Bacteriological Examination. 171 



intake; whether pollution of a stream has or has not 

 been removed by a certain period of flow. Evidence 

 upon these points must be obtained from a careful study 

 of the characteristics of the water in question, and this 

 study can be carried out along two lines, chemical and 

 bacteriological. 



A chemical examination of water for sanitary purposes 

 is mainly useful in throwing light upon one point the 

 amount of decomposing organic matter present. It also 

 gives an historical picture which may be of some value 

 or suggestiveness. Humus-like substances may be abun- 

 dant in surface-waters quite free from harmful pollution, 

 but these are stable compounds. Easily decomposable 

 bodies, on the other hand, must obviously have been 

 recently introduced into the water and mark a transi- 

 tional state. "The state of change is the state of danger," 

 as Dr. T. M. Drown once phrased it. Sometimes the 

 organic matter has been washed in by rain from the sur- 

 face of the ground, sometimes it has been introduced in 

 the more concentrated form of sewage. In any case, it 

 is a warning of possible pollution, and the determination 

 of free ammonia, nitrites, carbonaceous matter, as shown 

 by "oxygen consumed," and dissolved oxygen yield 

 important evidence as to the sanitary quality of a water. 



Furthermore, nitrates, the final products of the oxida- 

 tion of organic matter, and the chlorine introduced as 

 common salt into all water which has been in contact 

 with the wastes of human life, furnish additional informa- 



