26

QUANTITATIVE AGRICULTURAL AXALYX/X

which would be left as an ash on burning. To obtain the
latter condition, paper is subjected to a preliminary extrac-
tion with hydrochloric and hydrofluoric acids, thus dissolving
all but a small trace of ordinary ash-forming matter, Such paper
is usually called "ashless."

Inorganic Filters.— To avoid the reducing action of the
filter either an alundum crucible or a Caldwell crucible may
be used. Alundum is a porous form of aluminium oxide, partly
fused together with a hinder. A crucible of this material may
be placed in a rubber holder placed in a funnel, as shown in Fig. 5,

and the liquid drawn
through by suction. The
precipitate is then washed
and ignited directly in the
crucible*

The Oaldwell crucible
(usually known an a
"(Joodh") is n tall crucible*
of porcelain whose? bottom
in perforated by small holes.
This is used in a manner
similar to that described for
the alundum crucible, with
the exception that ft pad
of asbestos (see page 158)
i& formed over the bottom

*™I t/his provides the ttWK-

sary filtering Hurfa,c<*. For

high temperatures the platinum form m better. This in the*
original Gooch crucible.

Washing.— Wash bottles like Fig. 6 should be provided. A
fine stream of water, hot or cold, may be blown on to the? filter,
the precipitate and filter being thus washed free from Holuhkt
impurities. To avoid unpleasant effects due to blowing bark
of steam from the hot water bottle, or of volatile liquids when
these are used for washing in special cases, a pressure bulb and
vent may be provided.

Great care must be used to avoid mechanical low of precipitate,
It is well to remember also that the* most efficient wantiing in

6.— Common form of wiwh bottle,