326 CARNEGIE INSTITUTION OF WASHINGTON. 



atures. The spectrum of nitrogen shows no lines of selective absorption or 

 emission within the range of temperature of the lower atmosphere. Any 

 ability that nitrogen may have to absorb such short vibrations as penetrate 

 the uppermost atmosphere only is not here considered. Oxygen and argon 

 are here classed in the same group but only with some reservations, for oxygen 

 and argon have slight powers of absorption and radiation under ordinary 

 atmospheric conditions. The oxygen derivative, ozone, is more notably 

 absorbent of solar radiation, but it is essentially confined to the little-known 

 tracts of the higher atmosphere, which are outside the scope of this report. 

 It is not important to the merely general analysis here attempted that the 

 nitrogen group shall be regarded as rigorously non-absorbing and non- 

 radiating, but simply that they are so to an extent that differentiates them 

 strongly from such absorbers and radiators as carbon dioxide and water-vapor. 

 The climatic functions of this group may be suggestively pictured by 

 imagining an atmosphere wholly made up of elements that neither absorb 

 solar rays nor radiate their own heat, however received. The entrance and 

 exit of heat, in this case, would be confined substantially to contact with the 

 heated or cooled surface of the earth (neglecting, of course, such internal 

 sources as expansion and contraction, friction, etc.). In considering such 

 obstinate retentiveness, it becomes obvious that heat, once it has been in- 

 corporated in constituents of this class, would remain embodied in them for 

 relatively long periods under average conditions because of lack of means of 

 escape. This holds equally of their assembled state as a gas, for assembled 

 states do not radiate as such. The time-factor would thus usually be dis- 

 tinctly large. This extreme theoretical picture is not fully realized in the 

 atmosphere, but there is a sufficient approach to it to justify classing the main 

 mass of the atmosphere as an effective source of detention or short storage of 

 heat.^ This is a property of the first order of importance in the maintenance 

 of the thermal state of the earth's atmosphere. This property takes on con- 

 siderable importance also in its working relations with the absorbing and 

 radiating constituents, carbon dioxide and water-vapor. 



PROLONGATION OF THERMAL ACTION BY CARBON DIOXIDE. 



Since the main mass of the atmosphere is thus thermally conservative, the 

 simplest step toward a concrete view of the working relations of its pro- 

 longing property is found in the study of a constituent of its own "permanent" 

 type, but which readily absorbs terrestrial radiation and emits wave-lengths 

 of its own type, carbon dioxide (CO2). 



While atmospheric CO2 probably varies widely in amount from one geo- 

 logic era to another, it is nearly constant during short periods and is dis- 

 tributed with approximate uniformity throughout all parts of the lower 

 atmosphere. In these respects it differs widely from water-vapor, which 

 enters and leaves the atmosphere with great freedom and varies in amount 

 several hundreds per cent in different parts of even the lower atmosphere. 

 While both CO2 and H2O in the air are absorbers and radiators and share 

 these functions in common, their other differences place them in distinct 

 classes. While CO2 has some little power of absorbing the incoming solar 

 rays, it is so little as to be negligible in a general analysis of this kind; only 



* "Storage of heat" in this report is used only in the sense of the retention of sensible or true 

 heat. It is not used in the sense of transformed heat, such as "latent heat" or "potential tempera- 

 ture," which are not strictly thermal at all. 



