CHAPTER XV 



OTHER IMPORTANT FACTS ABOUT AIR 



So much is said about the difference between city 

 and country air, that we might think the latter has 

 much less carbonic acid gas and far more oxygen than 

 the former. Sea and mountain air have been found 

 to contain more of a bracing form of oxygen called 

 ozone than have other regions. The atmosphere is 

 so vast, however, and its movements are so far-reach- 

 ing that the law of diffusion usually keeps the percen- 

 tage of the important elements in outdoor air very 

 nearly normal. Yet there is a great difference in 

 cleanliness between air in the city and air in the country. 

 The amount of smoke and dust is far greater in city 

 air. It also naturally follows that where so many 

 people live, much more decaying animal and vegetable 

 matter and other waste materials are present to give 

 off their noxious gases. 



Another important difference is the far greater 

 percentage of bacteria or microbes in city air. In 

 previous chapters we have learned of these little 

 organisms as destroyers of food, but there are many 

 different kinds. It has been found that some cause 

 various diseases with which our bodies can become 



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