54 ELEMENTS OF WATER BACTERIOLOGY 



ammonium sulphate and 66Xio~^^ to 66X10"^^ per 

 cent ammonium phosphate. Similar minute amounts 

 of organic matter are found in the purest of natural 

 waters and under exceptional conditions certain species 

 of bacteria may therefore multiply in bottled samples, 

 or, at times, in a well or the basin of a spring. In 

 normal surface-waters, such growths of the prototrophic 

 forms do not apparently occur. Here it is found as a 

 matter of practical experience that the number of bac- 

 teria present depends upon the extent to which the 

 water has been contaminated with decomposing organic 

 matter, either by pollution with sewage or by contact 

 with the surface of the ground. The bacterial content 

 varies as the extent and character of the contamination 

 varies. It measures not merely organic matter, but 

 organic matter in a state of active decay, and like the 

 ammonias and other features of the sanitary chemical 

 analysis, indicates fresh organic pollution, with the added 

 advantage that the presence of the stable nitrogenous 

 compounds often present in peaty waters introduces 

 no error in the bacteriological analysis. 



Bacterial Content of Surface-waters. In judging of a 

 surface-water the student will be aided by reference to 

 the figures given for certain normal sources in Chapter 

 I; the Boston tap water with 50 to 200 bacteria per 

 c.c. (Philbrick, 1905) and the water of Lake Zurich 

 with an average of 71 in summer and 184 in winter 

 (Cramer, 1885) may be taken as typical of good potable 

 waters; and numbers much higher than these are open 

 to suspicion, since all contamination whether contributed 

 by sewage or by washings from the surface of the 



