52 ELEMENTS OF WATER BACTERIOLOGY 



bacteria, " very pure," with loo to looo bacteria, 

 " pure," with looo to 10,000 bacteria, " mediocre," with 

 10,000 to 100,000 bacteria, " impure," and with over 

 100,000 bacteria, " very impure." Few sanitarians 

 would care to dispute the appropriateness of the titles 

 applied to waters of the last two classes; but many bac- 

 teriologists have placed the standard of " purity " much 

 higher. The limits set by various German observers 

 range, for example, from 50 to 300. Dr. Sternberg 

 (Sternberg, 1892) in a more conservative fashion, 

 has stated that a water containing less than 100 bacteria 

 is presumably from a deep source and uncontaminated 

 by surface drainage; that one with 500 bacteria is 

 open to suspicion; and that one with over 1000 bac- 

 teria is presumably contaminated by sewage or surface 

 drainage. This is probably as satisfactory an arbitrary 

 standard as could be devised, but any such standard 

 must be applied with great caution. The source of 

 the sample is of vital importance in the interpretation 

 of analyses; a bacterial count which would condemn 

 a spring might be quite normal for a river; only figures 

 in excess of those common to unpolluted waters of the 

 same character give an indication of danger. Fur- 

 thermore, the bacteriological tests are far more delicate 

 than any others at our command, very minute addi- 

 tions of food material causing an immense multiplica- 

 tion of the microscopic flora. This delicacy necessarily 

 requires, both in the process of analysis and the inter- 

 pretation of results, a high degree of caution. As 

 pointed out in the previous chapter, the touch of a 

 finger or the entrance of a particle of dust may wholly 



