The commonest method of separating solids suspended in a liquid is by filtration, i.e., by passing the liquid through the pores of some substance called a filter. The liquor that passes through is called a filtrate, and the material that remains, the residue. Various kinds of material, such as, paper, cloth, cotton, wool, asbestos, slag, sand, and other porous substances, are used as filters. Cotton cloth is often used by fastening it onto a wooden frame in such a way that a shallow bag is formed into which the liquid to be filtered is poured. The first portion of the filtrate that comes through is cloudy, but the rest soon becomes clear, and then the first portion may be returned to the filter. Filtration cannot be hastened by scraping or stirring the precipitate on the cloth, as this action will merely cause the filtrate to run turbid.