Vapor Pressure of Mercury, 0 to 35° C.—The disagreement among the results obtained by previous observers for this range of temperature suggested the need for a direct determination of the vapor pressures with a Knudsen gauge. Impurities were eliminated by numerous distillations in a system cut off from the pump by a liquid air trap, and the slight amount of residual gas was corrected for. The readings obtained at 19 temperatures lie near a smooth curve which, it is believed, gives the vapor pressures to within 3 per cent. The values for 0, 10, 20, and 30° C. are, respectively,.000350,.000775,.00182 and.00407 mm. of Hg, considerably higher than those obtained by Knudsen in 1909, but agreeing fairly well with Morley's results up to 15°, and at higher temperatures with values extrapolated from the results of Ramsey and Young.