CHAPTER VIII 

 DUSTS, MOLDS, AND BACTERIA OF THE AIR 



87. Dust in the air. There is another part of the atmosphere 

 of which we have said nothing. It is the dust. It is a part of 

 the atmosphere that we might very well get along without, it 

 often seems, but it is always present. Unlike other parts of 

 the atmosphere, dust is a solid, not a gas. It consists of very 

 small, solid particles, which are borne along by the air. The 

 larger particles settle very rapidly, but the finer ones fall 

 through the air so slowly that they remain floating for a long 

 time. If a wind is blowing, they may be carried great distances. 



The abundance of dust in the air becomes obvious when 

 we see a beam of sunlight crossing a darkened room. All the 

 bright points dancing in the light are bits of dust, but most 

 of the particles are too small to be seen in this way. The 

 housewife has an equally good demonstration when she finds a 

 covering of dust on the furniture which was carefully cleaned 

 but a few hours before. 



88. Dust is everywhere. Dust is a very troublesome thing. 

 It accumulates on articles in our homes to such an extent that 

 much time is spent by the housekeeper in sweeping and dust- 

 ing. When we go out of doors, it gets into our mouths and 

 nostrils, irritates our eyes, and clings to our clothing. Indoors 

 we cannot escape it, for, even though we close the house, it 

 filters through the smallest cracks. It is upon the food we 

 eat, particularly if we purchase articles from street stands or 

 in stores where the food is displayed uncovered. In our cities 

 the air is often so filled with solid particles, whether called dust 

 or smoke, that it is impossible to see very far. Even in the clear 

 air over the desert or the ocean, dust is not wholly absent. 



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