370 



FILTRATION 



is poured by means of tho funnel E, lias also a stopcock which opens or shuts, at the 

 same time, tho small side tube ut, through which, during the entrance of tho fluid, 

 the air is let off from tho receiver. A glass tube (j, >sh<>\\s tho level of tho liquor in 

 tho body of the apparatus. In using it, tho cock B must be first closed, and tho cock 

 s must bo opened to fill tho receiver. Then tho filter is sot a-going by reopening the 

 cock E, so as to keep the fluid in tho filter upon a level with the opening of the tuboc. 

 Both these pieces of apparatus are essentially the same. 



In many manufactures, self-acting filters are fed by tho plumber's common con- 

 trivance of a ball-cock in which tho sinking and rising of the ball, within certain limits, 

 torvo to open or shut off tho supply of liquor as it may bo required or not. Dumont 

 has adopted this expedient for his system of filtering syrup through a stratum of 

 gramilarly ground animal charcoal or bone-black. Fig. 861 is a front view of this 



861 



862 



apparatus with 4 filters c ; and^ ; 862 is a cross section. The framework B supports 

 the cistern A, in which the syrup is contained. From it tho liquor flows through the 



stopcock b, and tho connection- 

 tube a, into the common pipe c, 

 which communicates, by the short 

 branch tubes e, with each of tho 

 four filters. The end of the 

 branch tube, which is inside of 

 tho filter tub, is provided with a 

 stopcock d f, whose opening, and 

 thereby the efflux of the liquor 

 from the cistern through tho tube 

 a, is regulated by moans of tho 

 floating-ball g. Upon tho brick- 

 work D the filter tub stands, fur- 

 nished at h with a false bottom 

 of zinc or copper pierced with 

 fine holes ; besides which, higher up at i there is another such plate of motal furnished 

 with a strong handle k, by which it may be removed, when the bono-black needs to 

 be changed. In tho intervening space I, the granular coal is placed, o is tho cover 

 of the filter tub, with a handle also for lifting it. One portion of it may be raised 

 by a hinge, when it is desired to inspect the progress of the filtration within, m in 

 is a slender vertical tube, forming a communication between tho bottom part h, and 

 the upper portion of tho filter, to admit of tho easy escape of tho air from that 

 space, and from among the bone-black as the syrup descends ; otherwise the filtration 

 could not go on. p is tho stopcock through which the fluid collected in the space 

 under h is let off from time to time into the common pipe q,fig. 861. r is a trickling 

 channel or groove lying parallel to tho tube q, and in which, by means of a tube s, 

 inserted at pleasure, tho syrup is drawn off in case of its flowing in a turbid state, 

 when it must be returned over the surface of tho charcoal. 



The celerity with which any fluid passes through the filter depends, 1, upon tho 

 porosity of the filtering substance ; 2, upon the pressure exercised upon it ; and 3, upon 

 tho extent of the filtering surface. Fine powders in a liquor somewhat glutinous, or 

 closely compacted, admit of much slower filtration than those which aro coarse and 

 free ; and the former ought, therefore, to be spread in a thinner stratum and ov.-r a 

 more extensive surface than tho latter, for rqual effect: a principle well exemplified in 

 tho working of Dumont's apparatus, just described. 



In many cases filtration may bo accelerated by tho increase of hydrostatic or pneu- 

 matic pressure. This happens when we close tho top of a filtering cylinder, and con- 

 nect it by a pipe with a cistern of fluid placed upon a higher level. Tho pressure of tho 



