134 



Elements of Water Bacteriology. 



test is in the control of the operation of municipal water 

 filters. It has been used for this purpose for ten years or 

 more at Lawrence, and Fuller laid stress upon its results 

 in his classic experiments on water purification in the 

 Ohio valley. At Cincinnati he records the presence of 

 colon bacilli in 60 per cent of the i-c.c. samples from the 

 Ohio River, while the effluent from either slow sand or 

 mechanical filters gave positive results only half the time 

 in samples of 50 c.c. The results of the examinations 

 carried out at Lawrence for six years are brought together 

 in the table below from the Annual Reports of the Massa- 

 chusetts State Board of Health. 



B. COLI IN MERRIMAC RIVER AND LAWRENCE FILTER 



EFFLUENT. 



* Not given. 



Effluents of better character are obtained by the filtration 

 of less polluted streams, though the per cent purification 

 effected is not so great as at Lawrence. Thus, at Harris- 

 burg, Pa., with one cubic centimeter samples, positive 

 tests were obtained 72 per cent of the time in the raw 



