312 HISTORICAL GEOLOGY 



as a planet, it may have had no atmosphere. If that was true, 

 the great processes of erosion could not then have been in 

 operation, since they are largely dependent upon the pres- 

 ence of an atmosphere. 



FIG. 315. Spiral nebula. (Photograph by Ritchey at the Yerkes 

 Observatory.) 



Gases, and therefore air, have a marked tendency to expand 

 and diffuse themselves through space. It is known that the 

 earth is able to keep its atmosphere only because of its strong 

 gravity, and that a few gases, notably free hydrogen, are too 

 active to be held by it. Since in small bodies gravity is much 



