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 loi c.c. of 

 distilled water, and these are sterilized in the autoclav at 

 120° 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.' 



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 quaUty 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 



' If work is carried on with dilutions of i : loo only, the flask may 

 conveniently be filled with lOO c.c. water instead of loi c.c. It is then not 

 necessary to remove i c.c. of water, as only 09 c.c. are left after sterilization. 



