Quantitative Bacteriological Examination. 49 



Kohn (1906) determined the minimal nutrient material re- 

 quisite for certain of the latter, and found that they could 

 develop in the presence of 198 X 10 - " to 198 X 10 - ^' 

 per cent of dextrose, 66 X 10 - " to 66 X 10 - " per cent 

 ammonium sulphate and 66 X 10 - " to 66 X 10 - " per 

 cent ammonium phosphate. Such 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 bacteria 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 bac- 

 teriological analysis. 



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 



