SIGNIFICANCE OF COLON GROUP IN WATER 167 



effluent in 72 per cent of the samples examined. In 

 January and February the organisms were found in 

 54 per cent and 62 per cent of the samples, respectively, 

 while in March the number fell to a normal value of 

 8 per cent. Corresponding to this excess of B. coli 

 in the city water, there were 12 cases of t>^hoid fever 

 in December, 59 cases in January, 12 in February, 

 and 9 in March, all during the early part of the month. 

 The authors conclude that "when filtering a river- 

 water as polluted as that of the Merrimac, it is safe 

 to assume that when B. coli is found only infrequently 

 in I c.c. of the effluent, the typhoid germs, necessarily 

 fewer in number and more easily removed by the 

 filter, have been eliminated from the water." 



The results of the daily tests carried out at mimicipal 

 filter plants are frequently expressed in monthly or 

 yearly averages, as in some of the cases quoted above. 

 It must be remembered, however, that averages of 

 this sort are accepted only by courtesy and with the 

 implied assumption that conditions are approximately 

 constant during the period averaged. WTien it is 

 said that an acceptable effluent may show B. coli in 

 3 or 4 per cent of the samples tested the statement is 

 true only for a series of samples collected and examined 

 at the same time. If in a given month 3 per cent of 

 the I c.c. samples tested show B. coli, the effluent may 

 or may not be safe. If on each of 20 days 3 B. coli 

 or thereabouts were present in 100 c.c. of the water 

 it is probably a safe one. If on 19 days no B. coli 

 were present, and on the twentieth day 100 c.c. showed 

 60 B. coli, the average result would be the same, but 



