FUNDAMENTAL' PROBLEMS OF GEOLOGY. 235 



than the moon and justifies the provisional conclusion that if the 

 young earth had no more than one-twentieth of its present mass it 

 probably possessed no atmosphere of appreciable geological efficienc}-, 

 but that when it had gained one-tenth of its present mass (radius 

 probably about 2, 100 miles) an appreciable, though relatively slight, 

 atmosphere surrounded it. 



When the growing earth reached a 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. 



By hypothesis, all the atmospheric and hydrospheric material of 

 the parent nebula which was not gathered into the aggregated plan- 

 etesimals remained as free-molecular planetesimals. While the plan- 

 etary nucleus was small it probably 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 the}' did not 

 form an atmosphere ; but when the growing earth reached the requi- 

 site mass these free atmospheric molecules were gathered about it and 

 retained as an atmospheric envelope. This would be a more abun- 

 dant source of suppl}' during the nebular stages than afterward, 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. 



In the later stages of organization, and thence down to the present 

 time, the molecules discharged from all the bodies of the solar system 

 were possible sources of atmospheric accretion. Of these the most 

 important were probably volcanic and similar discharges from the 

 small bodies that could not hold gases permanently and discharges 

 from the sun by virtue of the enormous explosive and radiant energies 

 that are there resident. 



As the planetesimals were gathered into the growing earth-nucleus 

 they carried their occluded gases in with them, except as the super- 

 ficial portion might be set free by the heat of impact. There was 

 thus built into the growing earth atmospheric material. So, also, 

 while the nucleus was growing it was subjected to the bombard- 

 ment 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 molecules as were driven into the 



