58 METHODS OF CULTIVATION OF BACTERIA. 



absolutely level while the medium is solidifying, and can be 

 readily handled afterwards without admitting impurities. 

 Whether plates or capsules are used, they are washed, dried 

 with a clean cloth, and sterilised for one hour in dry air at 

 170 C, the plates being packed in sheet-iron boxes made 

 for the purpose (see Fig. 18). 



i. Glass Plates (Koch). When plates of glass are to be 

 used, an apparatus on which they may be kept level while 

 the medium is solidifying is, as has been said, necessary. 

 An apparatus devised by Koch is used (Figs. 1 8, 1 9). This 



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. 



consists of a circular plate of glass (with the upper surface 

 ground, the lower polished) on which the plate used for 

 pouring out the medium is placed. The latter is protected 

 from the air during solidification by a bell jar. The 

 circular plate and bell jar rest on the flat rim of a circular 

 glass trough, which is filled quite full with a mixture of ice 

 and water to facilitate the lowering of the temperature of 

 whatever is placed beneath the bell jar. The glass trough 

 rests on corks on the bottom of a larger circular trough, 



