38 ELEMENTS OF WATER BACTERIOLOGY. 



are aware. Here the numbers of bacteria present will 

 depend, other things being equal, upon the extent to 

 which he water has been contaminated with decomposing 

 organic matter, either by pollution with sewage o: by 

 contact with the surface of the ground. The bacterial con- 

 tent will therefore vary 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 

 thus, like the ammonias and other features of the sanitary 

 chemical analysis, clearly indicate, 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. 



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 bac- 

 teria per c.c. (Whipple, 1896) 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 ground 

 is a possible source of danger. The excess of bacteria 

 in surface-waters during the spring and winter months is 

 by no means an exception to the general rule that high 

 numbers are significant, since the peril from supplies of 

 this character is clearly shown by the spring epidemics of 

 typhoid fever which at the times of melting snow visit 

 communities making use of unprotected surface-waters. 



