389. _Laplace's Hypothesis._--Laplace was led to the nebular hypothesis by considering the remarkable uniformity in the direction of the rotation of the planets. Believing that this could not have been the result of chance, he sought to investigate its cause. This, he thought, could be nothing else than the atmosphere of the sun, which once extended so far out as to fill all the space now occupied by the planets. He begins with the sun, surrounded by this immense fiery atmosphere. Since the sum total of rotary motion now seen in the planetary system must have been there from the beginning, he conceives the immense vaporous mass forming the sun and his atmosphere to have had a slow rotation on its axis. As the intensely hot mass gradually cooled, it would contract towards the centre. As it contracted, its velocity of rotation would, by the laws of mechanics, constantly increase; so that a time would arrive, when, at the outer boundary of the mass, the centrifugal force due to the rotation would counterbalance the attractive force of the central mass. Then those outer portions would be left behind as a revolving ring, while the next inner portions would continue to contract until the centrifugal and attractive forces were again balanced, when a second ring would be left behind; and so on. Thus, instead of a continuous atmosphere, the sun would be surrounded by a series of concentric revolving rings of vapor. As these rings cooled, their denser materials would condense first; and thus the ring would be composed of a mixed mass, partly solid and partly vaporous, the quantity of solid matter constantly increasing, and that of vapor diminishing. If the ring were perfectly uniform, this condensation would take place equally all around it, and the ring would thus be broken up into a group of small planets, like the asteroids. But if, as would more likely be the case, some portions of the ring were much denser than others, the denser portions would gradually attract the rarer portions, until, instead of a ring, there would be a single mass composed of a nearly solid centre, surrounded by an immense atmosphere of fiery vapor. This condensation of the ring of vapor around a single point would not change the amount of rotary motion that had existed in the ring. The planet with its atmosphere would therefore be in rotation; and would be, on a smaller scale, like the original solar mass surrounded by its atmosphere. In the same way that the latter formed itself first into rings, which afterwards condensed into planets, so the planetary atmospheres, if sufficiently extensive, would form themselves into rings, which would condense into satellites. In the case of Saturn, however, one of the rings was so uniform throughout, that there was no denser