H4 ELEMENTS OF WATER' BACTERIOLOGY 



two instances in which the presumptive test revealed con- 

 tamination not shown by the chemical analysis: 



"A certain driven- well station was located in swampy 

 land along the shores of a stream, and the tops of the 

 wells were so placed that they were occasionally flooded 

 at times of high water. The water in the stream was 

 objectionable from the sanitary standpoint. The wells 

 themselves were more than ioo feet deep; they pene- 

 trated a clay bed and yielded what may be termed arte- 

 sian water. Tests for the presence of Bacillus coli had 

 invariably given negative results, as might be naturally 

 expected. Suddenly, however, the tests became positive 

 and so continued for several days. On investigation it 

 was found that some of the wells had been taken up to 

 be cleaned, and that the workmen in resinking them had 

 used the water of the brook for washing them down. 

 This allowed some of the brook- water to enter the system. 

 It was also found that at the same time the water in the 

 brook had been high, and because of the lack of packing 

 in certain joints at the top of the wells the brook-water 

 leaked into the suction main. The remedy was obvious 

 and was immediately applied, after which the tests for 

 Bacillus coli once more became negative. During all 

 this time the chemical analysis of the water was not suffi- 

 ciently abnormal to attract attention. On another occa- 

 sion a water-supply taken from a small pond fed by 

 springs, and which was practically a large open well, 

 began to give positive tests for Bacillus coli, and on exam- 

 ination it was found that a gate which kept out the water 



