14 GLASS APPARATUS IN COMMON USE 



4. Adjust a rubber teat to the pipette and by pres- 

 sure on the bulb gradually drive the mercury in an 

 unbroken column down the capillary tube until it is 

 stopped by the filiform extremity. 



5. Cut off the capillary tube exactly at the upper 

 level of the column of mercury, invert it and allow 

 the mercury to run out. 



6. Snap off the remainder of the capillary tube from 

 the broad upper portion of the pipette which is now 



destined to form the covering tube or 

 air chamber, or what we may term the 

 " barrel." This barrel now has the lower 

 end in the form of a truncated cone, the 

 upper end being cut square. Remove 

 the teat. 



7. Introduce the capillary tube into 

 this barrel with the filiform extremity 

 uppermost, and the square cut end pro- 

 jecting about 0.5 cm. beyond the taper- 

 ing end of the barrel. 



8. Drop a small pellet of sealing wax 

 into the barrel by the side of the capil- 

 lary tube and then warm the tube at 



FIG. 1 8. the gas flame until the wax becomes 

 Throttle pipette so ftened and makes an air-tight joint 



small capacity. e J 



between the capillary tube and the end 

 of the barrel. 



9. Fit a rubber teat to the open end of the barrel, 

 and so complete a pipette which can be depended upon 

 to always aspirate and deliver exactly 5 cm. of fluid. 



Slight modification of this procedure is necessary in 

 making tubes to measure larger volumes than say 

 75 c.mm. Thus to make a throttle pipette to measure 

 100 c.mm.: 



i. Take a short length of quill tubing and draw out 

 one end into a roomy capillary stem, and again draw 

 out the extremity into a fine hair point, thus forming 



