e._, each division = 0.1 c.c.) may be selected.
Burettes may be fixed in any convenient stand; they must be vertical and
should be so placed that the assayer can read any part of the graduated
scale without straining. When not in use, they should be kept full of
water. When using a burette, the water must be run out; the burette is
next rinsed with some of the solution to be used, and drained; and then
it is filled with the solution. Next squeeze the india-rubber tube so as
to disentangle air-bubbles and, by smartly opening the clip, allow the
tube and jet to be filled; see that no bubbles of air are left. Then
run out cautiously until the level of the liquid in the burette stands
at zero. In reading the level with very dark-coloured liquids it is
convenient to read from the level A (fig. 27), and, provided it is done
in each reading, there is no objection to this. The accuracy of the
reading of a burette is sensibly increased by the use of an Erdmann
float. This is an elongated bulb, weighted with mercury, and fitting
(somewhat loosely) the tube of the burette. It floats in the solution,
and is marked with a horizontal line; this line is taken as the level of
the liquid. If the burette is filled from the top, the float rises with
aggravating slowness, and this is its chief disadvantage. The float must
come to rest before any reading is made.
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