4 EXPERIMENTAL DAIRY BACTERIOLOGY 



inner dishes; hence heating the oven to 175 C. and at once 

 turning off the heat, as is sometimes recommended, is likely 

 to result in incomplete sterilization. The sterilizer should not 

 be opened until it has cooled to 60 C. If cold air strikes the 

 hot glass, many dishes are likely to be broken. 



After sterilization Petri dishes should not be opened until 

 used. They should be handled as little as possible, as mold 

 spores are likely to gain entrance ; and in case they are to be 



FIG. 2. PIPETTE CASE AND PIPETTES 



Case of sheet copper with slip cover ; pipettes wrapped in tissue paper, on one 

 of which the paper is torn to show the cotton plug in the mouth end 



carried from the laboratory or are to be kept some time before 

 using, they should be wrapped, before sterilizing, in pieces of 

 newspaper. 



The mouth end of the pipettes should be plugged with 

 cotton, and the projecting fibers burned off so as not to in- 

 terfere in using the pipette. The pipettes should be placed 

 in the metal case, points down, the end of the case plugged 

 with cotton, and the whole sterilized. In place of the metal 

 pipette case the pipettes may be prepared for sterilization 

 by wrapping each in tissue paper, the paper at the bottom 

 being turned in and the mouth end twisted, so that the 

 ends may be distinguished before removing the wrapper. 



Exercise. Sterilize Petri dishes and pipettes. 



Preparation of media. The media used in the bacteriologi- 

 cal laboratory bears the same relation to bacteriology as does 



