Quotation from: General Science

Written by: Bertha M. Clark

(_c_) _Filtration_. In filtration, the water is forced through
porcelain or other porous substances which allow the passage of water,
but which hold back the minute foreign particles suspended in the
water. (See Laboratory Manual.) The filters used in ordinary dwellings
are of stone, asbestos, or charcoal. They are often valueless, because
they soon become choked and cannot be properly cleaned.