PETRI DISH CULTURE. 69 



This process is repeated with each of the two other tubes, and 

 the dishes labelled 1, 2, 3, respectively. In dish No. 3 the 

 colonies will be much fewer in number than in either No. 2 

 or No. 1, and the dilutions may be used for comparison. 



Cultures of pathogenic bacteria must be kept at a tempera- 

 ture approximating that of the body. To attain this tem- 

 perature, a special warming-oven or incubator is used. 



FIG. 24. 



Laboratory incubator. 



The incubator (Fig. 24) resembles the hot-air oven. The 

 space between the walls is filled with water, the amount of 

 which is registered by a water-gauge placed on one side 

 of the box. The roof is perforated by three holes, one of 



