Filter
Aiton's Encyclopedia (1910)
a device designed to strain the solid particles out of a liquid. The word is akin to felt. The filter most frequently used in the chemical laboratory is a sheet of asbestos paper. It is folded usually in the form of a funnel and is of such a texture that solid particles are held back, while the liquid in which they were trickles through drop by drop. Filters for domestic purposes are packed usually with a layer of powdered charcoal or boneblack. Charcoal in particular makes one of the best filters known, as it absorbs noxious gases as well. Other filter materials are a sponge, mineral wool, powdered glass, etc. Even a piece of muslin cloth will arrest many of the impurities contained in water. Filters are used by the manufacturers of sirups, vinegar, oils, wine, beer, fruit-juice, and the like. Many city systems of water supply include enormous filters, acres in extent, having a bottom of fine sand, through which the water percolates and is purified before entering the pipes of the consumers. After a filter of this sort has been in use for a time, the sand becomes clogged and foul. Another filter must be employed while this sand is removed, and clean sand put in its place. There are air-filters also. Several hundred of these are in use at the outlets of sewers in London to destroy odors. They are also used in windows of rooms opening onto territory objectionable to the sense of smell. These air-filters consist of cages filled with fragments of charcoal. See Water Supply; Waste
What Has Changed in the Last Century
When Aiton's wrote this in 1910 the sheet of asbestos paper was a routine laboratory filter and charcoal was called "one of the best filters known"; asbestos has since been recognised as a carcinogen and abandoned for lab work in favour of cellulose filter papers, sintered glass and glass-fibre discs. Municipal practice moved even further: the acres of slow sand beds described here were largely superseded by rapid sand filtration combined with chlorination, first applied to a US public supply at Jersey City in 1908, and typhoid deaths in filtered-and-chlorinated cities collapsed within a generation. Filtration on a scale the entry could not imagine now works at the molecular level, through membranes, ultrafiltration and reverse osmosis used for desalination. Air filtration too was transformed by the HEPA filter, developed in the 1940s, which traps particles rather than merely absorbing smells as the charcoal cages at London's sewer outlets did.
Written for this edition, 2026.