EXAMINATION OF WATER 587 



bottle in ice. As a rule in water of good quality liquefying 

 organisms are scanty, while in a polluted water they are 

 numerous. 



Number of colonies on the agar plates. As mentioned 

 before (see p. 581), it is the ratio of the number of organisms 

 developing on the agar plates to the number of those 

 developing on the gelatin plates that is of importance. 



Number of B. coli. The detection and enumeration of 

 B. coli are regarded by all as perhaps the most important 

 part of water examination. The number of B. coli is esti- 

 mated from the amounts of water that have been added 

 to the tubes of media, which, however, assumes that the 

 organism is regularly distributed throughout the sample, 

 and this must so far as possible be ensured by thorough 

 mixing. The results generally come out fairly concor- 

 dantly, though irregularities exceptionally occur which 

 can only be obviated by making duplicate sets of cultures. 



It is better to state the result as " B. coli present in 



c.c. of water " rather than to say that so many B. coli are 

 present, though as a matter of fact the latter statement 

 is approximately correct. Adopting the writer's method 

 for B. coli (p. 584), if none of the tubes contains B. coli, 

 we say that " B. coli is absent from 50 c.c. " ; if the 25 c.c. 

 tube contains B. coli, but not the remainder, " B. coli is 

 present in 25 c.c. but not in less," and so on. 



If nothing is known about the water, the following 

 standards may be adopted : 



(a) Waters of good quality. B. coli absent in 50 c.c. 

 of the water. 



(b) Waters of medium quality. B. coli present in 50 c.c. 

 but absent in 25 c.c. 



(c) Waters of poor quality. B. coli present in 50 c.c. and 

 25 c.c., but absent in 10 c.c. 



(d) Waters of suspicious quality. B. coli present in 

 50 c.c., 25 c.c., and 10 c.c., but absent in 1 c.c. 



