Experiments heve been carried out on the flow of different gases and vapours through an unconsolidated porous medium consisting of analcite of uniform particle size (50mu diameter). Since analcite crystals are almost spherical, necessary simplifying assumptions could be made. Rates of flow were measured near room temperature both for the inert gases and hydrogen, nitrogen, and oxygen which are little adsorbed by analcite at this temperature, and for sulphur dioxide, ammonia, and carbon tetrachloride vapours which are considerably adsorbed. measurements were made both in the Khudsen (low pressure) and Poiseuille (medium pressure) regions of flow, and data were obtained which support Adzumia's theoretical treatment for the region where they overlap. A new treatment of the transient state of flow is presented. Emphasis is laid upon the importance of the "time-lag" obtained when the flow in the steady state is extrapolated back to the time axis. Experimental results indicate the validity of the relationship, derived theoretically by Barrer, between the time-lag and the diffusion coefficient. Using this relationship, the average radius of the pores in the medium was estimated from the time-lag. Although this was somewhat smaller than the pore-radius calculated from the steady state of flow, good agreement was found between the values obtained using different gases. For the vapours, it was found that the relationships broke down, both for the steady and transient states of flow.The theoretical equations require modification if the molecules of the vapour adsorbed on the surface of the analcite remain there for an appreciable time, or if they are mobile on the surface. It has been shown that the data obtained enable the average lifetime of the molecules on the surface to be calculated, and from measurements at different temperatures, the heat of adsorption has been deduced.<p
