9° ELEMENTS OF WATER BACTERIOLOGY. 



diluting factor, the numbers were so reduced that 

 positive results were obtained only on eleven days out 

 of twenty in one-thousandth of a cubic centimeter, on 

 twenty days out of thirty in one-hundredth of a cubic 

 centimeter, and on twenty days out of twenty-three in 

 one-tenth of a cubic centimeter. At Averyville, one hun- 

 dred and fifty-nine miles below Chicago, colon bacilli were 

 isolated on only four days out of twenty-seven in one- 

 tenth of a cubic centimeter, and on thirteen days out of 

 thirty-one in one cubic centimeter. A comparison with 

 certain neighboring rivers showed this to be about the 

 normal value for waters of that character, as the following 

 table extracted from Professor Jordan's paper will show. 



NUMBER OF B. COLI PRESENT IN CERTAIN RIVER WATERS. 

 (Jordan, 1901.) 



.1 c.c. 1 c.c. 



No. Days No. Days No. Days No. Day 

 Source of Sample. Water B. Coli Water B. Coli 



Examined. Found. Examined. Found. 



Illinois River, Averyville 27 4 31 13 



Mississippi River, Grafton. . 34 10 35 23 



Fox River 22 2 23 6 



Sangamon River 25 14 27 21 



Missouri River 32 13 31 21 



These results harmonize rather closely with those pre- 

 viously recorded by Brown and Fuller and indicate that 

 in the larger rivers where the proportionate pollution is 

 not extreme, colon bacilli may be isolated in about half 

 the i-cc. samples examined. Such rivers are of course 

 inadmissible as sources of water-supply, according to 

 modern sanitary standards, unless subjected to pre- 

 liminary treatment. 



