112 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. 22), 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. 23. 



Glass benches for supporting plates. 



As the contents of each tube are emptied upon a plate 

 the cover of the cooling-stage is quickly replaced and 

 the plate allowed to stand until the gelatin or agar-agar 

 is quite solid. This takes longer with gelatin than with 

 agar. When quite solid they are placed upon little 

 glass benches (Fig. 23), and each bench is labelled with 

 the number of the plate in the series of dilutions. The 

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

 above the other in a glass dish, the so-called u culture- 

 dish," in which the plates are to be kept during the 



