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If a tall vessel or tube, having a wooden bottom, be filled with quicksilver, the liquid metal will be forced by its own weight through the pores of the wood, and will be seen escaping in a silver shower from the bottom.

(21.) The process of filtration, in the arts, depends on the presence of pores of such a magnitude as to allow a passage to the liquid, but to refuse it to those impurities from which it is to be disengaged. Various substances are used as filtres; but, whatever be used, this circumstance should always be remembered, that no substance can be separated from a liquid by filtration, except one whose particles are larger than those of the liquid. In general, filtres are used to separate solid impurities from a liquid. The most ordinary filtres are soft stone, paper, and charcoal.

(22.) All organised substances in the animal and vegetable kingdoms are, from their very natures, porous in a high degree. Minerals are porous in various degrees. Among the silicious stones is one called hydrophane, which manifests its porosity in a very remarkable manner. The stone, in its ordinary state, is semi-transparent. If, however, it be plunged in water, when it is withdrawn it is as translucent as glass. The pores, in this case, previously filled with air, are pervaded by the water, between which and the stone there subsists a physical relation, by which the one renders the other perfectly transparent.