the design popularized by liebig thus consisted of an inner, straight tube surrounded by an outer straight tube, with the outer tube having ports for fluid inflow and outflow, and with the two tubes sealed in some fashion at the ends (eventually, by a blown glass "ring seal"). its simplicity made it convenient to construct and inexpensive to manufacture, the higher heat capacity of the circulating water (versus air) allowed for maintaining near to constant temperature in the condenser, and so the liebig type proved to be the more efficient condenser—capable of condensing liquid from a much greater flow of incoming vapor—and therefore replaced retorts and air condensers. an added benefit of the simplicity of the straight inner tube design of this condenser type is that it can be "packed" with materials that increase the surface area (and so the number of theoretical plates of the distillation column, see section below), e.g., plastic, ceramic, and metal beads, rings, wool, etc. see fractional distillation.