Figures are given, for the helium content of air both on the earth's surface and at heights up to 25 km. No variations exceeding the limits of the analytical error ($\pm 0\cdot 2$%) have been found in air samples from all over the surface of the globe, and no significant changes have been observed in air up to 20 km. height; however, a small and varying surplus of helium averaging about 3% above the normal helium content was present in samples from 20 to 25 km. height. It follows from these results that at least up to 25 km. height, gravitational separation of the constituents has no appreciable effect on the composition of atmospheric air.