An examination of the existing data of the vapour pressures of mercury shows that at the higher temperatures a most astonishing lack of precision characterises all the measurements. The existing tables, especially above 300°, are therefore completely untrustworthy, a fact which might be guessed from the disagreement between the values they give (see table below). Regnault made his measurements with the thermometer bulb completely immersed in 50 kilos of the violently “bumping” metal. Hence his results are much too high. Ramsay and Young's well-known values were based on observations at four points. The two lowest of these were original: one was the boiling-point, taken from discordant data of Regnault's; and one was a measurement of the pressure at the boiling-point of sulphur.