6 Elements of Water Bacteriology. 



relation to the rich layers of bacterial growth upon the 

 surface of the globe. There are first the atmospheric- 

 waters which have never been subject to contact with 

 the earth; second, the surface-waters immediately exposed 

 to such contamination in streams and pools; third, the 

 lakes and large ponds in which storage has reduced 

 bacterial numbers to a state of comparative purity; 

 and fourth, the ground-waters from which previous con- 

 tamination has been even more completely removed by 

 filtration through the deeper layers of the soil. 



Even rain and snow, the original 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 rain- 

 fall these figures were reduced to 6 mg. and .25 mg., 

 respectively. 



With regard to what may be considered normal values 

 for rain, satisfactory data are not abundant. Those 

 obtained by Miquel (Miquel, 1886), during the period 

 1883-1886, showed on the average 4.3 bacteria per c.c. in 

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



