254 Stephen DeM. Gage 



varied data regarding the bacterial contents of a water could be 

 obtained, the apparent discrepancies would cease to exist, and the 

 chemical and bacteriological analyses would supplement and con- 

 firm one another, rendering the correct interpretation of the quality 

 of the water a comparatively simple matter. The object of the present 

 paper has been to supply a portion of the information necessary for the 

 etablishment of bacteriological procedures by which a more thorough 

 knowledge of the bacterial content of the water may be obtained. 

 Nearly all of the information desired concerning the bacterial content 

 of water may be obtained by the use of selective media, by the use 

 of selective temperatures, or by a proper combination of the two. 

 In the present investigation the selective action of four different 

 temperatures, 20°, 30°, 40°, and 50° C, and two different media, 

 regular agar, and litmus-lactose agar, in determining the bacterio- 

 logical contents of a number of different kinds of water, have been 

 studied; and while the results obtained have been in many cases 

 inconclusive, and in other cases too few in number to warrant the 

 drawing of any far-reaching conclusions, they indicate in a measure 

 the procedures which must be followed in order to place the bacterio- 

 logical analysis of water on the same plane as the chemical analysis. 



The results of the investigation may be summarized as follows: 

 The numbers of bacteria determinable upon agar or gelatin are 

 very closely approximated by the numbers determined upon litmus- 

 lactose agar, while the substitution of the latter medium for the 

 former allows of the simultaneous determination of the total bacteria 

 and of the acid-producing bacteria without appreciably increasing the 

 labor involved in the determination. The numbers of the two classes 

 of bacteria so determined indicate more completely the character of 

 the water than would the numbers of either class determined alone. 



It is, of course, unnecessary to discuss the significance of the 

 numbers of bacteria determined at 20" C. The number of acid- 

 producing organisms determined at 20° C, however, is an important 

 check upon the total numbers. In one case we saw that with two 

 well waters, one polluted and one not polluted, the numbers of bac- 

 teria in the pure well water were about twice as great as in the pol- 

 luted water, but the numbers of acid-producing bacteria showed 

 the high numbers for the pure water to be misleading. 



