146 BACTERIOLOG Y . 



When required for use, the plug is removed and held between the 

 fingers, which must not touch the part which enters the neck of 

 the flask. About 30 cc. of the water to be examined are intro- 

 duced into the flask, and the plug must be quickly replaced and 

 covered with a caoutchouc cap. If collected from a tap, the water 

 should first be allowed to run for a few minutes, and the sample 

 should, be received into the flask without the neck coming into contact 

 with the tap. From a reservoir or stream, the flasks may be filled 

 by employing a sterilised pipette. During transport contact between 

 the water and cotton-wool plug must be avoided, and if likely to 

 occur the sample must be collected and forwarded in a Sternberg's 

 bulb. 



Examination ~by Plate-cultivation. The apparatus for plate- 

 cultivation should be arranged as already described. Crushed ice 



FIG. 74. APPARATUS FOR ESTIMATING THE NUMBER OF COLONIES IN A 

 PLATE-CULTIVATION. 



may be added to the water in the glass dish to expedite the setting 

 of the gelatine, so that the plate may be transferred as quickly 

 as possible to the damp chamber. The caoutchouc cap 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 measured quantity (y 1 ^ or -^ cc.) of the sample is 

 transferred to a tube of liquefied nutrient gelatine, and the plugs of 

 the flask and of the 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, and the gelatine finally poured on a plate. 

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

 chamber, which should be carefully labelled and set aside in a place 



