APPENDIX. 239 



is removed from the flask, and the cotton-wool plug 

 singed in the flame to prevent contamination from 

 adventitious germs on the outside of the plug. The 

 flask is then held slantingly in the hand, and the plug 

 twisted out and retained between the fingers. With a 

 graduated pipette a drop of the sample is transferred to a 

 tube of liquefied nutrient gelatine, and the plug of the 

 flask and tube quickly replaced. If the water is very 

 impure, it may be necessary to first dilute the sample 

 with sterilised water. The inoculated tube must be 

 gently inclined backwards and forwards and rolled as 

 already explained, to distribute the germs throughout the 

 gelatine (p. 70), and the gelatine finally poured on a plate. 



FIG. 42. APPARATUS FOR ESTIMATING THE NUMBER OF COLONIES IN A 

 PLATE-CULTIVATION. 



When the gelatine has set, the plate is transferred to a 

 damp chamber, which should be carefully labelled and set 

 aside in a place of moderate temperature. In about two 

 or three days the cultivation may be examined. In some 

 cases the colonies may be counted at once ; more frequently 

 they are so numerous that the plate must be placed 

 on a dark background, and a special process resorted 

 to. A glass-plate, ruled by horizontal and vertical lines 

 into centimetre squares, some of which are again sub- 

 divided into ninths, is so arranged on a wooden frame that 

 it can cover the nutrient gelatine-plate without touching 

 it (Fig. 42). A lens is added to assist in discovering 

 minute colonies. If then the colonies are very numerous, 



