Petri's Dishes 



197 



the most important being that of Petri, who invented special 

 dishes to be used instead of plates. 



Petri's Dishes. These are small glass dishes (Fig. 42), 

 about 4 inches in diameter and ^ inch deep, with accurately 

 fitting lids. They greatly simplify bacteriologic technic by 

 dispensing with the plates and plate-boxes, the moist cham- 

 bers and benches, and usually with the leveling apparatus 



Fig. 42. Petri dish for making plate cultures. 



of Koch, though this is still employed in some labora- 

 tories, and must always be employed when an even distribu- 

 tion of the colonies is necessary in order that they can be 

 accurately counted. 



The method of using the Petri dishes is very simple. They 

 are carefully cleaned, polished, and sterilized by hot air, care 

 being taken that they are placed in the hot-air closet right 

 side up, and after sterilization are kept covered and in that 



Fig. 43. Petri dish forceps. 



position. They should be sterilized immediately before 

 using, or if they must be kept for a time should be wrapped 

 in tissue paper and then sterilized. The tissue paper pro- 

 tects the interior from the accidental entrance of dust be- 

 tween dish and lid, keeps the dish closed, and need not be 

 removed until the last moment before using. 



Time can be saved by sterilizing the dish and cover in 

 the direct flame, instead of in the hot-air closet, special 



