134 BACTERIOLOGY. 



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 are retained in an iron box (Fig. 25), which is 

 especially designed for the purpose. They should never 

 be placed upon the stage until cold; otherwise they 

 crack. 



"When the plates which have been placed upon the 

 stages are quite cold the melted gelatin or agar-agar in 

 the tubes which represent the three dilutions should be 

 poured upon them, each tube being emptied upon a 

 separate plate. If the medium is quite fluid, it spreads 

 over the surface of the plates in a thin, even layer. 

 Sometimes it may be more evenly spread as it flows 

 from the tube by the aid of a sterilized glass rod. 



Fig. 26. 



Glass benches for supporting plates. 



As the content of each tube is emptied the cover of 

 the cooling-stage is quickly replaced and the plate al- 

 lowed to stand until the gelatin or agar-agar is quite 

 solid. This takes longer with gelatin than with agar. 

 "When the medium is quite solid these plates are placed 

 upon glass benches (Fig. 26), each labelled with the num- 

 ber of the plate in the series of dilutions. The benches, 

 with the plates upon them, are then piled one above the 

 other in a " culture-dish," in which the plates are to be 

 kept during the growth of the bacteria. The benches 



