Cooley's Cyclopaedia of Practical Receipts Volume I Part 241

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 loss possible. Linen filters are for this reason preferable for large quant.i.ties, 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.s.sisting 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 possibly 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.s.sing 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.s.s 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 quant.i.ties than are absolutely necessary, as they are apt to choke up the filter, and to absorb a large quant.i.ty of the liquid. The less filtering powder used, the more rapid will be the progress 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.s.s the fluid through a layer of the substance in coa.r.s.e 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, &c., to pa.s.s the mixture through some coa.r.s.er 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 grosser sediment, and to employ separate filters for the two. Granulated animal charcoal is used according to the first method, to decolour syrups, oils, &c.; 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 quant.i.ties 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 vessel containing the unfiltered liquid on a higher level than the filter, and by having the end of the supply-pipe fitted with a ball-c.o.c.k, to keep the liquid in the filter constantly at the same height.

[Ill.u.s.tration]

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 pressure or force by which the liquid is impelled through the pores of the filter, and--the porosity and fineness 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 pressure is exerted beyond that of the weight of the column of the liquid resting on the filtering medium, but in some cases additional pressure 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.s.s through the pores of substances sufficiently fine to remove their impurities in the ordinary way.

[Ill.u.s.tration]

One of the easiest means of employing pressure in filtration is to increase the height of the column of the filtering liquid. From the peculiar properties of fluids, by which they transmit pressure 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 c.o.c.ks (_k_) and (_l_) are closed, and the liquid poured into the funnel (_a_); the c.o.c.k (_k_) is next opened, and, in a few minutes after, the c.o.c.k (_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 pressure. Care must be taken first to pa.s.s 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 pressure in filtration is the withdrawal of the air from the receiving vessel, as in the vacuum filter, by which a pressure 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 vessel may be produced by the air-pump, by steam, or by the Bunsen or Sprengel pump.

A commoner method of applying pressure 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, pressure 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 pressure 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 pressure take a longer period to run clear than without pressure, and that ruptures of the media more frequently take place in the former case, or with pressure, than in the latter. Great pressure 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 pressure is employed. The construction depends upon the same principles as the common filter, and merely requires that the feeding vessel 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.s.s the filter, leaving the sediment behind. In cold weather hot water may be employed.

[Ill.u.s.tration:

_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 s.p.a.ce for operating is limited. For this purpose it is merely necessary to connect the bottom of an ascending filter with the top of a descending one, or the reverse; the proper pressure being in either case applied.

=Filtration, the Laws of.= The 'Revue Universelle des Mines,' 1874, pp.

469, 551 contains a paper by M. Paul Havre recording his investigations on the rapidity of the filtration of water through sand, wool, &c., which resulted in ascertaining and measuring the influences which may modify the flow of water. In all cases of filtration, the influences which are exerted are:--the pressure and temperature of the water, the thickness of the filtering medium, compression in the case of fibrous filters, the size of the grains and their mixture in the case of a filtering medium a.n.a.logous to sand. The influence of obstruction, due to the dirtiness of the filter, depends on circ.u.mstances too variable to be taken into account. The delivery of a filter per square metre per 24 hours is equal to two cubic metres multiplied by the pressure of water in metres, divided by the thickness of the filtering medium in metres. An application of this formula is made to existing filter beds, including those at Southwark and at the Chelsea waterworks.

The first experiments for ascertaining the influence of a head of water on the delivery led to the following results:--The delivery increases in a higher ratio than the square root of the pressure, due to the height (TORRECELLI'S LAW); the delivery increases in direct ratio to the height of the column of water above the filter, admitting a previous initial delivery, due solely to the pressure of water above the filter; the co-efficient of the increase of delivery is constant, and in this case of a filtering substance 8662 inches (22 centimetres) thick, is equal to 0106 pint (6 centilitres) for sand to 0528 pint (30 centilitres) for compressed wool, and to 0792 pints (45 centilitres) for wool only slightly compressed.

The subsequent experiments were made with graduated transparent cylinders, 328 feet (1 metre) high, with the ends perfectly level, the filtering substances being kept in place by a thick double cloth tied tightly under the bottom of the tube. This apparatus presented no other obstacle to the running of the water than the layer of filtering substance; it permitted experiments to be made at all temperatures, and the thickness of the filtering medium to be measured exactly.

In these experiments sand is 'taken as the type of pulverulent substances,' but an unexpected difficulty was encountered in the settling or partial agglomeration of the large and small grains of the unsifted sand, thus diminishing the delivery of water to one half, one third, and ultimately to one fifth of its previous volume. This led to the adoption of sand--the grains of which were uniform in size, and to the discovery of the fact that, other tissues being equal, the resistance of filtration is constant when the sand is coa.r.s.e, when the grains of fine sand are of nearly equal size, and when there is but little fine sand mixed with the coa.r.s.e. From experiments in filtering through a layer of coa.r.s.e sand approximately 4 inches (10 centimetres) thick, it was found that the higher the temperature the more rapid was the delivery, and by filtering through a layer of coa.r.s.er sand 118 inches (30 centimetres) thick, the conclusion was arrived at that the temperature exerts an influence in proportion to the thickness of the layer.

See AIR-PUMP, BUNSEN'S WATER-AIR PUMP; CLARIFICATION; DEFECATION; FININGS, &c.

=FI'NINGS.= Substances used by publicans, brewers, wine merchants, &c., to clarify their liquors.

_Prep._ 1. (BREWER'S FININGS; COOPER'S F.) Isingla.s.s (finely shredded), 1 lb., and sour beer or cider or vinegar, 3 or 4 pints, are macerated together, and more of the sour liquor added as the isingla.s.s swells, until about a gallon has been used, agitation with a whisk or a small bundle of twigs being occasionally had recourse to, for the purpose of promoting the solution. As soon as the whole of the isingla.s.s is dissolved, the mixture is reduced to the consistence of thin syrup, with weak mild beer, or cider, or any other liquid that the finings are intended for. The whole is next strained through a tammy cloth or a hair sieve, and at once reduced to a proper state of dilution, by the addition of more liquor. _Product_, 6-1/2 to 7 galls. "A pound of good isingla.s.s will make about 12 galls. of finings." (Ure.) Used to clarify fermented liquors, especially beer. 1 to 1-1/2 pint is the usual dose for a barrel of ale or porter; and a quart for a hogshead of cider or wine.

2. (SPIRIT FININGS.)--_a._ Alum (ord. cryst.), 1 lb.; powder, and divide it into 12 equal portions, which are to be separately wrapped in blue paper, and marked No. 1. Next take of carbonate of soda (sesquicarbonate of the shops), 6 oz.; divide this as the last, wrap it in white paper, and mark each parcel No. 2. Keeps dry anywhere.

_b._ From alum, 1 lb.; salt of tartar (dry), 1/4 lb.; proceed as before.

The white papers containing the salt of tartar must be kept in a dry, well-corked, wide-mouthed bottle or jar. Both of the last two are used to clarify gin and cordials. The contents of one of the blue papers are dissolved in about a pint of hot water, and the resulting solution is well 'rummaged up' with the liquor. A solution of the contents of one of the white papers, in about 1/2 pint of hot water, is then added, and the agitation continued for some minutes longer; after which the cask is 'bunged' close and the whole allowed to repose until the next day. This is sufficient for a barrel (say 30 to 36 galls.), but many persons use double the quant.i.ty. The effect is not only to clarify, but also to 'blanch' the liquor.

_Obs._ Good liquors, either fermented or spirituous, need no artificial 'fining,' as they always clarify themselves by repose. With those, however, which are out of 'condition,' or of inferior quality, it is often necessary, as, without such a proceeding, they remain unsaleable. This is particularly the case with malt liquor. "Attempts to clarify it in the cask seldom fail to do harm. The only thing that can be used with advantage for fining foul or muddy beer is isingla.s.s." (Ure.) The disadvantages resulting from the artificial clarification of fermented liquors are--that they do not afterwards 'stand well on draught,' that much of the conservative astringent matter which they contain is precipitated with the 'finings,' that their piquancy and flavour is more or less diminished, and that they are more than usually liable to become flat and vapid, whether in cask or bottle. The larger the proportion of 'finings' used, the more marked are their injurious effects, and the shorter the interval which elapses before the accession of the several symptoms referred to. We have seen the most disastrous consequences follow the injudicious use of 'finings,' more especially in respect to those liquors in which a certain amount of piquancy, astringency, and briskness, is an essential condition. In one instance which came under our notice upwards of 30 barrels of 'underground' (a very strong old ale) was thus reduced in value to less than 1-3rd its original cost; and in another, a large bottled stock of the 'finest old Burton' was found to be utterly unsaleable. In both cases the 'spoiled liquor' was got rid of by mixing it in and selling it with 3d. and 4d. beer.

Liquors which 'refuse to fine' or become clear, when treated with 'finings' in the usual manner, are called 'stubborn' by coopers and cellarmen. See BREWING, GIN, MALT, LIQUORS, WINES, &c.

=FIRE.= The calamities resulting from this destructive agent are of such frequent occurrence, as to justly claim a notice of the subject here. The causes of fires are numerous, and of a varied character, and, in most instances, difficult to determine, because it is the interest of those concerned to suppress all evidence connected with the matter. Accident, that convenient word given to the imaginary hack to which so many fires are referred, if truthfully interpreted, will, in general, be found to be equivalent to carelessness, recklessness, or guilt. We believe that there are few fires which have happened that might not have been prevented by the exercise of common prudence, and that a vast number have been caused by direct negligence, arising from sheer laziness and indifference, to use no harsher terms. As familiar instances, may be mentioned--allowing sparks to fall on the ground and remain there without extinguishing them; carrying a naked candle into rooms containing inflammable substances; smoking carelessly and in dangerous places, as workshops, warehouses, on shipboard, &c.; keeping instantaneous light matches in improper places, and neglecting to pick up those that may happen to fall on the ground, &c.

&c. The list might easily be extended, but we believe every reflecting reader can do so for himself. The great increase in the number of fires since the introduction of lucifer matches, and the almost general use of tobacco, cannot fail to have attracted the attention of every one. The danger of matches falling about might be avoided by the use of those which can only be ignited by rubbing them on the prepared surface of the box.

These 'safety matches' are coming into general use, and must eventually supersede all the more dangerous kinds.

The late Mr Braidwood cla.s.ses the causes of fires under the following heads:--1. Inattention in the use of fires and lights. 2. Improper construction of buildings, &c. 3. Furnaces or close fires, for heating buildings, or for mechanical purposes. 4. Spontaneous ignition. 5.

Incendiarism.

Amongst many other causes of fire, too numerous to specify, may be noticed--incautiously approaching window- and bed-curtains with a candle or lamp, airing linen before the fire, allowing children to play with fire, women's dresses taking fire, and taking off the burning coals from a fire and laying them on the hearth. Another very common cause of fire is covering up a fire-place when not in use with wood, or paper and canvas, &c. The soot falls either from the flue itself or an adjoining one into the grate; a neighbouring chimney takes fire, a spark from this falls down the blocked-up flue, ignites the soot in the grate, which smoulders until the covering is burnt through, and thus sets the building on fire.

Another cause of fire, and one which cannot be too strongly condemned, is the dangerous practice of reading in bed by candle-light. A very serious annual loss of property is also caused by want of proper care in hanging up or removing the goods in linendrapers' shop windows when the gas is burning. Another frequent cause of fire is the employment of young children in lighting fires, from their propensity to play with flame.

The employment of close fires with brick flues is also a frequent source of danger. Frequently, from various causes, the furnace almost always cracks, thus giving egress to smoke and flame. When this occurs no time should be lost in thoroughly repairing the defect, or building a new furnace; merely plastering over the surface will be found an ineffective and dangerous remedy.

To guard against the dangers arising from the ignition of wearing apparel many methods have been suggested for rendering fabrics flame proof, all of them consisting in soaking the dress in a weak solution of a non-inflammable substance, such as chloride of zinc, alum, tungstate of sodium, sulphate of ammonia, &c. Of these alum has the advantage of greatly improving the appearance of the fabrics, especially if they be coloured.

Fire-guards, particularly where there are children, ought to be adopted much more generally than they appear to be.

_Prev._ This consists of the exercise of those ordinary precautions which the good sense of every careful and trustworthy man, be he taskmaster or servant, cannot fail to suggest. It would be useless to enumerate them.

Immediately on the fire being discovered, secure an alarm being given to the nearest of the fire escape stations, not delaying an instant; do not wait "to see if it is wanted." Life is more valuable than property, and events have often proved how fatal even a moment's hesitation is in sending for the fire-escape.[307]

[Footnote 307: 'Handbook for Emergencies,' Ca.s.sell.]

The late Mr Braidwood's advice was, "that if the fire appears at all serious, and there are fire-engines within a reasonable distance, that it is best to wait until they arrive; many buildings have been destroyed from opening doors, and trying to extinguish fires with insufficient means. If no engines are within reach it is advisable to keep a hand-pump. If that is not to be had, the next best thing is to collect as many buckets outside the room on fire as can be obtained, keeping the door shut; then to creep into the room on hands and knees (if the heat and smoke are considerable), and throw the water as nearly in the direction of the fire as possible, keeping the door shut while more water is being collected.

"The police of the metropolis understand shutting up fires so well, that they have in many instances kept fires two or three miles distant from the engine-stations, shut up till the fireman arrived in time to extinguish them."

Fires might often be readily extinguished when first discovered by the timely application of a few buckets of water. When an apartment is found to be on fire, the door, chimney, and windows should be immediately closed, if possible, and only opened for the purpose of projecting water on the flames. By this means the supply of air will be cut off, and rapid combustion prevented. The same applies to the lower doors and windows of a house (especially the shop window), which are often injudiciously kept open or removed, under the pretence of rendering a.s.sistance. The neglect of this precaution has often caused a mere smouldering fire, that might have been easily put out, to burst into an unextinguishable ma.s.s of flame.

It has been proposed at various times to make certain additions to the water used for the purpose of extinguishing fires, in order to render its action more certain and effective. It is found that sal ammoniac (5 oz.

to the gall.) exerts this property in a remarkable degree. Several other articles, as common salt, pearlash, and kitchen soda, act in the same way, though less effectively. A few buckets of such water will speedily arrest the progress of a fire before it has much extended itself. Such a plan is easily applied, by adding the saline matter to the buckets of water, which are either used by hand, or to feed the engine for the first few minutes of its working. When, however, a fire has made much progress, the action of such substances becomes scarcely perceptible.

Chimneys on fire are readily extinguished in several ways, without having recourse to throwing water down them from the top, by which much damage is frequently done to the furniture in the rooms. One of the simplest methods is, to cautiously scatter a handful of flowers of sulphur over the dullest part of the burning coals; the sulphurous vapours, being incapable of supporting combustion, rapidly extinguish the flames. Another method is, to shut the doors and windows, and to stop up the bottom of the chimney with a piece of wet carpet or blanket, throwing a little water or flowers of sulphur, or even common salt, on the fire immediately before doing so.

By this means the draught is stopped, and the burning soot extinguished for want of air. In many of the first-cla.s.s houses recently erected, 'fire-place shutters' are provided, which, when partly drawn down, act as powerful bellows or 'blowers' and which, when wholly drawn down, so as to touch the hearth-stone, entirely close up the fireplace, and instantly extinguish the combustion of the fuel in the grate, or that of the soot in the chimney. This simple arrangement, the advantages of which were pointed out in an early edition of this work, renders fires in chimneys of little moment, as it is only necessary to draw down the shutter to put them out.

If a chimney is stopped at top, instead of at the bottom, the whole of the smoke must, of necessity, be driven into the apartment.

In France, M. Marateuh has successfully applied the principle of Davy's safety lamp for the prevention of fires in chimneys. He places fire-frames of iron work near the base of the chimney, one above the other, about one foot apart; no flame pa.s.ses through them, whilst the draught in the chimney is not interfered with, the result being that no fire can happen in the chimney.