THE GEOLOGIC FUNCTIONS OF LIFE. 643 



antly for a long period, but was measurably protected from the organisms 

 that preyed upon it and from inorganic decomposition, as by falling 

 into water or by prompt burial under sediment, the atmosphere might 

 be growing richer in oxygen. If, on the other hand, vegetation were 

 being relatively reduced, as perhaps it is being reduced now by man, 

 and if previous organic products were being reoxidized at an unusual 

 rate, as they are now in the burning of timber, coal, natural oil and gas, 

 the carbon dioxide of the atmosphere might be relatively increasing, 

 while the oxygen might be relatively diminishing. The possible fluctu- 

 ations of the atmosphere as the result of organic action are, therefore, 

 matters of vital importance, and invite attention in the historical study 

 of the earth and in the outlook into its future. 



The climatic effects of organic action. — Interest does not, however, 

 rest at this point. The researches of physicists have made it probable, 

 if they have not altogether demonstrated, that the composition of the 

 atmosphere has much to do with the climatic conditions at the surface 

 of the earth. The atmosphere blankets the earth and equalizes its tem- 

 perature. Acting as a screen, it subdues in some measure the intensity 

 of the sun's rays by day, while it retards the radiation of the earth's 

 acquired heat at night. This is in some measure the function of all the 

 constituents of the atmosphere, but by no means of all equally. The 

 oxygen and nitrogen are relatively diathermous, letting the sun's ra3^s 

 pass in freely, and the earth's rays pass out freely; but carbon dioxide 

 and the vapor of water are much less diathermous, particularly to rays 

 of low intensity, such as are thrown out by non-luminous bodies like 

 the earth. It follows that while the solar ra^^s come in rather freely and 

 heat the surface of the earth, the dark rays Avhich the earth radiates 

 back are measurably arrested by the carbon dioxide and vapor of water, 

 and serve to keep the air warm. The influence of the vapor of water 

 is vividly shown in the different degrees to which cooling takes place 

 at night in a dry and in a moist atmosphere, respectively, where other 

 conditions are the same. Ice is said to form at night in desert regions 

 where the air is extremely dry, even within the tropics, while in humid 

 regions of the same latitude and altitude oppressively hot nights are 

 common. The influence of the carbon dioxide is not thus famiharly 

 demonstrated, since its amount varies but slightly in different locali- 

 ties, but physical experiment indicates that it has a similar function. 



If the amount of carbon dioxide in the atmosphere varies from age 



