172 CHEMICAL TECHNIQUES 



paper. In the assembly shown in Figure 50, the pipette is filled by- 

 applying gentle suction through the tube S with H closed and K 

 open. When the liquid is a little above the mark, K is closed and the 

 slowly falling meniscus is observed through the low-power micro- 

 scope (M). The moment the meniscus reaches the mark, the vessel 

 of liquid is quickly lowered away from the tip and the capillary 

 forces will prevent the liquid from running out of the pipette. The 

 vessel into which the liquid is to be delivered is brought up so that 

 the pipette tip touches the vessel wall near the bottom and H is 

 opened. P leads to a source of compressed air, and the pressure 

 regulator ( T) enables the hquid to be forced out of the pipette under 

 constant pressure. Usually a 20 cm. column of water gives the 

 required pressure; the emptying time should not be less than 5 sec. 

 H is not to be closed until the delivered liquid has been lowered away 

 from the pipette. With pipettes having a capacity of 7 ix\. the error 

 of pipetting was found to be less than 0.3%. 



Hand Pipettes. A hand pipette (A. H. Thomas Co. and E. Peter- 

 sen, Carlsberg Laboratory), Figure 51, having an accuracy of about 

 1 % was also used by the Carlsberg group. The instrument is filled or 

 emptied by sucking or blowing through the attached rubber tubing. 

 The tip of the pipette is fine enough to prevent liquid from running 

 out unless a slight pressure is applied through the rubber tubing. 

 Hand pipettes, in which the suction or pressure is applied by a glass 

 syringe, have been used by Kirk's group, Kirk and Craig (1932), 

 Sisco, Cunningham, and Kirk (1941) (Figure 52) {Microchemical 

 Specialties Co.). A rubber gasket fixed to the end of the syringe 

 barrel receives the large end of the pipette, or the metal syringe 

 fitting of a hypodermic needle is cemented to the pipette in order to 

 permit easy attachment to, and separation from, the syringe. 



Constriction Pipettes. The preceding types of pipette have been 

 displaced very largely by the constriction pipette (Levy, 1936; 

 Linderstr0m-Lang and Holter, 1940) shown in Figure 53 (A. H. 

 Thomas Co. and E. Petersen, Carlsberg Laboratory) . In this pipette 

 the calibration mark is replaced by a constriction in the lumen of 

 the capillary. Liquid is first sucked up over the constriction and a 

 slight pressure is then applied which causes the liquid to fall down 

 to the constriction but not past it. To deliver the charge, a momen- 

 tary greater pressure is applied to force the meniscus through the 

 constriction and then gentle pressure can be employed to empty the 



