interstices or capillaries between the structural elements of a solid. Open pore canals, which permeate the entire volume of a porous body, are differentiated from closed pore cells. The first are characteristic of adsorbents, catalysts, and various filtering materials; the second are prevalent in such substances as foam materials and volcanic glass.

According to the classification system developed by M. M. Dubinin, pores are classed by size into micropores (mean effective radius r < 15–16 Å), intermediate pores_o (16–20 < r < 2,000 À), and macropores (from r > 2,000 Á to a pore size visible to the naked eye). There are two types of micropores: fine (r > 6–7 A), as in zeolites and activated carbon, and large (6–7 < r < 15–16 Å), as in certain silica gels. Each pore class has its own principles of adsorption and of other physical phenomena. Micropores correspond to the sizes of molecules or atoms in a solid and do not disturb its homogeneity. For example, the absorption of gases or liquids by microporous sorbents involves permeating the pores of these sorbents; in many way such absorption is similar to diffusion.

Macropores and intermediate pores, found in typically heterogenous materials, substantially exceed molecular size. In polymers, they exceed the size of monomer units in the macromolecule. A reduction in pressure causes a viscous, mass flow of liquids and gases in connecting macropores, whereas only diffusion is possible in micropores.

The aggregate of the size and number of pores in a body, that is, the total space occupied by pores, is termed porosity. Porosity is expressed by the volume of pores per unit volume or mass of the porous body, as well as by the body’s specific surface. Controlled and natural porous substances are widely used in technology. The porosity of rocks and soils determines the intensity and nature of many processes associated with heat and mass exchange in the earth’s crust and on the earth’s surface.