84 



BACTERIOLOGY. 



at a temperature not detrimental to the organisms with 

 which the tubes may have been inoculated 



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. 11) 

 upon which it is to be subsequently 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 indicated by 

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

 of screws, Avith which they are provided. Three stages 

 are usually employed. When ready for use they should 

 be exactly level. 



The Glass Plates. — On the stages are to be placed 

 the glass plates upon which the liquefied gelatin or agar- 

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



Fig. 12. 



therefore necessary that the plates should not only be 

 sterile when placed upon the stages, but should be 

 carefully protected by a cover against dust and bacteria 

 from outside sources during manipulation. 



A number of plates at a time are usually sterilized in 

 the dry sterilizer at a temperature of 150° to 180° C. 

 for one hour. During sterilization and until used they 



