It was thought to be of interest to find out whether the polarizability of a molecule is affected by changes of density of the substance, and in the case of a polar molecule, whether the moment is changed. It seemed best to start with a gas, as with this the largest changes in density can be obtained. Previous measurements of the influence of change of density on the dielectric constant of gases have been carried out amongst others by :— (1) Tangl*—Air, hydrogen and nitrogen up to 100 atms. at 20° ; (2) OCCHIALINI and BODAREU—Air up to 350 atms. at room temperature ; (3) WAIBEL—Air up to 40 atms. at 14° ; (4) KEYES and KIRKWOOD—C02 at 0°, 35°, 70° and 100° and NH3 at 100°, 125°, 150° and 175° up to 200 atms.; (5) BROXON—Air and nitrogen at 18° up to 170 atms.; (6) The Authors—Nitrogen up to 150 atms. at 25°, 75° and 125°.