Bacteriological Examination. 177 



sensitive as the methods of chemical analysis; and studies 

 of the self-purification of streams have confirmed their 

 results on a practical scale. Thus in the Sudbury River 

 it was found that while the chemical evidences of 

 pollution persisted for six miles beyond the point of 

 entrance, the bacteria introduced could be detected for 

 four miles further (Woodman, Winslow, and Hansen, 

 1902). 



The statement is sometimes made that while bac- 

 teriological methods may be more delicate for the detec- 

 tion of pollution in surface-waters, contamination in 

 ground-waters may best be discovered by the chemical 

 analysis. That such is not the case has been well shown 

 by Whipple (Whipple, 1903), who cites the following two 

 instances in which the presumptive test revealed con- 

 tamination not shown by the chemical analysis: 



" A certain driven- well station was located in swampy 

 land along the shores of a stream, and the tops of the 

 wells were so placed that they were occasionally flooded 

 at times of high water. The water in the stream was 

 objectionable from the sanitary standpoint. The wells 

 themselves were more than 100 feet deep; they pene- 

 trated a clay bed and yielded what may be termed arte- 

 sian water. Tests for the presence of Bacillus coli had 

 invariably given negative results, as might be naturally 

 expected. Suddenly, however, the tests became positive 

 and so continued for several days. On investigation it 

 was found that some of the wells had been taken up to 



