230 ON THE PROCESSES FOR CLARIFYING LIQUIDS. 



tion ; but their nature and their purity muft firft be examiner! 

 more particularly when the fluid to be filtered is of a faline 

 nature. 



require feleaion. It is incumbent on the chemift to felect from among the 

 different filters, that which, by producing the clarification of 

 the fluid in the beft manner, fliall not at the fame time effect 

 any change in its conflituent parts. Now the choice to be 

 made in this refpeft mult be determined from the knowledge 

 we polTefs of the nature of the fluid, and that of the kind of 

 filter proper to be employed. 



Paper If the fluid be aqueous, vinous, alcoholic, or oily, paper 



may be ufed without inconvenience, provided it be of good 

 quality. This lafi: condition is abfolutely indifpenfible, for 

 without this the filtration would frequently prove defective. 



a&s mcchanl- \y e know that paper is a mechanical texture of vegetable 



V fibres, which have undergone different preparations. The 



particles of this fibre, by intertwining, leave pores, the te- 

 nuity of which is always governed by the ffate in which the 

 pafle or fluff exifled at the moment when it was converted 

 into paper. If this tenuity was confiderable, the pores be- 

 come fpeedily obftru&ed by the fediment depofited from the 

 liquor under filtration, and at this period the filtration ceafes. 

 On the contrary, if the pores be very open, the filtration is 

 made with rapidity in an incomplete manner, becaufe at the 

 fame time that the fluid paffes through the pores it carries 

 along with it the moll minute particles which are fufpended, 

 and there are only the coarfer which remain on the furface of 

 the filter. 



The great art is therefore to choofe fuch paper as has its 

 pores of a requifite magnitude to admit the fluid intended to 

 be filtered, without fuffering any of the particles which pror 

 duced the turbidncfs to pafs through. 



Different kinds Two forts of paper are found in commerce, which nearly 



of paper for fil- produce this effect ; and though they are not always as per- 

 fect as might be defired, thefe have hitherto been preferred 

 to every other. The one, which is of an imperfect white, is 

 known in France particularly by the name of papier Jofeph. 

 The other is a kind of grey paper, lefs coarfe than that which 

 ferves for wrapping up cheap goods. Neither of thefe have 

 any lize. 



The 



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