Note on the adsorption of gases by solids and the thickness of the adsorbed layer

I. Langmuir ('J. A. C. S.' (1918), xl, 1361) claims to have shown that when gases are accorded at solid surfaces the thickness of the adsorbed layer of gas is never greater than the diameter of a molecule, and he concludes that the forces which bind the molecule to the surface cannot, therefore, extend to a greater distance from the surface than the diameter of a molecule. This conclusion is the basis for the theory which be has developed of the mechanism of chemical reactions at solid surfaces ('Trans. Faraday Soc.,' vol. 17, May, 1922). The experiments on which Langmuir's generalization is based were conducted by him with gases at low pressures. In no case examined by him did the pressure exceed 172 bars. (0·129 mm. mercury), and was generally not greater than half this value.