GENERAL BACTERIOLOGY 85 



with cotton and sterilized in the hot-air oven. Then a 

 number of Erlenmeyer flasks are filled with 100 c.c. of 

 distilled water, and these are sterilized hi the autoclave 

 at 120 C. for 5 minutes. During sterilization a small 

 variable amount of water is lost. This has to be 

 disregarded. 



Method of procedure 



1. With a sterile pipette carry over to one of these 

 flasks i c.c. of the sample after shaking. The dilution 

 is now i : 100. Mark with a glass pencil. 



2. With a sterile 10 c.c. pipette remove 10 c.c. from 

 another dilution flask, and add to the remainder 10 c.c. 

 of the first dilution. We now have a dilution of i : i ,000. 

 (See appendix.) Make a number of dilutions in this 

 manner, carrying the dilutions higher in proportion 

 to the quality of the water to be examined. 



3. Melt a number of agar and gelatin tubes, corre- 

 sponding to the number of dilutions made, and cool to 

 43 C. 



4.. Transfer i c.c. of each dilution flask to a petri 

 dish. 



5. Pour the contents of one agar tube on the petri 

 dish and mix this with the i c.c. of water by tipping 

 the dish back and forth. 



6. Incubate the agar plates at 37 C. and keep the 

 gelatin plates at room temperature. 



Estimation of colonies. The colonies are then 

 counted after 48 hours, by means of a colony counter 

 (see back cover). Plates should be counted which 

 contain no more than 200-300 colonies. If it is neces- 

 sary to count plates with a large number of colonies, 

 an estimate must be made by counting different sec- 



