94 GEOLOGY. 



mass sufficient to control the flying molecules of atmospheric material, 

 there were two sources from which these could be supplied for the 

 accumulation of an atmosphere, an external and an internal one. 



1. External source of supply. — By hypothesis, all the atmospheric 

 and hydrospheric material of the parent nebula which was not gathered 

 into the aggregated planetesimals remained as free-molecular planet- 

 esimals. While the planetary nucleus was small, it could not gather 

 and hold the lighter molecules, even when they collided with it, except 

 as this was done by occlusion or surface tension, in which case they 

 did not form an atmosphere. But when the growing earth reached 

 the requisite mass, these free atmospheric molecules were gathered 

 about it and retained as an atmospheric envelope. This would be a 

 more abundant source of supply during the nebular stages than after- 

 wards, but, by hypothesis, it continues to be a source of some supply 

 even to the present time, for the very doctrine that postulates the loss 

 of such high-speed molecules implies their presence in space subject to 

 capture by bodies capable of capturing them. 



The planetesimals originated, by hypothesis, from gaseous matter 

 shot forth from the ancestral sun. Those portions which condensed 

 hux) aggregated planetesimals probably occluded within themselves 

 some atmospheric material, much as meteorites and crystalline rocks 

 of the earth do. These planetesimals were doubtless highly heated 

 on striking the earth's surface, or on entering its atmosphere after 

 it began to gather one, and were thus probably forced by the acquired 

 beat to give up a part of their gases, as meteorites and rocks do when 

 heated. These expelled gases would be an additional source of supply; 

 but the planetesimal material on cooling might be disposed to re-ab- 

 sorb gases, and the final result might be little or no gain; at least this 

 seems an uncertain source of supply. 



2. Internal source of supply. — As the planetesimals were gathered 

 into the growing earth-nucleus, they carried their occluded gases in 

 with them, except as the superficial portion might be set free by the 

 heat of impact. There were thus built into the growing earth atmos- 

 pheric materials. So also, while the nucleus was growing, it was sub- 

 jected to the bombardment of free molecular planetesimals of the 

 atmospheric substances. In its early stages it might not be able to 

 hold these as a free gaseous envelope, but to a certain extent, it could 

 hold, by virtue of capillary and subcapillary attraction, such mole- 



