EXAMINATION OF WATER AND SEWAGE 77 



regarded as accustomed to the same temperature found within 

 the animal body and the water consequently as suspicious. 



Collecting Samples. Care must be taken that a sample is 

 representative. If taken from a tap or pump the water in the pipe 

 must first be run off to obviate any effect the metal may have had ; 

 if from a lake or pond the surface scum or bottom mud should be 

 avoided or both may be examined separately ; if from a brook the 

 specimen should be taken some distance from the bank. 



The container must of course be sterile and the test made as 

 soon as possible, for an increase or a decrease in the number of 

 bacteria begins immediately. If a short delay is unavoidable 

 the sample should be kept on ice at a temperature of 5 C. 



Technique for Quantitative Analysis. The sample is vigorously 

 shaken about twenty-five times and then by means of a sterile 

 graduated pipette 1 c.c., 0.1 c.c., and 0.01 c.c. are placed respec- 

 tively in three sterile Petri dishes previously marked with the 

 amount to be received. Immediately a tube of liquid agar cooled 

 to 40 C. is poured into each dish and the water and agar are thor- 

 oughly mixed by a rotary movement of the dish before the agar 

 solidifies. The test should be made in duplicate, one set of plates 

 being placed in the incubator at 37 C. for twenty-four hours and 

 the other kept in the dark at room temperature for forty-eight 

 hours. Usually the number of colonies developing at room tem- 

 perature is far greater than that developing at 37 C. The colonies 

 are counted in the manner already described. 



Presumptive Test for B. Coli. To determine the presence of 

 the colon bacillus a medium such as the Conradi-Drigalsky is 

 employed, and the plating procedure already indicated is done 

 in triplicate, to the third series of plates being added the colored 

 medium instead of the plain agar. After twenty-four hours' 

 incubation if colon bacilli are present in the amount of water tested 

 red characteristic colonies will appear which stand out well against 

 the blue background. 



A second test is made by inoculating fermentation tubes con- 

 taining colored lactose peptone water with varying amounts of 

 water : 10 c.c., 1 c.c., and 0.1 c.c. After incubation at 37 C. for 



