The Bacteria in Natural Waters 7 



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 raindrop 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- 

 meter; and furthermore the amounts of organic and 

 mineral matters 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 these enormous bacterial numbers somewhat lower. 

 Ground-water containing little microbic life enters as a 

 diluting factor from below. The larger particles of organic 

 matter are removed by sedimentation; many earth 

 bacteria, for which water is an unfavorable medium, 

 gradually perish; and in general a new condition of 

 equilibrium tends to be established. It is difficult, how- 

 ever, to find a river in inhabited regions which does not 

 contain several hundreds or thousands of bacteria to the 

 cubic centimeter. Furthermore, heavy rains which intro- 

 duce wash from the surrounding watershed may at any 

 time upset whatever equilibrium exists, and surface- 

 waters are apt to show sudden fluctuations in their bac- 

 terial content. Particularly in the spring and fall high 

 numbers manifest themselves, and seasonal variations 

 arise, such as are shown in the appended table. 



