256 Bacteriology of Water 



allowed to stand for a short time. If the water to be exam- 

 ined must be transported any considerable distance before 

 the manipulations are performed, it should be packed in ice. 

 The greatest care must always be exercised that the unnat- 

 ural conditions arising from the bottling of the water, the 

 changes of temperature, and the altered relationship to light 

 and the atmosphere, do not modify the number of contained 

 bacteria. 



1OO 



Fig. 74. Frost's plate counter, for counting colonies of bacteria on 

 Petri dish or plate cultures. The cross- lines divide the figure into 

 square centimeters. The numbers at the top of the figure indicate the 

 area in centimeters of the various discs. The area of each sector (a and 

 b) is one-tenth of the whole area. 



The detection of such important bacteria as the colon and 

 typhoid bacilli, and the cholera spirillum, will be considered 

 in the chapters treating of those respective organisms. 



Unfortunately, the bacteriologic examination of waters 

 does not throw satisfactory light upon their exact hygienic 

 usefulness. Of course, if cholera or typhoid fever bacteria 

 are present, the water is dangerous, but the quality of the 

 water cannot always be gauged by the number of bacteria it 

 contains. 



