FILTRATION BY ASPIRATION 



23 



has been withdrawn the end B is sealed in a Bunsen flame thus effectually 

 excluding air from the bottle. 



It may however sometimes happen that on ceasing to work the syringe 

 some air will enter the bottle through B, and since this may 

 carry organisms with it there is a risk of the liquid in the bottle 

 becoming contaminated. The difficulty is easily overcome by 

 the following simple device. 



Before being sterilized the external limb of the tube B is cut 

 in the middle, and the two ends B and C (fig. 23) connected by 

 means of a piece of red rubber tubing, into which a short length 

 (1-2 cm.) of glass rod D has already been introduced. When 

 not in use the glass rod completely obliterates the lumen of the 

 rubber tubing, and cuts off all communication between the 

 outside air and the contents of the bottle. But if the rubber 

 tubing be pinched between the thumb and index finger, a 

 small channel is formed through which the liquid can be forced 

 by squeezing the syringe attached to A (fig. 22). The apparatus 

 would then be worked as follows : 



The end of the glass tubing C being flamed and the point 

 broken off, the syringe is squeezed a few times, and then the 

 india-rubber tubing between B and C pinched up. The liquid 

 will then run out from the open end. The flow of liquid is stopper for use 

 stopped by first releasing the finger and thumb from the india- with distribut- 

 rubber tube, thus cutting off all communication with the 

 outside air, and then but not till then relaxing the pressure on the syringe. 

 Finally, the broken end is sealed and the syringe disconnected. 



4. L. Martin's filtering apparatus (fig. 24). This consists of a porcelain 



FIG. 23. 



FIG. 24. -L. Martin's filter. 



(The upper tubulure of the bulb should have a wool plug between constrictions : 

 this has been accidentally omitted from the figure.) 



filter contained in a metal cylinder similar to that described before (p. 15). 

 The cylinder has a tap funnel screwed into the top to facilitate manipulation. 

 Technique. Connect the nozzle of a porcelain filter with a bulb of the shape 

 shown in the figure by a length of pressure-tubing. Sterilize in the autoclave. 

 Then fix the filter in its metal case, and connect the upper tubulure of the bulb 

 to a water pump with pressure-tubing. 



