FILTRATION 



369 



dilute muriatic acid, to remove some lime and iron that are generally present in it. 

 1'ilter papers are first cut square, and then folded twice diagonally into the shape of 

 a cornet, having the angular parts rounded off. Or the piece of paper, being cut into a 

 circle, may be folded fan-like from the centre, with the folds placed exteriorly, and 

 turned out sharp by the pressure of the finger and thumb, to keep intervals between 

 the paper and the funnel into which it is fitted, to favour the percolation. The diameter 

 of the funnel should be about three-fourths of its height, measured from the neck to 

 the edge. If it be more divergent, the slope will be too small for the ready efflux of 

 the fluid. A filter covered with the sediment is most conveniently washed by spouting 

 water upon it with a little syringe. A small camel's-hair paint-brush is much employed 

 for collecting and turning over the contents in their soft state. Agitation or vibration 

 is of singular efficacy in quickening percolation, as it displaces the particles of the 

 moistened powders, and opens up the pores which had become closed. Instead of a 

 funnel, a cylindrical vessel may be employed, having its perforated bottom covered 

 with a disc of filtering paper folded up at the edges, and made tight there by a wire 

 ring. Linen or calico is used for weak alkaline liquors ; and flannels, twilled woollen 

 cloth, or felt-stuff for weak acid ones. These filter bags are often made conical like a 

 fool's cap, and have their mouths supported by a wooden or metallic hoop. Cottou 

 wool put loose into the neck of a funnel answers well for filtering oils upon the small 

 scale. In the large way, oil is filtered in conical woollen bags, or in a cask with 

 many conical tubes in its bottom, filled with tow or cotton wool. Stronger acid and 

 alkaline liquors must be filtered through a layer of pounded glass, quartz, clean sand, 

 or bruised charcoal. The akarrhazas are vessels of porous biscuit stone-ware made in 

 Spain, which are convenient for filtering water, as also is the porous filtering stone of 

 Teneriffe, largely imported into England at one time, but now superseded in a great 

 measure by the artificial filters patented under many forms, consisting essentially of 

 strata of gravel, sand, and charcoal-powder. 



It is convenient to render the filter self-acting by accommodating the supply of 

 liquid to the rate of percolation, so that the pressure upon the porous surface may bo 

 always equally great. "Upon the small scale, the lamp-fountain or bird's-glass form 

 so generally used for lamps, will be found to answer. 



Fig. 859 represents a glass bottle A, partly filled with the fluid to be filtered, sup- 

 ported in the ring of a chemical stand, and having its mouth inverted into the same 

 liquor in the filter funnel. It is obvious, that whenever this liquor by filtration falls 



860 





below the lip of the bottle, air will enter into it, let down a fresh supply to feed the 

 filter, and keep the funnel regularly charged. If larger quantities are to be operated 

 upon,the following apparatus may be employed. Fig. 860, A B, is a metallic vessel, 

 which may be made air-tight ; c is the under pipe provided with a stopcock E, for 

 letting down the liquor into the filter a b. The upper pipe t, through which the fluid 

 VOL. II. B B 



