36 ELEMENTS OF WATER BACTERIOLOGY. 



pure," with 100 to 1000 bacteria, "pure," with 1000 to 

 10,000 bacteria, " mediocre," with 10,000 to 100,000 bac- 

 teria, " 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 bacteriologists have placed the 

 standard of "purity" much lower. The limits set by 

 various German observers range, for example, from 50 to 

 .300. Even Dr. Sternberg (Sternberg, 1892), in a much 

 more conservative fashion, has stated that a water con- 

 taining 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 bacteria is presumably contaminated by 

 sewage or surface drainage. This is probably as satisfac- 

 tory 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 inter- 

 pretation of analyses; a bacterial count which would con- 

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

 figures in excess of those common to unpolluted waters 

 of the same character giye the indication of danger. Fur- 

 thermore, the bacteriological tests are far more delicate 

 than any others at our command, very minute additions 

 of food material causing an immense multiplication of 

 the microscopic flora. This delicacy necessarily requires, 

 both in the process of analysis and the interpretation of 

 results, a high degree of caution. As pointed out in the 

 previous chapter, the touch of a finger or a particle of 



