[Sidenote: Limit of the theory of gravitation.] The theory of gravitation, as delivered by Newton, thus leads us to a knowledge of the mathematical construction of the solar system, and inferentially likewise to that of other systems; but it leaves without explanation a large number of singular facts. It explains the existing conditions of equilibrium of the heavenly bodies, but it tells us nothing of their genesis; or, at the best, in that particular it falls back on the simple fiat of God. [Sidenote: Phenomena of the solar system.] The facts here referred to conduct us, however, to another and far higher point of view. Some of them, as enumerated by Laplace, are the following:--1. All the planets and their satellites move in ellipses of such small eccentricity that they are nearly circles; 2. The movements of the planets are in the same direction and nearly in the same plane; 3. The movements of the satellites are in the same direction as those of the planets; 4. The movements of rotation of these various bodies and of the sun are in the same direction as their orbitual motions, and in planes little different. [Sidenote: The nebular hypothesis.] The nebular hypothesis requires us to admit that all the ponderable material now constituting the various bodies of the solar system once extended in a rarefied or nebulous and rotating condition, beyond the confines of the most distant planet. That postulate granted; the structure and present condition of the system may be mathematically deduced. For, as the vast rotating spheroid lost its heat by radiation, it contracted, and its velocity of rotation was necessarily increased; and thus were left behind from its equatorial zone, by reason of the centrifugal force, rotating rings, the same result occurring periodically again and again. These rings must lie all in one plane. They might break, collapsing into one rotating spheroid, a planet; or into many, asteroids; or maintain the ring-like form. From the larger of these secondary rotating spheroids other rings might be thrown off, as from the parent mass; these, in their turn breaking and becoming spheroids, constitute satellites, whose movements correspond to those of their primaries. We might, indeed, advance a step farther, and show how, by the radiation of heat from a motionless nebula, a movement of rotation in a determinate direction could be engendered, and that upon these principles, the existence of a nebulous matter admitted, and the present laws and forces of nature regarded as having been unchanged, the manner of origin of the solar system might be deduced, and all those singular facts previously alluded to explained; and not only so, but there is spontaneously suggested the cause of many minor peculiarities not yet mentioned.