QUANnTATIVE BACTERIOLOGICAL ANALYSIS 59 



recorded at Washington and Lawrence; and these 

 may be taken as tj'pical, since the Harrisburg plant is 

 the latest of its type, as the Washington plant is the 

 newest and most perfectly equipped of slow sand 

 filters. 



In well-managed purification plants the bacteria 

 in the effluent are determined daily, and any deviation 

 from the normal value at once reveals disturbing factors 

 which may impair the efficiency of the process. In 

 Prussia official regulations demand such systematic 

 examinations and prescribe 50 as the maximum number 

 of bacteria allowable in the filtered water. In the 

 same way the condition of an unpurified surface supply 

 may be determined by daily bacteriological analyses 

 and warnings of danger issued to the public, as has 

 been done at Chicago and other cities. In general, 

 any regular determination of variations from a normal 

 standard furnishes ideal conditions for the bacteriological, 

 methods; and the detection by Shuttleworth (Shuttie- 

 worth, 1895) of a break in a conduit under Lake Ontario 

 by a rise in the bacteria of the Toronto water-supply 

 may be cited as a classic example of its application. 



Often, however, the expert is called to pass upon the 

 character of a water of which no series of analyses is 

 available. In such cases an inspection of the location 

 from which the water comes should be insisted on, as a 

 sound interpretation of a water analysis can only be made 

 with a reasonably full knowledge of the source of the 

 sample. After a careful sanitary inspection, however, 

 the comparison of the result of even a single examina- 

 tion with the normal range for waters of the same class 



