solution and solvent reservoirs are separated by a solvent permeable membrane . a path interconnects the reservoirs . solvent vapor from the solution reservoir is created upon heat applied thereto for condensation back to the solvent reservoir . a vent extends from the solvent reservoir into the solvent vapor path so that the amount of solvent pumped through the membrane , which is determined by the level of solvent wetting the membrane , will equal the amount of solvent evaporated from the solution .