CHAPTER XII. 

 DUST AND BACTERIA. 



The Air is washed by Rain or Snow. Every one will 

 recall how delightfully refreshing the air is after a rain or 

 a snowstorm. This is not due merely to the fact that the 

 air is cool. It is clean because it has been washed. The 

 rain and snow absorb most of the various impure gases 

 that are in the air. The raindrops and snowflakes also 

 bring down with them many particles of dust that were 

 floating in the air. Take some of the snow that has fallen 

 in a town. It looks pure in its almost dazzling whiteness. 

 But melt some of it, and you will usually find that the 

 water has an inky tinge, showing that as the flakes sifted 

 down through the air they caught myriads of particles of 

 .dust. 



The Sources of Dust Where soft coal is used to any 

 large extent it is one abundant source of this dust. In 

 summer dust has many sources. The dust that blows into 

 your face, and perhaps into your mouth, may be made of 

 dry soil. Take a dry clod and drop it ; it falls quickly to 

 the ground. Crush it in your hand before dropping it, and 

 much of it floats in the air for some time. Any substance 

 that is easily dried and reduced to powder may form part 

 of the common dust. The dust that you wipe from your 

 eye, or is caught by the mucus of the nasal passages, may, 

 instead of being made of clean soil, be from the excreta of 

 horses, decayed leaves, wood, grass, etc. Indoors we are 



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