The comets, Laplace assumed, probably immigrated at a later period into our solar system when the condensation had already advanced so far that the chief mass of the nebular matter had disappeared from interplanetary space. Chamberlin and Moulton have attempted to show that the difficulties of the hypothesis of Laplace may be obviated by the assumption that the solar system has evolved from a spiral nebula, into which strange bodies intruded which condensed the nebular mass of their surroundings upon themselves. We have pointed out examples of how the nebula seems to vanish in the vicinity of the stars, which would correspond to growing planets, located in nebulæ. In concluding this consideration, we may draw a comparison between the views which were still entertained a short time ago and the views and prospects which the discoveries of modern days open to our eyes. Up to the beginning of this century the gravitation of Newton seemed to rule supreme over the motions and over the development of the material universe. By virtue of this gravitation the celestial bodies should tend to draw together, to unite in ever-growing masses. In the infinite space of past time the evolution should have proceeded so far that some large suns, bright or extinct, could alone persist. All life would be impossible under such conditions. And yet we discern in the neighborhood of the sun quite a number of dark bodies, our planets, and we may surmise that similar dark companions or satellites exist in the vicinity of other suns and stars; for we could not understand the peculiar to-and-fro motions of those stars on any other view. We further observe that quite a number of small celestial bodies rush through space in the shapes of meteorites or shooting-stars which must have come to us from the most remote portions of the universe. The explanation of these apparent deviations from what we may regard as a necessary consequence of the exclusive action of gravity will be found under two heads—in the action of the mechanical radiation pressure of light, and in the collisions between celestial bodies. The latter produce enormous vortices of gases about nebular structures in the gaseous condition; the radiation pressure carries cosmical dust into the vortices, and the dust collects into meteorites and comets and forms, together with the condensation products of the gaseous envelope, the planets and the moons accompanying them. The scattering influence of the radiation pressure therefore balances the tendency of gravitation to concentrate matter. The vortices of gases in the nebulæ only serve to fix the position of the dust, which is ejected from the suns through the action of the radiation pressure.