72 



place. The principle remains the same whether the outshooting 

 particle is directed normally or obliquely. In either case the 

 molecule pursues theoretically an elliptical path. It is obvious 

 that the more attenuated the atmosphere the more prevalent 

 these escapes outward, followed by elliptical flig^hts, must become. 

 Gravity is the only barrier ou the outer side and must ultimately 

 take the place of collisions altogether in turning the molecules 

 backward toward the earth. It is therefore inferred as a logical 

 necessity that the outer face of the coUisional atmosphere takes 

 on this phase of action fully. Dr. Stone long since appro- 

 priately styled this phase of action, "fountain-like." 



Logical analysis can not, however, stop here. The extent of 

 the elliptical flght of any molecule is measured by the velocity 

 it derived from the last encounter matched against the gravity 

 of the earth. If the gravity overcomes the inertia of the mole- 

 cule before it reaches the limit of the earth's control, it returns 

 to the collisional atmosphere; if not, it passes beyond the sphere 

 of the earth's control into that of the sun, and is lost, temporarily 

 at least, to the atmosphere of the earth. Now under the Boltz- 

 mann-Maxwell law of distribution of velocities, some molecules 

 must acquire velocities greater than the gravity of the earth 

 can control. In addition to the velocities derived from the col- 

 lisional atmosphere through the law of distribution, there are 

 special agencies of agitation which give to some molecules of 

 the external atmosphere exceptionally high velocities, and hence 

 cause tliem to make excursions into the outer fields of the earth's 

 control or pass entirely beyond it. Among these agencies are 

 the impacts of meteorites, the ionizing action of ultra-violet light, 

 the probable bombardment of the outer atmosphere by both 

 electrons and alpha particles from the sun, the probable infall 

 of molecules from the sun's atmosphere, the strokes of particles 

 driven by light pressure, etc. 



