86 ELEMENTS OF WATER BACTERIOLOGY. 



either slow sand or mechanical filters, the effluent gave 

 positive results about one-half the time in samples of 

 50 c.c. 



The last three reports of the Massachusetts State 

 Board of Health contain reports of the abundance of 

 B. coli in a more highly polluted stream, the Merrimac 

 at Lawrence. In 1898 (Massachusetts State Board of 

 Health, 1899) the number of organisms found by making 

 litmus-lactose-agar plates directly and inspecting the 

 colonies varied from 20 per c.c. in May and June to 92 per 

 c.c. in August and September, the average for the year 

 being 47. Of 117 samples of the water which had passed 

 through the city filter, only 9 showed the organism in a 

 single colony. 



In 1899 (Massachusetts State Board of Health, 1900) 

 the study was considerably extended. The average num- 

 ber of colon bacilli in the river at the intake of the filter 

 was again 47, and in only 1 sample out of 180 was it absent ; 

 below the city at the Lawrence Experiment Station the 

 additional pollution raised the average number to 103. 

 The effluent as it came directly from the filter showed 

 B. coli in 24 per cent of the cubic centimeters examined, 

 but at the outlet of the reservoir, the proportion had 

 fallen to 7 per cent and at the Experiment Station, after 

 passage through the distribution pipes, to only 4 per cent. 

 The results obtained in the next year, 1900, were prac- 

 tically the same, but parallel tests were made in a larger 

 volume of water by incubating with the addition of phenol 



