424 • BACTBKIOIiOGY. 



excess of chlorine and nitrogenous matter would therefore 

 indicate the presence of animal excreta. An increase, for 

 instance, in the amount of chlorine might be due to the 

 presence of sodium chloride derived from urine. Similarly, 

 large quantities oi free and albuminoid ammonia are 

 usually taken to indicate the presence of urea. Unchanged 

 urea would be represented by albuminoid ammonia. But, 

 as is known, urea readily undergoes ammoniacal fermenta- 

 tion and yields ammonia and carbonic acid. Other bacteria 

 attack the ammonia and convert the nitrogen present into 

 nitrous and nitric acids. For these reasons an increase in 

 free and albuminoid ammonia, and in nitrates and nitrites 

 is taken as indicating a pollution with organic, nitrogenous, 

 waste matter. 



The assumption of the existence of a pollution is not 

 always justifiable whenever the limits, as given above, are 

 exceeded. Thus, in Michigan it is not uncommon, on ac- 

 count of the peculiar geological formation, to meet with a . 

 marked increase in the amount of chlorine or salt. More- 

 over, in many, especially low lying sections, the water will 

 be rich in nitrogenous constituents which in this case are 

 derived from vegetable and not animal matter. * 



It is evident from what has been said that a chemical 

 analysis may, or may not, indicate the presence of pollution 

 with animal excreta. Furthermore, the important fact 

 should not be overlooked that a specific organism may be 

 present, and hence render a water extremely dangerous 

 without any appreciable increase in the chemical consti- 

 tuents of such water. A chemical analysis of water is use- 

 ful and it should always be carried out, but the results ob- 

 tained should, as a rule, be subordinated to those obtained 

 by a bacteriological examination. 



A bacteriplogical examination of water is intended to 

 reveal the number and kind of bacteria that may be pres- 

 ent. As a rule, the smaller the number of bacteria in a 



