FILTER and FILTRATION. In chem ical technology the process of filtration consists in passing a liquid through a porous or fibrous body such as paper, cloth, felt, or biscuit pot tery, for the purpose of removing certain solid substances from the liquid. In chemical analy sis in the wet way, filtration is resorted to con tinually, in order to separate precipitates from the liquids in which they are floating. The filters used for this purpose consist of sheets of a special kind of bibulous paper, which con sist of almost absolutely pure cellulose, so that it will leave practically no ash when burned. The precipitate, when it has all been deposited upon the filter paper, and has been thoroughly washed, is transferred, together with the paper, to a platinum crucible, in which it is heated until. the paper has become entirely burned away, and nothing is left but the precipitate that is to be weighed. Gelatinous precipitates filter very slowly, and for this reason it is usual for chemists to pay careful attention to such little details of manipulation as tend to cause the precipitate to take as granular a form as possible, so that the filtration may be per formed rapidly. When the object of the filtra tion is merely to remove solid particles that are suspended in a liquid, and it is the liquid and not the solids which is important, other mate rials than paper are often used as filtering media with advantage. Thus the liquid may be passed through plugs of asbestos fibres, or of glass wool, these particular substances being very useful when the liquid to be filtered is of a corrosive nature, like a strong mineral acid. Filter are in common use in chemical factories. These are of pottery make the clay being mixed with sawdust or cork filings.
When the stones are °fired° these combustible substances are burned away leaving large porosities. Filtration through porous plugs is often hastened by creating a partial vacuum on that side of the plug toward which it is desired that the liquid shall flow, and aspirators, or special filter suction pumps, are constructed for this purpose. Filtration is also carried on under pressure, the filter being placed in a pres sure box, and the liquid fed to it from a height corresponding to the pressure desired. This idea is utilized on a large scale in the filter press, which is a series of many individual filters, exposing a very large area of filtering material at once in a small space. Rapid filtra tion is also accomplished by centrifugal motion, the revolving drum being lined with paper or cloth, as may be needed. When it is desired to remove such minute bodies as bacteria from a liquid in which they are suspended, it is com mon to pass the liquid through a solid septum of unglazed porcelain, though it has been found that nearly all such bodies can be re moved by filtration through a sufficiently thick mass of sand. Cities which must obtain their drinking water from rivers polluted by sewage from other towns and cities are now supplied with water of very good quality by filtering the river water through beds of sand several feet in thickness, and statistics, in such cases, have shown that the installation of such filtration plants is often attended by a remarkable de crease in the death rate from typhoid fever and other diseases the germs of which may be conveyed by drinking water. See WATER Sup PLY.