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 anunonia, 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,- 



