FILTRATION is a process purely me chanical, for separating a liquid from the undissolved particles floating in it, which liquid may be either the useful part, as in vegetable infusions, or of no use, as the washings of mineral precipitates. The filtering substance may consist of any porous matter in a solid, foliated, or pul vernlent form ; as porous earthenware, unsized paper, cloth of many kinds, or sand. The white blotting paper sold by the stationers, answers extremely well for filters in chemical experiments, pro vided it be previously washed with dilute muriatic acid, to remove some lime and iron that are generally present in iL Fil ter papers are first cut square, and then folded twice diagonally into the shape of a cornet, having the angular parts round ed off. Or the piece of paper being cut into a circle, may be folded fan-like from the centre, with the folds placed exteri orly, and turned out sharp by the pres sure of the finger and the thumb, to keep intervals between the paper and the fun nel into which it is fitted, to favor 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 sedi ment 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 percola tion, as it displaces the particles of the moistened powders, and opens up the pores which had become closed. Instead of a funnel, a cylinder vessel may be em ployed, having its perforated bottom covered with a disc of filtering powder folded up at the edges, and made tight there by a wire ring. Linen or calico is used for weak alkaline liquors ; and flan nels, 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 woollen or metallic hoop. Cotton wool put loose into the neck of a funnel an swers well for filtering oils upon the small scale. In the largo 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 al earrhazas are a porous biscuit-of stone ware made in Spain, which are conve nient for filtering water, as also the po rous 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.