Quantitative Bacteriological Examination. 47 



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

 1000 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 bacteriologists have placed the 

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

 various German observers range, for example, from 50 to 

 300. Dr. Sternberg (Sternberg, 1892) in a more conserva- 

 tive fashion, has stated that a water containing less than 

 100 bacteria is presumably from a deep source and uncon- 

 taminated 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 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 the indi- 

 cation of danger. Furthermore, 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 Tiigh degree of caution. 

 As pointed out in the previous chapter, the touch of a 



