480 



THE RESPIRATORY EXCHANGE 



air, measured at standard temperature and pressure, and 

 760 mm., is : 



Oxygen 



Carbon dioxide ...... 



Nitrogen ........ 



Argon ........ 



Helium, Krypton, Neon, Xenon, and Hydrogen 



Volumes 

 per Cent. 



20-94 

 0-03 



78-09 

 0-94 



Traces 



In addition, air contains aqueous vapour, 1 which, varies greatly 

 in amount in different places and at different times. The tem- 

 perature of the air is important not only as regards its direct effect 

 upon the respiratory exchange of animals, but also as one of the 

 most important factors which determine the amount of moisture 

 in the air. The nitrogen and argon appear to be inert as far as 

 the higher animals are concerned, but it must be remembered that 

 some bacteria are able to fix nitrogen, and thus enrich soil ; cultures 

 of such bacteria have even been used upon a large scale to render 

 soil more fertile. If a fully grown pea plant be uprooted, tubercles 

 will be found upon its roots ; these in microscopic sections are seen 

 to be composed of vegetable tissue with very large cells and to con- 

 tain large numbers of bacteria, Bacterium radicicola. These bacteria 

 stand in symbiotic relationship to the plant, and enable it to obtain 

 nitrogenous compounds at the cost of the atmospheric nitrogen. 



The proportion of carbon dioxide in pure country air appears 

 to be 3 volumes per 10,000, but it is liable to a variation in summer 

 owing to the influence of vegetation ; it may rise by night to 3-5 

 volumes and fall by day to 2*6, for, as is well known, vegetation 

 under the influence of sunlight decomposes carbon dioxide. These 

 facts are well shown in the following table, which gives the average 

 results of exact determinations of the carbon dioxide in air, made by 

 J. S. Haldane and E. S. Haldane in the Ochil Hills, Perthshire. 



1 See p. 258. 



