Abstract
An historical account is given of concepts regarding the mechanism of osmosis and imbibition, starting with Lord Kelvin's gravitational column, where he pointed out that a capillary standing in a dish of water within an isothermal enclosure must have a lowered vapor pressure at its elevated meniscus so as to match that emanating from the surface in the dish, otherwise distillation would violate the Second law. A brilliant sequence to this simple idea followed through Poynting, Arrhenius, Noyes and culminated with Hulett, who in 1901 formulated the “solvent tension theory” of osmosis, stating in essence that the thermal motion of the solute molecules by impact with the free solvent surface put the solvent under tension. This lowers the vapor pressure and thereby also its freezing point. Perrin, in famous experiments on Brownian motion, demonstrated solute‐solvent independence within a solution and further support came through Herzfeld, Mysels and Duclaux. We measured negative pressures in saltfree sap of mangroves and other plants matching the osmotic pressure in the leaf cells. A series of measurements on magnetic and gravitational effects on osmotic pressure likewise bore out the tension theory. The fashionable “water concentration theory” is left experimentally contradicted and in violation of the Second law.