QUANTITATIVE EXAMINATION OF WATER 43 



at 20 degrees and at 37 degrees and a presumptive 

 test for B. coli in lactose bile." 



This action of the section responsible for the appoint- 

 ment of the Standard Methods Committee appears to 

 supersede the report of the committee itself and makes 

 the combination of the 20 and 37-degree counts the 

 standard American procedure. The 20-degree count 

 may be made on either gelatin or agar; but it is the 

 20-degree count which will be discussed in this chapter, 

 leaving the body temperature count for consideration 

 in Chapter IV. 



The exact composition of the medium is, of course, 

 of prime importance in controlling the number of 

 bacteria which will develop. The figures pre\dously 

 cited in connection with the discussion of Hesse's 

 Nahrstoff agar show how bacterial counts may vary 

 with media of widely different composition. The 

 table quoted on page 44 from Gage and Phelps (1902), 

 shows the considerable differences which may be due 

 to the presence or absence of meat infusion, peptone, 

 etc., in media of generally similar character (compare 

 the figures for plain gelatin, peptone, gelatin, and meat 

 gelatin). Much slighter variations than this, however, 

 are significant. The reaction of the medium was found 

 as early as 1891 to be important, for Reinsch (Reinsch, 

 1891) showed in that year that the addition of one 

 one-hundredth of a gram of sodium- carbonate to the 

 Uter increased sixfold the number of bacteria develop- 

 ing. Fuller (Fuller, 1895) ^^^ Sedgwick and one of 

 us (Sedgwick and Prescott, 1895), working indepen- 

 dently, established the fact that an optimum reaction 



