(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 If, however, it be plunged in water, when it is withdrawn it is as translucent as glass.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. Larger mineral masses exhibit degrees of porosity not less striking. Water percolates through the sides and roofs of caverns and grottoes, and being impregnated with calcareous and other earths, forms stalactites, or pendant protuberances, which present a curious appearance. (23.) _Compressibility._--That quality, in virtue of which a body allows its volume to be diminished without diminishing its mass, is called _compressibility_. This effect is produced by bringing the constituent particles more close together, and thereby increasing the density and diminishing the pores. This effect may be produced in several ways; but the name “compressibility” is only applied to it when it is caused by the agency of mechanical force, as by pressure or percussion. All known bodies, whatever be their nature, are capable of having their dimensions reduced without diminishing their mass; and this is one of the most conclusive proofs that all bodies are porous, or that the constituent atoms are not in contact; for the space by which the volume may be diminished must, before the diminution, consist of pores. (24.) _Elasticity._--Some bodies, when compressed by mechanical agency, will resume their former dimensions with a certain energy when relieved from the operation of the force which has compressed them. This property is called _elasticity_; and it follows, from this definition, that all elastic bodies must be compressible, although the converse is not true, compressibility not necessarily implying elasticity.