portion to attract the rest around it; and thus the ring of Saturn retained its annular form. [Illustration: Fig. 456.] Such is the celebrated nebular hypothesis of Laplace. It starts, not with a purely nebulous mass, but with the sun, surrounded by an immense atmosphere, out of which the planets were formed by gradual condensation. Fig. 456 represents the condensing mass according to this theory. 390. _The Modern Nebular Hypothesis._--According to the nebular hypothesis as held at the present time, the sun, planets, and meteoroids originated from a purely nebulous mass. This nebula first condensed into a nebulous star, the star being the sun, and its surrounding nebulosity being the fiery atmosphere of Laplace. The original nebula must have been put into rotation at the beginning. As it contracted and became condensed through the loss of heat by radiation into space, and under the combined attraction of gravity, cohesion, and affinity, its speed of rotation increased; and the nebulous envelop became, by the centrifugal force, flattened into a thin disk, which finally broke up into rings, out of which were formed the planets and their moons. According to Laplace, the rings which were condensed into the planets were thrown off in succession from the equatorial region of the condensing nebula; and so the outer planets would be the older. According to the more modern idea, the nebulous mass was first flattened into a disk, and subsequently broken up into rings, in such a way that there would be no marked difference in the ages of the planets. The sun represents the central portion of the original nebula, and the comets and meteoroids its outlying portion. At the sun the condensation is still going on, and the meteoroids appear to be still gradually drawn in to the sun and planets. The whole store of energy with which the original solar nebula was endowed existed in it in the potential form. By the condensation and contraction this energy was gradually transformed into the kinetic energy of molar motion and of heat; and the heat became gradually dissipated by radiation into space. This transformation of potential energy into heat is still going on at the sun, the centre of the condensing mass, by the condensation of the sun itself, and by the impact of meteors as they fall into it. It has been calculated, that, by the shrinking of the sun to the density of the earth, the transformation of potential energy into heat would generate enough heat to maintain the sun's supply, at the present rate of dissipation, for seventeen million years. A shrinkage of the sun which would generate all the heat he has poured into space since the invention of the telescope could not be detected by the most powerful instruments yet constructed.