or strainers provide the means of separating liquids from solids. The common laboratory form consists of a circular piece of porous paper made of a special quality which yields a minimum amount of ash when burned. By folding it first of all into halves and then again into quarters, it will be found, when opened out, to fit into a funnel of appropriate size, the paper lying against the sides of the funnel. In common practice, the paper filter when so fitted and before use, is first of all moistened with a liquid of the same nature as that contained in the mixture to be filtered. Thus, for aqueous mixtures, water is applied from a washbottle for alcoholic mixtures, alcohol of the same strength should be used and so forth. Whatmans Extraction Thimbles are made of specially prepared filterpaper, are seamless, and can be used repeatedly for the extraction of soaps, fats, foods, rubber, etc. Paper filters can, in some cases, be used for separating two liquids of different gravities that are not miscible that is, cannot be mixed together so that they will not separate again after shaking and upon standing; for example, a mixture of turpentine and water or olive oil and water. In such cases, if the filter be first of all wetted with water and then used, the watery part will pass through the filterpaper, while the oily bodies will be retained more or less permanently on the filter. There are filters or strainers of solid porous porcelain or earthenware which are sometimes employed for straining liquids from magmas or thick, moist mixtures of crystalline or other solid substances. These filters sometimes take the fo,rm of solid fiat plates on which the magma is placed, and the liquid part gradually soaks or flows away through the texture of the filter block, leaving the more or less dry crystalline or other solid body on its surface. In other cases, strainers are constructed of funnels having their necks plugged more or less tightly with various materials known not to Lie chemically affected by the materials to be filteredas, for example, glasswool, slagwool, asbestos fibres, cottonwool, flannel, etc. Buchner Funnel is a type of filter designevl for the filtration of bulky precipitates, usually made of stout porcelain, and consists of a cylindrical cup of diameter 5 to 15 cms. wide and j to 5 cms. deep. The flat bottom is pierced with a number of holes and below it the funnel narrows rapidly to an outlet tube 1 to 2 cms. in diameter. A circular sheet of filterpaper or other filtering medium is laid down on the perforated bottom so as to cover it completely, and the cup is filled up with the liquid to be filtered. It is always used in conjunction with a suction filter pump. In manufacturing and industrial operations, appliances constructed on these principles are extensively employed, particularly where large volumes have to be dealt with, and when it is difficult to otherwise separate liquids from the mixtures to be dealt with. In laboratory practice, some such preliminary filtration has often to be made before the clouded filtrate thus prepared can be more perfectly filtered say through an ordinary paper filter as previously described. Sand is used in waterworks for the filtration of water on a large scale, and there are many types of domestic filters, variously prepared for the purification of water. In some, animal charcoal pressed into blocks, is used and no doubt it assists the oxidation of organic matters, but does not sterilize the water. In the ChamberlandPasteur filter a cylinder of unglazed porcelain is used, and in the Berkfeld filter a block of baked siliceous earth is employed. Both of these remove the microorganisms from the water, but they require periodical sterilization. In the Bischof filter, iron prepared in a spongy form is used as the purifying agent and is very useful in many circumstances.
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