CHAPTER III. 



THE INTERPRETATION OF THE QUANTITATIVE BACTERI- 

 OLOGICAL ANALYSIS. 



The information furnished by quantitative bacteri- 

 ology as to the antecedents of a water is in the nature 

 of circumstantial evidence and requires judicial interpre- 

 tation. No absolute standards of purity can be estab- 

 lished which shall rigidly separate the good from the bad. 

 In this respect the terms "test" and "analysis" so univer- 

 sally used are in a sense inappropriate. Some scientific 

 problems are so simple that they can be definitely settled 

 by a test. The tensile strength of a given steel bar,' for 

 example, is a property which can be absolutely deter- 

 mined. In sanitary water examination, however, the 

 factors involved are so complex, and the evidence neces- 

 sarily so indirect, that the process of reasoning much more 

 resembles a doctor's diagnosis than an engineering test. 



The older experimenters attempted to establish arbi- 

 trary standards, by which the sanitary quality of a water 

 could be fixed automatically by the number of germs 

 alone. Thus Miquel (Miquel, 1891) published a table 

 according to which water with less than 10 bacteria per 

 c.c. was "excessively pure," with 10 to 100 bacteria, 



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