176 CHEMICAL TECHNIQUES 



capillary at both ends so that, with one atmosphere pressure, the 

 fineness of the tips will prevent liquid from running out. 22 is a 

 siphon arm connecting to a reservoir of the liquid to be pipetted 

 which is placed about 50 cm. above the instrument. The pipette is 

 filled as follows: Close L and open K and H to fill the outer cham- 

 ber. When the level is a few mm. over the upper tip of the pipette, 

 close H; the pressure from the reservoir will fill the pipette. Then 

 close K and open // and L to bring the level of the liquid in the outer 

 chamber below the upper tip. Deliver the pipette charge by closing 

 H and L and opening K, which compresses the air in the chamber and 

 forces the liquid out. A pipette of this type having a capacity of 30 

 ^il. was found to have an error of measurement of less than 0.1%. 



Accurate syringe pipettes have been employed which use a screw 

 (Krogh and Keys, 1931; Krogh, 1935) or a micrometer spindle 

 (Trevan, 1925) to move the plunger of a small hypodermic syringe. 

 The micrometer syringe pipettes are essentially the same as the 

 micrometer burettes of Dean and Fetcher (1942) and Hadfield 

 (1942) (page 255). The Krogh-Keys instrument is manufactured 

 by Macalister Bicknell Co. It has a delivery precision of 0.1 ix\. 



Devices for Drawing Finer Pipettes. Finer pipettes which are 

 used under a microscope may be drawn by hand, but mechanical 

 devices for making them are considerably more efficient. DuBois 

 (1931) described an automatic device for drawing very fine micro- 

 pipettes and microneedles which has been made available commer- 

 cially (Leitz). A capillary tube is clamped in two arms of the device, 

 and between the arms the tube passes through a small electric heater. 

 When the glass softens in the heater the spring tension on the arms 

 pulls them back, thus drawing out the tube into a pair of pipettes. 

 Rachele's device, described by Benedetti-Pichler and Rachele ( 1940) , 

 operates in a similar manner except that gravity is used to pull out 

 and lower the arms when the glass is softened by the electric heater. 



C. FILTERS 



Sintered-glass filters for small volumes of liquid were used by 

 Kirk and co-workers for the quantitative collection of precipitates 

 for various determinations. The filter described by Cunningham, 

 Kirk, and Brooks (1941b) is made of capillary tubing (2 mm. in- 



