96 



MANUAL OF BACTERIOLOGY. 



the medium up into foam, and also avoid 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 convenient 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 

 form. This dish should be cleaned and sterilized 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. 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 



Fig. 32. — Petri dish. 



to be labeled Nos. i, 2 and 3.* 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 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 carbolic acid. They should be sterilized for an hour in the 

 steam sterilizer on each of three days. 



*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 bac- 

 teriological laboratory it is best to make it a rule that no object is to be put in the 

 mouth. 



