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.

[Illustration: FIG. 30.]

A convenient plan for filling a burette from below is shown in fig. 30.
The diagram explains itself. The bottle containing the standard solution
is connected with the burette by a syphon arrangement through the glass
tube and T-piece. The flow of liquid into the burette is controlled by
the clip. When this clip is opened, the burette fills; and when it is
closed, the burette is ready for use in the ordinary way.

~Measuring Gases.~--Lange's nitrometer (fig. 69) is a very convenient
instrument for many gasometric methods. It requires the use of a fair
quantity of mercury. In fig. 31, there is a representation of a piece of
apparatus easily fitted up from the ordinary material of a laboratory.
It is one which will serve some useful purposes. It consists of a
wide-mouthed bottle fitted (by preference) with a rubber cork. The cork
is perforated, and in the perforation is placed a glass tube which