The level of a curved
surface of liquid is at first somewhat difficult to read: the beginner
is in doubt whether the surface should be taken at A, B, or C (fig. 27).
It is best to take the lowest reading C. In some lights it is difficult
to find this; in such cases a piece of white paper or card held behind
and a little below, so as to throw light up and against the curved
surface, will render it clear. In reading, one should look neither up at
nor down upon the surface, but the eye should be on the same level with
it. It must be kept in mind that flasks _contain_ the quantity
specified, but deliver less than this by the amount remaining in them
and damping the sides. If it is desired to transfer the contents say of
a 100 c.c. flask to a beaker, it will be necessary to complete the
transfer by rinsing out the flask and adding the washings; otherwise
there will be a sensible loss. Graduated cylinders (fig. 28) are
convenient for preparing standard solutions.
[Illustration: FIG. 27.]
[Illustration: FIG. 28.]
[Illustration: FIG. 29.]
~Pipettes~ and burettes are graduated to _deliver_ the quantities
specified. The principle of the pipette, and the advantages and
disadvantages of its various forms, may be understood by considering the
first form shown in fig. 29. It is essentially a bulbed tube drawn out
to a jet at its lower end, and having on each side of the bulb a mark so
placed that when the surface of the liquid falls from the upper to the
lower mark the instrument shall deliver exactly 100 c.
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