The Rideal absorption hygrometer has been studied with a view to its possibilities as a laboratory instrument for the measurement of the hygroscopic condition of gas in small and confined spaces. The theoretical errors due to adsorption of moisture upon the containing walls of the apparatus, and the error introduced by the presence of stagnant air in the connecting tubes of the hygrometer, have been derived. The nature of the theoretical error due to adsorption is supported by experiments. The temperature changes occurring in the hygrometer, during the process of an experiment, have been followed and corrections applied. The results, thus corrected, show reasonable agreement with the published data upon the saturated vapor pressure of water. Finally a hygrometer has been built suitable for immersion in a thermostat. Measurements with this instrument have been compared with reliable dew point determinations taken in a hygrostat of special design.