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Since bubble growth rate varies from bubble to bubble, it is not possible by the present method to determine how bubble growth rate varies with the surface temperature. This will require obtaining several bubble photographs during any one temperature drop. CONCLUSIONSThe following conclusions have resulted from this study.

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Steady state heat and mass transfer between a submerged evaporative interface in a porous medium and an external gas stream were analyzed theoretically. Local and average evaporation rates for laminar and turbulent flow over a flat plate as well os for fully developed laminar or turbulent flow near the thermal entry region of a duct were obtained as a function of the position of the evaporative interface. Calculated local evaporation rates are significantly lower than values computed by Luikov, who did not take into account the variation of evaporative surface temperature with recession, indicated both by the present analysis and by observations related to the drying of thick porous materials. Application to the analysis of the drying process is discussed.This investigation is concerned with heat and mass transfer between an evaporating liquid in a porous medium and an external gas stream, an area of research which is of particular relevance to the field of drying. An analysis is presented of steady state heat and mass transfer at low mass transfer rates between a submerged evaporative interface parallel to the surface of a porous medium and an external gas stream. The results of the analysis permit calculation of both local and average evaporation rates for different geometries and flow regimes, provided that the location of the evaporative interface is known.The use of a receding plane of evaporation to analyze drying processes was suggested by Cilliland in 1938 (8). Nissan, George, and Bolles (19) used the concept of a submerging plane of evaporation in order to develop a complete description of drying but their analysis neglects to take into account the influence of the fluid boundary layer. The direct precursor to this paper is a 1962 study by Luikov (15, la), who applied boundary-layer theory to the problem of evaporation of liquid from a porous body into a flowing air stream, Based on earlier experimental results ( 1 7 ) , Luikov as well as Nissan had concluded that most of the evaporation during the drying process takes place in a very narrow layer at the junction between wet and dry regions. Accordingly, Luikov's analysis was based upon the hypothesis that all evaporation takes place from a submerged surface which is parallel to the surface of the porous medium. In contrast to Nissan's analysis (19), the effects of the fluid boundary layer as well as those of the porous medium on the evaporation process were taken into account.There are two main distinctions between Luikov's previous analysis and the analysis reported in this paper. Whereas Luikov's solution depends upon the assumption that the velocity is constant across the fluid boundary layer, our analysis employs an integral representation that can be used for the more general flow situation in which the heat and mass transfer coefficients past a single step in wall temperature and concentration at x = 5 are of the form Robert P. Morgan is with the University of Missouri, Columbia, Missouri.

Distillation has been and continues to be a separation method of widespread use in the chemical and petroleum industries. The design of distillation equipment to effect separations or to recover solvents requires a knowledge of vapor-liquid equilibrium compositions. With this in mind, the purpose of the investigation reported here was to obtain the necessary data for the construction of the equilirium diagrams of two binary systems. Equilibrium data for the ethyl alcohol-toluene system at 756 mm. of mercury and the ethyl alcohol-benzene system at 760 mm. of mercury are reported.