Cooley's Cyclopaedia of Practical Receipts Volume I Part 240

_b._ A false bottom fitting in perfectly water-tight.

_c._ A perforated wooden or metallic vessel 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 vessel, 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 c.o.c.k _e_. By uniting the c.o.c.k _e_ with a tank or casks, and by keeping the upper portion _B_ always full by means of a ball-c.o.c.k, a considerable quant.i.ty 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, &c., 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 'PLATYLITHIC 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.s.sociation,' "Take any common vessel, perforated below, such as a flower-pot, fill the lower portion with coa.r.s.e pebbles, over which place a layer of finer ones, and on these a layer of clean coa.r.s.e 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 coa.r.s.ely-powdered charcoal;[304] before, however, having access to this, the water has to pa.s.s through a sponge, to remove the coa.r.s.er portion of the impurities. Among the many new kinds of portable filters now offered for sale, which claim special notice, are the following, viz.--

[Footnote 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.]

[Footnote 304: Frankland and Byrne have shown that animal is greatly superior to vegetable charcoal when employed for water-filters.]

[Ill.u.s.tration]

=1.= The MOULDED CARBON FILTER, consisting of a spherical or cylindrical vessel formed of compressed 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,' Professor w.a.n.klyn says that it will render river water containing a considerable amount of free and alb.u.minoid 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 alb.u.minoid ammonia are always much reduced after filtration through spongy iron.

_d._ That a minute quant.i.ty of iron is dissolved 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 dissolved 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 Commission on Rivers' Pollution. The Commissioners, 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 haemat.i.te with sawdust. This filter has a good repute.

? The Royal Commission "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.s.sed 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 pharmacopial solution of pota.s.sium 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._

------------------+---------------------------------------------------------- Dissolved Matters.

---------+-------+---------+--------+-----------+--------- Description. Total Organic Organic Ammonia. Nitrogen, Total solid carbon. nitrogen. as nitrates combined impurity. and nitrogen.

nitrites. ------------------+---------+-------+---------+--------+-----------+--------- As delivered from Chelsea Waterworks 2804 198 042 0009 117 220 The same water filtered through spongy iron 168 069 018 019 018 049 ------------------+---------+-------+---------+--------+-----------+--------- The mean of the 14th and 15th taken after the spongy iron filter had been in operation in the Rivers Commission Laboratory for upwards of eight months.[305] As supplied from Waterworks 2447 170 055 001 098 154 After filtration through spongy iron 1426 083 016 0 0 016 ------------------+---------+-------+---------+--------+-----------+---------

---------------+------------------------------------------------------------- Dissolved Matters.

+---------+---------+-------------------------------+--------- Hardness. +----------+----------+---------+ Description. Previous Chlorine. Temporary. Permanent. Total. No. of Sewage samples or Animal a.n.a.lysed.

contamin- ation. ---------------+---------+---------+----------+----------+---------+--------- As delivered from Chelsea Waterworks 1464 201 155 62 217 15 The same filtered through spongy iron 177 200 68 49 117 15 ---------------+---------+---------+----------+----------+---------+--------- The mean of the a.n.a.lysis 14th and 15th of the samples taken 15th after the sample. spongy iron filters had been in operation in the Rivers Commission Laboratory for upwards of eight months.[305] As supplied from Waterworks 675 195 --- --- 191 --- After filtration through spongy iron 0 195 --- --- 96 --- ---------------+---------+---------+----------+----------+---------+---------

[Footnote 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 hardness.]

If sponges are at all used, they should be removed from time to time, and thoroughly washed in hot water.[306]

[Footnote 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 bottomless casings, or bags of coa.r.s.e canvas, about 5 to 6 or 8 inches in diameter, for the purpose of condensing a great extent of filtering surface into the smallest possible s.p.a.ce. 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration:

_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 process 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 quant.i.ties 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.

[Ill.u.s.tration]

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 vessel, 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 c.o.c.k 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 5/6ths of the vessel, 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 usefulness. The same filter may be removed from cask to cask, with the facility of a common c.o.c.k.

The filtration of SYRUPS is now generally effected on the large scale by pa.s.sing them through the 'CREASED BAG FILTER' just described. On the small scale, as employed by confectioners and druggists, they are usually pa.s.sed 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 vessels 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 sc.u.m rises, which must be removed as soon as it becomes consistent, and the skimming continued until the liquid becomes clear. Any floating portions of sc.u.m that may have escaped notice are easily removed by running the syrup through a coa.r.s.e flannel strainer, whilst hot. The most extensive application of the process 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 coa.r.s.e medium, to remove any floating substances, as pieces of straw, &c. Spirits which are largely loaded with essential oil, as those of ANISEED, &c., 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 possible, 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 quant.i.ty may be filtered through coa.r.s.e or woollen filtering paper, supported on a piece of coa.r.s.e calico placed on a funnel; when the quant.i.ty 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 alb.u.men 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, &c.) 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 possible, 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 across 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 quant.i.ties of liquid, is effected by laying the flannel across the mouth of a coa.r.s.e 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.s.s the filter rapidly if a very small quant.i.ty of acetic, tartaric, sulphuric, or other strong acid, is previously added.

[Ill.u.s.tration]

Corrosive liquids, as the STRONG ACIDS, are filtered through powdered gla.s.s, or SILICEOUS SAND, supported on pebbles in the throat of a gla.s.s 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.s.s or sand. Weak alkaline lyes may be filtered through fine calico, stretched across the mouth of a funnel. Many corrosive liquids, as solution of pota.s.sa, &c., 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.s.ses. The tube (_c_) may also be fixed by placing pebbles and powdered gla.s.s 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 quant.i.ty of air pa.s.ses up the tube into (_a_). LIQUOR POTa.s.sae may be always obtained fine by depuration in close vessels, when the sediment of lime only need be filtered, which may be effected with calico fixed across the mouth of a funnel.

[Ill.u.s.tration]