86 MANUAL OF BACTERIOLOGY. 



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 Kttle larger, but of the same form. This 

 dish should be cleaned and steriHzed in a hot-air sterilizer at 

 150° C. or higher for an hour. When it is cool it may be used. 

 Such dishes having previously been prepared, the contents 

 of tube No. 1 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.* In pouring proceed as follows: 



Fig. 30. — Petri Dish. 



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, hft 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. 

 Tubes Nos. 2 and 3 are to be treated in the same manner. 

 Burn the plugs, and fill the empty tubes with 5 per cent, solution 

 of carboUc acid. They should be sterilized for an hour in the 

 steam steriHzer on each of three days. 

 The culture-medium in the Petri dish will soon solidify. 



* 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. 



