THE ATMOSPHERE 29 



detected. The quantity present in the atmosphere is increased 

 by many processes, among the most important being ; 



(1) Emission from volcanoes, deep springs, and other sub- 

 terranean sources. 



(2) Oxidation of carbonaceous matter — e.g., the combustion 

 of most fuels, the decay of animal and vegetable matter, and 

 the respiration of animals and plants. - 



(3) The dissociation of carbonates by heat — e.g., in lime- 

 burning. 



CaCOj = CaO + CO,. 



The chief process tending to diminish its quantity is its 

 decomposition by the green portions of plants under the influ. 

 enc3 of sunlight. In this case the carbon is retained, being 

 converted into various complex organic compounds, and used 

 in forming the tissues of the plant, while the oxygen is returned 

 in the free state to the atmosphere. 



The magnitude of this process of removal of carbon dioxide 

 from the air by plants is enormous. Some conception of it 

 can be formed when it is remembered that about half the dry 

 portion of most plants consists of carbon, and that the whole 

 of this is obtained from the atmosphere. Thus an acre of 

 an average crop of mangolds will abstract from the aii* before 

 reaching maturity about 3500 lb. of carbon, which repre- 

 sents the carbon dioxide in a 200-feet layer of air covering 

 some 180 acres. The amount of carbon dioxide present in the 

 atmosphere is continually being affected by the two kinds of 

 actions described, oxidation of carbon compounds and decom- 

 position of carbon dioxide by plants, and these processes 

 approximately balance each other. 



Ammonia and Nitric Acid, or some oxide of nitrogen, are 

 often present in the air, but in exceedingly small quantities. 

 Near Paris the average amount of ammonia is estimated at 

 1-7 milligrammes per 100 cubic metres in winter, and 21 milli- 

 grammes per 100 cubic metres in summer. Other observers 

 have found three times as much in Juno as in February. 



