58 Elements of Water Bacteriology. 



under such conditions, while most of those derived from 

 other sources do not. Savage classifies the bacteria 

 which may be found in water under three headings: 

 normal inhabitants, like B. fluorescens; unobjectionable 

 aliens (from soil), hke B. mycoides, and objectionable 

 ahens (from excreta), like B. coli. The first sort and 

 many of the second sort are generally unable to grow at 

 37 degrees. This criterion is not an absolute one. Savage 

 (1906) reports an experiment in which unpolluted soil, 

 which had not been manured or cultivated for at least 

 three years, was added to tap water, with the result that a 

 20° count of 76 was increased to 1970, and a 37^ count of 3 

 was raised to 1630. In this case most of the bacteria in 

 the soil were capable of development at body tempera- 

 ture. Experience shows, however, that the numbers of 

 such bacteria which actually reach natural waters from 

 such sources are seldom large. The count at 37 degrees, 

 therefore, helps to distinguish contamination by wash of 

 the soil of a virgin woodland from pollution by excreta, 

 since in the former case the proportion of blood-tempera- 

 ture organisms is much smaller than in the latter. Further- 

 more, this method is free from much of the error introduced 

 by the multiplication of bacteria after the collection of a 

 sample, as most of the forms which grow in water during 

 storage cannot endure the higher temperature and conse- 

 quently do not develop upon incubation. Recently, for 

 example, water from a spring of good quality was shipped 

 to the laboratory from a considerable distance. Gelatin 



