LABORATORY WORK IN DAIRY BACTERIOLOGY 273 



"No. 6. Determination of the Number of Bacteria in Milk. 

 It is always first necessary to dilute the milk with sterilized 

 water, since ordinary milk contains so many bacteria that, un- 

 less diluted, the numbers will be too large to be estimated prop- 

 erly. Milk contains such a widely varied number, however, that 

 it is difficult to determine beforehand how much to dilute the 

 material ; and one must be guided, when possible, by knowledge 

 as to the age of the milk. Where a miscellaneous lot of milk 



FIG. 74 HARDENING AGAR SLANTS 



is to be studied for this purpose, probably the best method is 

 to dilute it 1,000 times. 



In order to dilute milk 1,000 or any other definite number of 

 times, it is necessary to have flasks and vials measured to hold 

 certain definite amounts. It is most convenient to have some 

 holding 4 c.c., 9 c.c., 19 c.c. and 99 c.c. (Fig. 75.) From them 

 the desired dilution can readily be obtained. 



a. Fill a number of these vials to the mark with water, plug 

 \vith cotton and sterilize. This can best be done in an autoclav. 

 (Fig. 76.) In using this autoclav place a small amount of water 

 within it and then the vials to be sterilized. Replace the cover, 

 fastening it down tightly with the screws designed for the 

 purpose, and light the gas. Leave the cock open until steam 

 begins to come out, and then close. Allow the steam pressure 

 to rise to 15 Ibs. and maintain this pressure for one hour. Allow 

 the autoclav to cool before opening. The vials may then be 



