232 



STEPHEN DEM. GAGE 



24 hours. At 30 C., however, our 24-hour counts show us only 

 30 to 50 per cent of the bacteria which would be able to produce 

 colonies if the period of incubation were increased to two or three 

 days. In Table i is shown the per cent of samples in which bac- 

 teria and acid-producing types occurred in waters from different 

 sources when examined by the various methods. The results of deter- 

 minations of acid-producing bacteria at 20 divide themselves nat- 

 urally into groups. All of the samples of sewage and effluents 

 from sewage filters, and over 80 per cent of the polluted waters 

 and effluents from sewage filters, contained bacteria of this type 

 the percentage for shallow wells dropping to 70, for pond waters 

 to 60, and for deep wells to 40. The springs differed from the other 

 relatively pure waters in that all samples contained acid-producing 

 bacteria. At 30 C. the percentage of samples showing growth 

 and acid-producing bacteria is generally indicative of the quality 

 of the water, becoming less as the quality of the water improves. 

 The same rule holds for the determinations made at 40 C., the 



TABLE i. 



RELATIVE OCCURRENCE OF BACTERIA AND OF ACID-PRODUCING TYPES ON PLATES INCUBATED AI DIF- 

 FERENT TEMPERATURES WITH VARIOUS CLASSES OF WATERS. 



distinction between samples known to have been polluted recently, 

 even although they have been subjected to purification, and samples 

 from sources whose chance of pollution is more remote, being espe- 

 cially well marked. The driven- well waters showed entire absence 

 of bacteria growing at this temperature. The distinction between 

 sewage and polluted water before filtration, and the effluents from 



