180 BACTEEIOLOGY. 



paratus, are more or less expensive, and are too cumber- 

 some to be transported from place to place, in the case of 

 out-door investig'ations. Even in the laboratory, the latter 

 apparatus requires time to prepare and to level it, and, not 

 infrequently, ice may be lacking. Furthermore, the moist- 

 chamber when placed in an incubator, or in a cooler, takes 

 up an unnecessary large amount of space. In the plate 

 method as given, more or less contamination may result 

 from exposure to the air, while the plates are on the ice 

 apparatus, or subsequently, when they are kept in the 

 large moist chamber. Whenever it is desired to examine 

 the growth on the plates, it is necessary to expose these 

 freely to the air. Even if but one plate is to be examined, 

 it necessitates the exposure of all to contamination from 

 the air. Furthermore, it may happen that the gelatin on 

 an upper plate liquefies, either by heat or by the growing 

 bacteria, and then drips over the edge of the plate on those 

 which are below it. To overcome these difficulties, Petri 

 introduced the use of the shallow dishes (Fig. 28 c) which 

 bear his name. The cover to the dish should be 10 cm. in 

 diameter, and the bottom dish should be as flat as possible, 

 and should not be more than 1 cm. deep. A greater depth in- 

 rterferes in the subsequent examination with the microscope. 



Petri dish culture. — The Petri dishes (Fig. 28 c) are placed 

 in a wire basket, and sterilized in the dry-heat oven (Fig. 

 25), and then allowed to cool. If the dishes are, to be kept 

 for some time before use, or are to be transported, they 

 ;should be wrapped in paper, and then sterilized. Three 

 gelatin tubes are inoculated with the organism to be 

 plated, the dilutions being made in exactly the same man- 

 ner as described in connection with the ordinary plate 

 method (p. 174). The contents of each tube are then 

 poured into one of the cooled, sterile Petri dishes. The 

 precaution of flaming the neck of the tube (p. 176) before 

 pouring out the gelatin must, of course, be observed. 



