MUNICIPAL WATER SUPPLIES. Ill 



Operating a water supply sj'stem because any sudden change 

 in the bacterial count, unaccompanied by some cause, as for 

 instance a rainfall,indicates something calling for examination. 

 The presence of Bacillus coli communis in a given water 

 can only be absolutely demonstrated by the bacteriologist. 

 There is, however, a certain series of preliminary tests which 

 can be made in routine work, which tests decide whether 

 or not the water is to be looked upon with suspicion. If 

 these tests are responded to positively, the water should be 

 immediately turned over to the expert bacteriologist for care- 

 ful determination of species of bacteria. If the reactions are 

 negative we may rest assured that the water is reasonably free 

 from sewage contamination. There are some eight or ten 

 common tests which ma}' be used, but for practical routine 

 study of the operation of a given water system the following 

 only need be applied : — 



1. A certain number of small fermentation tubes are 

 charged with dextrose bouillon (ordinary culture bouillon 

 with a small amount of dextrose added) and each inoculated 

 with I cc. of the given sample of water. These are then set 

 aside to ferment and if Bacilhis coli communis is present gas 

 will form abundantly in some of the tubes. An idea of the 

 number of bacteria present may be formed by observing the 

 relative number of tubes which show the reaction. 



2. The amount of gas produced may be expressed in 

 percentages of the length of the closed limb of the fermenta- 

 tion tube. If Bacillus coli communis is present the closed 

 limb will be about half full of gas by the end of the third 

 day. The gas will usually all be evolved by the end of the 

 second day. Twentj^-five per cent, of gas or seventy-five per 

 cent, of gas will not point to Bacillus coli communis , espe- 

 cially if the other tests are negative. 



3. The gas formed by Bacilhis coli communis is usually 

 nearly all evolved during the first twenty-four hours. Read- 



