For fuller information on the subject of laboratory filtration, the reader is referred to the following papers (which are too long
for quotation here) in ‘The Chemical News’:—
“On a New Mode of Filtration,” by J. B. Cooke, May 30th, 1873; “Filtering Apparatus,” by John F. Kerr, February 6th, 1874; “Implements for Filtration,” by P. Casamajor, July 23rd, 1875, and 30th, 1875; Ibid, by W. Jago, February 4th, 1876; “On Rapid Filtration,” by E. C. H. Hildebrand, August 11th, 1876; also to ‘Journal of the Chemical Society,’ for papers on:—“Simple Suction arrangement for Rapid Filtering,” by C. Holthof, vol. xxxii, part 2, p. 508; “Employment of Compressed Air on Filtering Solutions,” by W. Leübe, vol. xxxii, part 1, p. 270.
When solid substances, as porous stone or earthenware, are used as the media for filtrations, vessels of metal, wood, or stone-ware, are employed to contain them and the supernatant liquid. In these cases the filtering medium is usually arranged as a shelf or diaphragm, and divides the vessel into two compartments; the upper one being intended to contain the dirty liquid, and the under one to receive the same when filtered. Such an apparatus is set in operation by merely filling the upper chamber, and may at any time be readily cleared out by reversing it, and passing clean water through it in an opposite direction. Small arrangements of this kind, intended to be screwed on to the water supply-pipe by either end, and which answer the purpose intended in the most satisfactory manner, have been manufactured and vended under the name of ‘REVERSIBLE’ or ‘SELF-CLEANING FILTERS,’ When pulverulent substances, as sand, coarsely powdered charcoal, &c., are employed, a similar arrangement is followed; but in this case the shelf or diaphragm must consist of any convenient substance pierced with numerous holes, over which must be placed, first a stratum of coarse pebbles, next some of a finer description, and on this a proper quantity of the sand, charcoal, or other medium. Over the whole should be placed another layer of pebbles, or a board or plate of metal or earthenware, pierced with a number of holes, to allow the liquid to be poured into the filter without disturbing its arrangement. Apparatus of this kind, of a permanent description, and arranged for filtering large quantities of liquids, are properly denominated ‘FILTERING MACHINES,’
Among the liquids usually submitted to filtration, the following may be mentioned as the principal—water, oils, syrups, tinctures, vegetable juices, infusions, and decoctions.
The filtration of water may now be considered. The water of our wells is presented by nature ready filtered to the hand of man, and often exhibits an admirable degree of transparency and purity. It acquires this state by percolating through the mineral strata of the earth, which deprives it of the organic matter it derives from the soil and subsoil, but, at the same time, it dissolves a portion of the saline and earthy media through which it passes, and hence acquires that peculiar ‘hardness’ which is constantly found in spring water. On the large scale, this natural system of filtration has been imitated by some of the commercial companies that supply our cities and towns with water. Extensive beds of sand and gravel have been employed, with variable success, as the filtering media; and were it not that filters gradually lose their porosity by the accumulation of the retained matter in their pores, such a method would be excellent. But the great expense of such filters precludes the possibility of frequently cleaning or renewing them, by which means they can alone be kept in an efficient state.
A filter which possesses the advantages of being easily and cheaply cleaned when dirty, and which frees water from 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. Water, under gentle pressure, flows with such rapidity through the pores of compressed sponge, that it is said that a few square feet of this substance will perfectly filter several millions of gallons of water daily. This method of filtration has been made the subject of a patent, and has been favorably noticed by the legislature.
A few barrels or hogsheads of water may be easily filtered daily, by the arrangement represented in the engraving.
A. A common water-pipe or cock.