[Illustration: FIG. 17.]

[Illustration: FIG. 18.]

[Illustration: FIG. 19.]

~Filtration.~--Solutions are best filtered hot whenever the assay allows
of this being so done. The precipitate should be allowed to settle, and
the clear liquid decanted on the filter with the aid of a glass rod if
necessary. The filter-paper must not be too large, but at the same time
it must not be overloaded with the precipitate. There should be ample
room for washing. For general use three sizes of filter-paper are
sufficient. Common quick filtering-paper (English) is best for most work
in assaying. The specially prepared paper (Swedish or Rhenish) is used
for collecting those precipitates which have to be weighed. The papers
are folded as shown in fig. 16, and should not project above the funnel.
The filter-paper works better if damped with hot water. In special cases
filtering is hastened by means of an air-pump. The apparatus used
consists of a water-jet (fig. 17), which is connected with the tap, as
also with a bottle fitted as shown in fig. 18. The pump draws the air
out from the bottle, and atmospheric pressure forces the liquid through
the filter-paper. The bottom of the funnel is provided with a platinum
cone, which supports the filter-paper, and prevents its breaking. The
pump is only used in exceptional cases; nearly all the filtrations
required by the assayer can be made without it. The usual methods of
supporting the funnel during filtration are shown in fig. 19. Where the
filtrate is not wanted, pickle bottles make convenient supports. After
the precipitate has been thrown on the filter, it is washed. In washing,
several washings with a small quantity of water are more effective than