8 ELEMENTS OF WATER BACTERIOLOGY. 



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

 figures were reduced to 6 mg. and .25 mg., respectively. 



With regard to what may be considered normal values 

 for rain we have no very satisfactory figures. Those 

 obtained by Miquel (Miquel, 1886) during the period 

 1883-1886, showing that rain contains on the average 4.3 

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

 c.c. in Paris, are probably lower than would be yielded by 

 the present methods of examination. 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, and his results indicate that the num- 

 ber is independent of the temperature at the time of 

 snowfall. 



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 centimeter; and 

 furthermore the amounts of organic and mineral mat- 

 ters which serve as food materials, and thus become a 

 factor in later multiplication of organisms, are greatly 

 increased. 



In the larger streams several conditions combine to 

 make the bacterial numbers lower. Ground-water con- 

 taining little microbic life enters as a diluting factor from 

 below. The larger particles of organic matter are re- 

 moved from the flowing water by sedimentation; many 

 earth bacteria, for which water is an unfavorable medium, 

 gradually perish ; and in general a new condition of equi- 



