The measurement of the adsorption of vapours at plane crystalline surfaces is attended by the inherent difficulty of packing into a reasonably small dead-space a sufficient quantity of the solid to give an appreciable effect. The use of powders is open to the objection that the superficial area of the particles cannot be directly measured with accuracy; moreover, their surfaces may contain a network of fine cracks, the effect of which is to multiply the area by an unknown factor (Smekal 1925; Joffé 1928; McBain 1932). Plates of mica, used first by Langmuir (1918) and later by Bawn (1932), are free from these objections. The peculiar laminar structure of this substance renders it highly unlikely that the sheets of atoms in its exposed cleavage planes are subject to a strong distortional influence such as affects the surface lattices of sodium chloride and similar crystals (Lennard-Jones and Dent 1928). When cleaved, a plate of mica parts along the plane of the potassium atoms; and though the exposed face may comprise a number of “steps”, over each of these the surface is “true” to a molecular thickness (W. L. Bragg 1937).

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