32 ELEMENTARY AGRICULTURAL CHEMISTRY 



very small ; in towns or over marshes it is rarely found. It 

 is most abundant in Europe during May or June, especially 

 after violent thunderstorms or gales. 



In addition to the substances already mentioned as occurring 

 in air^ there are others which may be described as accidental. 

 Near towns or wherever much coal is burnt, air is found to 

 contain sulphur dioxide^ which, on oxidation, passes into sul- 

 phuric acid. The rain of towns is usually distinctly acid from 

 this cause, and to this acidity is mainly attributable the diffi- 

 culty of growing plants, particularly grasses, in towns. 



Air also contains suspended matter of various kinds. Parti- 

 cles mainly composed of common salt, NaCl, are very abundant, 

 resulting from the evaporation of the tiny droplets of sea- water 

 sent up as spray from breaking waves. These particles are 

 carried immense distances b}^ the wind, and are the chief source 

 of the chlorides found in rain-water. The amount is greatest 

 at places near the coast, but even far inland rain often contains 

 considerable quantities of chlorides. 



In addition to the inanimate solid matter, air usually con- 

 tains micro-organisms or their spores. These are most abundant 

 in towns or wherever organic substances are undergoing decay ; 

 on mountain-tops they are very rare. The existence of these 

 micro-organisms is very important, since to them are attribut- 

 able many diseases and many formg of putrefaction and fer- 

 mentation. They are of particular interest and importance 

 with reference to dairy work, fermentation in breweries, the 

 manufacture of wines and spirits, and the preservation of all 

 forms of organic matter. 



