We have thus considered the Laplacian hy pothesis somewhat fully because of its historical importance and because for so long a time it was regarded as furnishing a very probable out line of the successive steps by which our solar system developed from the original nebula into its present form. It should not be overlooked, however, that much less importance was as cribed to this theory by its originator than by his followers, who, attracted by its completeness and seeming simplicity, raised it almost to the position of a proved and fundamental doctrine.
Laplace himself, a master of rigorous mathe matical deduction, could not have been deceived as to the weakness of its foundation. Indeed, he put it forth as a speculation merely, which (in his own words) was advanced °with that distrust which everything ought to inspire that is not a result of observation or of calculation' Modern Hypotheses.— In all modern hy potheses as to the development of the solar system it is recognized at the outset that the original nebula must have been of a very het erogeneous structure. Its material in the begin ning may bave been arranged in a more or less spiral form, the curved branches extending outward from a common centre, and thus re sembling the thousands of spiral nebula; seen in the sky but whether this be so or not, it is ,assumed that there existed here and there 'throughout the cloud many smaller clouds, or knots, of the nebulous material, denser than the nebula immediately surrounding them. The original nebula is supposed to have been a great cloud, composed of meteorites or meteoric particles, cosmic dust, gases, and probably some matter in a radiant condition ; in short, to have had exactly the same constitution as we believe is possessed by the same class of nebulae which we see in the sky. The whole meteoric cloud thus constitutes a Resisting Medium, through which its separate particles and the denser clouds within it must move. The question concerning what such a heterogeneous cloud will develop into as it shrinks together under its own gravitation has recently received a great deal of attention from certain astron omers. The result of a most thorough mathe matical analysis is to indicate that the outcome must be a solar system more or less closely resembling our own. The analysis has been
applied, not only to a broad outline of the de velopment of the system as a whole, but also to the separate planet-satellite systems, the rings of Saturn, the retrograde or apparently too rapid motions of certain satellites, and even to the reason for the present situations and rotations of the planets, for all of which it seems to account in a satisfactory manner. The principal contributions to this subject will be found by the reader as follows: Thomas C. Chamberlain and F. R. Moulton, 'Year-book No. 3> of the Carnegie Institution of Washing ton ; The Astrophysical Journal (Vol. XXII, 1905) ; See, T. . 'Evolution of the Stellar System> (Vol. I, Lynn, Mass.), and numerous N papers in the Astronomische and the Journal of the American Philosophical So ciety. By the former authors, the new theory is called the Planetesimal Theory, the separate particles of the original nebula being called Planetesimals; by the latter it is named the Capture Theory, attention being thus drawn to the fact that the lesser bodies of the completed system, instead of having originally formed a part of the central body and having afterward separated from it, were actually drawn nearer to this body from without, and so in a sense °cap tured.* In the present article we can only indicate in briefest outline the successive steps of the development which the original nebula will undergo.
In the first place any separate particle or any cloud of parades in the original nebula will (unless there is no motion at all) find itself moving, not in empty space, but in a re sisting medium. It can readily be shown by mathematics that the certain effect of this re sistance will be to cause the particle to fall con tinually nearer the centre of the whole, gather ing up and adding to itself much of the material through which it moves, until finally, if the re sisting medium extends quite to the centre, the particle or cloud will fall onto the central mass and become a part of it. Thus the central mass will continually grow by the absorption of a great pay of the cloud around it, nor will this action cease until the resisting material has been quite completely swept away.