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 in 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 1 : 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, 

 ?.n estimate must be made by counting different sec- 



