The New Student's Reference Work/Filter
Fil'ter, a vessel arranged for purifying water or other liquids. When water or some other liquid is passed through a substance, the pores or openings of which are too small to allow the solid particles to go through, they, of course, are kept back and the water is cleared. The familiar process of straining is a good illustration of the principle of a filter. For home use, a simple water filter is often made by stuffing a sponge in the hole of a flower-pot, then a layer of pebbles, then one of coarse sand and one three or four inches deep of powdered charcoal, with a second layer of pebbles on top. The pebbles and gravel form the strainers, while the charcoal purifies by absorbing impurities, for which it has a special affinity. The charcoal needs to be renewed occasionally, as it loses its power, and the sand and gravel need cleansing. All filters, therefore, made on this principle, should be easily opened, so as to reach all parts. Water is often filtered, before entering a cistern, by an external filter, or in the cistern itself by means of a division-wall of brick, the water passing through the pores of the bricks. An excellent method of filtering water, devised by Pasteur, consists in forcing the liquid through unglazed porcelain. The process is slow but very effective. Special filters are required for syrups, oils and the like. Oil is passed through bags made of horsehair or canton-flannels; syrups through bags of flannel. Ale, beer and such liquids are difficult to filter, and usually need an addition of some clearing substance, as gelatine. The use of filters in the laboratory is important and extensive. Here the material used for filtering is usually paper which is manufactured for the purpose.