410 THOMAS C. CHAMBERLIN 
organization. They were subject to collision with one another, 
indeed, but their sparse distribution made the contingency less 
immanent than might easily be imagined. When collisions did 
occur, rebounds were more probable than mutual coherence; 
permanent unions were only probable when their relative speeds and 
other conditions were specially favorable. After union, they were 
liable to be driven apart again by succeeding collisions. Ultimate 
capture by the earth core was probable, but even in this case only 
as their orbits favored. ‘They were perturbed by all the attractions 
of the solar system. ‘These, on the whole, favored capture by the 
earth nucleus but not in all cases. The process of collection was 
intricate, indirect, and slow." 
63. As a step toward realizing the sparseness of the plane- 
tesimals and the time required for their collection, let them be 
supposed to stand still as at first distributed, while the earth 
nucleus, taken as a net 6,000 miles in diameter, sweeps through 
them at 18 miles per second. To expedite the work, let the path 
of this net be so shaped and shifted by some demon that it will clean 
up an entirely new swath at each revolution. Even then it would 
take about 100,000,000 years to sweep up all the planetesimals.’ 
64. To try, as a next step, the most rapid natural way, let the 
planetesimals act as though particles of a gas, collapsing on the 
track of the nucleus after each sweep—though that is far from 
their habit—and let each sweep, as before, clear a path 6,000 miles 
in diameter at the rate of 18 miles per second. It would then 
take about 260,000,000 years to gather in go per cent of the plane- 
tesimals; to sweep up all would require an indefinite period. 
65. The real case was much less favorable. The planetesimals 
and the nuclei were moving in the same general direction, at some- 
what similar speeds. They could thus come together, as a rule, 
only as one overtook the other, or as their paths converged, a 
relatively slow method. They attracted one another but this did 
not necessarily bring them together, for their orbits might become 
so adjusted to one another that they merely became traveling 
companions, like the earth and the moon. Mutual attractions 
1““The Intimations of the Planetesimal Mechanism,” Article XIII, Jour. Geol., 
Vol. XXVIII (1920), pp. 677-78. 
2 Ibid., p. 678. 3 [bid., p. 679. 
