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To Filter Precipitates

When filtration is employed to separate precipitated matter from the solution in which it is suspended, the filtering medium should be such that the powder may be easily reclaimed from it with as little loss as possible. Linen or smooth bibulous paper are the best for this purpose. A camel-hair pencil should be used, if needed, in preference to a knife, to remove adhering powder from a filter, and the precipitate should be first washed down the sides of the filter by a small stream of water, SO as to collect the most of it to one spot at the bottom.

The first runnings in filtration should al. ways be returned to the filter. 3838. Bunsen's Method of Rapid Filtration. A great deal of time is frequently lost in washing precipitates, by having to wait for the liquid to pass through a filter.

Bunsen's improvement consists in fixing the filtering funnel air-tight, by means of a perforated cork in the neck of a bottle which has an opening connected with the receiver of an air-pump. By exhausting the air in the bottle, the liquid will run faster through the filter in proportion to the diminution of the pressure in the bottle. Comparative experiments, some made according to the old, and others according to the new method, showing that the filtration, washing, and drying of a precipitate, which took 7 hours by the old plan, could be performed, by filtration into an exhausted bottle, in 13 minutes.