the same or contrary to that in which the sun rotates, the orbit may be a circle, an ellipse, a parabola, or an hyperbola; and yet the planet shall revolve, subject to the law of gravitation. It may rotate on its own axis either with or against its revolution in its orbit; and in case we give to this planet a satellite, the same statements are true with reference to this attendant. So that, so far as the law of gravitation is concerned, there might have been among the planets all the diversity in the form of their orbits, in the angles of their inclination to a fixed plane, and in the direction of their motions, as are found among the comets, and yet each object would have been subject to the great law of universal gravitation.

We cannot, therefore, affirm that the peculiar structure of the solar system results from the laws of motion and gravitation, without pre-supposing a condition of matter entirely different from that now recognized as existing in the planets and their satellites. We have already noticed the wonderful constitution of the comets. In these bodies is found a kind of matter which has been termed nebulous, in which the minute particles are separated by some repulsive force, and the entire mass is but a vapour of the most refined tenuity.

Among the stellar regions the telescope has revealed objects whose light is so faint and whose forms are so ill defined, that they have been regarded by many astronomers of high reputation to be analogous to the comets in their material, exhibiting the primitive or primordial condition of the matter composing the physical universe. This conjecture (for it is nothing more) may be true or false; but its truth or falsehood cannot in any way affect the credibility of the theory or hypothesis we are about to present. The present condition of matter cannot in any way be assumed to be the only condition in which it ever existed, since we

now know it to be subject to extraordinary changes and most wonderful modifications.

Let us, then, suppose that a time once was, when the sun and all its planets and their satellites existed as one mighty globe of nebulous matter, whose diameter far exceeded the present diameter of the orbit of Neptune; that to this stupendous globe a motion of rotation was given; and that its heat is slowly lost by radiation; and let us endeavour to follow the changes which must flow from the loss of heat and the operation of the laws of motion and gravitation, and learn whether from this parent-mass a scheme of planets and satellites such as now exist can be generated. We prefer to present the reasoning in the language of M. Pontecoulent, one of the most eminent of the illustrious disciples of Newton -merely premising that, in case the central rotating mass contracts by loss of heat, a time must come when, in consequence of the increased velocity of rotation, the force of gravity of a particle at the equator will be overcome by the centrifugal force generated by the velocity of rotation; and hence flat zones or rings of vapour or nebulous matter must eventually be formed in the plane of the equator of the revolving globe :

"These zones must have begun by circulating round the sun in the form of concentric rings, the most volatile molecules of which have formed the superior part, and the most condensed the inferior part. If all the nebulous molecules of which these rings are composed had continued to cool without disuniting, they would have ended by forming a liquid or solid ring. But the regular constitution which all parts of the ring would require for that, and which they would have needed to preserve whilst cooling, would make this phenomenon extremely rare. Accordingly, the solar system presents only one instance of this, that of the rings of Saturn. Generally the ring must have broken into several parts, which have continued to circulate round the sun, and with almost equal velocity, while at the same time, m consequence of their separation, they would acquire a

rotatory motion round their respective centres of gravity; and as the molecules of the superior part of the ring, that is to say, those furthest from the centre of the sun, had necessarily an absolute velocity greater than the molecules of the inferior part which is nearest it, the rotatory motion, common to all the fragments, must always have been in the same direction as the orbitual motion.

This

"However, if, after their division, one of these fragments has been sufficiently superior to the others to unite them to it by its attraction, they will have formed only a mass of vapour, which, by the continual friction of all its parts, must have assumed the form of a spheroid flattened at the poles and elongated in the direction of its equator. Here, then, are rings of vapour left by the successive retreats of the atmosphere of the sun, changed into so many planets in the condition of vapour circulating round the sun, and possessing a rotatory motion in the direction of their revolution. must have been the most common case; but that in which the fragments of some ring would form several distinct planets possessing degrees of velocity must also have taken place, and the telescopic planets discovered during the present century seem to present an instance of this; at least if it is not admitted, with Olbers, that they are the fragments of a single planet, broken by a strong interior commotion. It is easy to imagine the successive changes produced by cooling on the planets whose formation has been just pointed out. Indeed, each of these planets, in the condition of vapour, is, in every respect, like one of the nebulæ in the first stage; they must, therefore, before arriving at a state of solidity, pass through all the stages of change we have just traced in the sun. At first, the condensation of their atmosphere will form round the centre of the planet a body composed of layers of unequal density, the densest matter having, by its weight, approached the centre, and the most volatile reached the surface, as we see in a vessel different liquids ranged one above another, according to their specific gravity to arrive at a state of equilibrium. The

atmosphere of each planet will, like that of the sun, leave behind it zones of vapour, which will form one or several secondary planets, circulating round the principal planet as the moon does round the earth, and the satellites round Jupiter, Saturn, and Uranus; or else they form, by cooling without dividing, a solid and continuous circle, of which we have an instance in the ring of Saturn. In every case the direction of the rotatory and orbitual motion of the satellites or the ring will be the same as that of the rotatory motion of the planet; and this is completely confirmed by observation.

"The wonderful coincidence of all the planetary motions (a phenomenon which we cannot, without infringing the laws of probability, regard as merely the effect of chance) must, then, be the result even of the formation of the solar system on this ingenious hypothesis; we see also why the orbits of the planets and satellites are so little eccentric, and deviate so little from the plane of the solar equator. A perfect harmony between the density and temperature of their molecules in a state of vapour would have rendered the orbits rigorously circular and made to coincide with the plane of this equator; but this regularity could not exist in all parts of such large masses; there has resulted the slight eccentricities of the orbits of the planets and satellites, and their deviation from the plane of the solar equator.

"When in the zones abandoned by the solar atmosphere there are found molecules too volatile either to unite with each other or with the planets, they must continue to revolve round the sun, without offering any sensible resistance to the motions of the planetary bodies, either on account of their extreme rarity, or because their motion is effected in the same way as that of the bodies they encounter. These wandering molecules must thus present all the appearances of the zodiacal light.

"We have seen that the figure of the heavenly bodies was the necessary result of their fluidity at the beginning of time. The singular phenomenon presented by the rigorous equality