Abstract: Distilling tube apparatus for converting salt water such as sea water into fresh water is provided. The apparatus includes a first tube through which a feed stream of heated salt water is caused to flow, the first tube being constructed of a porous material which is both water-vapor permeable and liquid water impermeable, which tube allows water vapor to permeate through its walls thereof when heated salt water flows therethrough, the vapor then condensing outside of the first tube upon cooling of the vapor thereat. This first tube is preferably constructed of porous, expanded polytetrafluoroethylene. A second tube oriented in generally parallel relationship and in close proximity to the first tube is provided through which cooling water is caused to flow. The cooling water is maintained at a temperature lower than that of the heated salt water, thereby resulting in a lowering of the temperature of the surroundings of the second tube and providing an enhanced driving force for distillation of the salt water.