FILTRATION 41 



1. Take a length of glass tubing of, say, 1.5 cm. 

 diameter, in the centre of which a bulb has been blown, 

 fill the bulb with dry cotton-wool (Fig. 32), wrap a 

 layer of cotton-wool around each end of the tube, and 

 secure in position with a turn of thin copper wire or 

 string; then sterilise the piece of apparatus in the hot- 

 air oven. 



2. Prepare the cultivation in a Ruffer or Woodhead 

 flask (Fig. 33) the inlet tube of which has its free 

 extremity enveloped in a layer of cotton- wool, secured 



FIG. 33. Ruffer's flask. 



by thread or wire, whilst the exit tube is plugged in 

 the usual manner. 



3. Sterilise a short length of rubber tubing by boil- 

 ing. Transfer it from the boiling water to a beaker 

 of absolute alcohol. 



4. When all is ready remove the rubber tube from 

 the alcohol by means of a pair of forceps, drain 

 it thoroughly, and pass through the flame of a Bunsen 

 burner to burn off the last traces of alcohol. 



5. Remove the cotton-wool wraps from the entry 

 tube of the flask and from one end of the filter tube 

 and rapidly couple them up by means of the sterile 

 rubber tubing. 



