28

Q UA N TIT A TIVE A GHIC ULTI IRA L A A' .1L) ',S7N

confining the moist substance in a space which contains some
hygroscopic material.

Certain types of ovens make use of methods (a) or (/>}, above.
In such ovens an air-tight chamber is provided and a dried gas is
passed through this or the chamber is exhausted by means of an
air pump.

Desiccators.—Method (c) is employed in the various types of
desiccators, used at ordinary temperatures. Figure 7 illustrates

a small desiccator suitable
for carrying about flic*
laboratory. In Fig. H in
shown a desiccator in which
are used the principles of
methods (a) and (c). This
is what is known as a
"vacuum desiccator." In
both illustrated forms of
apparatus the dry ing agent,
which may be calcium chlo-
ride, sulphuric acid, or, in
certain special cases, phos-
phorus pentoxide, is placed
in a layer on the bottom.

Ignition of Precipitates.
The term "ignition*1 in
used in thin connection in
a sense somewhat beyond
its ordinarily accepted
meaning, since it is applied to the heating to high temperatures of
substances that are entirely incombustible. The* purjxwes of
ignition are to destroy the filter, if paper has been used, to exjx'1
the last traces of moisture and volatile impurities flint have not
been removed by washing and to cause the precipitate to change
in a definite manner, if a change is to be made. If a pai>er filler
has been used it is carefully removed from the funnel by slipping
up the side. It is then folded as indicated in Fig. 9, the object,
being so to enclose the precipitate that loss in impoHHible, If
it is to be dried and removed it is then placed in the overt on n
cover gla8s. .

FIG. 8.—-" Vacuum " dcHimitor.