20 



STERILIZATION BY FILTRATION 



[Technique. Stand the filter F in a tall glass cylinder C which must be 

 rather larger than the filter, and fill up the cylinder with the unfiltered liquid. 

 Tighten the clip K on the vertical limb of the T-piece and also the clip H of 

 the delivery tube, and turn on the water. The liquid is thus aspirated 



C 



F 



FIG. 18. Cobbett's bulb fitted up for filtration by aspiration. 



through the filter into the bulb. When the bulb is nearly full, gently release 

 the clip K on the vertical limb of the T-piece, and then turn off the water. 

 Then, as before (p. 18), remove the test-tube E and draw off the filtrate into 

 suitable and previously sterilized vessels. Having emptied the bulb replace 

 the test-tube E, tighten the clips K and H, turn on the water and exhaust 

 again. In case the filtrate or a part of it has to be stored for future use, the 

 vessels in which it has been collected may be sealed off in the flame of the 

 blow-pipe, or the wool plugs can be made air-tight to prevent evaporation 

 by sealing them with paraffin or sealing-wax (p. 30).] 



[2. Instead of a Cobbett's bulb, an Erlenmeyer flask may be used, but the 

 procedure is a little more complicated (fig. 19). The filtrate is aspirated into 

 an Erlenmeyer flask, and then blown out with a bicycle pump. 



[Technique. (a) Take an Erlenmeyer flask of sufficient size to contain the 

 filtrate. Plug the lateral tubulure with wool between the constrictions. 



FIG. 19. A convenient arrangement for filtration by aspiration. 



Fit the mouth with an india-rubber bung with two holes ; through one hole 

 pass a piece of glass tubing bent at a right angle, the vertical limb of which 

 is long enough to reach to the bottom of the flask, and through the other 



