42 Elements of Water Bacteriology^ 



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 ventilated 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. The English Committee appointed to 

 consider the standardization of methods for the Bac- 

 terioscopic Examination of Water (1904) fixed the time at 

 72 hours. French bacteriologists, and some Germans 

 (Hesse and Niedner, 1906), still recommend longer periods, 

 and the table on p. 43 from Miquel and Cambier (Miquel 

 and Cambier, 1902) shows that many bacteria fail to 

 appear in our ordinary procedure. It is, however, in the 

 main, the characteristic water bacteria which develop 

 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 difference between good and bad waters. 



Counting. The number of bacteria is determined by 

 counting the colonies developed upon the plate, with the 

 aid of a lens magnifying at least five diameters. For 

 convenience in counting, the plate may be placed upon a 

 glass plate ruled in centimeter squares and set over a black 

 tile, or the tile itself may be ruled. As has already been 

 said, it is desirable that the number of colonies should not 



