THE ATMOSPHERIC GASES 



189 



humidity receptors in the tarsi of spiders 

 (Blumenthal, 1935) and in the antennae of 

 the beetle, Tenebrio molitor (Pielou and 

 Gunn, 1940). When ant hons, which 

 characteristically construct their pit traps 

 in dry sand or soil, are placed in a humidity 

 gradient, they react by trial and error and 

 finally collect and remain quietly in the dry 

 end of the gradient.** 



Relative humidity is readily determined 

 with fair exactitude even in the field. As 



" A. E. Emerson, unpublished material. 



with pH, this is not necessarily a sound 

 reason for the amount of study devoted to 

 it. The analysis of Leighly (1937) and 

 Thornthwaite (1940) show that much of 

 the work will need to be repeated when 

 suitable methods are developed for approxi- 

 mating the vapor tension of evaporating 

 surfaces, whether Hving or nonUving. Pend- 

 ing the development of gross methods 

 suitable for larger habitats and for larger 

 organisms, micro-methods that will reveal 

 the intimate relations of small organisms in 

 their habitat niches must wait. 



13. THE ATMOSPHERIC GASES 



The total atmosphere of the earth consists 

 of an estimated 56,328 X 10" tons of mat- 

 ter, of which an average of about 14,615 X 

 10" tons are water vapor. Since the water 

 vapor varies, it is customary to disregard 

 it and base the analysis of gases on air that 

 is theoretically dry. If the total amount of 

 dry air is regarded as 100 volume per cent 

 and as making a total pressure of 760 mm. 

 of mercury, then the different constituent 

 gases are present in the amounts indicated 

 in Table 14. 



duces no confusion if one remembers that 

 both carbon dioxide and ozone are highly 

 important elements of the environment of 

 living organisms, even though each repre- 

 sents but a small percentage of the whole 

 Carbon dioxide, drawn largely from the air, 

 is a basic ingredient in the process of the 

 photosynthesis of carbohydrates by green 

 plants (p. 199), and ozone screens out ul- 

 traviolet radiation from the sun that other- 

 wise might destroy nearly all the hfe we 

 know (p. 74). 



Table 14. Gases Present in the Atmosphere (Data from Humphreys, 1931, and Sverdrup, 



Johnson, and Fleming, 1942) 



Gas 



Nitrogen 



Oxygen 



Argon 



Carbon dioxide 



Hydrogen 



Neon 



Helium 



Krypton 



Ozone 



Xenon 



Volume % 



78.03 



20.99 

 0.9323 

 0.03 

 0.01 

 0.0018] 

 0.0005 J 

 0.0001 

 . 00006 

 . 000009 



Partial Pressure in mm. Mercurv 



593.02 

 159.52 

 7.144 

 0.228 



0.088 



Xenon, with the lowest percentage, has a 

 total estimated mass in the atmosphere of 

 21 X 10* tons. Nitrogen and argon are com- 

 bined in common usage and regarded as 

 forming 79 per cent of the atmosphere, 

 and oxygen is then said to make up the 

 other 21 per cent. Such a simplifying as- 

 sumption has much justification and intro- 



The percentages of the gases in the lower 

 atmosphere are practically constant, except 

 for rare death valleys and where carbon 

 dioxide accumulates as it escapes from some 

 underground source. One such valley occurs 

 on the Dieng plateau in Java (Hesse, Allee, 

 and Schmidt, 1937). The chemically un- 

 stable ozone is more abundant in the higher 



I 



