CERTAIN methods of measuring the surface tension of mercury, especially the `big-drop' method as used by Popesco (Ann. de Physique, 3, p. 402; 1925), appear to show that the presence of air or other gas increases the surface tension of a freshly formed surface by about 100 dynes. The `drop-weight' method, on the other hand, shows a difference of only a few dynes between the values in air and in vacuo, and this indeed in the opposite sense to that indicated above (Harkins, Jour. Am. Chem. Soc., Dec. 1920). Observations by the writers using the same mercury in order to measure its surface tension by the above as well as other methods show that the differences in the measured values of the surface tension are due to differences in the method and not to differences in the quality of mercury (Trans. Faraday Soc., May 1927).