I 



GELATIN AND POTATO MEDIA. 



177 



may be used to moisten the paper which covers the bottom 

 of the dish. On three small pieces of paper, write 

 the name of the germ or material, the number of 

 the plate, and the date. This label may be written 

 on the giass bench (Fig-. 28 a). Now, place a glass 

 bench on the bottom of the moist chamber, and, on 

 it the label for plate 1. Transfer the gelatin plate from 

 the ice apparatus to the bench. Pour the contents of the 

 remaining 1 gelatin tubes on plates, in the same manner as 

 described; and, when cool, transfer to the benches which 

 are arranged one above the other, in the moist chamber. In 

 doing this, the utmost care should be taken to avoid 

 touching the gelatin, either with the fingers or with a 

 glass bench. Each chamber can, or should hold a stack 

 of six plates. 



FIG. 32. Water apparatus for cooling plates (F. G. N.). 



The moist chamber is now set aside. It should not be 

 placed near the steam coil, or where the sun-light may pos- 

 sibly strike it. The rapidity with which colonies develop 

 in the gelatin will depend upon the temperature of the room, 

 and, to a certain extent, upon the organism itself. The best 

 temperature for the development of a growth on gelatin 

 plates is about 18-20. Under this condition, excellent col- 

 onies will form, as a rule, in from 36 to 48 hours. The lower 

 the temperature the slower will be the development. If the 

 temperature exceeds 20 the growth will be more rapid, but, 

 in that case, there is always danger of melting the gelatin 

 and thus spoiling the outcome. The ordinary, 10 per cent. 



12 



