BACTERIOLOGICAL EXAMINATION. 113 



the litmus-lactose-agar plate serve for what Whipple has 

 well called presumptive tests. 



The results of the bacteriological examination have, in 

 several respects, a peculiar and unique significance. 

 First, this examination is the most direct method of sani- 

 tary water analysis. What we dread in drinking-water is 

 the presence of pathogenic bacteria, mainly from the 

 intestinal tract of man, and it is quite certain that the 

 related non-pathogenic bacteria from the same source 

 will behave more nearly as these disease germs do than 

 will any chemical compounds. In the second place, the 

 bacteriological methods are superior in delicacy to any 

 others. Klein and Houston (Klein and Houston, 1898) 

 showed by experiment with dilutions of sewage that the 

 colon test was from ten to one hundred times as sensitive 

 as the methods of chemical analysis; and studies of the 

 self-purification of streams have confirmed his 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 



