442 BACTERIOLOGY. 



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able to the growth of bacteria. In a given time, a much 

 larger number of bacteria will be formed on the surface by 

 multiplication then will be deposited by sedimentation. 

 Moreover, the currents of the cold spring-- water from be- 

 low will tend to keep the suspended matter in the surface 

 layers. 



The mountain lakes, on the other hand, are primarily 

 fed by the cold, crystal-clear water derived from glacier 

 streams. The water, whether on or below the sur- 

 face, has a minimum low temperature, and, moreover, the 

 rapid onward and downward now prevents the warming of 

 that on the surface. Sedimentation in such cases may 

 occur, whereas the reverse, so far as numbers are con- 

 cerned, will be met with at lower altitudes. 



River waters, according to the conditions mentioned 

 above, will necessarily vary greatly in the number of bac- 

 teria which they contain. Thus, the river Seine as it enters 

 Paris has only about 300 bacteria in a c.c., whereas when 

 it reache's its suburb St. Denis, after receiving the sewage 

 of Paris, it contains 200,000 per c.c. This number in itself 

 is small owing to the rapid flow of the water and the con- 

 sequent enormous dilution of the sewage. A low altitude, 

 warm climate, abundance of organic matter and a slow cur- 

 rent will necessarily favor the multiplication of bacteria in 

 such river-water. The number of bacteria in river-water 

 under these conditions may easily rise to 100,000 bacteria 

 or more per c.c. The water of rapidly flowing rivers may, 

 as a rule, be said to contain less than 500 bacteria in a c.c. 



The ice formed on rivers and lakes will necessarily con- 

 tain, like the water itself, a variable number of bacteria. 

 The surface "snow-ice" will always contain a larger num- 

 ber than what is found in the clear ice. Thus, Prudden 

 found the clear ice obtained from the Hudson river, a few 

 miles below Albany, to yield 398 bacteria; whereas, the 

 snow-ice gave 9,187 bacteria per c.c. of the water obtained 



