FILTERING; COLLECTING PRECIPITATES 7phere can be obtained. If the pressure is insufficient, an equally-
good vacuum can be obtained by means of the suction of the escap-
ing water. To this end the escape pipe must be prolonged by a
tube sufficiently constricted to prevent the sections of the descend-
ing water column from breaking and thus allowing air to enter
from the bottom.
To keep the suction pump working continuously, however, is
extravagant of water as well as being a nuisance in the laboratory
on account of the unnecessary noise. Consequently this rule is
made and must be observed:
The suction pump must never be kept in operation more than two
minutes at one time.
If suction must be applied for more than that length of time,
the vacuum which is produced inside of the two minutes may be
maintained in the suction bottle by closing the screw cock. (See
Fig. 3.) Thus, if all the joints of the bottle are tight, a slimy
precipitate may be left filtering under suction over night, or even
longer.
Trap. The use of the trap shown in the diagram is always
necessary, as otherwise dirty water may be sucked back acciden-
tally and contaminate the solution in the filter bottle.
(c) Filtering without Suction. A slimy or gelatinous precipitate
can be collected much better without suction. Suction drags the
solid matter so completely into the pores of the filter that in most
cases the liquid nearly ceases to run. A filter funnel and filter
should be chosen large enough to hold the entire precipitate. The
filter paper should be folded twice and then opened out in the form
of a cone and fitted into the funnel. The upper edge of the filter
should come about £ inch below the rim of the funnel. It is best
to fit the paper carefully into the funnel, to wet it and press it
up against the glass all around, so that there will be no air
channels.
For slow-running liquids, if a large filter is used, it may be filled
at intervals and left to take care of itself while other work is being
done.
If a considerable weight of liquid is to come on the point of
the filter, this may be reenforced by means of a piece of linen
cloth, which should be placed under the middle of the filter paper
before it is folded, and should then be folded in with it so as to
strengthen the point.