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The sorption isotherms of a number of vapours on a sample of ammonium phosphomolybdate outgassed at 180" have been determined ; the vapours comprised the normal hydrocarbons from methane to nonane, isopentane, cyclohexane, benzene, carbon tetrachloride, ammonia, water and nitrogen. Except with ammonia (and possibly with nitrogen) the isotherms obtained were approximately of the Langmuir type, becoming almost horizontal over a wide range of pressures, and turning upwards again as the saturated vapour pressure was approached ; and the amount, ~0 . 5 , sorbed along the flat portion was approximateIy constant for the different adsorbates (Gurvitch rule). For the n-hydrocarbons the reciprocal of the number of molecules sorbed, also along the flat portion, was a linear function of the number of carbon atoms in the chain.The results are best interpreted in terms of "condensation" of the vapour in fine channels within the grains of solid, to a liquid-like condition. With ammonia, lattice penetration probably occurs.Such loss of weight as did occur on outgassing at 180" may be reasonably accounted for as being due to evolution of water. PROCEDUREAll the isotherms (including that of nitrogen at -183") were measured on the electromagnetic sorption balance previously described by Gregg,J which was housed along with the system for handling the adsorbate vapours in an air thermostat maintained at 29". A sample of solid, about 3 g in weight, was placed in the bucket of the sorption balance 1355

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