THE BACTERIA IN NATURAL WATERS. 15 



of the food-supply is to be found, I believe, the main rea- 

 son for the bacterial self -purification of streams." 



In general we have seen then that surface-waters tend 

 continually to decrease in bacterial content after their 

 first period of contact with the humus layer of the soil. 

 In that other portion of the meteoric water which pene- 

 trates below the surface of the earth to join the reservoir 

 of ground-water, later to reappear as the flow of springs 

 and wells, this diminution is still more marked since 

 the filtering action of the earth removes not. only most 

 of the bacteria but much of their food material as well. 

 Indeed many observers formerly believed that all ground- 

 waters were nearly free from bacteria, because often no 

 colonies appeared on plates counted after the ordinary 

 short periods of time. If, however, a longer period of 

 incubation be adopted considerable numbers may be 

 obtained. 



For convenience we may divide ground- waters into two 

 groups, namely: first, springs and shallow open wells, and 

 second, "tubular" (driven) or deep wells. This division is 

 a convenient one because ordinary springs and wells form 

 a group by themselves in respect to the possibility of 

 aerial and surface contamination, their water often being 

 fairly rich in bacterial life. Egger (Wolff hugel, 1886) ex- 

 amined 60 wells in Mainz and found that 17 of them 

 contained over 200 bacteria to the cubic centimeter. 

 Maschek (Maschek, 1887) found 36 wells out of 48 exam- 

 ined in Leitmeritz which had a bacterial content of over 

 500 per c.c. Fischer (Horrocks, 1901) reported 120 wells 



