CHAPTER XIII 



MICROORGANISMS IN RELATION TO MAN 

 I. THE DUST OF THE ATMOSPHERE 



270. Composition of the air. The atmosphere of the earth 

 is a mixture of various substances. From the time of Lavoisier, 

 chemists have known of its chief constituents, nitrogen, oxygen, 

 and carbon dioxid. Recently, however, they have discovered 

 in it several relatively unimportant inert gases, such as argon, 

 helium, neon, and krypton. Traces of ammonia, hydrogen, and 

 hydrogen peroxid are also found in the air. 



The volumes of the four chief gases found in a liter, or 1000 

 cubic centimeters, of dry air are as follows : 



Nitrogen 784 cubic centimeters 



Oxygen 209.4 cubic centimeters 



Argon 6.3 cubic centimeters 



Carbon dioxid 0.3 cubic centimeter 



1000.0 cubic centimeters 



271. Dust in the air. It is a matter of common experi- 

 ence that the atmosphere we breathe contains besides these sub- 

 stances a quantity of water vapor and of more or less minute 

 dust particles. Water vapor in condensation is supposed to 

 collect about dust particles in forming cloud, mist, fog, or rain. 

 That dust is abundant in the air may be observed in a ray of 

 sunshine passing through a darkened room ; it is seen to form 

 a veritable cloud. The particles observed are comparatively 

 coarse, the vastly greater bulk of suspended dust being in- 

 visible to the unaided eye. 



The air we breathe is obviously not always clean or pure, 



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