Filters are contrivances for separating substances from liquids
which are not dissolved in them; but in the most common acceptation
of the term, filters are vessels used for separating the impurities
from water. Filters on the very large scale required by the water
companies consist of sand or gravel so contrived that the water shall
drain through them. This, indeed, is the natural way in which well
or spring water is filtered; for the rain falling on the surface of
the earth sinks down through such substances as gravel and sand, and
lies in beds at the bottom, when it meets with stone or clay, through
which it cannot sink (see “Artesian Wells”). This water when drawn
up is in most cases very bright, as it has been strained through the
sand or gravel in passing downwards. The best substance through which
to filter water for household use is sponge pressed together with
some force, and this is the usual plan adopted in all the earthenware
filtering vessels sold; but there is usually a layer of sand or some
other substance placed below, which is useless or worse, as it often
becomes foul and taints the water. If the water has a bad odour, a
few pieces of newly-burned charcoal placed in it above the sponge
will purify it (see “Charcoal”). Filters for other purposes, and
for any small quantity of liquid, may be made by cutting a piece
of white blotting paper round, and then folding it into quarters
and partially opening it (fig. 1); this if put into a funnel forms
a convenient filter for any substance to be brightened, as water,
vinegar, or wine. A particular kind of porous sandstone used to be
hollowed out and used as a filter, but these filtering-stones are
now but seldom used, except in the case of self-filtering cisterns,
which are made by enclosing the inner opening for the tap in slabs
of porous stone, so as to form a box within the cistern (fig. 2);
by this contrivance, when the tap is turned, only that water escapes
which has been filtered. It is necessary to have an air-tube to let
the air in as the filtered water runs out, and to let the air out as
the water filters in from the cistern. Even in these cisterns a box
of slate or other substance having several holes with sponges pressed
into them would answer much better, as these could be removed from
time to time, washed, and returned. Filters, of course, can only
separate mechanical impurities, such as dust, insects, &c., for if
sugar or salt were put into the water, all the filtering that could
be used would not separate them when dissolved, and thus it is that
well and spring water, although perfectly bright, are still very
impure, containing much lime and carbonic acid dissolved in them,
together with other matters, as iron, &c., which are not separable
by filtration; if it be desirable to separate them, distillation must
be had recourse to (see “Distillation”). Some of these, however, as
lime, may be separated by boiling the water for some time, which
causes the lime to fall down in the form of chalk, and adhere to the
bottom of the vessel--hence the “fur,” as it is called, in kettles.
Water containing lime, although quite “hard” and unfit for washing
purposes, is made sufficiently “soft” for use by boiling.