FILTRA′TION

Syn. Filtratio , L. The separation of liquids from substances mechanically suspended in them, by pa ing them through media having pores sufficiently fine to retain or keep back the solid matter. Filtration is one of the most common and useful of the chemico-mechanical operations of the arts, and its succe ful performance in an economical and expeditious manner is therefore a matter of the highest importance in the laboratory, and, indeed, in almost every branch of human skill and industry, in which liquids are employed. Simple in principle, and apparently easily performed, it is, neverthele , one of those operations which require no le of care than of tact and experience to conduct it with certainty and succe . The lo es sustained in the laboratory, by defective manipulation in this particular, often exceed those arising from ignorance and accidents in every other department conducted in it. Filtration is generally resorted to for the purpose of freeing liquids from feculence, dirt, and other foreign matter, and for obtaining them in a clear or transparent state; but, in some cases, it has for its object the collection of the suspended substances, as precipitates, ., and in others both these intentions are combined. The word ‘filtration’ is absolutely synonymous with ‘straining,’ but in the language of the laboratory it is usually applied to the operation of rendering liquids transparent, or nearly so, by pa ing them through fine media, as filtering paper, sand, and the like; whilst the term ‘straining’ is employed to designate the mere separation of the gro er portion, by means of coarse media, flannel, horsehair cloth, ., through which they flow with considerable rapidity. Filtration is distinguished from ‘clarification’ by its mere mechanical action, whereas the latter operates by depuration, or the subsidence of the suspended substances or fæces, arising from their gravity being naturally greater than the fluid with which they are mixed, or being rendered so by the application of heat, or by the addition of some foreign substance. The apparatus, ve els, or media, employed for filtration, are called ‘ FILTERS ,’ and are technically distinguished from ‘ STRAINERS ’ by the superior finene of their pores. Both strainers and filters act on the same principles as the common sieve on powders; they all, in like manner, retain or hold back the coarser matter, and permit the liquid or smaller and more attenuated particles to pa through. The term ‘medium’ (pleural ‘media’) is applied to the substance or substances through the pores of which the liquid percolates. IMG:596405606043057898_i738-1.png: Fig. 1. The form of filters, and the substances of which they are composed, are various, and depend upon the nature of the liquids for which they are intended. On the small scale, funnels of tin, zinc, copper, wedgwood-ware, earthenware, gla , or porcelain, are commonly employed as the containing ve els. (See engr. ) The filtering medium may be any substance of a sufficiently spongy or porous nature to allow of the free percolation of the liquid, and whose pores are, at the same time, sufficiently small to render it limpid or transparent. Unsized paper, flannel, linen, calico, cotton wool, felt, sand, coarsely powdered charcoal, porous stone, or earthenware, and numerous other substances of a similar kind, are employed for this purpose. IMG:596405606043057898_i738-2.png: Fig. 2. For many liquids that filter easily, and in which the suspended matter is of a coarse and porous nature, it is often sufficient merely to place a little cotton wool or tow, or a small piece of sponge, in neck of the funnel, as at ( a , fig. 1) in the above engr.; but such an apparatus, from the small extent of the filtering surface, acts either slowly or imperfectly, and soon gets choked up. Filters of unsized paper are well suited for all liquids that are not of a corrosive or viscid nature, and are universally employed for filtering small quantities of liquids in the laboratory. A piece of the paper is taken of a size proportionate to the quantity of the liquid to be filtered, and is first doubled from corner to corner into a triangle (see engr. a ), which is again doubled into a smaller triangle b , and the angular portion of the margin being rounded off with a pair of sci ors c , it constitutes a paper cone, which is placed on a funnel of proportionate capacity, and is then nearly filled with the liquid. A piece of paper so cut, when laid flat upon the table, should be nearly circular. Filtering paper is now sold ready cut in circles of various sizes, which simply require doubling for use. Another method of forming a paper filter, preferred by some persons, is to double the paper once, as above, and then to fold it in a similar way to a fan, observing so to open it and lay it on the funnel that a sufficient interval be left between the two to permit of the free pa age of the filtered liquid on its descent towards the receiver. The ‘plaited filter,’ as thus formed, is exceedingly useful for general purposes; it exposes the entire surface of the paper to the liquid, and allows filtration to proceed more rapidly than a ‘plain filter’ does. (See Fig. 3.) IMG:596405606043057898_i738-3.png: Fig. 3. Mr Rother takes objection to the ordinary plain paper filter employed in the laboratory, because of the superfluous fold which in two thickne es lies under one half of the extended surface of the filter. He says the interposition of these two extra layers compels the liquid to pa through three thickne es of paper on the half side of the extended filter, whilst the other half side presents only a single thickne . It is evident that the two hidden layers are a very appreciable impediment to the current, aside from the more important fact that the liquid will traverse this side le rapidly than the other, and thus occasion an imperfect washing of the precipitate, or at least prolong the operation beyond reasonable limits. Recognising these objections to the old filter, Mr Rother has invented a very simple modification of the plain filter which, whilst saving 50 per cent. of the paper, he states, removes all the defects of the old form. This new filter practically presents but a single thickne of paper to penetrate, at the same time preserving an even surface, equal in all other advantages to the plain filter. The filtrations are said to be more rapid than with the usual form, and the absence of the superfluous half sheet admits of more rapid drying. To make the new filter:—Cut the circular disk of filtering paper in two through the line of its diameter, take either half disk, and fold it acro the line of the radius, then turn down the double edge of the cut side and fold it over several times—finally, run a hard smooth surface along the seam thus produced, to compre it, and spread the finished filter into an appropriate funnel, first moistening it with water before the liquid to be filtered is poured in. In reference to funnels, it may be remarked that those employed for filtering rapidly should be deeply ribbed on the inside, or small rods of wood or gla , or pieces of straw, or quills, should be placed between them and the paper. The neck or tubular part of the funnel should, in like manner, be deeply ribbed or fluted on the outside, to permit of the free pa age of the air, when it is placed in a narrow-mouthed bottle or receiver. When this is not the case, filtration proceeds but slowly, and the filtered liquid is apt to be driven up the outside of the neck of the funnel by the confined air, and to be continually hi ing and flowing over the mouth of the ve el. The breadth of a funnel, to filter well, should be about three fourths its height, reckoning from the throat ( a ). When deeper, the paper is liable to be continually ruptured, from the pre ure of the superincumbent fluid; and when shallower, filtration proceeds slowly, and an unnece arily large surface of the liquid is exposed to the atmosphere, and is lost by evaporation. To le en this as much as po ible, the upper edge of the gla is frequently ground perfectly smooth, and a piece of smooth plate-gla is laid thereon. When paper filters are of large dimensions, or employed for aqueous fluids that rapidly soften the texture of the paper, or for collecting heavy powders, or metallic precipitates, it is usual to support them on linen or calico, to prevent them breaking. This is best done by folding the cloth up with the paper, and cutting the filter out of the two, in the same way as would be done with doubled paper, observing so to place it in the funnel that the paper and calico may remain close together, especially towards the bottom. IMG:596405606043057898_i739-1.png: The filtration of small quantities of liquid, as in chemical experiments, may often be conveniently performed by merely placing the paper on the circular top of a recipient (see engr. ), or on a ring of gla or earthenware laid on the top of any suitable ve el. A filter of this kind that will hold one fluid ounce will filter many ounces of some liquids in an hour. Good filtering paper should contain no soluble matter, and should not give more than 1 ⁄ 250 to 1 ⁄ 230 of its weight of ashes. The soluble matter may be removed by washing it, first, with very dilute hydrochloric acid, and secondly, with distilled water. The ‘Munktell’ Swedish filtering paper [301] is composed of flax fibres very much crushed and broken, and owes its value to the broken pieces of the fibres filling up the pores, and thus preventing solids from pa ing through the paper. Rhenish filtering paper is also made from flax, but in consequence of the more perfect condition of its fibres, is more porous than Munktell’s, and therefore inferior to it for filtering purposes. Another kind of Rhenish paper, also of flax, in which the fibres are much torn, is manufactured and is said to be a useful article, and to allow the rapid pa age of fluids through it. The white filtering papers of English make have a small quantity of cotton mixed with the flax; and the fibres are much torn and crushed; hence they make serviceable filters. [301] Dr F. Mohr says that Swedish filtering paper is now undeserving its traditional reputation, and that it contains soluble alumina. The grey, circular cut filtering paper of varying sizes, of foreign make, as well as the grey sheet filtering paper of Dutch and English manufacture, contains a large quantity of wool, much of which is coloured; as well as jute and esparto gra , both of these latter in an unbleached state. The amount of ash in the Munktell paper has of late increased in quality. [302] [302] Greenish. IMG:596405606043057898_i739-2.png: For filtering a larger quantity of a liquid than can be conveniently managed with a funnel, and also for substances that are either too viscid or too much loaded with feculence to allow them to pa freely through paper, conical bags made of flannel, felt, tweeled cotton cloth or Canton flannel, linen or calico, and suspended to iron-hooks by rings or tapes, are commonly employed. The first two of the above substances are preferable for saccharine, mucilaginous, and acidulous liquors; the third for oily ones; and the remainder for tinctures, weak alkaline lyes, and similar solutions. These bags have the disadvantage of sucking up a considerable quantity of the fluid poured into them, and are therefore objectionable, except for large quantities, or when they are to be continued in actual use as filters for some time. On the large scale, a number of them are usually worked together, and are generally enclosed in cases to prevent evaporation, and to exclude dirt from the filtered liquor that trickles down their sides. These arrangements will be noticed further on. IMG:596405606043057898_i739-3.png: A simple mode of filtering aqueous fluids, which are not injured by exposure to the air, is to draw them off from one ve el to another, by means of a number of threads of loosely twisted cotton or worsted, arranged in the form of a syphon. (See engr. ) The little cotton rope at once performs the operations of decantation and filtration. This method is often convenient for sucking off the water from a small quantity of a precipitate. 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. Casa major, 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 Compre ed 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, ve els 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 ve el 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 pa ing 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, ., 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 di olves a portion of the saline and earthy media through which it pa es, and hence acquires that peculiar ‘hardne ’ 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 succe , 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 po ibility of frequently cleaning or renewing them, by which means they can alone be kept in an efficient state. A filter which po e es 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 compre ed to any required degree. Water, under gentle pre ure, flows with such rapidity through the pores of compre ed 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. IMG:596405606043057898_i740.png: A. A common water-pipe or cock. b. A false bottom fitting in perfectly water-tight. c. A perforated wooden or metallic ve el 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 ve el, 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, ., 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 ‘ PLATY LITHIC 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 A ociation,’ “Take any common ve el, 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 acce to this, the water has to pa 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] Frank land and Byrne have shown that animal is greatly superior to vegetable charcoal when employed for water-filters. IMG:596405606043057898_i741.png: 1. The MOULDED CARBON FILTER , consisting of a spherical or cylindrical ve el formed of compre ed carbon. 2. The SILICATED CARBON FILTER , in which the medium is a compact substance, formed of animal charcoal and the ashes of Boghead coal. Of the many forms of this filter, we may mention the ‘Syphon Filter for Travellers,’ by means of which wholesome water may be drunk from any pond or stream by simply immersing the filter therein and drawing the water through the tube by suction. Of the ‘Silicated Carbon Filter,’ Profe or Wanklyn says that it will render river water containing a considerable amount of free and albuminoid ammonia as pure as deep spring water. 3. Bischoff’s Patent Spongy-iron filter. —This differs from one invented many years ago by Dr Medlock, in bringing the water into contact with spongy iron instead of thin iron rods, and thus effecting filtration much more rapidly. Medlock believed that the iron rods brought about the oxidation of the nitrogenous organic matter and its consequent conversion into nitrites and nitrates. Bischoff states that he has experimentally investigated the properties of spongy iron, and finds that it— a. Decomposes even distilled water, which has been previously boiled. b. That it reduces nitric acid to ammonia. c. That the amounts of organic nitrogen and albuminoid ammonia are always much reduced after filtration through spongy iron. d. That a minute quantity of iron is di olved by the carbonic acid contained in the water, ferrous bicarbonate being formed. The latter being soon oxidised and precipitated is easily removed by filtration. e. That the action of spongy iron on impure water is two fold, viz. chemical and mechanical. “The chemical action is clearly indicated by the decomposition of water. The readiest explanation for the decomposition of water, is, the intimate contact between the electro-positive and electro-negative bodies, such as metallic iron and carbon, or even metallic iron and any ferric oxide, which has escaped reduction, or which has been reoxidised by exposure to air or water; and it may well be supposed that, consequent to the galvanic current thus produced, the atmospheric oxygen di olved in water is ozonised, and caused to act as a powerful oxidising agent in organic matter.” We extract the tables on the next page from the Sixth Report of the Royal Commi ion on Rivers’ Pollution. The Commi ioners, we may here state, speak in high terms of this filter. 4. The so-called Magnetic carbide of iron filter . In this, the filtering material is said to be prepared by heating hæmatite with sawdust. This filter has a good repute. ⁂ The Royal Commi ion “on Rivers Pollution” strongly recommend filters of animal charcoal to be recharged every three to six months, “since they found that myriads of minute worms were developed in the animal charcoal, and pa ed out with the water when these filters were used for Thames water, and when the charcoal was not renewed at sufficiently short intervals.” Cleansing of Filters. —Every two or three months (according to the kind of water) air should be blown through, and if the charcoal be in the block form it should be brushed. Then four to six ounces of the pharma copœial solution of pota ium permanganate, or twenty to thirty grains of the solid permanganate in a quart of distilled water, and ten drops of strong sulphuric acid, should be poured through, and subsequently a quarter to half an ounce of pure hydrochloric acid in two to four gallons of distilled water. This plan would be useful on foreign stations where the filter cannot be sent home, or taken to pieces; if it can be taken to pieces, the charcoal should be spread out in a thin layer, and exposed for some time to air or sun, or heated in an oven. The Average Composition of Thames Water, before and after Filtration through Spongy Iron. | Description. | Di olved Matters. | | Total solid impurity. | Organic carbon. | Organic nitrogen. | Ammonia. | Nitrogen, as nitrates and nitrites. | Total combined nitrogen. | | As delivered from Chelsea Waterworks | 28·04 | ·198 | ·042 | ·0009 | ·117 | ·220 | | The same water filtered through spongy iron | 16·8 | ·069 | ·018 | ·019 | ·018 | ·049 | | The mean of the 14th and 15th taken after the spongy iron filter had been in operation in the Rivers Commi ion Laboratory for upwards of eight months. [305] As supplied from Waterworks | 24·47 | ·170 | ·055 | ·001 | ·098 | ·154 | | After filtration through spongy iron | 14·26 | ·083 | ·016 | 0 | 0 | ·016 | [305] The figures demonstrate that the purifying action of spongy iron, if at all altered, has been increased , as regards the most important impurities of water, viz., nitrogenous matters and hardne . | Description. | Di olved Matters. | | Previous Sewage or Animal contamination. | Chlorine. | Hardne . | No. of samples analysed. | | Temporary. | Permanent. | Total. | | As delivered from Chelsea Waterworks | 1·464 | 2·01 | 15·5 | 6·2 | 21·7 | 15 | | The same filtered through spongy iron | ·177 | 2·00 | 6·8 | 4·9 | 11·7 | 15 | | The mean of the 14th and 15th samples taken after the spongy iron filters had been in operation in the Rivers Commi ion Laboratory for upwards of eight months. As supplied from Waterworks | ·675 Analysis of the 15th sample. | 1·95 | — | — | 19·1 | — | | After filtration through spongy iron | 0 | 1·95 | — | — | 9·6 | — | If sponges are at all used, they should be removed from time to time, and thoroughly washed in hot water. [306] [306] Parkes ‘Practical Hygiene.’ Oils are filtered, on the small scale, through cotton-wool, or unsized paper, arranged in a funnel; and on the large scale, through long bags, made of tweeled cotton-cloth (Canton flannel). These bags are usually made about 12 or 15 inches in diameter, and from 4 to 8 feet long (see engr. ), and are inclosed in bottomle casings, or bags of coarse canvas, about 5 to 6 or 8 inches in diameter, for the purpose of condensing a great extent of filtering surface into the smallest po ible space. A number of these double bags (from 1 to 50 or 60) are connected with corresponding holes in the bottom of a block-tin or tinned-copper cistern, into which the oil to be filtered is poured. The mode in which these bags are fastened to the cistern is of the utmost importance, as on the joint being close and secure depends the integrity of the apparatus. Three methods of doing this are figured in the engraving, which, with the references, will explain themselves, the same letters referring to the same parts of each. IMG:596405606043057898_i743-1.png: Filtering-bag of cotton-cloth. Cotton filtering-bag, ‘ creased ,’ or enclosed in its canvas envelope, ready for fixing. The second of the above arrangements is the least expensive, and certainly the most convenient in practice; and when the cylinder l fits the hole closely (allowing for the bag), is as safe, or safer, than an ordinary screw. IMG:596405606043057898_i743-2.png: a. Bottom of cistern. b. Filtering-bag. c. Screw of the conical nozzle fitting into the cistern. d. Binding cord connecting bag and nozzle. e. Binding cord connecting bag and lower nozzle. f. Bayonet-catch, connecting the lower portion of the nozzle fastened to the bag with the upper and fixed part, g . i. The thick hem at the top of the bag (purposely made large by enclosing a piece of thick cord therein), resting on the shoulders, k . l. A metallic cylinder, loosely fitting the hole in the cistern, and over which the top of the bag is drawn, before being put into its place; when fitted, as in the engraving, it retains the hem i securely in its place above the shoulder k . The bags are surrounded by a wooden screen fitted up with doors for the purpose of keeping off the dust; and the bottom of the apartment is furnished with large steam-pipes, by which a proper temperature may be kept up in cold weather. The use of heat should, however, never be had recourse to when it can be avoided, as although it vastly increases the rate of filtration, the oil so filtered is more apt to become opaque in cold weather than when the proce is conducted at the natural temperature of the atmosphere. This is particularly the case with castor oil and sperm oil. In the United States of America, where the latter is consumed in enormous quantities for illumination, the best is always ‘winter strained,’ as it is popularly called. In practice, it is more convenient to have a number of small cisterns at work (say 50 or 100 galls. each), than one or two larger ones, as any accident that may occur is more easily remedied, and that without stopping the whole operation. When cotton-cloth bags are employed without being ‘creased,’ or enclosed in others of canvas, they should not be longer than about 3 or 4 feet, and not wider than about 5 or 6 inches when filled. When larger they are dangerous. IMG:596405606043057898_i743-3.png: A convenient method of filtering a single cask of oil is, to insert the pipe of a two-way patent filter into the cork-hole, by which means the whole will be filtered as drawn off, without any trouble on the part of the operator. This filter consists of a porous bag stretched over a perforated metallic ve el, nearly the shape and size of the exterior casing, and its edge is tightly screwed between the sides and bottom of the latter, so as to be quite water-tight. The cock communicates with the interior of the perforated plate and filter, and the supply-pipe with the exterior. By this means the interior chamber, which occupies 5 ⁄ 6 ths of the ve el, rapidly fills with filtered oil, and continues full as long as any liquor remains in the cask. This arrangement is also well adapted to the filtration of wines, beer, cordials, porter, and various other liquors. It is unequalled in simplicity and usefulne . The same filter may be removed from cask to cask, with the facility of a common cock. The filtration of SYRUPS is now generally effected on the large scale by pa ing them through the ‘ CREASED BAG FILTER ’ just described. On the small scale, as employed by confectioners and druggists, they are usually pa ed through CONICAL FLANNEL BAGS . (See page 726.) The filtration of thick syrups is, however, attended with some difficulty, and it is therefore a good plan to filter them in a somewhat dilute state, and afterwards to reduce them to a proper consistence by evaporation in clean ve els of tinned copper, by steam heat. Syrups, when filtered in a heated state, run well for a time, but the pores of the fabric rapidly get choked, from the thickening of the syrup and partial crystallization of the sugar, occasioned by the evaporation of the aqueous portion from the surface of the bag. This may be partially prevented by enclosing the bag in a metallic casing. On the whole clarification is preferable for syrups to filtration on the small scale. They need only be well beaten up while cold with a little white of egg, and then heated; a scum rises, which must be removed as soon as it becomes consistent, and the skimming continued until the liquid becomes clear. Any floating portions of scum that may have escaped notice are easily removed by running the syrup through a coarse flannel strainer, whilst hot. The most extensive application of the proce of filtration in the arts is in the refining of sugars. Tinctures and dilute spirits are usually filtered, on the small scale, through BIBULOUS or UNSIZED PAPER placed on a funnel; and on the large scale, through thin and fine COTTON BAGS . In general, however, tinctures clarify themselves by the subsidence of the suspended matter, when allowed to repose for a few days. Hence it is the bottoms alone that require filtering; the supernatant clear portion need only be run through a small hair sieve, a piece of tow or cotton placed in the throat of a funnel, or some other coarse medium, to remove any floating substances, as pieces of straw, . Spirits which are largely loaded with e ential oil, as those of ANISEED , ., run rapidly through paper or calico, but usually require the addition of a spoonful or two of magnesia before they will flow quite clear. When po ible, tinctures, spirits, and all similar volatile fluids, are better and more economically cleared by subsidence or clarification than by filtration, as, in the latter way, a portion is lost by evaporation, and the strength of the liquid is thereby altered. Vegetable juices should be allowed to deposit their feculous portion before filtration. The supernatant liquid will then be often found quite clear. It is only when this is not the case that filtration should be had recourse to. A small quantity may be filtered through coarse or woollen filtering paper, supported on a piece of coarse calico placed on a funnel; when the quantity is large, one of the CONICAL BAGS before described should be employed. The bottoms from which the clear portion has been decanted should be placed on a separate filter, or else not added until the whole of the other portion has drained through. Vegetable juices are often rendered clear by simply heating them to about 180° or 200° Fahr., by which their albumen is coagulated; they are also frequently clarified by the addition of a little white of egg and heat, in the same way as syrups. Many of them (as those of hemlock, henbane, aconite, .) are greatly injured by heat, and must consequently be filtered, or only simply decanted after repose. In all cases they should be exposed to the air as little as po ible, as they rapidly suffer decomposition. Vegetable infusions and decoctions may be cleared by defecation followed by filtration. The conical bags of flannel before described are usually employed for this purpose. When the liquid is to be evaporated to an extract, they are commonly suspended by a hook over the evaporating pan. A convenient method of straining these fluids, practised in the laboratory, is to stretch a square of flannel on a frame or ‘horse,’ securing it at the corners by pieces of string. (See engr. ) Such a frame, laid acro the mouth of a pan, is more easily fed with fresh liquid than a bag, whose mouth is 40 or 50 inches higher. The same purpose, for small quantities of liquid, is effected by laying the flannel acro the mouth of a coarse hair sieve. The concentrated infusions and decoctions being usually weak tinctures, may be filtered in the same way as the latter. (See above .) Many vegetable solutions, that from the viscidity of the suspended matter can scarcely be filtered, may be readily clarified with white of egg in the cold, or pa the filter rapidly if a very small quantity of acetic, tartaric, sulphuric, or other strong acid, is previously added. IMG:596405606043057898_i744-1.png: Corrosive liquids, as the STRONG ACIDS , are filtered through powdered gla , or SILICEOUS SAND , supported on pebbles in the throat of a gla funnel, or through asbestos or gun-cotton placed in the same manner. Charcoal has also been employed for the same purpose, but is not fit for some acids. Strong caustic alkaline lyes are also filtered through powdered gla or sand. Weak alkaline lyes may be filtered through fine calico, stretched acro the mouth of a funnel. Many corrosive liquids, as solution of pota a, ., require to be excluded from the air during filtration. The simplest apparatus that can be employed for this purpose is that figured in the margin:—( a ) is a globular bottle fitted with the ground stopper ( d ), and having a perforated neck ( f ) ground to the bottle ( b ); ( c ) is a small tube, wrapped round with as much asbestos, linen, or calico, as is required to make it fit the under neck of the bottle through which it pa es. The tube ( c ) may also be fixed by placing pebbles and powdered gla or sand round it, as before mentioned. For use, the solution to be filtered is poured into the bottle ( a ) nearly as high as the top of the tube ( c ), and the stopper is replaced. The liquid then descends into ( b ), and a similar quantity of air pa es up the tube into ( a ). Liquor pota æ may be always obtained fine by depuration in close ve els, when the sediment of lime only need be filtered, which may be effected with calico fixed acro the mouth of a funnel. IMG:596405606043057898_i744-2.png: When a precipitate, or the suspended matter in a liquid, is the object of the filtration, the filter should be of such a nature that the powder may be easily separated from it, when dry, and that with the least lo po ible. Linen filters are for this reason preferable for large quantities, and those of smooth bibulous paper for small ones. The powder should be washed down the sides of the filter, and collected, by means of a small stream of water, in one spot at the bottom, a isting the operation with a camel-hair pencil; and, when the whole is dry, it should be swept off the paper or cloth with a similar pencil or brush, and not removed by a knife, as is commonly done, when it can be po ibly avoided. The ‘first runnings’ of liquid from a filter are commonly foul, and are pumped back or returned until the fluid runs perfectly limpid and transparent, when it is ‘turned into’ the ‘filtered liquor cistern,’ or proper receiver. In many cases the liquid does not readily become transparent by simply pa ing through the filter; hence has arisen the use of FILTERING POWDERS , or substances which rapidly choke up the pores of the media in a sufficient degree to make the fluid pa clear. In the employment of these powders care should be taken that they are not in too fine a state of division, nor used in larger quantities than are absolutely nece ary, as they are apt to choke up the filter, and to absorb a large quantity of the liquid. The le filtering powder used, the more rapid will be the progre of the filtration, and the longer will be the period during which the apparatus will continue in effective action. For some liquids these substances are employed for the double purpose of decolouring or whitening, as well as rendering them transparent. In such cases it is preferable first to pa the fluid through a layer of the substance in coarse powder, from which it will ‘run’ but slightly contaminated into the filter; or, if the powder is mixed with the whole body of the liquid, as in bleaching almond oil, ., to pa the mixture through some coarser medium to remove the cruder portion before allowing it to run into the filter. Another plan is, after long agitation and subsequent repose, to decant the clearer portion from the gro er sediment, and to employ separate filters for the two. Granulated animal charcoal is used according to the first method, to decolour syrups, oils, .; and filtering powder by the second and third, to remove a portion of the colour, and to clarify castor and other oils. The common plan of mixing large quantities of filtering powder with castor oil, and throwing the whole into the filter, as adopted by the druggists, is injudicious. When simple filtration is required, it is better to use little or no powder, and to continue returning the oil that ‘runs’ through, until, by the swelling of the fibres of the filter bags, it flows quite clear. By this plan the same filters may be used for a long period of time (for many years), and will continue to work well; whilst, by the usual method, they rapidly decline in power, and soon deliver their contents slowly, and after a short time scarcely at all. It is often of great advantage to render a filter ‘self-acting,’ or to construct it in such a way that it may ‘feed itself,’ so that it may continue full and at work without the constant attention of the operator. On the small scale, this may be readily effected on the principle of the common fountain lamp (see engr. ); and on the large scale, by placing the ve el containing the unfiltered liquid on a higher level than the filter, and by having the end of the supply-pipe fitted with a ball-cock, to keep the liquid in the filter constantly at the same height. IMG:596405606043057898_i745-1.png: The rapidity of filtration depends upon—the porosity of the filtering medium—the extent of the filtering surface—the relative viscidity or mobility of the filtering liquid—the pre ure or force by which the liquid is impelled through the pores of the filter, and—the porosity and finene of the substances it holds in suspension. The most efficient filter is produced when the first two or the first three are so graduated to the others that liquid filters rapidly, and is at the same time rendered perfectly transparent. In the common method of filtration no pre ure is exerted beyond that of the weight of the column of the liquid resting on the filtering medium, but in some cases additional pre ure is employed. This is had recourse to for the purpose of producing a more rapid filtration, and more especially for filtering liquids that, from their viscidity, will scarcely pa through the pores of substances sufficiently fine to remove their impurities in the ordinary way. IMG:596405606043057898_i745-2.png: One of the easiest means of employing pre ure in filtration is to increase the height of the column of the filtering liquid. From the peculiar properties of fluids, by which they transmit pre ure in an equal degree in all directions, this column need not be of equal diameter throughout, but may be conveniently contracted to the size of a small pipe, as in the accompanying engraving, which represents a small filter on this construction at work. ( a ) Is the funnel or reservoir of foul liquid; ( b ) a small pipe conveying the liquid to the filter; ( c c ) a chamber, of which the upper portion ( d ) is filled with the descending liquid, and the lower portion ( e ) with the filtering media; ( i i ) are screws by which the bottom plate is fastened on, which plate is removed to clean out or renew the filter. For use, the cocks ( k ) and ( l ) are closed, and the liquid poured into the funnel ( a ); the cock ( k ) is next opened, and, in a few minutes after, the cock ( l ), when an uninterrupted flow of filtered liquor will be obtained as long as any fluid remains in the funnel ( a ) and the tube ( b ). The length of the tube determines the degree of pre ure. Care must be taken first to pa the foul liquid through a hair sieve, or some other strainer, to remove any substance that might choke up the pipe ( b ). Another method of employing pre ure in filtration is the withdrawal of the air from the receiving ve el, as in the vacuum filter, by which a pre ure of about 14 1 ⁄ 2 lbs. to the square inch becomes exerted on the surface of the liquid by the atmosphere. The vacuum in the receiving Sprengel pump. A commoner method of applying pre ure than either of those already mentioned is to condense the air over the surface of the liquid by means of a forcing-pump, or by steam. On the small scale, pre ure may be applied to filtration by means of a syphon, whose shorter leg has its mouth blown into the shape of a bell or funnel, over which filtering paper or fine calico may be stretched. The application of pre ure to filtration is not always advantageous, and beyond a certain limit is generally attended with inconvenience, if not with absolute disadvantage. It is found in practice that fluids under pre ure take a longer period to run clear than without pre ure, and that ruptures of the media more frequently take place in the former case, or with pre ure, than in the latter. Great pre ure is in no case advantageous. The filters already noticed are those that act by the fluid descending through the media; but in some cases the reverse method is employed, and the liquid filters upwards, instead of downwards. These are called ascending filters, and are often preferable to those on the descending principle, because the suspended matters that require removal by filtration usually sink, and thus a portion escapes being forced into the pores of the filter. They are also more convenient when pre ure is employed. The construction depends upon the same principles as the common filter, and merely requires that the feeding ve el should be higher than the upper surface of the filtering media. Oils are conveniently filtered in this way, because of their little specific gravity. By fixing a small filter on this principle into the head of a cask, and pouring in water through a funnel, whose neck reaches nearly to the bottom of the cask, the oil will float up and pa the filter, leaving the sediment behind. In cold weather hot water may be employed. IMG:596405606043057898_i746.png: a. Cask of oil. b. Stand. c. Funnel for water. d. Filter. In some cases the upward and downward systems of filtration are united in the same apparatus, and this plan is advantageous where the space for operating is limited. For this purpose it is merely nece ary to connect the bottom of an ascending filter with the top of a descending one, or the reverse; the proper pre ure being in either case applied.