BACTERIOLOGICAL TECHNIC 27 



5. Cool to about 60 C. Dissolve the whites of 

 two or three eggs, or about 10 grams of pure powdered 

 egg albumin in about 100 c.c. of tap water. Mix this 

 solution with the gelatin and heat slowly to the boiling 

 point, placing a piece of asbestos under the pan unless a 

 double boiler is used. Boil gently until the egg white 

 or egg albumin has coagulated and a solid film has 

 formed which mechanically incloses the impurities. 



6. Adjust the weight to 1,000 grams, allowing for 

 the weight of the pan. Filter and tube, as in the 

 preparation of agar. 



7. Sterilize in the autoclave for 5-8 minutes at 

 120 C., or in the arnold for three successive days. 



If sterilized in the autoclave, care should be taken 

 uot to allow the temperature to go beyond i2oC., 

 and gelatin should not be permitted to remain at this 

 temperature beyond the prescribed time. Gelatin is 

 readily decomposed by heat and then does not solidify 

 after cooling. 



EXERCISE 5. PREPARATION OF LITMUS MILK 



Milk is one of the most important culture media. 

 Only the cleanest milk obtainable should be used. 

 "Certified milk" is most suitable. In many cases com- 

 mercial milk powder may be used. If certified skim- 

 med milk or fat-free milk is available step i is omitted. 



1. Separate five-sixths of the cream from the milk. 



2. Add a sufficient quantity of tincture of litmus to 

 impart a decided blue color to the milk. If a solution 

 of Merck's pure extract of litmus 1:100 is at hand 

 about 5 per cent of this will be sufficient. 



3. Distribute in culture tubes and sterilize in the 



