These essentially consist of porous bodies which mechanically remove the solids because the pores are too small to permit any but liquids to pass. These may consist of textile or felted fabrics of every description, the pores of which are fine enough - coarse pottery or earthenware in the biscuit or unglazed state; sandstone in various forms, particularly that called dripstone, which is a sandstone of an open texture; carbon diaphragms formed of powdered coke, cemented together by means of carbon deposited by heat from sugar or tar in closed moulds; wood in thin sheets; leather; layers of finely-powdered substances such as glass, sponge and paper, sand, coke, asbestos, etc. The water in passing through these porous substances leaves the solid matters behind.

Chemical Filters

These essentially consist of platinum black (the most active of all), animal charcoal, various kinds of clay, silicate of magnesia, spongy iron, hydrate of alumina. Platinum black for practical purposes may be classed in the category of chemical curiosities. The other substances, which are used as chemical filters for the purification of water, are animal charcoal and spongy iron. Animal charcoal is usually employed in the form of granules about the size of barley.

Fig. 12.   The National Filter

Fig. 12. - The National Filter.

Spongy iron is used in the form of an aggregated mass of particles in a porous form like a sponge. Neither animal charcoal beds nor spongy iron can be considered as good mechanical filters.

Various Patent Filters

We now annex the illustrations and descriptions of some of the principal patent filters, various filtering arrangements and home-made filters, for low and high pressure, leaving it to the intelligent reader to make his selection to suit his purpose.