A filter which possesses the advantages of being easily and cheaply cleaned when dirty, and which frees solutions from suspended matter or mechanical impurities with immense rapidity, may be formed by placing a stratum of sponge between two perforated metallic plates, united by a central screw, and arranged in such a manner as to permit of the sponge being compressed to any required degree. Solutions flow with such rapidity through the pores of compressed sponge, that it will perfectly filter a large quantity of liquid. This method of filtration has also been adopted for purifying water from mechanical impurities, as in Part First, where we have illustrated a sponge filter.

If sponges are at all used, they should be removed from time to time, and thoroughly washed in hot water.

A few barrels or hogsheads of aqueous solutions may be easily filtered daily by the arrangement represented in the engraving.

Fig. 368.   Barrel Filter

Fig. 368. - Barrel Filter.

Fig. 369.   Diaphragm Filter

Fig. 369. - Diaphragm Filter.

A, A common water-pipe, or cock. b. A false bottom fitting in perfectly water-tight, c. A perforated wooden or metallic vessel or boz, covered with a bag of felt or other filtering substance (not shown in the engraving), d. A small tube, fitting water-tight into the false bottom and uniting the interior of the filter with the lower portion of the cask.

It is evident that when the solution is poured into the upper portion B, of a vessel, so arranged, it will sink through the filter c, and pipe d, into the lower chamber C, and this filtration will go on as long as the supply continues, and the filtered liquid is drawn from the cock e. By uniting the cock e with a tank or casks, and by keeping the upper portion By always full by means of a ball-cock, a considerable quantity may be thus filtered. The advantage of this plan is, that the filter c, can be always readily got at, and easily cleaned or renewed.

For filtering aqueous solutions on the small scale, diaphragms of porous earthenware and filtering-stone and layers of sand, already referred to, are commonly employed as filtering media. The filtering power of porous stone or earthenware may be greatly increased by adopting the arrangement represented in Fig. 369, which consists in making the diaphragm of the shape of a disk (d), supporting plates of the same material, the whole forming but one piece.