44 ELEMENTS OF WATER BACTERIOLOGY. 



case the proportion of blood- temperature organisms is 

 much smaller than in the latter. Furthermore, this 

 method is free from much of the error introduced by the 

 multiplication of bacteria after the collection of a sample, 

 since most of the forms which grow in water during 

 s'.orage cannot endure the higher temperature and conse- 

 quently do not develop upon incubation. 



The body-temperature count must, of course, be made 

 upon agar plates, otherwise the technique is the same 

 which has already been described for the routine quan- 

 titative bacteriological analysis. The period of incuba- 

 tion ordinarily adopted by the writers is twenty-four 

 hours, as little development occurs after that time. Diffi- 

 culty is sometimes caused by the spreading of colonies 

 of certain organisms over the surface of the plate in the 

 water of condensation which gathers; this may be avoided 

 by inverting the plates after the agar is once well set. 



Additional evidence as to the character of a water 

 sample may be obtained with little extra labor by adding 

 a sugar and some sterile litmus to the agar medium and 

 observing the fermenting powers of the organisms present, 

 as first suggested by Wurtz (Wurtz, 1892) for the separa- 

 tion of B. coli from B. typhi. It happens that the most 

 abundant intestinal organisms belonging to the groups 

 of the colon bacilli and the streptococci decompose dex- 

 trose and lactose with the formation of a large excess of 

 acid, while many other organisms, even if they grow 

 abundantly at the body temperature, are not favored by 

 the presence of the sugar and litmus. The decomposi- 



