HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 97 



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. 



•hj , , Molecular weights Average molecular velocities 



Moiecmes. (in round numbers). at 0° C. in cm. per sec. 



C0 2 44 33,259 



2 32 39,155 



N 2 28 41,735 



H 2 18 56,522 



H 2 2 169,611 



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. 



Carbon dioxide would be held some appreciable time before oxygen, 

 and still longer before nitrogen, and all these a notable time before 

 the vapor of water. The inference is that the initial atmosphere was 

 very rich in carbon dioxide, for an abundant supply was correlated 

 with a superior power of retention. 



The amount of nitrogen occluded in rocks and meteorites is rela- 

 tively small, and it was perhaps a small constituent of the early atmos- 

 phere. Owing to its chemical inertness, it may be supposed to have 

 been increasing ever since, and thus to have attained its present domi- 

 nance. A similar history may be assigned to the other, and even 

 more inert, elements, argon, neon, zenon, krypton, and helium, of 

 which the supplies seem to have always been very limited. 



After the earth acquired the power of holding water-vapor, the 

 supply being abundant, accession doubtless went on for a time as 

 fast as the capacity to hold increased. 



The amount of oxygen in the early atmosphere is more uncertain 

 from doubt as to the sources of supply. Crystalline rocks and meteor- 

 ites are not known to contain it in a free state. As above remarked, 

 it occurs among volcanic gases, but it is not known that it comes from 

 the deep interior. It is detected in the sun, and not improbably existed 

 in the nebula, from which it might have been gathered shortly after 

 the accretion of carbon dioxide began. The safer inference seems 

 to be that it was not very abundant, relatively, in the' very earliest 

 atmosphere There are two ways in which it might have been effectively 

 increased: (1) The impact of the planetesimals would be felt at their 



