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The effect of pressure P on the rate of diffusion D has been measured for a large number of gas-metal systems. In a previous paper we have pointed out that the experimental results are more nearly represented by an equation D =k . θ . √P (1) than by the usual equation D =k . √P, (2) where θ is the fraction of the surface covered by adsorbed gas atoms. Whilst equation (1) represented within the limits of experimental error all the systems so far investigated, its theoretical basis was obscure. The factor θ was introduced on the hypothesis that the departure from the √P relation at very low pressures was due to the surface being incompletely covered with adsorbed gas. It has generally been assumed that the rate of diffusion is determined by three processes :— (i) Adsorption on the surface, (ii) Penetration of the surface, (iii) Diffusion through the metal, each of which requires an activation energy.

The diffusion of gases through metals - IV—The diffusion of oxygen and of hydrogen through nickel at very high pressures