Presumptive Tests for B. Colt. 



143 



and Group VI waters too much polluted to be safely used 

 for domestic purposes. 



PERCENTAGE OF SAMPLES OF WATERS OF VARIOUS 

 SANITARY GRADES GIVING POSITIVE TESTS FOR B. 

 COLI WHEN DIFFERENT AMOUNTS WERE EXAMINED. 

 (WHIPPLE, 1903.) 



In view of these results Whipple suggested the following 

 provisional scheme of interpretation: 



Sanitary Quality. 



Presumptive Test for Bacillus Coli. 



o.oi 

 c.c. 



10. o 

 c.c. 



100 

 c.c. 



Safe 



Reasonably safe 

 Questionable . . 

 Probably unsafe 

 Unsafe . . . . 



+ 



+ 



o 



+ 

 + 

 + 



+ 

 + 

 + 

 + 



+ 

 + 

 + 

 + 

 + 



It is undoubtedly true that a negative presumptive 

 test is generally obtained with unpolluted waters. For ex- 

 ample, in a study previously cited, Winslow and Nibecker 

 (1903) reported that of 775 dextrose-broth tubes inotu- 

 lated from 259 unpolluted sources, only 41 showed gas. 



