LABORATORY AND TEACHING METHODS 



605 



Sugar Gelatin (Another formula). — Prepare nutrient gelatin and weigh out 

 glucose 20 grams ( = 2 per cent.) and dissolve in the hot gelatin. Filter, tube and 

 sterilize as for nutrient gelatin. In certain cases, lactose, maltose or saccharose in 

 similar percentages is substituted for glucose. 



Litmus Gelatin. — Prepare nutrient gelatin, add sterile litmus solution, sufficient 

 to tint the medium a deep lavender color, tube and sterilize as for nutrient gelatin. 



LESSON 12 



Agar-agar. — To make i liter of agar-agar take 



A. Dried peptone (i per cent.), grams. . 10 

 Common salt (0.5 per cent.), grams. . 5 

 Liebig e.xtract (0.5 per cent.), grams. 5 



Water, c.c 500 



Boil for three minutes and neutralize. 



B. ' Agar-agar ( 1 . 2 percent.), (in shreds, or as 



flour) grams 12 



Water, c.c 500 



Chop the agar and put into autoclave (Fig. 

 213). Run autoclave up to two atmospheres of 

 pressure, giving i2i.4°C. of heat. As soon as 

 this pressure is reached, turn out the flame, and 

 allow the autoclave to cool until below ioo°C. be- 

 fore opening. The two solutions A and B are 

 then mixed, cooled to 6o°C., the whites of two 

 eggs beaten in 50 c.c. of water added, well stirred 

 in, and the whole then boiled, the solidified albumen 

 and precipitate skimmed ofT and the residue filtered 

 through paper. 



The whole process requires only an hour and 

 a quarter to an hour and a half, and the result 

 is a most excellent jelly. Instead of .the white of 

 egg, blood-serum may be used, which seems to 

 add also to the nutritive value of the medium. 



Agar made with meat extract will often form a precipitate during the sterilization, 

 which is objectionable if one wishes to use it in the pouring of Petri dishes, or the 

 making of Esmarch's roll-tubes. 



Agar with Fresh Meat.— To make an absolutely and permanently clear agar, 

 fresh meat should be used as follows: 



To make i liter' take: 



A. Chopped meat, grams 500 



Water, c.c 5^*° 



Mix and place in cool place over night, then strain through towel. 



B. Agar-agar (1.2 per cent.), grams 12 



Water, c.c 5°° 



Fig. 213. — Usual form of 

 laboratory autoclave for sterili- 

 zation with steam under pres- 

 sure. (Fig. 16, p. 40, Schneider, 

 Pharmaceutical Bacteriology, 

 1912.) 



