Ill SOILS 23- 



really only serves to dilute it and prevent it having too 

 active an effect. Besides these two gases, the air contains 

 carbonic acid, ammojiia, nitric acid, chlorine, sulphuric acid 

 and water in the form of vapour. The quantities of these 

 substances, in comparison with the oxygen and nitrogen, are 

 so small as to be scarcely perceptible, but then the atmo- 

 sphere is so vast in its extent that even an extremely minute 

 proportion in a given quantity is really an enormous mass 

 in the whole. 



Carbonic Acid, as we have seen, is taken up by the plants 

 in order to build up their tissues, for the roots, stem, leaves, 

 flowers and fruit are, for the most part, composed of the 

 carbon supplied by the carbonic acid of the air. 



The Animo7tia in the atmosphere comes from the decay of Formation of 

 animal and vegetable bodies, from active volcanoes, and from ^'^"^°"^^- 

 the combustion of substances by heat, as when wood or 

 coal is burnt in a fire. It would naturally appear, therefore, 

 that ammonia would be more abundant in the air of large 

 cities than in country districts, and careful experiments 

 have shown that this is really the case. 



Nitric Acid is formed principally by the electricity, which Formation of 

 is so often manifested to all by means of lightning. After a "'^"*^ ^^^ ' 

 thunder storni, the air always contains a much larger quantity 

 of nitric acid than it did before. 



Chlorine, in the form of chloride — commonly called chlo- Salt in the 

 ride of sodium or salt— is found in the atmosphere, and its ^t"^°^P^^''^- 

 presence is due almost wholly to the influence of the sea. 

 Small quantities of salt, as fine spray, are carried by winds 

 and currents of air for wonderful distances inland. 



The Sulphuric Acid in the air is due to the same cause as 

 nitric acid, and it is also given off in large quantities from 

 active volcanoes. 



The Watery Vapour of the atmosphere is often precipi- Sources of 

 tated in the form of rain and dew in hot and temperate J'hJatmo" 

 countries, and as snow or hail in colder regions. It is con- sphere. 

 stantly given off by the leaves of plants and in the breath of 



