34 BACTERIOLOGICAL EXAMINATION OF WATER. 



was thought that the bacteria which appeared in the filtrate were 

 derived solely from the inferior system and had no relation to 

 the bacteria applied to the surface of the bed. FrankePs experi- 

 ments with B. violaceus and the Lawrence experiments with B. 

 prodigiosus showed conclusively that applied bacteria do find 

 their way into the filtrate. Friinkel poured water charged with 

 B. violaceus on to a ripe filter, and found colonies of the organ- 

 ism in the filtrate. In order to avoid the objection that the 

 B. violaceus might have gradually grown through the filter owing 

 to the nutriment supplied by the broth culture, Frankel made 

 his cultures in broth so dilute that their addition increased the 

 nutritive powers of the water only in a very infinitesimal degree. 

 The State Board of Health, Massachusetts, has carefully inves- 

 tigated the working of sand filters by means of experimental 

 beds at Lawrence. The Board employed the terms " bacterial 

 efficiency," " bacterial purification," and " hygienic efficiency " to 

 express the results of the action of sand filters. "Bacterial 

 efficiency " is the percentage of the number of bacteria in the 

 applied water which fails to appear in the effluent. The number 

 of bacteria in the effluent, however, includes those which have 

 their origin in the lower portions of the filter and its under-drains, 

 as well as those which have passed directly through the filter from 

 top to bottom. At the present time there are no well-defined 

 methods by which the two classes of bacteria can be readily 

 separated. " Bacterial purification " is the percentage removal by 

 filtration of the applied bacteria. The expression bears no relation 

 to those bacteria in the effluent which have their origin within the 

 filter or under-drains; consequently to gain an idea of the bacterial 

 purification produced by a filter it is necessary to employ a special 

 micro-organism. The Board employed the B. prodigiosus because 

 it appeared incapable of growth within the filters and their under- 

 drains and could be readily detected in the effluents. The following 

 table shows some of the results obtained at Massachusetts : 



Percentage of bacteria which 



Dentil of sind Form of Ratc pcr appeared in the nitrate. 



/na - nitration. acre daily. , * N 



B. Trodigiosus. Water bacteria. 



5 feet continuous 7,660,000 0'171 1'48 



7,700,000 0-148 M9 



12 inches ., 3,700,000 0'337 2'34 



5 feet intermittent 3,600,000 0'463 3- 10 



continuous 5,550,000 0'183 T04 



