BACTERIA 225 



color. Pour about one and one-half inches of gelatin into each 

 test tube and plug with cotton. Sterilize the tubes twenty 

 minutes for three consecutive days, so as to kill all spores. 



Agar-agar medium. Mix the same as the gelatin medium, 

 using 15 grams of agar-agar in place of the 100 grams of 

 gelatin. The preparation of agar-agar medium, however, is . 

 more troublesome than the gelatin. Agar-agar does not dis- 

 solve easily and is difficult to filter. To obtain a quick result it 

 is best to perform the filtration in parts. If the funnel, lined 

 with absorbent cotton, is well heated, about one half of the 

 agar-agar mixture will have passed through the filter in fifteen 

 minutes. Remove the funnel and reboil the remaining agar- 

 agar and pass through a fresh filter. Repeat the process until 

 the mixture is filtered. 1 



Potato medium. Pare the potatoes and cut with a cork borer 

 of suitable size for the test tube. Divide the cylinders into 

 two-inch lengths and then cut diagonally across. Place the 

 " potato slants " thus prepared in water for several hours, to 

 extract the product which turns them black when exposed 

 to air. 



Put into test tubes, slant side uppermost, plug, and sterilize 

 in a steam sterilizer for twenty-five minutes at 100 C. for three 

 successive days. A small piece of glass rod placed in the bottom 

 of the test tube holds the potato above the condensed steam. 



Rules and methods of manipulation. (1) Learn as early in 

 the course as possible that all dishes should be washed and 

 sterilized in the hot-air sterilizer before using. All micro- 

 organisms are killed when they are heated as follows : three 

 hours at 150 C., or until paper is brown ; one half hour at 

 160 C. ; one fourth hour at 170 C. ; one minute at 190 C. 

 (2) Before sterilizing, wrap the Petri dishes in paper and 



1 If time is limited, obtain the prepared gelatin or agar-agar from a local 

 hospital laboratory or board of health, or order from a regular dealer in 

 such supplies. 



