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. 
“Tf” 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? This notion of a Zuriickstossungskraft, 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. 
