CHAPTER III. 



THE INTERPRETATION OF THE QUANTITATIVE BAC- 

 TERIOLOGICAL 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 analysis, however, the factors 

 involved are so complex and the evidence necessarily so 

 indirect that the process of reasoning much more re- 

 sembles 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 numbers of germs 

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

 according to which water with less than 10 bacteria per c.c. 

 was "excessively pure," with 10 to 100 bacteria, "very 



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