FILTRATION.

47a

’ and the connexion-tube «, into the common pipe c, which communicates, by the shortranch tubes e, with each of the four filters. The end of the branch tube, which is inside°1 the filter tub, is provided with a stop-cock d /, whose opening, and thereby the efflux,?. ttle liquor from the cistern through the tube a, is regulated by means of the float-ball g.u P»n the brickwork D the filter tub stands, furnished at h with a false bottom of zinc oranpper pierced with fine holes; besides which, higher up at i there is another such platenietal furnished with a strong handle k, by which it may be removed, when the boneac k needs to be changed. In the intervening space Z, the granular coal is placed, o ise /=over of the filter tub, with a handle also for lifting it. One portion of it may be rais-■ * Wnge, when it is desired to inspect the progress of the filtration within, vi m is a

nnder vertical tube, forming a communication between the bottom part h, and the upper!, rtl °n of the filter, to admit of the easy escape of the air from that space, and from amonge hone black as the sirup descends; otherwise the filtration could not go on. p is the■ ,>Coc k through which the fluid collected in the space under h, is let off from time to timetuh° t *' e CHmmon P‘P e 7 tfig- 388. r is a trickling channel or groove lying parallel to thein 6 arK * * n which, hy means of a tube s, inserted at pleasure, the sirup is drawn offc ase of its flowing in a turbid state, when it must be returned over the-surface of thec har coa l.

'pi

t v Il e ce lerity with which any fluid passes through the filter depends, 1. upon the porosi-teni t *' e Bering substance ; 2. upon the pressure exercised upon it; and 3. upon the ex-c °h Ihe filtering surface. Fine powders in a liquor somewhat glutinous, or closelyfor Pacte( l> admit of much slower filtration than those which are coarse and free; and thef ac nie J- ought, therefore, to be spread in a thinner stratum and over a more extensive sur->no n t * le ^ atler > f° r e 1 ua l effect; a principle well exemplified in the working of Du-s apparatus, just described.

m a[ j Dla >iy cases filtration may be accelerated by the increase of hydrostatic or pneu-n ec[ c . P'essure. This happens when we close the top of a filtering cylinder, and con-air ^7 a Pipe with a cistern of fluid placed upon a higher level. The pressure of thev es „ . a y be rendered operative also either by withdrawing it partially from a closet°p 1[Uo which the bottom of the filter enters, or by increasing its density over thecr eat liquor to be filtered. Either the air pump or steam may be employed toor s , 6 a Partial void in the receiver beneath the filter. In like manner, a forcing pumpc °ttu am ma I' be ern Pl°y e( ito exert pressure upon the surface of the filtering liquor. Aits Un ° a s J'Phon may; on the same principle, be made a good pressure filter, by makingth e wll 6 ] * e =. trumpet-shaped, covering the orifice with filter paper or cloth, and fillingpre Ss „ ao e with liquor, the lower leg being of such length so as to create considerableOn ^ re hy the difference of hydrostatic level. This apparatus is very convenient eitherTh e . sma U or great scale, for filtering off a clear fluid from a light muddy sediment.taiu s P r essure °h Ihe atmosphere may be elegantly applied to common filters, by the appa-ca Se ^ e P>esented in fig. 390, which is merely a funnel enclosed within a gasometer. The‘ n( lricai n „ 6ais an annu ^ ar hollow vessel a b, filled with water, in which receiver the cyl-edg e t 0 = as ? m eter d, e, f, i, is immersed. The filter funnel is secured at its upperthe Sa le Inner surface of the annular vessel a b. In consequence of the pressure ofequ a q °® eter regulated by the weight g, upon the air enclosed within it, the liquid is°Uter su P r Sed ’ and l * le water in the annular space rises to a corresponding heighten the-hp(_q • r ace of the gasometer, as shown in the figure. Were the apparatus made ofIn op,?’ U ,‘ e a,| nular space might be charged with mercury,that i® eia > relatively to the application of pressure to filters, it may be remarked,len derini> , ' t I k 0l: P us hed very far, without the chance of deranging the apparatus, or

s l’ ea kin.? tti 6 ®* terei l liflnor muddy. The enlargement of the surface is, generally? s Pecial|Ip e safest and most efficacious plan of increasing the rapidity of filtration,“ag filt er n ° r “fiords of a glutinous nature. This expedient is well illustrated in the creasedt- t nany°" r * n u ? e - n most 'he sugar refineries of London . See Sugar.

“fiaid shall Cases rt is convenient so to construct the filtering apparatus, as that theaav antag es . descend, but mount by hydrostatic pressure. This method has two? ta jic pressiir ’ taat w ‘ l .h° ut much expensive apparatus, any desired degree of hydro-er ‘ n S surfacp 6 IP veni > as also that the liquid may be forced up through several fil-

® e Par a t e ^ ® Placed alongside of each other; 2. that the object of filtering, which is to

ec 'ly attained in ^ oat ‘ n § la the fluid without disturbing the sediment, may be per-Orfa Ce _ > and thus very foul liquids be cleared without greatly soiling the filtering

^^hined^hh^tn 11 *, s P ecu Harly applicable to the purification of water, either alone,C s hown in qq. e downwards plan of filtration. Of the former variety an exampleth; ted bottoms nr- .* The ' V °° den or zinc conical vessel is provided with two per-for the format; S16V 5 S l ^ eLw ^ xt the filtering: substance is packed. Over

° u §h which the | 0l k° j !f e * s P ace h hy there is a third shelf, with a hole in its middle,u o d b is passed, so as to be water tight. This places the upper