96 - GEOLOGY. 



The gases chiefly occluded in meteorites and in the crystalline 

 rocks (Vol. I, pp. 619-620) are hydrogen, carbon dioxide, carbon mon- 

 oxide, in leading amounts, and marsh-gas and nitrogen, in small quan- 

 tities. It is assumed that the gases of the aggregated planetesimals, 

 and hence those of the interior of the early earth , were of the same 

 order of abundance. There is experimental ground for believing that, 

 at the right temperatures and pressures, hydrogen would take oxygen 

 from ferric oxide (which, from the analogy of igneous rocks and mete- 

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

 therewith 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 volatile 

 ingredients of the rock-material might contribute through volcanic 

 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. 



Summary of available material. — The material of internal deriva- 

 tion available for the atmosphere, therefore, embraced chiefly water- 

 vapor, carbon dioxide, and nitrogen. 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 formed) 

 might possibly give free oxygen. 



The material of external derivation might probably embrace all 

 the atmospheric constituents, but in proportions unknown. 



The actual proportions of the atmospheric gases determined by the 

 earth's gravity. — In determining the actual proportions of the con- 

 stituents of the early atmosphere, the abundance of the supply was 

 probably less decisive than the power of the earth to hold the individual 

 gases. As the gravity gradually increased by the earth's growth from 

 an incompetent minimum, its power to control the heaviest mole- 

 cules 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- 



