io8 



BACTERIOLOGY 



to hold some material containing bacteria, and a little of this 

 material may be removed on the tip of the wire from the first 

 tube to the second. When the needle is introduced into or re- 

 moved from either tube it should not touch the side of the tube 

 at any point, and should only come in contact with the region 

 desired. After inoculation of the second tube has been effected, 

 the wire is to be heated to a red heat in the flame, the necks of 

 the tubes are to be passed through the flame, and the plugs are to 

 be returned to their respective tubes. In the same manner 

 transfer three loopfuls from tube No. 2 into tube No. 3. 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 convenient 

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



FIG. 40. Petri dish. 



about 10 cm. in diameter and 1.5 cm. in height, provided with a 

 cover which is a little larger but of the same form. This dish 

 should be cleaned and sterilized for an hour in a hot-air sterilizer 

 at 1 50 C. or higher. When it is cool it may be used. 



Such dishes having previously been prepared, the contents 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 pouring 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 



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 object is to be put in the mouth. 



