116 



BACTERIOLOGY. 



organisms with which the tubes may have been inocu- 

 lated. 



THE COOLING-STAGE AND LEVELLING-TRIPOD. 

 While the medium of which the plates are to be made 

 is melting, it is well to arrange the cooling-stage (Fig. 

 24) upon which the gelatin or agar-agar is to be subse- 

 quently solidified. 



This stage consists of a glass dish filled with ice- 

 water and covered with a ground-glass plate, which in 

 turn has a dome-shaped cover. The dish rests upon a 

 tripod which can be brought to an exact level, as indi- 

 cated by the spirit-level, by raising or lowering its legs 

 by means of thumb-screws, with which they are pro- 

 vided. Three stages are usually employed. When 

 ready for use they should be exactly level. 



THE GLASS PLATES. On each of the stages is to be 

 placed a glass plate upon which the liquefied gelatin or 



FIG. 25. 



Russia iron box for holding plates, etc., during sterilization in dry heat. 



agar-agar is to be poured and allowed to solidify. It 

 is ; therefore, necessary that the plates should not only 

 be sterile when placed upon the stages, but they should 

 be carefully protected by a cover against dust and bac- 

 teria from outside sources during manipulation. 



A number of plates at a time are usually sterilized in 



