BACTElJiA 225 



color. r<>ur about one and one-lialf inches of gelatin into each 

 test tube and plug with cotton. Sterilize the tubes tAventy 

 minutes for three ('(Hisccutivc days, so as to kill all spores. 



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

 usiiu»" 15 o-rams of ao-ar-aiiar in phice of tlic ItM) o-ranis of 

 gelatin. 'I'he })re[)aration of agar-agar medium, however, is 

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

 solve easily and is ditlicult to lilter. To obtain a (pi irk 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 tlu'ough a fresh filter, liepeat the process until 

 the mixtuie is filtered.^ 



Potato medium. Pare the potatoes and cut \\'ith a cork borer 

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

 two-incli 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, ping, and sterilize 

 in a steam sterilizer for twenty-five minutes at 1<>0 C. for three 

 successive days. A small piece of glass rod placed in the l)ottom 

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



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

 the course as possible that all dishes should be washed and 

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

 orofanisms are killed when thev are heated as follows: three 

 hours at 150° C, or until pai)er is brown : one half hour at 

 160° C; one fourth lumr at 170°C. ; one minute at 190° ('. 

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



1 If time is limited, obtain the prepared p;elatiii vr agar-agar from a local 

 hospital laboratory or board of health, or order from a regidar ilealer in 

 such supplies. 



