54 METHODS OF CULTIVATION OF BACTERIA 



levelled ground glass (as in Figs. 18, 19). The bell jar of the levelle 

 being now lifted a little, the gelatin in tube a is poured out on thi 

 surface of the sterile plate, and while still fluid, is spread by stroking 



FIG. 18. Koch's levelling apparatus for use in preparing plates. 

 Hands shown in first position for transferring sterile plate from iron 

 box to beneath bell jar, where it subsequently has the medium poured 

 out upon it. 



with the rim of the tube. After the medium solidifies, the plate is 

 transferred to the moist chamber as rapidly as possible, so as to avoid 



FIG. 19. Koch's levelling apparatus. Hands shown in second 

 position just as the plate is lowered on to the ground glass surface. 

 By executing the transference of the plate from the box in this way, 

 the surface which was undermost in the latter is uppermost in the 

 leveller, and thus never meets a current of air which might con- 

 taminate it. 



atmospheric contamination. In doing this, it is advisable to have an 

 assistant to raise the glass covers. Tubes b and c are similarly treated, 



