28 ELEMENTS OF WATER BACTERIOLOGY. 



packed in ice for forty-eight hours fell off from 535,000 to 

 54,500; from 412,000 to 50,500, and from 329,000 to 

 73,000, respectively. It is, therefore, necessary to adhere 

 strictly to the recommendations of the A. P. H. A. Com- 

 mittee that the interval between sampling and examina- 

 tion should not exceed twelve hours in the case of rela- 

 tively pure waters, six hours in the case of relatively 

 impure waters, and one hour in the case of sewage. 



Plating. The bottle containing the sample of water is 

 first shaken at least twenty-five times in order to get an 

 equal distribution of the bacteria. If the number of 

 bacteria present is probably not greater than 200, i c.c. is 

 then withdrawn with a sterile i c.c. pipette and delivered 

 into a sterile Petri dish of 10 cm. diameter. To this is 

 added 5 c.c. of standard 10 per cent gelatin at a tempera- 

 ture of about 30 C. or standard agar (7 c.c.) at 4o-42 C 

 Should the number of bacteria per c.c. probably exceed 

 200, dilution is necessary. This is best accomplished by 

 adding i c.c. of the water in question to 9, 99, or 999, etc., 

 c.c. of sterile tap water according to the amount of dilution 

 required. The diluted sample is then shaken thoroughly 

 and i c.c. taken for enumeration. In order to determine 

 the number of bacteria originally present it is only neces- 

 sary to multiply by the factor 10, 100, or 1000, etc. 



When a sample of water from an unknown source is to 

 be examined it is generally desirable to make a series of 

 plates at each of the above dilutions, selecting those which 

 give nearest to 200 colonies on the plates after incubation 

 as the ones on which to rely for the count. A much 



