Abstract: A gas can be separated by condensation from a gaseous mixture by exposing the mixture to reduced temperature in a chamber having an outlet. The outlet contains a restrictive structure, such as a sintered glass filter disc, on which a thin film of the condensed liquid forms. Liquid is drawn off through the restrictive structure while the film is replenished so that a barrier layer of the liquid which seals the outlet is continuously maintained. The gas may be ozone made from electrical treatment of oxygen and the resulting liquid ozone may be used for water purification.