b. A false bottom fitting in perfectly water-tight.
c. A perforated wooden or metallic vessel or box 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 water 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 water is drawn from the cock e. By uniting the cock e with a tank or casks, and by keeping the upper portion B always full by means of a ball-cock, a considerable quantity of water 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 water on the small scale, and for domestic use, ‘alcarazzas,’ diaphragms of porous earthenware and filtering-stone and layers of sand and charcoal, &c., 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 the margin, which consists in making the diaphragm of the shape of a disc (d), supporting plates of the same material, the whole forming but one piece. The ‘PLATYLITHIC WATER-FILTERS,’ which are formed of porous stone cut on this plan, present 200 to 300 square inches of filtering surface. A cheap, useful form of portable filter, is the following, given in the ‘Proceedings of the British Association,’ “Take any common vessel, perforated below, such as a flower-pot, fill the lower portion with coarse pebbles, over which place a layer of finer ones, and on these a layer of clean coarse sand. On the top of this a piece of burnt clay, perforated with small holes, should be put, and on this again a stratum of three or four inches thick, of well burnt pounded animal charcoal. A filter thus formed will last a considerable time, and will be found particularly useful in removing noxious and putrescent substances held in solution by water.”[303] The ‘PORTABLE-FILTERS,’ set up in stone-ware, that are commonly sold in the shops, contain a stratum of sand, or coarsely-powdered charcoal;[304] before, however, having access to this, the water has to pass through a sponge, to remove the coarser portion of the impurities. Among the many new kinds of portable filters now offered for sale, which claim special notice, are the following, viz.—
[303] A very similar filter to this was invented by the late Mr George Robins, the celebrated auctioneer. Mr Robins’ filter differed from the above in having a lid with a hole in the centre in which a sponge was placed; an arrangement which by keeping back the suspended matter contained in the water, prevented the filter from being clogged up.
[304] Frankland and Byrne have shown that animal is greatly superior to vegetable charcoal when employed for water-filters.
1. The MOULDED CARBON FILTER, consisting of a spherical or cylindrical vessel formed of compressed carbon.
2. The SILICATED CARBON FILTER, in which the medium is a compact substance, formed of animal charcoal and the ashes of Boghead coal.