LABORATORY WORK IN DAIRY BACTERIOLOGY 



275 



to a third similar water blank. This last vial will contain i/ioo 

 of a c.c. of milk and one c.c. of it will contain 1/1,000 of a c.c. 



It will readily be seen 

 that any modification of 

 the dilution may easily be 

 made. If two water blanks 

 only are used, the dilution 

 will be 100. If four are 

 used, it will be 10,000. If 

 very high dilutions are 

 desirable, for example, 

 1,000,000, six vials can be 

 used, or it is better to 

 proceed somewhat differ- 

 ently. Place 99 c.c. of 

 water in each of three 

 flasks or bottles, and ster- 



FIG. 76 AUTOCLAY, FOR STERILIZING 

 UNDER STEAM PRESSURE 



ilize. To one add I c.c. of 

 the milk to be tested, and, 



after a thorough shaking, transfer I c.c. into a second flask con- 

 taining 99 c.c. Transfer I c.c. of this to a third flask and I c.c. 

 of this last will contain 1/1,000,000 of a c.c. of milk. By very 

 simple calculation any desired dilution may be obtained. 



c. Melt four test tubes of the agar medium in hot water and 

 cool to a temperature of about 50. Into each tube place, with 

 a sterilized pipette, I c.c. of the diluted milk and then mix 

 thoroughly by gentle shaking. After the mixing pour the con- 

 tents of the tube into a sterilized petri dish (Fig. 77) and cover 

 immediately. As soon as the agar hardens place the petri dish, 

 now called an agar plate, in an incubating oven at a temperature 

 of 37. It is always well to make at least four such plates in 

 order that an average may be taken for the final result, since 

 no one plate can be relied upon for very great accuracy. After 

 24 hours at 37 examine the plates; they will be covered with 



