BACTERIOLOGICAL INVESTIGATION OF WATER. 197 



introduction of non-pathogenic bacteria occasionally so dimin- 

 ishes the animal resistance that animals ^resistant to inocula- 

 tions by some pathogenic bacteria are subsequently rendered 

 susceptible. 



This quantitative analysis is especially useful in cases in 

 which the mean quantity of bacteria in a given body of 

 water is already known, and as a matter of comparison to 

 ascertain whether any new source of contamination has been 

 introduced. 



The examination is made as follows : A sample of the 

 water is collected in clean sterilized bottles or tubes. If the 

 water is from a pump, well, or from a cistern, it should be 

 allowed to run for a few minutes before the sample is taken. 

 If the water is from a spring, river, or any collection of 

 water, the sample for examination should be taken a foot 

 or two below the surface of the water. Agar and gelatin 

 plates should be immediately inoculated with the water. 

 When this is not practicable the plates should be made at 

 as early a time as possible, the samples meanwhile being 

 kept on ice, near the freezing-point, to prevent the further 

 development of bacteria. 



For the purpose of collecting water for examination, glass 

 bulbs after the pattern of Sternberg (Fig. 70) are very use- 

 ful. These consist of a sphere blown on the end of a glass 



FIG. 70. 



Glass bulb for collecting samples of water. (Abbott.) 



tube, the stem at the other end terminating in a capillary 

 tube. After thoroughly cleaning these bulbs, a negative 

 vacuum is established therein by introducing in each tube 

 a few drops of water, allowing same to boil over a gas-flame, 

 and when the water has completely vaporized into steam the 

 capillary end of the bulb is brought into the flame and the 

 apparatus sealed. When it is desired to collect a sample of 



