FUNDAMENTAT, PROBLEMS OF GEOLOGY. 237 



oxide (which, from the analogy of igneous rocks and meteorites, 

 may be presumed to have abounded in the earth material) and there- 

 with form water. The gases extruded from the interior should 

 therefore have been largely water-vapor and the carbon oxides, with 

 minor quantities of hydrocarbons and nitrogen. To these might be 

 added such chlorine, sulphur, and other temporary gases as the vola- 

 tile ingredients of the rock material might contribute through vol- 

 canic action ; but these chemically vigorous constituents would 

 doubtless soon disappear by union with the rock material. It is 

 probable that carbon monoxide would pass into carbon dioxide, as 

 it does not now accumulate in the atmosphere, although abundantly 

 produced. The marsh gas also disappears in some way. 



The material of internal derivation available for the atmosphere, 

 therefore, embraced chiefly water-vapor, carbon dioxide, and nitro- 

 gen. Oxygen is now given forth in some abundance by volcanoes, 

 but it is not known whether it really comes from the interior or has 

 merely been carried down from the surface. The reduction of ferric 

 oxide under certain conditions (the reverse of the process by which 

 water is assumed to have been found) might possibly give free oxygen. 



The material of external derivation might probably embrace all 

 the atmospheric constituents, but in proportions unknown. 



In determining the actual proportions of the constituents of the 

 earlv atmosphere, the abundance of the suppl)' was probably less 

 decisive than the power of the earth to hold the individual gases. 

 As gravity graduallj^ increased b}^ the growth of the earth from an 

 incompetent minimum, its power to control the heaviest molecules 

 with the lowest velocities was acquired before its ability to hold the 

 lighter ones of higher velocities. According to the kinetic theory, 

 molecular velocities vary inversely as the square root of the molecu- 

 lar weights. Assuming this to be correct, the leading constituents 

 would be held in the following order, it being noticed that molecules, 

 not atoms, must be dealt with : 



Molecules. 



CO, 



o,. 



N,.... 

 H,0. 

 H,... 



The commingling of the gases introduced some modifications of 

 the limitations of retention, and these were favorable to the lighter 

 gases ; but the refinements of the case are of no moment here. 



