An Apparatus for Liquid Measurement by Drops. 199 



the passage of air and to prevent effectively the escape of any of a column of 

 pure dry mercury, 20 cm. long, which is introduced to act as a plunger. 



The tube is arranged as shown in the figure. The right-hand end is joined 

 by a short rubber tube to the upper end of a pipette, which is supported by a 

 clamp, and the left-hand end, attached to a screw-clamp, can be moved up or 

 down the tall stem of a retort stand. 



The stem of the pipette has an internal diameter of 1 mm. or so, and the 

 lower of its two calibration marks is 1 or 2 cm. below the bulb. Then 



Fig. 1. 



1 or 2 cm. below this the pipette is joined by a short piece of cleaned bicycle 

 valve-tubing to the upper end of the capillary dropping nozzle. For special 

 work the junction may be made by means of a sleeve of good cork or by 

 grinding or fusing the nozzle on to the pipette point. 



The liquid friction in a small dropping nozzle ought to be such that the 

 head of mercury employed may be great enough to render negligible the loss 

 of 2 or 3 cm. head of water as the pipette empties. The acceleration of the 

 falling mercury down the well-throttled sloped tube is negligible. 



To carry out a dropping experiment the mercury is first brought to the 



