[¦freŋk·əl ¦hȯlz·ē ¦hil ′ī·sō‚thərm i‚kwā·zhən] (physics) An equation for the volume v of a gas adsorbed on a surface at a given temperature, ln (p / po)= k / vs, where p is the pressure of the gas, pois the vapor pressure, and k and s are constants.