64 METHODS OF CULTIVATION OF BACTERIA. 



each corner, the height of which can be adjusted, and 

 which thus constitutes the levelling apparatus. A spirit 

 level is placed where the plate is to go, and the level of the 

 ground glass plate thus assured. There is also prepared a 

 " damp chamber," in which the plates are to be stored 

 after being made. This consists of a circular glass trough 

 with a similar cover. It is sterilised by being washed 

 outside and inside with perchloride. of mercury i-iooo, 

 and a circle of filter paper moistened with the same is 



FIG. 1 8. Koch's levelling apparatus for use in pre- 

 paring 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. 



laid on its bottom. Glass benches on which the plates 

 may be laid are similarly purified. 



To separate organisms by this method three tubes, a, b, 

 r, are inoculated as in using Petri's capsules. The hands 

 having been washed in perchloride of mercury i-iooo and 

 dried, the plate box is opened, and a plate lifted by its 

 opposite edges and transferred to the levelled ground glass 

 (as in Fig. 19). The bell jar of the leveller being now 

 lifted a little, the gelatine in tube a is poured out on the 

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

 stroking with the rim of the tube. The plate is now trans- 



