670 GEOLOGY. 



of the earth, but any change it is likely to have undergone was probably not 

 sufficiently large to make it needful to dwell upon it here. But if the carbon 

 dioxide in the air and in the ocean was in the critical state of reduction we have 

 inferred from previous losses, and was being absorbed by the ocean on account 

 of the cooling experienced from the above changes of condition, the accelerated 

 carbonation induced by the deformation may well have made serious inroads 

 upon it. It is accordingly inferred that its quantity was reduced appreciably, 

 and that the earth was thereby deprived of some large part of the thermal blanket- 

 ing which it had previously effected. 



The vicarious action of carbon dioxide.— Before considering the amount of 

 this reduction, let it be noted that, in so far as it reduced the temperature, it re- 

 duced the moisture in the atmosphere, for this is immediately dependent upon 

 temperature, and any agency which reduces the temperature reduces the moisture, 

 and hence reduces the thermal effects due to moisture. Probably this secondary 

 effect of a loss of carbon dioxide is greater than its primary effect. To realize 

 this, it is important to recognize the fundamental difference between the rela- 

 tively permanent constituents of the atmosphere, such as nitrogen and oxygen, 

 and such an exceedingly dependent and fluctuating one as the vapor of water. 

 No temperature effect arising from moisture seriously affects the amount of 

 oxygen, nitrogen, or carbon dioxide in the air, while any reduction of these which 

 involves reduction of temperature, effectually reduces the water- vapor. While 

 a reduction of temperature does not immediately reduce the carbon dioxide, 

 it leads indirectly to its absorption in the ocean, and so a reduction of either 

 carbon dioxide or water-vapor reduces the other, and their mutual reaction 

 intensifies their common temperature effect. Saturation being assumed, the 

 theoretical amount of water varies with the temperature, but unequally. In 

 the vicinity of 0° C, the amount of vapor varies roughly about 7% for every 

 degree Centigrade, when computed from one degree to another, or, roundly, 

 the vapor is about doubled or halved for every temperature change of 10° C 



The amount of vapor in the air at —30° C, when completely saturated, about 

 equals the normal amount of carbon dioxide in the air. The air is, however, 

 rarely saturated with moisture. While, therefore, the amount of vapor in the 

 air is. on the whole, much greater than the carbon dioxide, there are consider- 

 able high-latitude areas at the ground surface, in addition to the whole upper 

 zone, in which the carbon dioxide exceeds the vapor of water. 



The thermal functions of the atmospheric constituents. — Although not a little 

 experimental work has been done on the competency of the atmospheric 

 gases to absorb heat, it has not furnished an adequate basis for the satisfac- 

 tory discussion of their relative serviceability in maintaining the temperature 

 of the earth's surface. In part this has been due to the great difficulties of accu- 

 rately determining the absorbent effects of the chief thermal rays, which have 

 long wave-lengths (.4/* to 30//) and are invisible. These long wave-lengths are 



1 For exact data, see Landolt and Bornstern's Physikalisch-Chemische Tabellen, 

 or Evans' Physico-Chemical Tables. 



