EXOSMOSE, and ENDOSMOSE, are relative terms derived from the Greek ὅρος signifying impulsion, and applied to the attraction of one gas, vapour, or liquid for another of less density, operating through a vegetable or animal membrane, or other porous medium. When two fluids of different densities are separated from each other by such medium, the denser will attract the lighter fluid. When the transmission thus effected is from within outwards, it is termed exosmose; and when the action is in the contrary direction, it is termed endosmose. This subject is treated in more detail under BOTANY, vol. v., p. 68.