156 



Elements of Water Bacteriology. 



first became apparent after ten to fifteen hours and 

 reached their maximum after twenty to sixty hours, accord- 

 ing to the number originally present. 



Applying the same method to polluted waters, similar 

 periodic changes were observed; pure cultures of B. coli 

 were first obtained, then the gradual displacement of one 

 form by the other took place, and at length the streptococci 

 were present either in pure culture or in great predomi- 

 nance as shown by the accompanying tables. The samples 

 of water were plated directly upon litmus lactose agar and 

 the plates were incubated at 37 for twenty-four hours, 



TABLE I. 



RELATIVE GROWTH OF B. COLI AND SEWAGE STREPTOCOCCI 

 FROM POLLUTED WATERS IN DEXTROSE BROTH. 

 (PRESCOTT AND BAKER, 1904.) 



