62 ELEMENTS OF WATER BACTERIOLOGY 



be found in water under three headings: normal inhab- 

 itants, like B. fluorescens; unobjectionable aliens 

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

 (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 

 3 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 temperature. Experience shows, however, that 

 the numbers of such bacteria which actually reach 

 natural waters from such sources are seldom large. 

 The count at 37, therefore, helps to distinguish con- 

 tamination by wash of the soil of a virgin woodland 

 from pollution by excreta, since in the former case the 

 proportion of blood-temperature organisms is much 

 smaller than in the latter. Furthermore, this method 

 is free from much of the error introduced by the mul- 

 tiplication 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 plates showed 4200 bacteria per c.c., but agar 

 plates at 37 were sterile. 



Significance of the 37 Count. A majority of the 

 English Committee appointed to consider the standard- 



