g6 Elements of Water Bacteriology. 



ing-water is not advantageous. In comparing the results 

 of the tests in i c.c. and in loo cc, it will be noted that the 

 proportion of lactose fermenting organisms and of colon 

 bacilli in the unpolluted waters was more than doubled 

 in the latter; thus waters of good quality are more likely 

 to be condemned by the use of large samples. On the 

 other hand, in the polluted waters a considerable propor- 

 tion of the colon bacilli originally present were lost during 

 the incubation of the large samples, so that waters of 

 bad quality actually appeared to better advantage by the 

 use of loo cc. with preliminary incubation in phenol 

 broth." 



Whipple (Whipple, 1903) notes that 2.9 per cent of 

 some samples of water examined by him gave positive 

 tests with .1 c.c. but not with i cc, while 4.3 per cent 

 gave positive tests with ,t cc or i cc and negative tests 

 with 10 c.c. . Again, in another series of samples exam- 

 ined, of those which gave positive tests in smaller portions 

 5.3 per cent were negative in 10 cc, 4.7 per cent in 100 

 cc, and 7.7 per cent in 500 cc 



In our ordinary routine at the Institute we use one cubic 

 centimeter sample only, inoculating five or ten tubes in 

 dupHcate with that amoimt. In this way we ascertain 

 whether B. coli is generally or rarely present in one cubic 

 centimeter of the suspected water; and this is the infor- 

 mation of greatest practical value. If absent from one 

 cubic centimeter the presence of the organism in ten cubic 

 centimeters would not lead to the condemnation of the 



