CULTURES, AND THEIR STUDY. 121 



Petri's Dishes. These small dishes (Fig. 22), about 

 4 inches in diameter and y 2 inch deep, with accurately 

 fitting lids, are about as convenient as anything that has 

 been devised in bacteriological technique. They dis- 



FIG. 22. Petri dish for making plate-cultures. 



pense with plates and plate-boxes, with moist chambers 

 and benches, and usually with the levelling apparatus, 

 though this is still employed in connection with the 

 Petri dishes in some laboratories. 



The method of the employment of Petri dishes is very 

 simple. The dishes 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 position. The dilution of 

 the material under examination is made with gelatin or 

 agar-agar tubes in the manner described above, the plugs 

 are removed, the mouth of the tube is cautiously held 

 for a moment in the flame, then the contents of each 

 tube are poured into one of the sterile dishes, whose top 

 is elevated just sufficiently to allow the mouth of the 

 tube to enter. The gelatin is spread over the bottom 

 of the dish in an even layer, is allowed to solidify, 

 labelled, and then stood away for the colonies to develop. 



Esmarch Tubes. This method, devised by Esmarch, 

 converts the walls of the test-tube into the plate and dis- 

 penses with all other apparatus. The tubes, which are 

 inoculated and in which the dilutions are made, should 

 contain less than half the usual amount of gelatin or 

 agar-agar. After inoculation the cotton plugs are pushed 

 into the tubes until even with their mouths, and then 

 covered with a rubber cap, which protects them from 

 wetting. A groove is next cut in a block of ice, and 



