BACTERIA OF WATER AND SEWAGE 295 



watershed than from a virgin soil ; more, and particularly 

 more dangerous kinds, when the watershed is inhabited than 

 when it is uninhabited. Many of the bacteria which get into the 

 water are capable of growing there and forming what is known 

 as the water bacteria, or the normal bacterial flora of the water. 

 Many of these bacteria are able to grow even in waters that 

 contain very little organic matter. Certain species of bacteria 

 can practically always be found in a water supply. In spite 

 of the fact, however, that they seem to be well adapted to 

 life in water they do not grow beyond certain limits. One 

 would naturally think that if bacteria existed in water fitted 

 for their development that they would soon increase to enor- 

 mous numbers, but this is not true. Protected from the 

 contamination which comes from the soil, the bacterial content 

 of a surface water remains quite constant and would give a 

 comparatively low count. When these same waters are sepa- 

 rated from their natural condition, as, for instance, when they 

 are bottled ready for analysis, the number of bacteria in them 

 increases rapidly to an enormous extent. This indicates that 

 there is in natural waters a germicidal substance which hinders 

 the excessive multiplication of water bacteria. Just what this 

 bactericidal property is due to is an open question. 



The Species of Bacteria in Water. Certain bacteria 

 find their optimum conditions for development in water. 



These are the bacteria which are ordinarily spoken of as " water 





 bacteria." They have been extensively studied by a number 



of investigators, but are not important, so far as we know, in 



