EXAMINATION OF WATER, AIR, AND MILK 115 



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

 number of Erlenmeyer flasks are filled with 101 c.c. of 

 distilled water, and these are sterilized in the autoclav at 

 1 20 for 5 minutes. About i c.c. of water is lost by evapo- 

 ration during this process, so that the sterile flasks con- 

 tain 100 c.c. each. 1 



Method oj procedure. 



1. With a sterile pipette remove i c.c. of water from 

 one of the sterilized dilution flasks. 



2. Carry over to this flask i c.c. of the sample after 

 a thorough shaking. The dilution is now i : 100. Mark 

 with glass pencil. 



3. 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 : 1000. 

 (See dilution table, p. 117.) Make a number of dilutions 

 in this manner, carrying the dilutions higher in proportion 

 to the quality of the water to be examined. 



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

 sponding to the number of dilutions made, and cool to 

 43. Read in reference book carefully the chapters on 

 the importance of using both agar and gelatin media. 



5. Mix carefully i c.c. of each dilution flask with a 

 tube of liquified medium, respectively. 



6. Pour into Petri dishes. 



7. Incubate the agar-plates at 37, and keep the 

 gelatin-plates at room temperature. The mixing of the 

 diluted sample and agar may also be effected by pouring 

 the sample into a Petri dish and mixing it with the liquid 

 medium. This method is not as convenient, but slightly 



1 If work is carried on with dilutions of i : 100 only, the flask may 

 conveniently be filled with 100 c.c. water instead of 101 c.c. It is then not 

 necessary to remove i c.c. of water, as only 99 c.c. are left after sterilization. 



