174 Elements of Water Bacteriology. 



truly indicative of purity or pollution. Furthermore, as 

 several days must elapse before the bacterial tests can be 

 completed, the results when obtained may have passed 

 their usefulness. If, however, we can so modify our 

 procedure that the varied character of the bacteria in 

 waters of different classes may be quickly and accurately 

 recognized, the value of bacterial water analysis will be 

 enormously increased. Much of this information may 

 be obtained by the use of selective media, selective tem- 

 peratures, or by a proper combination of the two. 



" By the use of litmus lactose agar in place of agar or 

 gelatin we obtain similar counts of total bacteria, and in 

 addition are able to separate those bacteria into two 

 groups, which do and do not produce acid fermentation 

 of lactose, and the numbers of the two classes of bacteria 

 so obtained indicate more completely the character of the 

 water than would the numbers of either class alone. 

 By incubating our plates at temperatures of 30 or 40° C. 

 we are able to obtain counts in twelve to eighteen hours, 

 which counts, while smaller than those on plates incubated 

 for a Ipnger period at a lower temperature, appear to be 

 fully as significant. If we increase our number of deter- 

 minations by incubating duplicate plates at two or more 

 temperatures, the various results and the ratios between 

 them furnish a check upon one another in addition to 

 increasing the available data upon which to base an 

 interpretation. 



" The distinction between polluted waters and waters of 



