There are, of course, very material differences between the contemporary and the Kantian form of the hypothesis; notably, our contemporary geologists ascribe "the gathering of the planetesimals to the nuclei, to form the planets, essentially to conjunctions in the course of their orbital motions, not," as does Kant, "to simple gravitation, except as gravitation was the fundamental cause of the orbital motions." But in the two cardinal points Kant's is a planetesimal theory: (1) it conceives the planets to have grown by gradual accretions from very small nuclei, not to have been condensed from large masses "abandoned" or thrown off by a rotating, gaseous sphere; (2) it also conceives these nuclei to have been in regular orbital revolution about a central body before the formation of planets as such. The first trait distinguishes both the planetesimal and the meteoritic hypotheses from the general type of theory to which the conjectures of Swedenborg, Buffon and Laplace alike belong; the second is the specific mark differentiating the planetesimal hypothesis in turn from the meteoritic. "If," in the words of Chamberlin and Salisbury, "the meteorites could be supposed to come together so as to revolve in harmonious orbits about a common center, on a planetary basis, the assemblage might be perpetuated, but this takes the case out of the typical meteoritic class, and carries it over to the planetesimal." It is precisely this that we find exemplified in the third stage of the Kantian cosmogony.
Whether, in view of the state of knowledge in his time, Kant had any good reasons for preferring his theory to those of the other type which Swedenborg and Buffon had already put forward, I shall not venture to discuss. In any case, the features of Kant's cosmogony which establish its kinship with the planetesimal hypothesis are closely connected with one of the most elusive and most questionable details of his system of dynamics—namely, his "force of repulsion." It is this and this alone which (to his mind) explains why particles, as they fall towards the center of attraction, are "deflected sideways" and thus have their rectilinear motion converted into movement of revolution. It is likewise the establishment of an equilibrium between repulsive and attractive forces that, as he conceives, gives shape and determinate limits of size, not only to planets, but to all coherent and individuated masses of matter.[1] This notion of a Zurückstossungshraft, which he took over from Newton, but the use of which to explain revolutional motion Newton would never have sanctioned, was a favorite one with Kant from the beginning of his career to the end; he reverts to it so late as 1786, in his "Metaphysical Foundations of Natural Science." It is in the "Physical Monadology," 1756, that we get the most definite account of it. We there learn the quantitative formula for this force, when acting between any two bodies; while attraction decreases in proportion to the square of the distance, repulsion decreases
- ↑ "Monadologia Physica," X.
