Porosity is a term in physics, opposed to density, and signifies the relative proportion of matter and space included within the exterior superficies of a body. The volume of a body is the quantity of space included within its external surfaces. The mass of a body is the collection of atoms or material particles of which it consists. Two atoms or particles are said to be in contact when their nearer approach is resisted by their mutual impenetrability. If the component particles were in contact, the volume and mass would be identical; but there is good evidence to prove that the particles of no known substance are in contact. Hence it follows that the volume of a body consists partly of material particles, and partly of interstitial spaces, which are either empty or filled with some other different substance: these interstitial spaces are called pores. In bodies uniformly constituted, the component particles and pores are uniformly distributed through the volume; that is, a given space in one part of the volume will contain the same quantity of matter, and the same quantity of pores as an equal space in another part.

The proportion of the quantity of matter to the magnitude of a body is called its density: if, of two substances, one contains in a given space twice as much matter as the other, it is said to be "twice as dense." The density of bodies is, therefore, proportionate to the closeness or proximity of their particles; and, consequently, the greater the density the less will be their porosity. The pores of a body are frequently filled with another body of a more subtile nature. If the pores of a body on the surface of the earth, and exposed to the atmosphere, be greater than the atoms of air, then the air will pervade the pores: this is found to be the case in many sorts of wood which have open grain. If a piece of such wood, or of chalk, or sugar, be pressed to the bottom of a vessel of water, the air which fills the pores will be observed to escape in bubbles, and to rise to the surface. 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.

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 filters; but whatever be used, this circumstance should always be remembered, that no substance can be separated from a liquid by filtration, except that whose particles are larger than the pores of the filtering substance. In general, filters are used to separate solid impurities from a liquid. The most ordinary filters are soft stone, paper, and charcoal. When the liquid is of a corrosive nature, as some of the stronger acids, pounded glass is frequently employed.

All organized substances in the animal and vegetable kingdoms are, from their very nature, porous in a high degree. Minerals have various degrees of porosity. Among the silicious stones is one called hydrophane, which manifests its porosity in a very remarkable manner. The stone in its ordinary state is semitransparent; if, however, it be plunged into water, when it is withdrawn it is transparent 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 transparent. Oil or water placed on paper has a somewhat similar effect. A good method of observing the extreme porosity of woods, is to place a piece at the bottom of a vessel of water placed under the receiver of an air pump; during the exhausting of the receiver the air will be seen to issue from a thousand pores on the surface of the wood, and this emission will continue for hours. As the water enters the spaces previously occupied by the air, the body becomes heavier; and even charcoal treated in this way becomes heavier than water.

Large masses of minerals, by their porosity, produce most important results: thus the rains which fall, and the snows that melt on the mountains, pass through the pores of the various substances they meet with, and issue forth to refresh the plains in spring which are the origin of the various magnificent rivers that at once fertilize and adorn our globe.