TECHNIQUE OF MAKING PLATES. 85 



are retained in an iron box (Fig. 12), 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. 



As the contents of each tube is 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. 13), and each bench is 



FIG. 13. 



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

 during the growth of the bacteria. The benches are 

 sterilized before using, in the way given for the plates. 



CULTURE-DISH. This dish, which is about 22 cm. in 

 diameter and has vertical sides of about 6 cm. in height, 

 5 



