CHAPTER VIII 

 DUST, MOLDS, AND BACTERIA OF THE AIR 



98. Questions for Discussion. 1. Is there commonly more dust in 

 the air over cities or over the open country? 2. AVhat causes milk to 

 sour? meat to decay? fruit juice to ferment? 3. If milk does not 

 sour when allowed to stand for several days in a warm place, does 

 this indicate that it is of good quality? What might prevent it from 

 souring? 4. If a bacterium should divide and thus produce two bac- 

 teria once in each thirty minutes, and continue to do so for twenty- 

 four hours, how many will there be at the end of that time as the 

 result of the successive divisions? 5. Why are objects less likely to 

 mold in a house which is carefully and frequently cleaned than in 

 one which is not well cared for? 6. What is the source of the mold 

 which sometimes appears upon bread if the bread is left for some 

 days in a tightly closed box? 7. What are the bluish-green patches 

 that appear on gloves or shoes which have been left in a warm, damp 

 closet? 8. Would surface wells in cities and other densely inhabited 

 places commonly be safe sources of supply for drinking water? 9. Are 

 the water supplies at summer resorts and picnic grounds in your 

 vicinity safe sources for drinking water? 10. Should we be able to 

 get on better if there were no bacteria or other organisms which pro- 

 duce decay? 11. Why should schoolroom floors be cleaned and oiled 

 frequently ? 



99. Abundance of dust. Dust 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 



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