Porosity (Gr. , a passage), the condition of open structure in which the particles of matter are arranged in all bodies, leaving between them pores or interstices that are supposed to be vacant or filled with air. The existence of such spaces even in the most solid bodies is proved in various ways. When wood or stone of the most compact structure is immersed in water under the receiver of an air pump and the air is exhausted from the surface, that contained in these bodies immediately makes its appearance rising in a cloud of bubbles. Under great pressure water is forced through the pores of cast iron, even of 4 in. thickness. The porosity of this material is evidently increased by dissolving out the carbon disseminated throughout its substance, by which it becomes malleable iron without change of form, but the change of texture thus indicated is not apparent to the eye. Density, which is the opposite condition to porosity, is increased in most metals by pressure and hammering. Liquids are also supposed to be porous from the fact that mixtures are sometimes made which occupy less space than the sum of the volumes of the ingredients when separated.
This is the case with alcohol and water. (See Compressibilitt).