20 NITKOGEN OXYGEN 



oxygen under the influence of the high temperature of the electric 

 spark, 1 and such combination is doubtless brought about in the at- 

 mosphere by lightning discharges, with the production of oxides of 

 nitrogen and eventually of nitric acid. Similar combination between 

 nitrogen and oxygen has been observed to occur during the combus- 

 tion, in air, of certain substances which, in their burning, produce a 

 high temperature. 



The amount of free nitrogen removed from the atmosphere by 

 these various agencies is relatively small and probably quite equalled 

 by that yielded by processes of decay and putrefaction. 



It is found that organic nitrogenous bodies during decomposition 

 yield their nitrogen, partly as ammonia and, under certain circum- 

 stances, partly as free nitrogen. So, too, by combustion, organic 

 substances evolve practically the whole of their nitrogen 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 con- 

 siderable local variations. Its amount, on the average, is nearly 21 

 per cent by volume or 23-2 per cent by weight of dry air. As pro- 

 cesses of oxidation are taking place continually it might be expected 

 that the proportion of oxygen in air would show great variations from 

 place to place. In consequence, however, of diffusion, air currents, 

 and the compensating influences of vegetation, the variations which 

 have been observed, though well marked, are not great. 



The extremes noticed by various observers are 



Bunsen 20-84 20-97 



Kegnault 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. 



In 1886 a series of daily analyses of air were made simultaneously 

 at Dresden, Bonn (Germany), Cleveland (U.S.A.), Para (Brazil) and 

 Tromsoe (Norway), from 1 April to 16 May. The mean values 

 for the amount of oxygen were 



Para 20-92 per cent, by volume 



Bonn 20-92 



Cleveland 20-93 



Dresden 20-93 



Tromsoe 20-95 



The maximum was 21-0 at Tromsoe and the minimum 20*86 at 

 Para. The mean percentage of oxygen of the whole series was 20'93. 2 



1 McDougal and Howies (Jour. Chem. Soc., 1900, Abstracts, ii. 651) found that, 

 by a large electric discharge in air, as much as 303 grammes of nitric acid per 

 12 horse-power hours could be produced. 



2 Hempel, Ber., 20, 1864. 



