formations which are the seeds of the future planets. For the particles, as they move round the sun in parallel circles and at not too great a difference of distance from the sun, are, by the equality of their parallel motion, almost at rest with respect to one another, and thus the attraction of those particles which are of a higher specific attraction immediately produces an important effect, namely, the collection of those nearest one another so as to form a body which, in proportion to the growth of its mass, extends its attraction farther and draws elements from a wide region to unite with it in its further formation."
It must be left to mathematicians and astronomers to assess the precise merits of these speculations in comparison with those of Kant's predecessors and successors in the same undertaking. But as to the historic affinities of Kant's hypothesis the facts seem so clear that even a layman may pronounce upon them. The Kantian scheme is as different from Laplace's as any post-Newtonian cosmogony could well be. For it does not start with a gaseous, rotating, heated nebula; it does not explain the direction of revolution and rotation of the planets as derived from the rotation of a mass formerly cohering with that now constituting the sun; it does not regard the planets as having ever formed part of any such mass. It is well-known that the rings of Saturn suggested the most characteristic feature of Laplace's theory. Kant has a chapter explaining these rings much as Laplace does; but he expressly insists that "the ring which surrounds Saturn was not acquired in the general way, nor has been produced by the universal laws of formation which have ruled the whole system of the planets." On the other hand, it is not quite exact to identify (as does Hastie[1]) Kant's system of planetary evolution with the meteoritic hypothesis of Lockyer and G. H. Darwin. So far as I understand these matters, Kant's cosmogony most nearly resembles an extremely recent doctrine upon the subject—the planetesimal hypothesis of Chamberlin and Salisbury. In the words of those authors:
Under the typical form of that hypothesis it is assumed that the parent nebula of the solar system is formed of innumerable small bodies, planetesimals, revolving about a central gaseous mass much as the planets do today. The evolution of the system consisted in the aggregation of these innumerable small bodies into much fewer large ones. . . . The hypothesis, therefore, postulates no fundamental change in the system of dynamics after the nebula was once formed, but only an assemblage of the scattered material. The state of dispersion of the material at the outset, as now, was maintained by orbital revolution, or, more closely speaking, by the centrifugal acceleration arising from revolution.[2]
- ↑ "Kant's Cosmogony," 1900, p. lxxxiv. At this date, of course, the planetesimal type of hypothesis had hardly been differentiated from the meteoritic.
- ↑ Chamberlin and Salisbury, "Geology," 1906, II., p. 38. The authors of this theory have failed to recognize in Kant an early prophet of their own doctrine, and have referred to him, in the conventional manner, as having held a hypothesis "somewhat similar" to Laplace's (op. cit., p. 4).
