216 ELEMENTS OF WATER BACTERIOLOGY 



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. 



Sanitary Chemical Analysis. A chemical examina- 

 tion of water for sanitary purposes is mainly useful 

 in throwing light upon one point the amount of decom- 

 posing organic matter present. It also gives an his- 

 torical picture which may be of much value. Humus- 

 like substances may be abundant 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 transitional state. " The 

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

 Drown once phrased it. Sometimes the organic mat- 

 ter has been washed in by rain from the surface of the 

 ground, sometimes it has been introduced in the more 

 concentrated form of sewage. In any case, it is a warn- 

 ing 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 infor- 

 mation as to the antecendents of a sample. The results 

 of the chlorine determination are indeed perhaps more 

 clear than those of any other part of the analysis, for 

 chlorine and sewage pollution vary together, due allow- 



