Osmotic Pressure

Abstract

THE theory that osmotic pressure is due to bombardment of the walls of the containing vessel by the particles of solute has met with considerable criticism, both from the chemical and physical sides (compare, e.g., van Laar, Proc. Amsterdam Academy, vol. xvii., p. 1241; vol. XViii., p. 184; abstracted in NATURE, March 16, 1916). However, at the discussion on osmotic pressure before the Faraday Society on May 1, with Sir Oliver Lodge in the chair, the kinetic theory more than held its own. It was claimed by Prof. A. W. Porter that this theory is the only one which gives directly the experimentally obtained values for dilute solutions; that it has now been placed on a sound experimental basis as a result of Perrin's invest gations, which show that particles suspended in a liquid, and therefore also the molecules of the solute, are in rapid motion to the precise aount required by the theory; and that any other theory of osmotic pressure must not only be competent to account for the observed facts, but must explain the absence of the effects thai we have a right to expect from the molecular agitation of the solute. These claims were not seriously shaken by the criticisms of subsequent speakers, and towards the close of the meeting the chairman expressed his general agreement with the arguments put forward in favour of the kinetic theory.