668 T. C. CHAMBERLIN 



those shot out at lesser velocities, or sidewise at various angles, 

 would have intermediate velocities. All would therefore be 

 moving in the same general direction as the nucleus and their 

 orbits would still be similar. The molecules in these new orbits 

 would therefore be planetesimals, because they would revolve 

 about the sun in orbits similar to those of the planets. The 

 kinetic theory of gases requires us to suppose that molecules of 

 the lighter gases escaped from the outer border of the earth- 

 nucleus with some degree of frequency while it was hot and diffuse, 

 and that such molecules have continued to escape from the outer 

 border of the earth's atmosphere ever since, but much less fre- 

 quently. And so of all other planets that have atmospheres, and 

 of the sun as well. Practically all the molecules that thus escaped 

 into orbits still remained within the sphere of control of the sun 

 and were liable in time to be picked up again, so that this whole 

 system of escape and recovery constitutes a mode of exchange of 

 atmospheric material between the domains of the sun and the 

 planets. It contributes to the maintenance and equilibrium of 

 our atmosphere as elsewhere set forth.^ 



Let us now look to the gathering in of the planetesimals in this 

 typical case, for that is the vital point here. Under the laws of 

 mechanics the planetesimals shot forth in this way would come 

 back to the virtual points of their escape at the end of each revolu- 

 tion in their new orbits, unless they were diverted by some inter- 

 vening influence. From this it is easy to jump to the conclusion 

 that they would all soon be picked up again by the nucleus, but 

 not so, in general. They were nearly all thrown into larger or 

 smaller orbits and that determined the time at which they should 

 get back to the point of their origin. In most cases this was either 

 earlier or later than the return of the nucleus and hence recapture 

 was avoided. Figure i and the accompanying periodic data, pre- 

 pared by Dr. MacMillan, make this very clear. From the point A 

 at the top of the figure let one molecule be shot forward at a speed 

 lo per cent greater than that of the nucleus and let another mole- 

 cule be shot backward so that its velocity shall be lo per cent less 

 than that of the nucleus. The first molecule will take the outer 



' The Origin of the Earth (1916), pp. 13-17. 



