THE ATMOSPHERE 



41 



as a primary compound in the origin of life; it probably was 

 more widely utilized after the chlorophyllic stage of plant 

 evolution, for not until chlorophyll appeared was life equipped 

 with the best means of extracting large quantities of carbon 

 dioxide from the atmosphere. 



The stable elements of the present atmosphere, for which 

 alone estimates can be given, are essentially as follows:^ 



Oxygen. . 

 Nitrogen 

 Argon. . . 



By Weight 



23.024 



75-539 

 1-437 



I 00 . 000 



By Volume 



20. 941 



78. 122 



-037 



Atmospheric carbon dioxide (CO2), which averages about three 

 parts in every 10,000, and water (HoO) are always present 

 in varying amounts; besides argon, the rare gases helium, 

 xenon, neon, and krypton are present in traces. None of the 

 rare gases which have been discovered in the atmosphere, such 

 as helium, argon, xenon, neon, krypton, and niton — the latter 

 a radium emanation — are at present known to have any rela- 

 tion to the life processes. Carbon dioxide- exists in the atmos- 

 phere as an inexhaustible reservoir of carbon, only slightly 

 depleted by the drafts made upon it by the action of chloro- 

 phyllic plants or by its solution in the waters of the conti- 

 nents and oceans. Soluble in water and thus equally mobile, 

 of high absorption coefficient, and of universal occurrence, 

 it constitutes a reservoir of carbon for the development of 

 plants and animals, radiant energy being required to make this 

 carbon available for biological use. Carbon dioxide in water 



1 Clarke, F. W., letter of March 7, 1916. 

 "Henderson, Lawrence J., 1913, pp. 136-139. 



