OXYGEN. 25 



circumstances, partly as free nitrogen. So, too, by combustion, 

 organic substances evolve practically the whole of their nitro- 

 gen in the free state. 



Oxygen, the most important constituent, since it takes part 

 in so many of the reactions occurring in the atmosphere, is 

 liable to considerable local variations. Its amount, on the 

 average, is nearly 21% by volume or 23-2% by weight of dry 

 air. As processes of oxidation are taking place so continually 

 it might be expected that the proportion of oxygen in air would 

 show great variations from place to place. In consequence, 

 how r ever, of diffusion, air currents, and the compensating in- 

 fluences of vegetation, the variations which have been 

 observed, though w r ell marked, are not great. 



The extremes noticed by various observers are : 

 Bunsen ... 20*84 20-97 



Regnault ... 20-90 21-00 



Angus Smith ... 20-89 21-00 



Leeds ... 20-82 21-03 



Jolly ... ... 20-53 21-01 



As is to be expected, the air of towns is found to contain less 

 oxygen than that of the country or over the sea. The lower 

 numbers given in the above table were, in all cases, observed 

 in the air of large cities. In marshy places, too, the amount 

 of oxygen is generally lower than elsewhere. 



Comparatively few experiments on the composition of air 

 from great heights have been made, but the general result of 

 what is known is in agreement with theory, which indicates 

 that relatively less of the heavy constituent, oxygen, should be 

 present in such air.* In the Alps it has been observed that a 



* According to Hinrichs (Compt. Rend., 1900, 131, 442), if each constituent of the 



air were independent of the others the composition of air at various heights would 



be as given in the accompanying table. 



Altitude in Carbon .^ 



From the above table it is seen that the carbon dioxide becomes inappreciably 



