170 FILTER When the filter is in action, the grosser impurities are stopped by the porous stone, while the finer pass into the charcoal and are chemically acted on. Each time the unfiltered water tap is opened to obtain water, unfiltered water enters the inlet and scours out the impurities in the chamber. To clean the filter, once every six months or other period, a plug is drawn up by means of a galvan ized chain (attached as shown) from the top of the filter and dropped into the inlet. The unfiltered water tap is then turned on several minutes, so that the cistern water rashes down through the filter and out of the tap, carry ing out impurities from the filter. In the filtration of water for supply of towns, galleries of masonry are often constructed in the sand and gravel forming the river bed or banks ; the water percolates through and enters these tunnels at the bottom and by side channels ; it is thence pumped to the town. Genoa, Toulouse, Lyons, and Perth present examples of this system, which is apt to prove rather costly. The system of artificial filter-beds of sand resting on gravel, &c., is now more generally adopted. It was first introduced into London by Mr Simpson, in 1839, after a study of various works in the north, and especially of the costly experience of Glasgow. For the Chelsea Water Company he had a series of tunnels built of brick without mortar ; these were covered with a layer of fine gravel 2 feet thick, then a 2-feet stratum of fine gravel and coarse sand, and lastly 2 feet of fine sand, or 6 feet in all. The sand was periodically removed to the depth of about half an inch. To the filter bed, covering 1 acre, were attached two reservoirs, slightly higher ; into these river water was pumped, which, after time for subsidence, was admitted to the filter beds through small pipes. The eight Water Companies of London all use similar beds, increasing in coarseness downwards, of various depth and proportion of materials. A sharp siliceous sand is preferred for the upper bed (the true filtering agent), and the stratum is seldom made less than 2 or more than 3 feet thick. Sometimes this bed is laid immediately on a bed of small shells. It is considered that filtration through sand, to be effective, should not proceed more rapidly than 6 inches of descent per hour (in the London beds the rate varies from 2 5 to 10 7 inches), and that there should be about 1 square yards of filtering area for each 1000 gallons per day. The depth of water maintained on filter-beds varies between 1 foot and 7 or 8 feet. The inlet arrangements should be such as to produce little disturbance of the sand in charging ; thus the water may be admitted into a long trough from which it gently overflows, or through an inlet pipe carried to the centre of the bed and turned upwards (at Chelsea the water enters through a wall of gravel between two horizontal concentric arches of brickwork with vertical joints). The beds are drained variously, e.g., by means of perforated stoneware pipes, or pipes with open joints, sometimes leading into a brick culvert which traverses the bed. A good method of draining is that of Mr Muir, adopted by the New River Company. It con sists of two courses of bricks laid flat and dry ; in the lower, the bricks are placed end to end in series alternating with half-brick spaces which serve as drains leading to a central culvert ; in the upper course, the bricks are laid close together, forming a floor, on which a thin layer of fine gravel supports a bed of sand. Filter beds require to be cleaned, at intervals varying from one week to six or eight, by removal of about half an inch of sand. The clean sand remaining is loosened with a rake and exposed to the air some time, then smoothed over. The filter bed may be made with several compart ments, some of which may remain in action while others are being cleansed. There are various contrivances for washing sand previous to its replacement. Filter beds are some times arranged to be cleansed by a reverse current sent upwards with force, an operation which may be aided by stirring the surface sand after the water has come above it. Such a system is practiced, e.g., in the Green ock, Paisley, and Dunkirk water works. At the last-named place, washed coke is among the filtering agents used. (For an account of the works at Dunkirk see Engineering, vol. xiv. p. 206.) In the system of M. Maurraz the water is filtered both per ascensum et descensum, the two portions of water, which flow in opposite directions, uniting at the middle. The sand is retained in closed and perforated boxes. This filter also permits of cleansing by reversal of the current. Mr Spencer s carbide system has been applied successfully on a large scale at Wakefield and other places, the carbide layer being combined with others of sand and gravel. With carbide, the total thickness of bed may be considerably reduced. Filtering is frequently practised by the chemist. And whereas in the ordinary filtration of water above described the purified liquid is the object of the process, while the matter retained is merely to be got rid of and destroyed, the reverse may be the case in the laboratory, the retained matter being sought, while the " filtrate," as it is called, is disregarded. For most laboratory preparations the material used is unsized paper. Swedish filtering paper (which is prepared with very pure spring water) possesses the advan tage of filtering very rapidly, and of being singularly free from inorganic matter. Cloth is employed in the case of viscid liquids such as syrup or white of egg ; while corro sive liquids may be filtered through pounded glass. Asbestos is a valuable filtering material, since, by making it red hot, all organic matter may be destroyed, and acid and alkalies have scarcely any action in it. Being nearly indestructible, it can be repeatedly used. Glass-wool has also of late years been recommended. Paper filters, to be placed in a funnel, are sold ready cut of circular shape. The paper is folded twice to the form of a quadrant, and this, when half opened, forms a cone, whose edges meet at an angle of G0. To facilitate passage of the filtered liquid, small folds are sometimes made in the filter all round. In a filter devised by Bunsen, the neck of the filter is inserted in the caoutchouc stopper of a lower vessel, and through this stopper also passes a tube con nected with an exhausting apparatus. The production of a partial vacuum below accelerates the filtering process. Sometimes substances have to be filtered under the influence of heat, as they solidify at ordinary temperature. In such cases the funnel may be surrounded by a sleeve containing water, which is heated with a lamp. In Robinson s oil filter, oil is forced up from a cask into and through the filtering apparatus (containing charcoal or other medium) by water entering below from an upper reservoir. Sundry modes of filtration practised in the arts (sugar-refining, &c.) will be referred to elsewhere. In
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some of them centrifugal force is employed. Circumstances are not uncommon in which it is very desirable to remove impurities from air by a process of filtration. Cutlers and other grinders use respirators to arrest the small particles which would otherwise find their way into the lungs. For steel particles magnetic gauze is an efficient protective. Professor TyndalFs respirator for firemen consists of an iron cylinder, attached to a mask, and containing charcoal, and three layers of cotton wool, one moistened with glycerine ; the ends of the case are of wire gauze. With this respirator it is possible to enr p an atmosphere of dense smoke and remain in it over a quarter of an hour. The disinfecting properties of charcoal have been turned to good account by Dr Stenhouse for purifying
