296 APPLIED BACTERlOLOGy 



supplies have been repeatedly proved by chemical analysis 

 to be of high organic purity. Moreover, it has been shown 

 that the organisms which are the cause of typhoid fever 

 and cholera may, when introduced into potable water of 

 good quality, not only retain their vitality for a consider- 

 able period of time, but may multiply almost indefinitely. 

 Therefore the slightest contamination with the alvine dis- 

 charges from a case of typhoid fever or cholera may serve 

 to render dangerous millions of gallons of drinking-water. 

 Thus it will be seen that the virulence of contaminated 

 water is not necessarily dependent upon the organic im- 

 purity of the water, but upon the specific pollution. 



A very important point to keep in view in the bac- 

 teriological examination of water is the great increase in 

 the number of organisms which takes place on keeping 

 the samples for a short time. Frankland states that a 

 pure water containing, say, 5 organisms per c.c. when 

 freshly drawn, may, even if kept in a sterile flask free 

 from aerial contamination, contain after a few days 

 perhaps 500,000 in the same volume — or, in other words, 

 as many as are found in slightly-diluted sewage. In fact, 

 it is precisely these purest waters, which initially contain 

 only a very small number of bacteria, that exhibit this 

 remarkable phenomenon of multiplication in the most 

 pronounced manner. Less pure waters, such as those of 

 ordinary rivers, for instance, and which contain initially 

 a large number of bacteria {e.g., 20,000 in 1 c.c.) exhibit, 

 when similarly treated, a much less conspicuous increase 

 in their bacterial population. He also points out, however, 

 that whilst in sewage the number of organisms only 

 gradually diminishes, in these pure waters ' after the rapid 

 increase in numbers follows a correspondingly rapid decline, 

 so that the numbers again fall below those found in impure 

 surface-waters.' The above facts must be constantly before 



