over for assaying along with the next batch. If one filters slowly or is
in any way less forward than the rest, it may lessen the accuracy of the
other assays, owing to oxidation, &c., it should, therefore, be put on
one side. The assays are dealt with in batches of ten or twenty, so that
a large quantity of work can be quickly finished.

[Illustration: FIG. 11.]

When the assays are finished, it is the duty of the assistant to clean
the apparatus (with reagents, if necessary), and to put the vessels in
the place set apart for them. Flasks are best kept inverted on a rack,
so that they may be dry and clean by the next morning. Berlin crucibles
must be cleaned and ignited.

The amount of apparatus employed should be as little as is feasible. The
assay should be carried out as much as possible in the same flask. The
bench must be clean, and altogether free from apparatus not in actual
use. Crucibles and dishes in which weighings are made should be marked
with numbers or letters; and their weights recorded, together with the
date of weighing, in a small ledger, which is kept in the drawer of the
balance. By this means a record of the "wear" of each piece of apparatus
is obtained, and, what is more important, much weighing is saved, and
increased confidence is gained. The weight of each piece of apparatus
need not be taken daily. It will be seen from the record in the book and
a knowledge of the use it has been put to how often a checking of the
weight is necessary. The entries are made in black lead as follows:--

Dish, A. Feb. 3 9.4210 grams.
5 9.4225
6 9.4230