98 MANUAL OF BACTERIOLOGY. 



succession from this tube, which is No. i , into tube No. 2 ; 

 sterilize the needle; replace the plugs; mix thoroughly, 

 without wetting the plug. Carry three loopfuls from tube 

 No. 2 into tube No. 3 in the same manner. The original 

 material will obviously be diluted in tube No. I, more in 

 tube No. 2, and still more in tube No. 3. The most con- 

 venient form of plate is that known as a Petri dish, a small 

 glass dish about 8 cm. in diameter and 1.5 cm. in height, 

 provided with a cover which is a little larger but of the same 



FIG. 30. 



Petri Dish. 



form. This dish should be cleaned and sterilized for an hour 

 in a hot-air sterilizer at 150 C. or higher. When it is cool 

 it may be used. 



Such dishes having previously been prepared, the con- 

 tents of tube No. i are poured into one dish, and those of 

 tube No. 2 into another, and those of tube No. 3 into a 

 third. They are to be labeled Nos. i, 2, and 3. 1 In pour- 

 ing proceed as follows : remove the plug of tube No. i ; heat 

 the neck of the tube in the flame ; allow it to cool, holding it 

 in a nearly horizontal position. When the tube has cooled, 

 lift the cover of the Petri dish a little, holding it over the 

 dish; pour the contents of tube No. I into the dish, and 

 replace the cover of the dish. The interior of the dish 

 should be exposed as little and as short a time as possible. 



1 The labels should be moistened with the finger, which has been dipped 

 in water. They should not be licked with the tongue. While working 

 in the bacteriological laboratory it is best to make it a rule that no ob- 

 ject is to be put in the mouth. 



