GEOLOGY. 329 



if the water- vapor is a real substitute for CO2, it should increase the value of 

 the secondary series just as if it were so much additional CO2. 



So far as the initial absorption is concerned, it will be a matter of chance 

 whether a molecule of CO2 or of H2O first catches any given ray in its out- 

 going path. Complete absorption of so much as can be absorbed will ob- 

 viously take place all the sooner and all the nearer the earth than if CO2 

 alone did the work. Hence, the secondary series of absorptions and radiations 

 will begin closer to the surface and be the more likely to be prolonged because 

 of the greater depth of absorbents above. Thus, while the CO2 will be antic- 

 ipated in a part of the work of first absorption, it is merely a case of apparent 

 suffering from competition, in which the heating service gets the benefit of 

 two agencies instead of one. The carbon dioxide is not robbed of its function 

 but merely forced to effect it in a different way. 



Still another mode of prolonging the activity which starts in the form of 

 CO2 wave-length springs from the relations of CO2 to the nitrogen group in 

 the air. Each molecule of CO2 which has been heated by absorption of 

 terrestrial radiation is forced to collide with its neighbors several billion 

 times a second, according to standard computations. The colliding mole- 

 cules necessarily exchange energies. A part of the energy takes the form of 

 translatory motion and is distributed so rapidly and intricately that no 

 particular type or unit can be followed further. But it has become a factor 

 in the temperature of the air and can not be radiated away until it has become 

 a factor in the internal mechanism of an atom or molecule. It will suffice, 

 therefore, to follow the energy that remains embodied in the atomic or 

 molecular mechanism, for, notwithstanding their contribution to the common 

 transitory activities, each molecule still has its own internal activities, and 

 it is through these alone that radiation and related actions take place. The 

 nature of the thermal exchange which occurs when a molecule of CO2 collides 

 with a molecule of nitrogen depends on which molecule has the higher temper- 

 ature. In an average case near the earth's surface the molecule of CO2 should 

 be the warmer, for the processes that generate CO2 are exothermal, such as 

 the decay of vegetable matter, the respiration of animals, the combustion of 

 fuel, volcanic action, etc. Carbon dioxide thus habitually enters the atmos- 

 phere with an excess of genetic heat, while it is likely to have at least as 

 favorable contacts with the heated earth as nitrogen. As the air rises, the 

 nitrogen group have no further earth contacts, but the CO2 is constantly 

 absorbing terrestrial radiation and should be warmer than the non-absorbers 

 so long as the earth is a source of heating, which is the normal state. 

 A part of the excess of heat of CO2 is therefore conveyed to the molecules of 

 the nitrogen group, and as these have little power of radiating this increment 

 of heat away it is pocketed, as it were. Normally, its escape is delayed until 

 it is withdrawn again by a radiating molecule or until some other of the 

 limited and indirect means of escape available to this class becomes available. 

 The molecules of the nitrogen group thus serve as temporary pockets, lockers, 

 or reservoirs into which the CO2 sends a part of the excess of heat it absorbs 

 from the terrestrial radiation. Since the molecules of the nitrogen group 

 vastly outnumber the molecules of CO2, the presumption is that a large 

 fraction of the excess of heat gained by the latter from terrestrial radiation 

 will be conveyed by collisional action to the former and will be retained in 

 thermal activity longer than if it had taken the secondary radiative courses 

 sketched above. 



