AIR S5 



than on dry and windy days. There are more organisms 

 in the air during summer than during winter, and fewer 

 after than before rain;— there are practically none at sea. 

 There are none in the outside air to which any pathogenic 

 significance can be attached, while the number present, 

 about 25 to the cubic foot, is from ten to twenty times less 

 than the air of inhabited places. 



IMPURITIES IN AIB. 



These are caused (1) By combustion of coal, gas, and 

 artificial light. (2) By various ofl:ensive trades. (3) By 

 decomposition of organic matters. (4) By respiration of 

 men and animals. 



They are removed or kept under control by certain 

 natural processes. Diffusion and the action of the wind 

 cause rapid and considerable dilution of noxious gases. 

 Plants assist by utilizing carbonic acid and giving up 

 oxygen. The oxygen of the air oxidizes organic compounds 

 converting them into ammonia, nitric and nitrous acids. 

 The air is washed by means of rain and impurities carried 

 to the earth, where they are assimilated by plants and 

 destroyed. 



Vitiation by Combustion. — It was computed several years 

 ago that not less than 822,000,000 cubic feet of carbonic 

 acid passed daily into the atmosphere of London. This is 

 largely derived from the combustion of coal, which gives off 

 during the process of burning three times its weight of 

 carbonic acid, and small quantities of carbonic oxide, sul- 

 phurous acid, and sulphuretted hydrogen. Besides these, 

 about one per cent, is given off as soot and tarry products. 



It is these latter, which owing to their weight are unable 

 to ascend more than 600 feet, that give to London its 

 peculiar winter fogs. The particles of moisture in the air 

 are enveloped in a greasy hydrocarbonaceous covering 

 which is impenetrable to light, and produce the peculiar 

 yellow colour; while the sulphurous acid gas causes 

 the irritation present in the eyes and bronchial membrane. 



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