55° THE POPULAR SCIENCE MONTHLY 



There are, of course, very material differences between the contem- 

 porary and the Kantian form of the hypothesis; notably, our contem- 

 porary 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 " aban- 

 doned" or thrown off by a rotating, gaseous sphere; (2) it also con- 

 ceives 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 differ- 

 entiating 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 con- 

 nected 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. 17 This notion of a ZuriicTcstossungshraft, 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 



17 " Monadologia Physica," X. 



