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. — Ou 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 



