EXAMINATION OF WATER. 443 



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by melting the ice. The ice supplied in cities, when melted, 

 may contain as many as 25,000 bacteria per c.c. of water. 



The rain-water and snow bring down from the air a 

 large number of organisms. The water as it penetrates the 

 ground is filtered, and, in this way, all the bacteria are re- 

 tained by the upper layer of the soil. It follows, therefore, 

 that water coming from the deeper layers of the earth is 

 practically germ-free. This is often the case with sprmg- 

 water. As a rule, however, a small number of bacteria 

 from the surface soil re-enter the water as it reaches the 

 surface. Hence, spring- water usually contains less than 50 

 bacteria per c.c. 



The water of artesian or tubular wells, for reasons just 

 given, will likewise be free from bacteria. A small number 

 of these are usually present, but this is due to contamina- 

 tion at or near the surface. 



Ordinary wells, as might be expected, give the great- 

 est krtown variation in the number of bacteria. The well 

 water may contain practically no bacteria and on the other 

 hand they may be almost innumerable. Thus, a number of 

 wells have been found to contain as many as 800,000 bac- 

 teria per c. c. As a rule, a well-water, especially when it 

 is very cold, will contain about 1,000 bacteria per c.c. 



In sea- water, there is less variation in the number and 

 kind of bacteria present, as well as in their distribution 

 from the shore or from the bottom than is usually met with 

 in ordinary waters. Russell has shown that the ocean sur- 

 face wd!ter contains from a few to as many as 120 bacteria 

 per c.c. This number, however, may at times be consider- 

 ably increased an^ may even reach 28,000. In the deeper 

 layers of the water they are no more abundant than on the 

 surface. The slime at the bottom of the sea, at Naples^ 

 contained about 300,000 while at Wood'& HoU only about 

 17,000 bacteria were present in^a c.c The enormous differ- 

 ence between the number of bacteria in the slime and in the 



