QUANTITATIVE BACTERIOLOGICAL EXAMINATION. 31 



Incubation. — " Incubation should take place in a dark, 

 well-ventilated chamber where the temperature is kept, 

 substantially constant at 20 and where the atmosphere 

 is practically saturated with moisture." — A. P. H. A. 

 Report. 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 

 humidity 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- 

 tilated dishes and by the presence of a pan of water in 

 the incubating chamber. 



According to American and German practice, plates 

 made for sanitary water analysis are counted at the end of 

 forty-eight hours. French bacteriologists still recom- 

 mend longer periods, and the following table from Miquel 

 and Cambier (Miquel and Cambier, 1902) shows that 

 many bacteria fail to appear in our ordinary analysis. 

 It is, however, as we have before pointed out, certain 

 peculiar water forms which develop so slowly, sewage 

 bacteria almost without exception being rapid growers. 

 The longer period of incubation is, therefore, not only 

 inconvenient but undesirable, since it obscures the differ- 

 ence between good and bad waters. 



