FILTRATION BY ASPIRATION 



21 



another piece of glass tubing also bent at a right angle, the vertical limb of 

 which reaches only just below the level of the lateral tubulure. To the 

 horizontal arm of the latter attach a piece of red rubber pressure-tubing, 

 which at its other end is connected to the nozzle of the filter. To the other 

 piece of glass tubing attach another piece of pressure-tubing, into the distal 

 end of which is inserted a short piece of glass tubing drawn out to a narrow 

 orifice ; over this glass tube an india-rubber plug is slipped to fit a test-tube 

 which protects the end of the delivery tube. 



[(6) Sterilize in the autoclave at 120 C. 



[(c) Attach the lateral tubulure of the flask to the lateral tubulure of another 

 Erlenmeyer flask with pressure-tubing, and insert a three-way piece of glass 

 tubing between the two flasks, the vertical limb being closed with rubber 

 tubing and a clip. Pass a right-angled piece of glass tubing through an india- 

 rubber bung in the mouth of the flask, and attach this tubing to the pump 

 by means of pressure-tubing. 



[(d) Place the filter in a glass cylinder larger than the filter, and fill up the 

 cylinder with the fluid to be filtered. Secure the two clips. 



[(e) Turn on the pump, and the fluid is aspirated from the cylinder to the 

 Erienmeyer flask. When the fluid reaches nearly up to the level of the 

 lateral tubulure, release the clip on the vertical limb of the three-way piece 

 of glass tubing. Disconnect the second flask, and attach a bicycle pump to 

 the first flask. Clip the tubing attached to the filter. 



[By working the pump the flask can be filled with air filtered by passing 

 through the wool plug in the lateral tubulure and the contents of the flask 

 thus put under sufficient pressure to allow them to be drawn off through the 

 tube contained in the test-tube, as in the former case.] 



3. A third method is to attach one end of a piece of red rubber pressure- 

 tubing B (fig. 20) to the nozzle of a filter, and the other end to a piece of 



FIG. 20. Filtration by aspiration. 



glass tubing bent at a right angle. Pass the latter through one of the holes 

 in an india-rubber bung. Fit the bung tightly into the neck of a stout white- 

 glass bottle A the capacity of which is equal to the amount of fluid to be filtered 

 (thin-walled flasks will not withstand the pressure, and should therefore 

 never be used for this purpose). Through the other hole in the bung pass 



