. Manual of physico-chemical measurements. his and bent asis shown in the figure. The mercury collects in thelower portion, the drops coalesce, and the metal flowsdry through a at the same rate as it collects fromabove. The height of a is determined so that themercury column in the wide part of the tube standsat least 5 cm. high when the whole tube is filled withacid. The tube is tied to the upright rod of a stand,the foot of which is provided with a raised edge to-catch any mercury that may run over. Apparatus for measuring Volumes.—Pipettesare used for measuring off quantities of liquid up t
. Manual of physico-chemical measurements. his and bent asis shown in the figure. The mercury collects in thelower portion, the drops coalesce, and the metal flowsdry through a at the same rate as it collects fromabove. The height of a is determined so that themercury column in the wide part of the tube standsat least 5 cm. high when the whole tube is filled withacid. The tube is tied to the upright rod of a stand,the foot of which is provided with a raised edge to-catch any mercury that may run over. Apparatus for measuring Volumes.—Pipettesare used for measuring off quantities of liquid up to100 cc. Only those with a jet at least 5 cm. longshould be used (Fig. 67a); the form with a shortbeak (Fig. 67b) should in all cases be rejected onaccount of the indeterminate error caused by thedrops of liquid adhering externally to the lower partof the body of the pipette. The upper end should be allowed to falltogether in the flame until the opening is only 1-2 mm. wide. Pipettes are almost always used for delivery, they are so. VOLUME AND DENSITY 85 / 15 V Pios. 67a, 676. graduated that the determined volume of liquid flows out when theyare filled up to the mark and allowed to run out. This volumedepends on the quantity of liquid left behind on thewalls, and the following precautions must be taken tosecure accurate measurement. If a pipette has a verywide jet so that its contents flow out rapidly, the amountof liquid left behind is considerable and liable to varia-tion. If the speed of outflow is diminished, the quantityof liquid remaining also decreases, and under a certainspeed the wetting reaches a minimum which variesvery little. This minimum is reached with certaintywhen the time of free outflow is forty secondsor more. Since the lower openings of pipettes, assupplied, are usually too wide, they should be allowedto faU together in the flame until the above time ofoutflow is reached, the walls at the point being simul-taneously strengthened. Another point to be
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