PREPARATION OF CULTURE MEDIA 23 



the temperature to rise beyond 120°, nor to allow it to 

 remain at that point any longer than 5 minutes. Car- 

 bohydrates are easily broken up by heat. They are 

 then valueless in the medium, and also generate an acid 

 reaction as a result of chemical decomposition. 



In lihe preparation of agar more difficulties are met 

 with than in the preparation of any other medium. The 

 main difficulty is the fact that it takes a long time — from 

 30 to 45 minutes — for the agar to dissolve, and this 

 creates the danger of burning it by overheating. Great 

 care mus't therefore be exercised in the preparation 

 at all stages. 



EXERCISE in. PREPARATION OF PEPTONE GELATIN 



1. Weigh the saucepan and measure 350 c.c. of 

 tap-water into it. This amount includes 50 c.c. allow- 

 ance for evaporation. 



2. Dissolve 0.75 g. of extract of meat, and, when near 

 boiling, add 3 g. of peptone. 



3. When boiling, add during the cold season 10 per 

 cent. (30 g.), during the warm season 12 per cent. (36 g.), 

 of the best gelatin (Gold label) by dissolving two or 

 three leaves at a time, with constant stirring. 



4. When completely dissolved, adjust the reaction 

 as directed in the preparation of agar. As gelatin con- 

 tains considerable acid, it will take mere NaOH solu- 

 tion in proportion than for agar. 



5. Then cool to 60°, and stir into the mixture the 

 white of one egg dissolved in 30 c.c. of water. Slowly 

 heat over the flame on a piece of asbestos, without stir- 

 ring, until the egg albumen is completely coagulated and 

 forms a dry film on top. 



