SEPARATION BY GELATIN MEDIA 



59 



the third, and so on. There is thus effected a twenty-fold 

 dilution in each successive tube. After these steps have been 



FIG. 17. 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. 



carried out, a definite amount, say '05 c.c., is transferred from 

 each tube to a tube of melted gelatin, the gelatin being after- 



FIG. 18. 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. 



wards plated and the colonies counted when growth occurs. 

 The number of tubes required will vary according to the 



