CHAPTER III 



THE INTERPRETATION OF THE QUANTITATIVE 

 BACTERIOLOGICAL EXAMINATION 



Standards for Potable Water. The information fur- 

 nished by quantitative bacteriology as to the antecedents 

 of a water is in the nature of circumstantial evidence 

 and requires judicial interpretation. No absolute stand- 

 ards of purity can be established which shall rigidly 

 separate the good from the bad. In this respect the 

 terms " test " and " analysis " so universally 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 determined. 

 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 

 arbitrary 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 bac- 

 teria per c.c. was " excessively pure," with 10 to 100 



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