KOCH'S METHOD OF PREPARING PLATES. 



55 



through the bottom. From their characters, colour, shape, 

 contour, appearance of surface, liquefaction or non-liquefaction 

 of the gelatin, etc., the colonies can be classified into groups. 

 Further aid in the grouping of the varieties is obtained by 

 making film preparations and examining them microscopically. 

 Gelatin or agar tubes may then be inoculated from a colony of 

 each variety, and the growths obtained are then examined both 

 as to their purity and as to their special characters, with a view 

 to their identification (p. 112). 



2. 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. 20, 21). This con- 

 sists of a circular plate of 

 glass (with the upper sur- 

 face ground, the lower 

 polished) on which the 

 plate used for pouring out 

 the medium is placed. 

 The latter is protected 

 from the air during solidi- 

 fication by a bell-jar. The bell-jar, where it subsequently has the medium 

 , , , j , n . poured out upon it. 



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 large circu- 

 lar trough, which catches any water that may be spilled. This 

 trough in turn rests on a wooden triangle with a foot at 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 con- 

 sists of a circular glass trough with a similar cover. It is ster- 

 ilised by being washed outside and inside with perchloride of 

 mercury i-iooo, and a circle of filter paper moistened with the 



FIG. 20. Koch's levelling apparatus for use in 

 preparing plates. Hands shown in first position for 

 transferring sterile plate from iron box to beneath 



