Oh, very interesting. The linked TIME article from 1924 (!) gives a bit more detail: > While helium is exceedingly light as compared with air, it is somewhat h

Oh, very interesting. The linked TIME article from 1924 (!) gives a bit more detail: > While helium is exceedingly light as compared with air, it is somewhat heavier than hydrogen. The total lift of a helium-filled dirigible is accordingly some 10% less than that of the hydrogen-filled airship. The difference does not appear important at first sight, but the total lift of the gas carries the structure, the motors and the crew. It is only the last 20% or so that is available for carrying fuel, and hence a difference of 10% in the gross lift may spell a difference of 50% in the fuel-carrying capacity. On long-distance flights this difference is vital. > Nor is the danger of fire totally eliminated with the use of helium; the gas-tanks and the fuel system generally are still vulnerable. But when a ship is properly designed and carefully handled, the danger of fire is comparatively small, even with hydrogen. https://web.archive.org/web/20110209024932/http://www.time.com/time/magazine/article/0,9171,786149-2,00.html https://web.archive.org/web/20110209024932/http://www.time.c...