A vapor pressure apparatus has been developed which has certain manipulative and accuracy advantages over the well‐known isoteniscope. A reliable criterion for complete degassing is inherent in the apparatus. Since degassing is required only once for a series of determinations at various temperatures, the apparatus is well adapted where the amount of sample is limited. The useful temperature range of this apparatus is between the freezing point of mercury (−38.9°C) and the temperature at which the vapor pressure of mercury becomes appreciable (∼0.1 mm at 80°C). The apparatus may not be used if the sample reacts chemically with mercury.