of the
special apparatus called a Gooch filter referred to above.

FILTRATION AND WASHING OF PRECIPITATES

Solutions should be filtered while hot, as far as possible, since
the passage of a liquid through the pores of a filter is retarded by
friction, and this, for water at 100 deg.C., is less than one sixth of the
resistance at 0 deg.C.

When the filtrate is received in a beaker, the stem of the funnel
should touch the side of the receiving vessel to avoid loss by
spattering. Neglect of this precaution is a frequent source of error.

The vessels which contain the initial filtrate should !always! be
replaced by clean ones, properly labeled, before the washing of a
precipitate begins. In many instances a finely divided precipitate
which shows no tendency to pass through the filter at first, while the
solution is relatively dense, appears at once in the washings. Under
such conditions the advantages accruing from the removal of the first
filtrate are obvious, both as regards the diminished volume requiring
refiltration, and also the smaller number of washings subsequently
required.

Much time may often be saved by washing precipitates by decantation,
i.e., by pouring over them, while still in the original vessel,
considerable volumes of wash-water and allowing them to settle. The
supernatant, clear wash-water is then decanted through the filter,
so far as practicable without disturbing the precipitate, and a new
portion of wash-water is added. This procedure can be employed to
special advantage with gelatinous precipitates, which fill up the
pores of the filter paper. As the medium from which the precipitate
is to settle becomes less dense it subsides less readily, and it
ultimately becomes necessary to transfer it to the filter and complete
the washing there.

A precipitate should never completely fill a filter. The wash-water
should be applied at the top of the filter, above the precipitate.
It may be shown mathematically that the washing is most !rapidly!
accomplished by filling the filter well to the top with wash-water
each time, and allowing it to drain completely after each addition;
but that when a precipitate is to be washed with the !least possible
volume! of liquid the latter should be applied in repeated !small!
quantities.

Gelatinous precipitates should not be allowed to dry before complete
removal of foreign matter is effected. They are likely to shrink and
crack, and subsequent additions of wa