The filtering medium may be any substance of a sufficiently spongy or porous nature to allow of the free percolation of the liquid, and whose pores are, at the same time, sufficiently small to render it limpid or transparent. Unsized paper, flannel, linen, calico, cotton wool, felt, sand, coarsely powdered charcoal, porous stone, or earthenware, and numerous other substances of a similar kind, are employed for this purpose.

For many liquids that filter easily, and in which the suspended matter is of a coarse and porous nature, it is often sufficient merely to. place a little cotton wool or tow, or a small piece of sponge, in neck of the funnel, as at a, (Fig. 358), in the engraving; but such an apparatus, from the small extent of the filtering surface, acts either slowly or imperfectly, and soon gets choked up.