THE BACTERIA IN NATURAL WATERS. 7 



the surface-waters immediately exposed to such contami- 

 nation in streams and ponds; third, the ground- waters 

 from which previous pollution has been more or less 

 removed by filtration through the deeper layers of the 

 soil. 



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 



Fig. 1. 

 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 re- 

 sults 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 



