AbstractWater was vaporized into air, helium, and Freon‐12 in turbulent gas flow over a wet‐bulb thermometer and through a 1‐ft.‐diam. tower packed with 1‐in. Rasching rings.The wet. and dry‐bulb measurements indicate that the gas‐film mass transfer coefficient varies with the Schmidt number to the —1/2 power for flow perpendicular to single cylinders.Heights of a transfer unit, H.T.U., were measured in the packed tower for flow of the gases countercurrent to water over a range of gas and liquid rate. H.T.U. varied as the 0.9 power of the Schmidt group. When compared at equal values ofρu2(ρ = gas density,u= velocity), H.T.U. varied as the 0.47 power of the Schmidt group. With reference to the psychrometric study, the latter method of comparison of H.T.U.'s seems preferable and indicates that further study of criteria for dimensional similitude in packed colu