46 ELEMENTS OF WATER BACTERIOLOGY 



Even the standard procedure fails to ensure uniformity 

 in one important respect. The meat infusion which it 

 calls for is in itself a highly variable quantity. Gage 

 and Adams (1904), in the examination of fifteen lots 

 of beef infusion, found variations of nearly i per cent 

 in organic solids (calculated on the weight of the whole 

 infusions), after the final filtration. The organic 

 constituents of the meat infusion varied, therefore, 

 among themselves by nearly the total amount of pep- 

 tone added. It is to be hoped that the standard methods 

 may soon be so revised as to eliminate this necessarily 

 uncertain constituent of nutrient media. Criticisms 

 of detail must, however, give way to the importance 

 of securing fairly comparable results; and the con- 

 fusion which would follow the use by individual bac- 

 teriologists of media made without meat would out- 

 balance the errors inherent in the standard procedure. 



Incubation. Incubation should take place in a dark, 

 well-ventilated chamber where the temperature is 

 kept substantially constant at 20 degrees and where 

 the atmosphere is practically saturated with moisture. 

 It has been shown by Whipple (Whipple, 1899) and others 

 that the number of bacteria developing in plate cultures 

 is to a certain extent dependent upon the presence of 

 abundant oxygen and moisture. Thus, reckoning the 

 number of bacteria developing in a moist chamber at 

 100, the percentage counts obtained in an ordinary 

 incubator were as follows: 75 when the relative humid- 

 ity of the incubator was 60 per cent of saturation; 82 

 when it was 75 per cent; 98 when it was 95 per cent. 

 This source of error may be avoided by the use of ven- 



