In Part I* measurements of the dielectric coefficients of hydrogen and the rare gases a t two temperatures were recorded. The investigation has now been extended to a number of other gases with a view to determining the electric moment of their molecules and comparing the values of the dielectric coefficient with those of the refractive index at long wave-lengths. The results now given are those of a series of measurements dating from 1928, and during their progress several papers comprising the same subject have been published. These will be discussed later and the results compared.When dealing with all but the simplest gases a knowledge of the compres sibility is essential in order to reduce the values of the dielectric coefficient at different temperatures to those for equal density; it is also desirable to employ gases which have been purified with the greatest care. The com pressibilities of the gases under reference have been determined approximately by measuring the dielectric coefficients a t different pressures as for ammoniaf and several values have been confirmed by direct determination or by refractometric measurements, an account of which will be given elsewhere.Determinations have been made by the " series " method previously employed and also by the " parallel ", method using a condenser specially constructed for measuring very small capacity changes. The results given by the two methods are in satisfactory agreement. The following gases have been examined : methane, ethane, propane, ethylene, propylene, acetylene, silicane, silicoethane, carbon monoxide and dioxide, oxygen, nitrogen, air, nitrous oxide, nitric oxide, silicon tetrafluoride and sulphur hexafluoride. Of these only propylene, carbon monoxide and the two oxides of nitrogen have a measurable electric moment.

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