If the finger is wet, the
flow will be jerky, and good work impossible. The pipette is next held
over the vessel into which the 100 c.c. are to be put, and the liquid
allowed to run out. When the bulb is nearly empty, the flow should be
checked by replacing the finger, and the liquid allowed to escape slowly
until the lower mark is reached. The pipette is then withdrawn; it is in
the withdrawing that the disadvantage of this particular form[5] makes
itself felt. It must be withdrawn very steadily, as the slightest shock
causes the remaining column of liquid to vibrate, whereby air is drawn
in and the liquid is forced out.
This disadvantage is got rid of by making the mouth of the jet the lower
limit, or, in other words, allowing the instrument to empty itself.
There are two forms of such pipettes; in the one generally recommended
in Gay-Lussac's silver assay (the last shown in fig. 29) the nose is
replaced by a jet. This is most conveniently filled by stopping the jet
with the finger, and allowing the liquid to flow in a fine stream into
the neck until the pipette is filled, and then working as just
described. The other form is the one in general use; in fact, a long
nose to a pipette is so convenient that it may almost be said to be
necessary. But the accuracy is slightly diminished; a long narrow tube
makes a poor measuring instrument because of the amount of liquid it
finally retains.
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