BACTERIA 219 



Bacteria of water. Streams ordinarily contain about 500 

 bacteria per cubic centimeter, collected from the air and soil 

 over the area drained. The river Seine, as it enters Paris, has 

 about 300 bacteria per cubic centimeter, but after it receives 

 the sewage from this city it contains 200,000 bacteria per cu- 

 bic centimeter. The supposed self-purification of streams is 

 found to be mainly due to dilution. Experts are impounding 

 running water in reservoirs previous to supplying cities, since 

 bacteria disappear from still water. Microscopic organisms 

 (plankton) upon which young fishes feed are found in greater 

 abundance in quiet water, and it is thought that they in turn 

 feed upon bacteria. 



Water in wells varies greatly in number of bacteria. Arte- 

 sian wells are practically free from them ; ordinary wells may 

 contain from 1000 to 8000 bacteria per cubic centimeter. Ice 

 varies in number of bacteria according to water from which it is 

 taken. Clear ice from the Hudson River contained 398 bacteria 

 per cubic centimeter, while its snow ice contained 9187. Why ? 



Bacteria of the soil. The number of bacteria of the soil 

 varies with the amount of moisture and organic debris. Su- 

 perficial layers contain from 10,000 to 5,000,000 bacteria per 

 gram; if polluted with organic debris, they may contain as 

 high as 100,000,000 per gram. The number of bacteria di- 

 minishes rapidly as we pass down into the earth ; at a depth 

 of from ten to fifteen feet few if any can be found. This is 

 the reason that in many cities water is passed through sand 

 filters before it is used for drinking purposes. 



