GEOLOGY. 327 



its ability to absorb and re-radiate a certain part of terrestrial radiation need 

 be considered. 



Estimates of the total absorbing capacity of the CO2 of the atmosphere 

 have varied rather widely, but 14 per cent of the terrestrial radiation (Abbot 

 and Fowle)* may be taken as a conservative figure. The points here urged 

 are not especially affected by the closeness of this to perfect accuracy. It 

 has been urged^ that much less than the total amount of CO2 in the air can 

 absorb all the terrestrial radiation susceptible of absorption by CO2 and that 

 no additional amounts of CO2 in the air would absorb a larger percentage. 

 This seems to imply that the 14 per cent absorbed is all of the terrestrial 

 radiation whose wave-lengths are of the class susceptible of absorption by 

 CO2; otherwise additional CO2 would absorb them. As 14 per cent appears 

 an entirely reasonable amount, this view is accepted as a present working 

 basis. From this primary absorption a secondary series of absorptions and 

 radiations take their origin. The total value of the thermal work of CO2 in 

 the atmosphere includes this secondary series of absorptions as well as the 

 primary absorption. 



On the assumption that there is more than enough CO2 in the air to absorb 

 all the wave-lengths in the terrestrial radiation susceptible of absorption by 

 it, the first absorptions would obviously take place at levels below the upper 

 limit of CO2 and, if there were much more than enough CO2, at levels com- 

 paratively near the earth's surface. If a column of 50 centimeters of pure 

 CO2 at mean temperature and pressure can completely absorb all terrestrial 

 radiation that any amount of CO2 can absorb, and if the CO2 in the atmosphere 

 is equivalent to a column of pure gas 250 centimeters high, or proportions of 

 any such order as this, the first absorption would be complete at a low level 

 in the atmosphere. The mean level of this first absorption is the mean from 

 which the secondary series of radiations and absorptions start and thus 

 determines their chances of prolonged action in the atmosphere, for the first 

 set of the secondary series of radiations and absorptions is followed by still 

 another set of radiations and absorptions, and so on indefinitely until all these 

 secondary radiations escape from the atmosphere or are otherwise disposed of. 

 All these radiations of CO2 will have the CO2 wave-lengths and be readily 

 absorbed by other molecules of CO2. The successive sets of radiations will 

 presumably decline in thermal value as the repetition goes on, but the rate 

 at which they will decline will depend on the amount of CO2 (or other 

 absorbent and radiator of these wave-lengths if there be such) in the 

 atmosphere. To appreciate the full realities of the case, it is necessary to 

 carry the analysis more into detail. 



Since each of the molecules of CO2 that absorbs terrestrial radiation re- 

 radiates the energy thus acquired in all directions, these radiations encounter 

 different contingencies in different directions. For a representative case, 

 let a molecule be chosen that vibrates at the mean height of its class. The 

 excess of molecules that were not able to take part in the absorption of 

 terrestrial radiation of course lie mainly at high levels. Picture this repre- 

 sentative molecule as at the center of an imaginary cube, so that its radiations 

 will be tangibly and conveniently grouped according as they issue from one 

 or another of its six sides. The one sixth of the radiation directed skyward 



* Annals Astrophys. Obs. Smithsonian Inst., vol. ii, p. 172 (1908). 

 » W. J. Humphrey, Physics of the air, p. 567 (1920). 



