24 THE ATMOSPHEKE. II. 



COMPOSITION OF THE ATMOSPHEKE. 



Air is a mechanical mixture of various gases and vapours, 

 but it invariably contains suspended solid matter, some of 

 which consists of micro-organisms. 



The main gaseous constituents are oxygen, nitrogen, argon, 

 carbon dioxide, water vapour, ammonia, oxides of nitrogen or 

 nitric acid vapour, and ozone. All these are subject to varia- 

 tion, but to very different extents. 



Nitrogen, the largest constituent, is the least variable in 

 amount. It usually constitutes about 78% by volume or 

 75'5% by weight of dry air. Its function in the atmosphere 

 has usually been regarded as mainly that of a diluent. Ap- 

 parently very few reactions take place in the atmosphere in 

 which nitrogen takes part. Eecently, however, many obscure 

 changes in which the free nitrogen of the air enters into com- 

 bination with oxygen and hydrogen have been observed, some 

 of which are of the greatest importance in agriculture. 

 These changes occur by the agency of micro-organisms in the 

 soil or plant (r. Chap. IV.). 



Very high temperatures, such as obtain in the blast furnace, 

 will cause nitrogen to combine with carbon in the presence of 

 alkalies with the production of cyanides. Nitrogen may be 

 made to combine with oxygen under the influence of the high 

 temperature of the electric spark,* and such combination is 

 doubtless brought about in the atmosphere by lightning dis- 

 charges, with the production of oxides of nitrogen and eventu- 

 ally of nitric acid. Similar combination between nitrogen and 

 oxygen has been observed to occur during the combustion, 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 putre- 

 faction. 



It is found that organic nitrogenous bodies during decompo- 

 sition yield their nitrogen, partly as ammonia and, under certain 



* McDougal ami Howies (J.C.S., 1900, abst. ii., 651). found that by a large electric 

 discharge in air, as much as 303 grammes of nitric acid per 1-2 horse-power-hours could 

 be produced. 



