670 T. C. CHAM BERLIN 



with one another until they return to the vicinity of A . But one 

 of them would reach A two hundred and thirty-nine days earlier 

 than the other. Even in this specially favorable case when they 

 had a common node there was no immediate opportunity for union. 

 Of course cases of less divergence could be chosen in which there 

 was a nearer coincidence of orbits and of time schedules, and if 

 there were many orbits there would be some real crossings farther 

 from the nucleus, but those chosen illustrate the prevalent fact 

 that even though such bodies have similar orbits and sometimes 

 actual crossings, they may yet remain independent for long periods. 

 Their mutual attractions would, in general, aid in bringing this 

 about ultimately, but instead of this they might be brought into 

 co-ordinate orbits like those of the earth and moon and revolve 

 together in harmony indefinitely. At best the process was likely 

 to be a very slow one. The picture of molecules drawn directly 

 together, as in the case of static bodies or of gases, is very com- 

 monly substituted for the real case, and is very misleading. When 

 all possible cases are considered, as well as the multitude of planet- 

 esimals, there are enough chances of collision and coalescence, 

 especially with the nuclei, to make the process of ingathering effect- 

 ive in the course of long periods, but in its very nature it cannot 

 be a speedy process. When planetesimal molecules or even 

 precipitate aggregates collide, rebound would be more likely to 

 follow than coalescence, unless they were electrically charged. 

 Coalescence almost inevitably follows collision with nuclei but not 

 encounters between planetesimals. 



This simple example of the evolution and behavior of planet- 

 esimals illustrates the mechanism by which they are maintained 

 and the contingencies of their capture or their mutual coalescence, 

 where the conditions are exceptionally favorable. For the case 

 most important in the formation of the earth, we must turn to the 

 solar molecules which were driven directly into orbits by the original 

 propulsion from the sun under the stimulus and attraction of the 

 co-operating body. These were subject to the law of return to 

 the points of their origin, but they were greatly diverted by the 

 pull of the co-operating body and so largely lost all such systematic 

 relations to a given nucleus as those that made the previous case 



