Gravel, in thick beds, is the most perfect natural filter; and instances may be met with, of springs from gravel pro ducing water very nearly as pure as distilled water. Sand, when white, such as that of the seashore, is also very good; but if coloured, it generally contains iron ; and where the colour is deep, the iron is often in such excess, that it will be communicated to the water in passing through it. Beds of sandstone filter extremely well, and also some porous limestone.
The ordinary filtering apparatus is made from a porous stone, of which them'c are two kinds ; a sandstone procured in Northamptonshire, and a limestone found in Derbyshire. A thick bowl or bason is formed of the stone, and mounted in a frame. The foul water, being poured into the basin, drains slowly through the substance of the stone, and drops into a receptacle below, in a perfectly transparent state. When the water is foul, a small quantity of mud or slime is found to collect at the bottom of the basin, and must oc casionally be cleared out; but in the course of time, the more minute impurities will insinuate themselves into the pores of the stone, and at last clog up the passage of the water. This is remedied by chipping away the interior surface of the stone one-half or three-fourths of an inch, because the impurities do not penetrate deeper into the stone ; this will restore its action for a long time. We do not approve of the ordinary figure of the filtering stone, which is that of a bowl, because the pressure of the water is unequal in every part ; it would be a preferrable form to have a vessel of wood, lead, or pottery, and fitted with a bottom, formed from a flat slab of the filtering stone; and this method would admit of constructing a much larger fi- ter than can be done in the form of a basin of solid stone.
The Society of Arts have published a description of a filter by Mr Moult, which consists of a vessel in which the foul water is contained, and a basin of filtering stone be ing placed in it, the water will percolate through the stone into its cavity, from whence it can be taken up clear and fit for use ; or the filtered water may be drawn off, by means of a curved leaden pipe, to reach over the edge of the ba sin and act as a syphon, with a cock at the lower end. By this plan of filtering the water into the basin, the deposit is not left on the stone, and will therefore be less liable to clog up its pores. The same principle was proposed by Mr Col lier; his machine consisted of a cask, or other vessel to con tain the foul water. Within this vessel cylindrical tubes of earthen ware are fixed; these tubes are closed with a hemis pherical end, but the other end, which is open, is applied to the sides of the vessel, and closely luted thereto, so that no water can gain entrance into the vessel, without first penetrating through the substance of the tube. A cock was provided
in the side of the vessel, opening within the tube to draw off the pure water. Mr Collier proposed also to fill up the vessel with pieces of broken pottery, which, by presenting a great surface, would allow every facility to the deposition of the gross impurities of the water, and thus avoid the clogging up of the filter, and also render the process more expeditious. • A patent was granted in 1790 to Mr Hempel, for a me thod of making filtering vessels or basins from a species or pottery, the composition of which was four parts of to bacco pipe clay, mixed up with five of coarse sea, river, or drift sand. For large vessels, this composition is found liable to crack in the burning, equal parts of the clay and sand were therefore recommended; and if this was found insufficient to prevent the tendency to cracking, one-ninth part of Stourbridge clay, or of old cru cibles- broken down and pounded, was to be added ; the basins were turned in a potter's lathe, in the usual manner.
The filtering stones most commonly used arc not the best kind, as they arc too expensive, liable to be broken ; and will not produce so great a quantity of filtered water as others, which, in imitation of the great natural filters, have beds of gravel and sand for the water to pass through. A very simple apparatus for domestic usc, consists of a cy lindrical vessel of pottery, provided with a cock to draw off the clear water ; upon this is fitted another cylindrical ves sel, having a globular bottom, which is pierced with small holes; a stratum of coarse gravel is first spread in the ves sel, over this a stratum of fine gravel, and above this fine sand. If the bottom of the vessel is covered with a coarse cloth, to keep the sand from running through the holes in the bottom, the gradations or gravel and sand will be ren dered less necessary. A plate of earthenware, or a board, is laid upon the sand, and being pea finated with holes, al lows the water to pass, but prevents the disturbance of the sand when the foul water is poured in upon it, and in perco lating through the sand it is filtered. The fineness of the sand and the thickness of the bed will be regulated by ex periment; and it is one of the greatest advantages of this Construction for a filter, that it can be so readily adapted to the degree of filtration which the water requires, the bed be ing made thick if the water is very foul, and diminished in proportion to its purity, by which Incans the greatest quanti ty of water can be passed through this filter. The sand should be changed every fortnight or three weeks, to keep the apparatus perfectly sweet and clean.