FILTRATION 



371 





air may be rendered operative also either by withdrawing it partially from a close 

 vessel, into which the bottom of the filter enters, or by increasing its density over the 

 top of the liquor to bo filtered. Either the air-pump or steam may be employed to 

 create a partial void in the receiver beneath the filter. In like manner, a forcing-pump 

 or steam may be employed to exert pressure upon the surface of the filtering liquor. A 

 common siphon may, on the same principle, be made a good pressure-filter, by making 

 its upper leg trumpet-shaped, covering the orifice with filter-paper or cloth, and filling 

 the whole with liquor, the lower leg being of such length as to create considerable 

 pressure by the difference of hydrostatic level. This apparatus is very convenient 

 either on the small or great scale, for filtering off a clear fluid from a light muddy 

 sediment. The pressure of the atmosphere may be elegantly applied to common filters 

 by the apparatus represented in fig. 863, which is merely a 

 funnel enclosed within a gasometer. The case A B bears an 

 annular hollow vessel ab, filled with water, in which receiver 

 the cylindrical gasometer defi is immersed. The filter 

 funnel c is secured at its upper edge to the inner surface of 

 the annular vessel a b. In consequence of the pressure of the 

 gasometer regulated by the weight g, upon the air enclosed 

 within it, the liquid is equally pressed, and the water in the 

 annular space rises to a corresponding height on the outer 

 surface of the gasometer, as shown in the figure. "Were the 

 apparatus made of sheet iron, the annular space might be 

 charged with mercury. 



In general, relatively to the application of pressure to filters, 

 it may be remarked, that it cannot be pushed very far, with- 

 out the chance of deranging the apparatus, or rendering the 



filtered liquor muddy. The enlargement of the surface is, generally speaking, the 

 safest and most efficacious plan of increasing the rapidity of filtration, especially for 

 liquids of a glutinous nature. This expedient is well illustrated in the creased bag 

 filter now in use in most of the sugar refineries of London. See SUGAR. .: 



An improved method of rapid filtration has been introduced by Professor Bunsen. 

 He facilitates the operation by filtering into a vessel from which the air has been 

 partially exhausted by means of a water air-pump, on the principle of the Sprengel 

 pump. The liquid undergoing filtration is thus forced, by the pressure of the atmo- 

 sphere, through the pores of the filtering medium, and meets with but little resistance 

 on the opposite side, where it passes into a rarefied atmosphere. Bunsen's paper on 

 this -subject has been translated in the ' Philosophical Magazine' for January 1869, 

 p. 1 . See ASPIRATOR. 



In many cases it is convenient so to construct the filtering apparatus, as that tho 

 liquid shall not descend, but mount by hydrostatic pressure. This method has two 

 advantages : 1. that without much expensive apparatus, any desired degree of hydro- 

 static pressure may be given, as also that the liquid may be forced up through several 

 filtering surfaces placed alongside of each other ; 2. that the object of filtering, which 

 is to separate the particles floating in the fluid without disturbing the sediment, may 

 be perfectly attained, and thus very foul liquids be cleared without 

 greatly soiling the filtering surface. 



Such a construction is peculiarly applicable to the purification 

 of water, either alone, or combined with the downwards plan of 

 filtration. Of the former variety an example is shown in fig. 864. 

 The wooden or zinc conical vessel is provided with two perfo- 

 rated bottoms or sieves ee, betwixt which the filtering substance is 

 packed. Over this, for the formation of the space h h, there is 

 a third shelf, with a hole in its middle, through which the tube 

 d b is passed, so as to be water-tight. This places the upper 

 open part of the apparatus in communication with the lowest space 

 a. From the compartment h h a small air-tube I runs upwards. 

 The filtering substance consists at bottom of pebbles in the 

 middle of gravel, and at the top of fine sand, which may be mixed with coarsely- 

 ground bone-black, or covered with a layer of the same. The water to be filtered 

 being poured into the cistern at top, fills through the tube b d the inferior compart- 

 ment a, from which the hydrostatic pressure forces the water upward ^ through the 

 perforated shelf and the filtering materials. The pure water collects in the space 

 h k, while the air escapes by the small tube I, as the liquid enters. The stopcock i 

 serves to draw off the filtered water. As the motion of the fluid in the filter is slow, 

 the particles suspended in it have time to subside by their own gravity; hence 

 there collects over the upper shelf at d, as well as over the under one at a, a precipi- 

 tate or deposit which may be washed out of the latter cavity by means of the stopcock in. 



B B 2 



864 



