no ELEMENTS OF WATER BACTERIOLOGY. 



characteristics of the water in question, and this study 

 can be carried out along two lines, chemical and bac- 

 teriological. 



A chemical examination of water for sanitary purposes, 

 is mainly useful in throwing light upon one point the 

 amount of decomposing organic matter present. Humus- 

 like substances may be abundant in surface-waters quite 

 free from harmful pollution, but these are stable com- 

 pounds. Easily decomposable bodies, on the other hand, 

 must obviously have been recently introduced into the 

 water and mark a transitional state. "The state of 

 change is the state of danger," as Dr. T. M. Drown has 

 phrased it. Sometimes the organic matter has been 

 washed in by rain from the surface of the ground, some- 

 times 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- 

 tion as to the antecedents of a sample. The results of 

 the chlorine determination are indeed perhaps more clear 

 than thfcse of any other sanitary analysis, for chlorine 

 and sewage pollution vary together, due allowance being 

 made for the proximity of the sea and other geological 



