6 ELEMENTS OF WATER BACTERIOLOGY 



contamination in streams and pools; third, stored waters 

 in lakes and large ponds where storage has reduced 

 bacterial numbers and produced a state of comparative 

 purity; and fourth, the ground-waters from which pre- 

 vious contamination has been even more completely re- 

 moved by filtration through the deeper layers of the soil. 



Bacterial Content of Various Waters. Even rain and 

 snow, the sources of our potable waters, are by no 

 means free from germs, but contain them in numbers 

 varying according to the amount of dust present in 

 the air at the time of the precipitation. After a long- 

 continued storm the atmosphere is washed nearly free 

 of bacteria, so that a considerable series of sterile plates 

 may often be obtained by plating i-c.c. samples. These 

 results are in harmony with the observations of Tissandier 

 (reported by Duclaux, 1897), who found that the dust 

 in the air amounted to 23 mg. per cubic meter in Paris 

 and 4 mg. in the open country. After a rainfall these 

 figures were reduced to 6 mg. and 0.25 mg., respectively. 



With regard to what may be considered normal values 

 for rain it is difficult to give satisfactory figures. Those 

 obtained by Miquel (Miquel, 1886) during the period 

 1 883-1 886 showed on the average 4.3 bacteria per c.c. 

 in the country (Montsouris) and 19 per c.c. in Paris. 

 Snow shows rather higher numbers than rain. Janowski 

 (Janowski, 1888) found in freshly fallen snow from 34 

 to 463 bacteria per c.c. of snow-water. 



As soon as the rain-drop touches the surface of the 

 earth its real bacterial contamination begins. Rivulets 

 from ploughed land or roadways may often contain 

 several hundred thousand bacteria to the cubic centi- 



