270 PURiFiCATiojsr or public water supplies. 



quantity witb. no effort to determine its origin or character. 



In the latter there are sixteen determinations, three of 

 which relate to mineral substances and the remaining 

 thirteen to ascertaining the character of the organic mat- 

 ters. 



The nitrogen compounds are especially interesting be- 

 cause of the change which takes place in the process of 

 purification, called nitrification, wherein the ammonias 

 and the nitrites diminish and the nitrates increase. 



When the former analysis was made bacteriology, as 

 an aid to chemical analysis, as now practiced, was un- 

 known. The belief that all forms of disease transmitted 

 by water have their origin in those minute objects termed 

 bacteria, and the knowledge that the actual number, 

 and in some cases the precise kind, of bacteria contained 

 in a given water may be determined, has brought this 

 method of water examination into great prominence. Of 

 course it does not do away with the necessity for chemi- 

 cal analysis, for water may contain mineral constituents 

 very objectionable and which can only be determined by 

 chemical examination. Moreover, the differentiation of 

 specific bacteria is a tedious and costly operation, and 

 therefore when the number of bacteria in a water is 

 determined the probable presence of pathogenic germs is 

 usually inferred from the chemical analysis and the sur- 

 rounding conditions. 



When, however, the normal chemical constitution and 

 surrounding conditions of a water have been determined, 

 the efficiency of any process of purification can be reli- 

 ably observed by a comparison of the number of bacteria 

 present in the applied and effluent waters of the system 

 in use. 



Until within a very few years, in this country the 

 principal object aimed at in securing public water sup- 

 plies has been never-failing abundance. We desire 

 plenty. Having been endowed as a nation with plenty 



20S 



