[Illustration: FIG. 23.]

The crucible is removed from the muffle with the tongs and carried to
the desiccator. It is best, in an office, to have a large desiccator
permanently fixed alongside the balance, into which all substances may
be put before being weighed. The substance is removed from the bench or
muffle in the small hand apparatus generally sold, and carried to the
balance room to be transferred to the large desiccator, where it is
allowed to become thoroughly cold before being weighed. Twenty minutes
is generally the time allowed after ignition before it is advisable to
weigh. Bodies allowed to cool in the air after they have been ignited will
absorb moisture, and hot bodies placed in the balance-pan will disturb
the equilibrium and show false results. Compounds that absorb moisture
must be weighed quickly; they should, therefore, be weighed in covered
vessels. Such compounds are detected by their continually-increasing
weight. They should be ignited and weighed again
in a well-covered dish.

Substances that have been washed with alcohol, ether, or any readily
volatile liquid are dried in the water oven. They quickly dry if there
is no water present, and are generally fit for weighing in less than one
hour. Sometimes drying for a few minutes only will be sufficient.

The weight of the crucible and precipitate having been obtained, the
weight of the crucible and ash is deducted; for example--

Crucible and precipitate 10.183 grams.
Crucible and ash 9.527 "
------
0.656 "