30 ELEMENTS OF WATER BACTERIOLOGY 



ground. In this country Gage and Phelps (Gage and 

 Phelps, 1902) showed that the numbers obtained by 

 the ordinary procedure were only from 5 to 50 per cent 

 of those obtained by the use of Heyden's Nahrstoff 

 agar. For practical sanitary purposes, however, our 

 methods are fairly satisfactory. Within limits, it is 

 of no great importance that one method allows the 

 growth of more bacteria than another. When we are 

 using the quantitative analysis as a measure of sewage 

 pollution the essential thing is that the section of the 

 total bacterial flora which we obtain should be thor- 

 oughly representative of that portion of it in which 

 we are most interested- the group of the quickly 

 growing, rich-food-loving sewage forms. In this respect 

 meat-gelatin-peptone appears to be unrivalled; and it 

 is in this respect that such media as Nahrstoff agar fail. 

 Miiller (1900) showed that the larger counts obtained 

 by plating on the Nahrstoff medium are due to the 

 fact that it specially favors the more prototrophic 

 forms, among the water bacteria themselves. Intestinal 

 organisms and even the ordinary putrefactive germs, 

 when plated in pure culture, show no higher counts on 

 Nahrstoff agar than on gelatin. Gage and Adams 

 (1904) found by plating pure cultures of the common 

 laboratory bacteria, saprophytes and parasites, that 

 Nahrstoff counts were actually lower than those obtained 

 by the use of gelatin. When sewage and highly polluted 

 waters are examined counts are slightly higher on 

 Nahrstoff media, while with purer waters the Nahrstoff 

 numbers are far in excess of those obtained with gelatin. 

 Winslow (1905) found the ratio of Nahrstoff agar 



