A process for freeing liquids from particles held in suspension in them, by causing them to percolate through various porous substances, which intercept the insoluble matter, but allow a passage to the liquids, which are thereby rendered clear and transparent. The purpose to which filtration is most extensively applied, is the purification of water for domestic purposes; and from the importance of pure water as regards the preservation of health, and from the general complaints of the impurities abounding in the water supplied by the different companies, the subject has of late excited much attention, and a variety of filtering apparatus have been offered to the public, some few of which we propose to describe.

Filtration 447

The first of these machines which we shall notice is Messrs. White and Aveline's "artificial spring," in which the water is made to filtrate upwards by its pressure against the under side of a stone, the quantity filtered depending upon the area of the stone, and the height of the reservoir from which the water descends; but with a head of 35 feet, which can be obtained in most houses in London, a stone of 10 inches square will filter nearly thirty gallons per hour. The engraving on the opposite page exhibits a vertical section of the apparatus, a is the cistern which receives the water in its impure state; it has a ball float and lever to keep a constant head of water over the pipe b, and likewise to prevent any air passing down it. The pipe b b is shown broken off, that the space between may be considered as of any required length. To the lower end of the pipe there is a nozle c through which the pipe passes, which causes the water to shoot up against the under surface of the filtering stone f.

Through this stone the water oozes with great rapidity, leaving the animalculae and other impurities in the lower part or basin e of the machine, from whence they are drawn off occasionally by the cock g, and carried away by the waste pipe h.

When the filtered water rises in the reservoir above k to a certain height, the filtration is stopped by the rising of the float l, which by its lever or rod n, shuts a cock o in the supply pipe. When the stone has become charged with a deposit on its under surface, it is capable of being cleansed by the scraper s which is turned round by means of a handle shown at the bottom of the reservoir k, the axis passing through the stone; provision is thus made for reviving the filtering properties of the stone whenever required, and with very little trouble.

A very old contrivance for filtering water, but which has been the origin of most of the more recent apparatus for the purpose, consists in nearly filling the two legs of a pipe, formed either of metal as in Fig. 1, or of wood as in Fig. 2, with washed sand, leaving merely a space at b and e to receive the turbid water, and another at c or f for the filtered water to run off by. The chief objection to these machines is, that they soon become foul, and consequently useless, until restored by cleansing, and this task, as generally performed, is such a laborious, tedious, and slopping one, that these filters are usually abandoned in a short time. This objection seems to be obviated in the arrangement shown in the cut on the following page, a is a barrel capable of being turned round, but rendered stationary by pins passing through the extremities of their bearings at b b; c is a bed of sand occupying about one-third of the cask; d is the supply pipe or hose, (any flexible tube,) which conducts the turbid water from a reservoir above, into the cask; at e is a union joint and nozle piece, containing a sponge, which serves three purposes: it prevents the grossest impurities of the water from entering among the sand; it prevents the column of water from forcing up the bed of sand; and it prevents the sand from falling into the pipe.

The filtered water is drawn off at f. When the sand requires cleaning, the pins at the bearings of the axis are taken out, and the winch turned so as to bring the union joint to the top of the cask, previous to which the pipe should be detached by unscrewing. The sponge being now removed, the water may be let on freely at top, and the barrel turned by the winch g, by which means the sand is expeditiously washed, the water being let on and run off as often as necessary, which it is obvious may be effected with so much facility, that the filtering powers may be at any time renewed in a few minutes.

Fig. 1.

Filtration 448

Fig. 2.

Filtration 449

The following engraving, Fig. 1, represents an apparatus of a convenient form, by Mr. James, of Knightsbridge. It consists of two vessels (A and B,) of stone ware, placed upon a strong stand c. The upper vessel, which is covered, receives the impure water in a chamber d, at the lower part of which there is a large aperture, stopped by a sponge e, which detains the grosser impurities: hence the water passes through a finely perforated earthenware plate into a layer of six inches of prepared charcoal, through which the water filters, and is thereby purified from any noxious smells, as well as any floating impurities; it then passes through another perforated plate g, and is received at h into the separate vessel, which is a stone ware cask, from which it may be drawn off at pleasure by the cock.

Filtration 450

Ftg.l.

Filtration 451

Fig. 2.

Filtration 452

A very convenient filtering machine, from its portability, is Wiss's patent filter, which is shown in the preceding cut, Fig. 2. a is a force pump; b a suction pipe, to be inserted in a pail or other vessel of water; c the pipe which conducts the water out of the pump to the top of the vessel d; e is a receiver for the purified water; f a cock for drawing it off; g g g, screws for separating the receiver from the machine when required. The filtering substances used in this apparatus are of the same description as in the foregoing ones. The upper portion of the filter down to the letter d in the engraving, is left vacant for the dirty water which first passes through a thin bed of charcoal, and then through a bed of sand occupying the remainder of the vessel, and supported by a perforated metal plate, covered with a few layers of flannel.