FILTER, an instrument or apparatus contrived for clearing liquids from any impurities mechanically combined with them. Filters are of various kinds, according to the nature of the liquor requiring to be clarified; but they all act on the same general principle, namely, straining the liquor by passing it through earth or other porous material; the process being thus exactly analogous to that of sifting, as used for powder or other dry substances. The operation of filtering is extensively used in chemistry and the arts, and also in purifying water for domestic purposes, and for the use of ships in long voyages; and hence the construction of artificial filters on a large scale for the supply of towns and cities with pure water becomes a very nice and difficult operation, and now forms an important branch of civil engineering. The pureness of spring water arises entirely from a process of filtration. Such water generally issues from a considerable depth beneath the surface of the ground, as is evident from the force with which it is discharged, and the equable temperature which it preserves throughout the year, being cool in summer, and in winter never freezing. Hence, in its course from the surface downwards, it must pass through a variety of soils, subsoils, and porous rocky strata; and it is by the perfect straining which it receives in percolating through such vast masses of filtering materials as must occur in the bowels of the earth, that it comes out at last with that admirable purity and transparency which we observe. But notwithstanding of this, spring water contracts, often among the rocks and minerals with which it comes in contact, various impurities, which become chemically dissolved in it, and from which it cannot be freed by filtration or any other mechanical process. It becomes often, therefore, unfit on this account for domestic use or the purposes of the arts, and is rarely equal, and never superior, to rain or river water properly filtered. The great importance of this operation, therefore, is evident, more especially from the frequent and serious failures which have taken place. But it may be better to defer the consideration of this branch of the subject till it can be resumed along with other hydraulic operations. See Water-Works.

On a smaller scale, for domestic use, water filters have ^at different times been brought forward, and for many of ‘them patents taken out. But as they are all very similar in principle, and none appears greatly distinguished above the rest, it is unnecessary to enter into a minute description of them. They are all liable to the objection of clogging up, and getting foul in the course of time. But if regularly cleaned, any of them will answer the purpose sufficiently well, the process of filtration being so simple and so easily effected. Several ingenious plans have been proposed for self-cleaning filters; the water, for instance, being made to ascend, or to pass alternately in opposite directions, and thus carry off the impurities by the process itself. But as none or these have succeeded completely, it will be found most convenient in small filters to clean them regularly; and those are therefore to be preferred which afford the means of doing this most conveniently.

In these filters the filtering material generally used is gravel, sand, porous stone, or unglazed earthenware. Charcoal is sometimes added, but this is more for its antiseptic and purifying quality, and is only necessary in the case of fetid or very impure water. Gravel, though the best of all materials on a large scale, is rather coarse in itself for small filters. But where it occurs in nature in very thick beds, the large mass of material makes up for the coarseness of the grain; and many instances occur where springs from this source are scarcely inferior in purity to distilled water. But in small filters it requires a bed of sand above it to detain the impurities. Sand is perhaps, after all, the best filtering material; it should, however, be clean sea or river sand, with no tincture of iron; this last is indicated by its reddish colour, which, if deep, will communicate an iron taint to the water, a circumstance harmless enough in itself for most domestic uses, but very prejudicial for the purposes of bleaching, paper-making, &c. in the arts.

In filters constructed with gravel or sand, all the apparatus that is necessary is a vessel to contain the filtering material, with a space above for holding a quantity of the water to be filtered, and a vessel below for receiving the filtered water. A simple and convenient apparatus of this kind was contrived by Messrs Hearman and Dearn, potters, Redriff, describedin Nicholson’s Journal, vol. viii,, and represented in Plate CCXLV. fig. 1, which exhibits the whole apparatus, and shows, in fig. 2, a shaded section. A is a vessel of stone-ware perforated with holes, m, at bottom, upon which coarse gravel, h, is laid, and on that again a stratum of fine gravel, and, lastly, fine sand, g. Or, otherwise, the bottom may be covered with a coarse cloth, which will render the graduated fineness of the gravel and sand less necessary. Upon the top of the sand is laid a perforated and loaded board or plate of earthenware, to prevent the sand from being disturbed when the water is poured in. B is a lower vessel, into which the filtered water from A drops, together with any sand that may escape from above. The clear water flows out through the neck ac into the vessel d for use.

On this plan an excellent filtering apparatus may be constructed at little expense, and in various ways, according to the means or conveniencies most readily at hand; and the sand and gravel possess a decided advantage over filtering stones or other materials, from the great facility of cleaning, the latter being liable, from long use and the impurities lodging in their pores, to give the water an unpleasant taint.

A very simple arrangement in the above plan is to have a cylindrical jar of earthenware, as at fig. 5, with two partitions or false bottoms, dividing the height into three equal portions, the partitions resting each on a shoulder formed all round on the jar, or otherwise supported; the middle division for the filtering material, the upper one for the water to be filtered, and the lower one for containing the filtered water, with a stop-cock for drawing it off at pleasure, and a vent or opening at the top for the admission and discharge of air. The stop-eock must be placed three or four inches from the bottom, to allow full space for any sand or other material to deposit in the bottom; and the air vent must be carried up by a small pipe to the top of the jar, to prevent the water ever rising and overflowing by it. The partitions are made of earthenware, with a number of small holes to allow the water to descend. If sand only is used, it will be necessary to spread a layer or two of cloth under it, to prevent the sand getting through the openings. But it gravel can be had, it will be better to lay a bed of this, of two inches or more in thickness, on the bottom, and the sand above it. After the sand is filled in to the height of the upper partition, this is placed on as a cover, which having also a number of small openings in it, the water flow» from the upper division downwards. The cover should press on the top of the sand, in order to prevent its being disturbed by the entrance of the water, or a layer of grave should be placed on the top of the sand. If a layer or two of cloth be laid over the partition, it will serve to detain some of the impurities; and being frequently was ed, it will make the filter continue longer clean. In order to clean this filter it is only necessary to remove t θ upper partition, take out the upper stratum of sand to t ae depth of two or three inches, wash it thoroughly, and re place it; or, if sand can be had readily, let the whom e replaced with a fresh quantity, clean and pure. Thisιs> [7:9:581]on the whole, a very simple, perfect, economical, and convenient filter. It may be constructed of wood or other convenient material, and is both very easily formed, and easily kept clean and in order; and by varying the thickness of the bed of sand, it can be adapted to the exact degree of filtration required. The vessel also admits of any ornamental shape which, taste may direct; and several elegant figures of this kind, contrived by the manufacturers, arc shown at the bottom of the plate. If the lower division also were left unglazed and porous, an evaporation would go on from the outside, so that in summer it would act as a cooler as well as a filterer. Should much sediment be found collecting in the lower vessel, perhaps it would be better to place the whole apparatus upon a second jar, so wide as to allow the first to -stand over it, and having a pipe in the bottom rising nearly to the top of the vessel, and open at the top. The filtered water would flow over this, and descend into the lower vessel, which being detached, could always be kept perfectly clean in the bottom. The upper end of the pipe should be covered with a fold or two of cloth. An ingenious plan, introduced by Mr Parkes, chemist, for drawing off the water in the reservoir of a filtering or purifying apparatus, though applied rather on a larger scale, might often be used with advantage in smaller filters, such as those for bleachers, paper-makers, and others. It is represented at fig. 6, and consists in connecting the stop-cock for the discharge of the water with a pipe and flexible joint terminating in a rose-head and floating ball, which keeps the head always immersed a little under water, and thus discharges it from near the surface, although the level of the water may vary considerably. The water here is also let in, not directly into the reservoir, but by an upright pipe having a funnel at the top, and descending to near the bottom^ which causes the water to enter below, where it must deposit any impurity before this can rise to the surface.

In the sand and gravel filters here described, the water descends during the whole of the process, and this causes a slight tendency to carry minute particles of sand or other impurities along with it. To remedy this defect, various plans have been proposed for making the water,* after descending, again ascend; and this plan certainly possesses advantages, and is a decided improvement on the other. A very neat contrivance of this kind is that proposed by Professor Parrot of Paris, in Nicholson’s Journal, and is represented at fig. 3.

It consists simply of an inverted syphon, which could be easily made of lead, having the bend filled with sand, and being covered on the top with gravel in each branch, the water entering the longer branch from a reservoir, passes through the sand, and rising up, is discharged at the other end, which should be three inches below the level of the first. When the section of the tube is four inches by two, and eighteen inches the length of sand, it was calculated that six Paris pints of pure water would be passed in an hour.

Another plpn, proposed by Sir Henry Englefield, in the same work, is shown at fig. 4. It consists of a conical vessel placed within the one containing the sand, and allowing the water to descend on the inner vessel, and ascend again in the outer one.

But the simplest mode of effecting the ascent of the "ater is merely to divide the jar or filtering vessel by a vertical partition, open at the bottom. The water being then poured into the one side, descends to the bottom, passes under the partition into the other side, and ascends to the same level at which it entered, and can then be drawn oft by a stop-cock or other means at pleasure. In 1 at case a single vessel only is wanted, and contains in tself the means of holding separately the filtering material, the water to be filtered, and the water after filtration. Upon the whole, this appears by far the simplest, most perfect, most economical, and most convenient filter of this, or perhaps of any kind, that can be contrived. All that is necessary is to have a pretty deep vessel of earthenware, wood, or other material; a tub or barrel will answer perfectly. Put a vertical partition in the middle, reaching nearly to the bottom, fill it half or three fourths full of sand, with a layer of gravel on the top, and it is ready for use. It may be found convenient, to prevent the gravel being disturbed, to place a cover over it, with holes. The water may either be drawn directly from the pure side with jugs, or let off by a stop-cock. Instead of sand and gravel, some have proposed to use charcoal, either by itself or in combination with the others; and an apparatus of this kind was contrived by Mr John Isaak Hawkins. It is only, however, when the water has an impure taint, which cannot be removed by sand, that the charcoal should be resorted to, as it is more expensive, and more difficult to be cleaned. In long voyages, however, charcoal, as proposed by Hawkins, may, in conjunction with sand, be found very useful.

Instead of the water descending and then ascending, several ingenious methods have been contrived where the water only ascends through the filter, being let in below, and forced upwards by the pressure of an external head of water. The advantage of this is, that the filter can be cleaned by letting the water return and run out at the place where it usually enters. These plans, however, on a small scale, are hardly ever found to supersede cleaning. On this principle was the patent granted in 1791 to Mr Peacock, which he intended for small filters as well as those on a large scale. His apparatus consisted of a vessel of wood, lead, or other materials, of any shape or dimensions required, divided near the bottom by a horizontal grate or partition, upon which the filtering medium was spread in layers, beginning with the coarsest and ending with the fine. The lower division had a communication with a more elevated reservoir, from which the water flowed, and rose through the filtering bed. This consisted of gravel, sand, broken and pulverized glass or pottery, all well washed and sifted with sieves. On a similar principle Mr Bramah had a patent for various modes of conveying and drawing oft' beer and other liquors, in which he describes a filtering apparatus which he proposed to place in the pipes to conduct the liquor. This was a cylindrical vessel, made of copper tinned within, or of any other materials ;· the upper end was closed by a lid screwed on by a flaunch, and the lower and terminated in a cone; a pipe was inserted into the vessel near the lower part, to introduce the liquor; and from near the upper end a similar pipe proceeded to convey it away. Between these two was fixed the filtering floor, consisting of flannel, sand in bags, sponges or other similar substances, which were spread upon a horizontal plate pierced full of small holes, and fixed in the cylinder above the filtering substance; and another similar plate was fixed and screwed down to confine it. The liquor was introduced by the lower pipe by means of a pump or otherwise, and ascending through the filtering floor, passed off in a clarified state; the impurities which are separated collect in the conical bottom of the cylindrical vessel, whence they can be drawn by a cock. This contrivance is adapted to give a slight filtration to a great quantity of liquor, such as beer; and Mr Collier, in a patent for the purification of oils, claimed the principle of filtering per ascensum combined with hydrostatic pressure.

Such are the different contrivances for filtering water, where sand and gravel are the materials; but notwith-∣ standing their simplicity, filters of porous stone are those which have hitherto been most generally in use. 1he [7:9:582] stones are procured from beds of sandstone, and sometimes a kind of limestone. They are found in Northamptonshire, Derbyshire, and other parts of England, and are often imported from the island of Teneriffe. The construction of the filter is extremely simple. A thick bowl or basin is formed of the stone, and mounted in a frame. The foul water, being poured into the basin, passes 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 occasionally 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.

The basin shape used in these filters is obviously not the most proper, as it leaves an unequal thickness in the bottom for the water to penetrate, and also the pressure of the fluid is unequal. The bottom should be flat and of equal thickness, and in fact it would be much better not to make the stone a reservoir at all; but to use a filtering vessel such as above described, substituting for the sand and gravel a plate of the filtering-stone, of such thickness as maybe found best adapted for the nature of the water. If a thin plate also were placed on the top of a thick one, it might then be removed without any trouble when it got foul, and replaced by another, without disturbing the lower mass; or several plates could be laid one above another till the requisite thickness for filtering were obtained.

Instead of pouring the water into the basin and allowing it to filter downwards, the water may be contained in a vessel in which the basin is nearly immersed; and in this way the water will filter partly laterally, but chiefly per ascensum, and leave much of its deposit behind, so as to enable the stone to continue longer clean. A filter on this plan was contrived by Mr Moult, and is described in the Transactions of the Society of Arts. The filtered water runs in and fills the basin, from which it can be taken out or let off by a pipe. This principle was also proposed by Mr Collier, and it possesses all the advantages we have already described in filtering upwards with sand; and if it were used with plates of the stone instead of a basin, it would be still more effectual.

The unglazed earthenware for filtering has been introduced as a substitute for the filtering stones. It is made of different compositions of pottery. The original patent was granted to Mrs Johanna Hempel, potter, Chelsea, in the year 1790. The proportions were four out of nine of tobacco-pipe clay, and five out of nine of coarse sea, river, or pit sand. This was intended for basins not exceeding one gallon. For larger capacities it is liable to fly or crack in the fire. A second composition, therefore, consisted of equal parts of tobacco-pipe clay and sea or river sand; or if this was found insufficient to prevent the cracking, three parts tobacco-pipe clay, one of Stourbridge clay, or clay from the surface of the coal mines, one of Windsor loam, or other loam of equal quality, and four of coarse river or sea sand; or four parts of tobacco-pipe clay, three of sand, and one of the burnt ground clay of which crucibles are made. The above ingredients are brought to the proper basin or bowl shape on the wheel, and are then hardened in the furnace like other pottery ware. The same objection, however, applies here to the basin shape as to the stones; in fact, it would be better to have this composition formed into plates of different thickness, which could then be used, one or more in number, in the filtering vessels, instead of sand or stone; one could be taken out when foul, and another put in, with the utmost facility; and this would certainly be attaining the perfection of a filtering apparatus.

Another still simpler plan would be to have one filtering vessel, with a vertical partition of the porous ware reaching to the bottom. The one half of the vessel would then be filled with the impure and the other with the pure water· and if several of these plates were at hand, one partition could be taken out when foul, and another inserted in its place, or there might be two porous partitions, and a solid one between them. The one would then ascend and run over the top of the solid partition, and again descend and pass through the second porous partition, which would therefore hardly ever require renewal.

A filter by Mr Robins of London, and for which he has taken out a patent, has been lately much recommended to the public. The external appearance is varied according to taste , and figs. 13, 14, 15, already referred to, represent some of the neatest and most elegant patterns. We are not acquainted with the interior construction, but understand it consists of various strata of filtering material, such as we have above described, arranged so as to act with peculiar efficacy. At the top there is a sponge neatly inserted into a recess above the upper partition, which is a very judicious and useful mode of detaining any impurities, and preventing them from getting into the filterer, the sponge being easily taken out and washed from time to time. The patentee, however, claims far higher powers for his invention than that of mere filtration. He states that the interior process involves a voltaic action, which decomposes soluble substances, and renders the water equal in purity to distilled water. We confess we have great doubts in regard to this, and think it irreconcilable with any idea of voltaic or chemical action with which we are acquainted. Still we have no doubt, from the various testimonials brought forward, that the filter is a good one. Fig. 14 is about three feet high, and one and a half broad, and is calculated to filter fifty gallons a day; the others in proportion.

From the great success with which water has been purified by filtration, attempts have been made to purify oils in the same manner; and an ingenious apparatus for this purpose was contrived, and a patent taken out, by Mr Joshua Collier, applicable also to water and other liquids, and of which an account will be found in the Repertory of Arts, vol. X. But in oils there is generally so many impurities, chemically or otherwise united, that filtration can do little for their purification. Charcoal is the substance chiefly employed. A filtering machine contrived by Mr Innes of Edinburgh is said to answer well for oils and other fluids. By it the fluid is forced through the pores of wood by the action of a forcing-pump. Such a degree of pressure is unnecessary for water; but for oil, quicksilver, and other valuable fluids, it appears certainly well adapted.

Filtration is much used in chemical processes, the materials employed are unsized paper, cloth, flannel, tow, sponge, sand, pulverized glass, flints, porous stones, earthenware, &c. In fig. 12 is represented a neat little filtering apparatus constructed by Mr Parkes; and the following remarks and directions are taken from Mr Faradays excellent work on Chemical Manipulation. “Of the above substances, the first is almost exclusively 7 in use at t relaboratory, a few of the others now and then being resorted to only on particular occasions.

“Funnels are continually necessary to support the paper through which filtration is to take place. The ordinary funnels required in the laboratory for the passage of fluids answer the purpose very well, but they are of use for sup porting filters, when, from their necks being broken o , they are otherwise unserviceable. They may be out of glass or good Wedgewood or other earthenware; those [7:9:583]of glass are to be preferred, because the progress of the filtration and the state of the filter can be better ascertained in them. Metal funnels should not be allowed in the laboratory.

“The funnels containing filters may be frequently supported by the glass or jar intended to receive the filtered fluid, and at other times by the rings of retort stands. But notwithstanding these facilities, so frequently is the operation required, a laboratory should be supplied with at least one filtering stand of considerable size. The end of such a stand is represented in fig. 7. It should be fifteen inches wide, three or four feet long, and have an interval between the bottom and top of twelve inches. The top should have a number of round holes made in it at intervals of six inches, differing in diameter from one and a half to three inches; two or three small ones of half an inch or an inch in diameter being intermixed. The whole should be well made and firm, so as to support considerable weights, the top having frequently to answer the purpose of a table, and to sustain jars full of solutions, as well as to bear funnels and filters.

“lt is no easy matter for the chemist to obtain unobjectionable filtering paper, and yet it is of such importance in reference to the duties it has to perform, that he should not spare pains to procure the best possible. It should be so porous as to admit the free and ready passage of fluids; so close as to retain the finest solid particles; so strong as to bear the weight of a considerable quantity of fluid; and so pure as to give nothing to the solution, or if heated with the substance retained upon it, to occasion no mixture of ashes. Some chemists use plate paper, that is, the paper of copperplate printers. It is very porous, and yet there are few precipitates that will pass through it; at the same time it is often tender, generally yields a considerable quantity of ashes when burnt, and is inapplicable for minute filters when very small quantities of fluid only are to be worked upon, because of its thickness, and consequent waste of the portion of solution imbibed by it.

“It is amongst the thinner varieties of unsized paper, or white blotting paper, kept by some of the stationers, that the chemist will probably find the kind best suited to his purpose. It should be so strong, that a single filter of it, capacious enough to hold a pint of water, should not break with that quantity, even though some degree of agitation be given to the funnel containing it. Its porosity, that is to say, its comparative freedom from size, for it is mostly sized in a slight degree, may be judged of by holding it to the tongue, and observing how it absorbs moisture; and by a cautious pull its strength may be ascertained whilst in such moistened state. The student who is unused to the examination of papers will, however, better judge of its capability of allowing fluid to pass, by actual trial with water; a pint filter filled with clean water should allow the fluid to run in a considerable stream.

“The best method of judging of the purity of paper is to bum it and examine its ashes: the fewer it yields the better is it adapted for filters. A demy sheet should not yield more than one and a half or two grains of ashes altogether. If it contains more, their solubility or insolubility should be observed, that the student may be aware 0 ; the impurities that may probably be imparted to solutions in very delicate experiments. In minute cases of ι nvestigation, sulphuric acid may frequently be traced to the sulphate of lime existing in the filtering paper.

“Filtering paper should be cut ready for use into different sizes. A demy paper furnishes useful sizes for filters, w ιen the sheet is separated into four, or six, or nine parts; an d parcels of each of these sizes being prepared, they sι °uld have a string passed through the corners, and be preserved for use in a clean place. The previous cutting of the paper in this way is very convenient, as readily supplying the sizes that will be wanted, and in preventing the waste that would occur by carelessly tearing up a sheet each time a filter is required.

“On preparing a filter, the piece of paper should be first examined, by looking through it against the light, to ascertain that it is free from holes. The simplest filter is made by folding the paper twice in opposite directions, so as to bring the four corners together, and by opening one corner from the other three, so as to produce an irreo gular conical cavity, as in fig. 8. Such a filter being put into a funnel, and then filled with liquid, will immediately permit its passage; but from the similarity of form between the filter and the funnel, and the close adhesion of the former to the latter over by far the greater part of its surface, considerable obstruction is opposed to the passage of the fluid, and the operation is retarded. For this reason different contrivances have been recommended, to separate the filter here and there from the funnel, and allow passages for the fluid. Lavoisier recommended small glass rods inclined along the funnel before the filter is put in. Straws are used in a similar way, and they certainly open channels in their immediate neighbourhood, by which the fluid may flow down. By other chemists ribbed funnels are recommended; but it is difficult to find a funnel so deeply ribbed as to support the paper in such a manner that it shall not touch the glass in every part, and if they . do not perform this they are of no use.

“The best expedient by far is so to fold the filter that ribs may exist in the paper itself; and this may be done so as not only to allow numerous free passages for the fluid between the filter and the glass, but also to allow of ready transmission through its whole surface, and not of one half only, and even that imperfectly, as in the former case. For this purpose, the paper is first to be doubled, and in this state is again to be folded in half, each half folded into quarters, and each quarter into eighths, the folds being all on the same side, and radiating at equal distances from the middle of the folded edge to the other edges. Fig. 9 represents the doubled paper thus divided into eight parts. Each eighth is now to be divided into half by folds in the opposite direction, but in lines still originating at the same centre, which makes the doubled piece resemble a child’s paper fan, both when closed and when a little open: it is represented by fig. 10. Whilst in this state, the projecting corners should be taken off by a knife; folding the whole up tight like a closed fan, and making the section at about a. Being now allowed to expand a little, the originally doubled sides are to be separated from each other for the first time, but without disturbing the angles or bending the ridges or ribs which they form. Having opened it sufficiently to separate the cut edges from each other, it will be found that the paper is equally divided into parts forming alternate external and re-entering angles, except at the two edges bb, where two external angles come together. Here the intervening portion of paper between the two contiguous external angles should be folded, by bringing the latter together, and creasing the paper down, so as to form a re-entering angle between them: this should be done at both places. Then opening the paper sufficiently to bring the bottom into proper shape, by thrusting out the part which is convex within, so as to make it project externally, the filter is completed, and being put into a funnel, is ready for use. Its appearance, when perfectly formed, is represented at fig. 11.

“It is necessary, in making these filters, that the folds be not continued to the point, but that they should stop about half an inch short of it; for if completed to the bottom, the frequent action of the fingers in folding will [7:9:584] so far break down and destroy the texture of that part, ' that a hole probably will appear before the filter is finished; or if not, it will be so weakened as to be unable to bear a quantity of fluid without breaking. Hence that part of the filter will, during the folding, assume a concave form; and the regularity of the folds, which by practice may be easily attained, must be strictly attended to in the upper part, but may be dispensed with at the lower. When opened out, therefore, for the completion of that part, before the filter is put into the funnel, all that will be required is to push that side of the bottom outwards which is convex inwards, doing it so carefully as to cause no injury. The folds of the filter should be distinct and sharp, an effect which should be obtained by a single decisive pressure, and not by much fingering. No wrinkle or mark should appear in a well-made filter, except the folds described, and the portion of paper between the folds should be as stiff as when first taken up. The filter should be handled lightly during the whole of the operation, and never be opened out more than is represented in the last figure. It is best to keep the folds as close, and the whole filter as compact, as possible, dropping it loosely into the funnel, and permitting the fluid poured in to do the office of opening it out. It should be so regularly made, that when thus expanded by the fluid, the external angles of the folds should touch the glass at equal distances from each other, except at two opposite places where smaller divisions exist; and unless a large quantity of fluid be present, the angles at the upper part should remain nearly as sharp when the liquid is introduced as they were before. Below they gradually pass into the rounded surface, forming the centre or bottom of the filter, which will be about the size of a sixpençe or shilling, according to the dimension of the funnel in that part. These filters leave such free space between themselves and the glass, as to admit of the passage of a far greater quantity of fluid than is necessary; and no obstruction being placed against the external surface of the paper, the whole acts in the filtration, and that in the most favourable manner.

“Of these two kinds of filters, which may be distinguished as the plain and the folded, sometimes the one and sometimes the other is preferable. When the object is to cleanse and purify the fluid, that being the valuable part, it is most rapidly and effectually attained by a folded filter; but when the precipitate is the part required to be taken care of, it is generally desirable not to spread it over the irregular and extensive surface of such a filter, but, by using a plain one, to retain it on a surface of only half the extent. It is then likewise of such uniform thickness in every part, as best to admit of the operation of washing by having successive portions of water passed through it, and when the filter is opened out, the precipitate is delivered in one continuous portion, and not divided into many parts, as happens with the folded filter.

“When a single filter is judged too weak to hold the mass of fluid it is required to sustain, a double one should be used. If the filter is to be a folded one, then a double thickness of paper is to be taken; but if plain filters are used, the two should be made separately, and put one into the other, in such a manner that the three thicknesses of the one may come against the single thickness of the other. Occasionally it is proper to strengthen the bottom of the filter, which, not being supported by the glass, has to support the greatest column of fluid; but not so as to obstruct the upper part, where a filtration as free as possible is required. In such a case a smaller filter is to be added to the exterior of the large one, so that it may not interfere with the precipitate within, when it is necessary to remove it from the paper.

“The filter and funnel being ready, and placed on the stand in one of the holes before mentioned, over a glass ready to receive the liquid, the mixture to be filtered is to be poured in. It should not be poured from a great height, nor upon the middle of the filter, but down the side, the force of its descent being diminished as much as possible by good management, lest it break a hole through the paper: for the same reason it is better to pour it down the rod. If the first portions of fluid which pass be not clear, they should be returned into the filter, and a second glass should be placed beneath. The solid matter in the liquid to be filtered will soon, by adhering to the paper, cause clear filtration, except perhaps in one or two particular cases, as with precipitated oxide of tin, &c. on which occasions a double thickness of paper must be used. On changing the vessels beneath the filter for the removal of the clear solution, it should be done so that no drop be lost. By inclining the empty vessel, its edge may be brought under the funnel before the full one is removed; and in no case should a vessel be left under the filter, m which there is not sufficient room for the contents of the latter if it should break. Such an accident ought not to occur, but ought always to be provided against.

“Sometimes hot filtrations are to be performed. Oils filter better hot than cold: tallow and cocoa-nut oil may be passed through paper when hot; and many solutions must be filtered at high temperatures, because of the greater solubility of the ingredients under those circumstances. In these cases the fluids should be heated in a flask or basin before they are poured into the filters, and the filter funnels being placed in the glasses or jars, and, after the fluid is poured in, covered over with a basin or glass plate, the whole should be enveloped in a piece of flannel or a dry cloth. If, from the length of the operation, or other causes, the flannel or cloth be not sufficient, it may be dispensed with; and the glass and filter being placed on a warm part of the sand-bath, should be covered with a box or vessel large enough to rest on the sand, and thus form a hot air-chamber for the process. When the operations are upon a small scale, a paper cone is sufficient to cover the vessels on the sand-bath, and keep them hot.

“λVhen the object is hastily to filter a fluid for the purpose of removing pieces of dirt, then a little loose tow at the bottom of a funnel, or a piece of sponge slightly thrust in, is often sufficient for the purpose. Upon some few occasions filters for acids are required. These are generally recommended of powdered glass, being arranged somewhat in the manner described for lixiviation. Pieces of glass are put into the neck of the funnel; upon these smaller fragments, then again other layers of particles diminishing in size, until a moderately fine powder has been used, the top being finished with a layer of small fragments, to prevent disturbance by pouring. The pieces of glass should be well cleaned before pulverization, and, when arranged, some water should be passed through the filter, to remove alkali or other matlers that may be separable from the glass, which should be that of winebottles, and not flint glass.

“Another mode of separating a fluid from the finely-divided solid matter it may contain, is to allow the latter to deposit, and then to remove the former. This is called decantation, and is a process much superior to filtration in many analytical experiments, and recommended in pie-ference both by Lavoisier and Berzelius.”

For further information on this subject see Repertory of Arts ; Hempel, vol. ii. p. 230; Collier, vol. x∙ p∙ 3“ > Peacock, vol. xi. p. 221, vol. xiii. p∙ 140; Nichols° ns Journal ; Hearman and Dearn, vol. viii. p∙ 126; P 3rl0t, vol. ix. p. 40; Sir H. Englefield, vol. ix. p. 95; LxllacU, Phil. Mag. vol. vi. p. 240; Mem. Acad. Par. 1745, His∙
[7:9:585]

82; 1748, Hist. 121; Alachines Approuvées, tom. vii. p. 280; Gilbert’s Journal der Physik, vol. xiii. p. 108. (c.)