LABORATORY WORK IN DAIRY BACTERIOLOGY 



277 



base with a divided circle. It is made by the International 

 Scientific Instrument Co. 



It will be evident that the number thus obtained will always 

 be an under estimate of the number of bacteria, because some 

 bacteria will not grow in the agar culture medium. There are 

 also other facts which decrease the number of colonies, so that 

 the numbers thus obtained 

 will never be equal to the 

 total number of bacteria. 



*No. 7. Gelatin Culture 

 Media. 



a. Measure out and place 

 in a stew pan a definite 

 quantity of the bouillon 

 prepared as above de- 

 scribed and add to it 12% 

 of a first grade gelatin, 

 "Gold Label Gelatin" 

 being preferred. Allow 

 the gelatin to soak in the 

 bouillon till soft and al- 

 most melted. Weigh the 

 whole material, with its 

 containing dish, and record 

 weight. Boil briskly for 



five minutes, then add water to restore original weight. 



b. Determine acidity and bring reaction to 1.5% HC1, as 

 described in 2, b to e. 



c. Boil 15 minutes. Add the white of an egg and boil briskly 

 for 15 minutes. Replace the water of evaporation. 



d. Heat once more to boiling and filter through absorbent 

 cotton, as illustrated in Fig. 72. 



e. Fill 25 test tubes with about 10 c.c. each; sterilize the 

 tubes and the rest of the gelatin in a flask, by steaming in 



FIG. 78 COUNTING-BOX 

 (International Instrument Co.) 



