The length variations of two zinc chloride activated carbon rods caused by the adsorption of several vapors are reported. The relationship between percent extension of the rod and calculated spreading pressure of the adsorbed film suggested by Bangham has been tested. The relation agrees well with experiment for organic vapors above about frac13; saturation of the monolayer capacity. It does not agree in the low adsorption region nor along the desorption branch of the water isotherm. A marked contraction of the rod below its original length has been noted when water is desorbed, and this contraction may be considered as a proof of the establishment of concave menisci in the capillaries along the desorption branch.
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