134 BACTERIA, YEASTS, AND MOLDS 



wells, drawn from a depth of a hundred feet or more. 

 All surface waters are sure to contain them, for they 

 are present in lakes, ponds, pools, rivers, and in the ocean. 

 They are more abundant in flowing streams than in water 

 standing in lakes or reservoirs, quite contrary to the usual 

 belief. We commonly look upon running waters as purer 

 than standing waters, but so far as concerns bacteria 

 the reverse is usually the case. Rivers or brooks contain 

 them in large numbers ; lakes in which the water has stood 

 for a long time contain a smaller quantity. The reason 

 is that rivers and brooks collect the bacteria from the 

 surface of the ground, from sewage, etc., and the longer 

 they flow the greater the number of bacteria they con- 

 tain, since they are great draining agents for the country. 

 When water stands in lakes or ponds the bacteria, which 

 are slightly heavier than the water, have a chance to settle 

 to the bottom. This they do in the course of a few days, 

 and after a time the water in such standing reservoirs 

 becomes far purer than in the supply streams. It will 

 easily be understood that the greater the amount of 

 decaying matter entering any stream the larger will be 

 the number of bacteria in its waters. The rivers of civi- 

 lized countries, that receive the sewage from cities, not 

 only contain these little organisms in immense numbers, 

 but contain some of the most dangerous kinds, since they 

 are the disease germs discharged from human patients. 



From these general facts we reach the conclusion that 

 no water at our command upon the surface of the earth is 

 absolutely free from bacteria. Spring water is the purest, 

 and water from deep artesian wells is about equally pure. 

 Water from lakes and reservoirs is the next in purity, and 



