For something I'm writing I'm constructing a world with an early twentieth century level of technology, and Zeppelins are there, and I couldn't help but notice; Why did they use room-temperature hydrogen? Did they? Wouldn't it decrease the density, and thus increase the lifting capacity, if they heated the hydrogen? Seeing as it was never done, though, there must be some very good reason for it, I just can't think of why. The explosive nature of hydrogen aside, of course, but heating it to ~200*F or 300*F shouldn't make it any more dangerous than it normally is, right? What about helium? Why not heat that? Thanks in advance. EDIT: I do realize that "Zeppelin" actually means a German airship, I guess I mean to apply this to rigid-hulled airships in general, though. With non-rigid hull, it's fairly obvious that the increase in volume would rupture the container. But with a rigid hull, excess hot hydrogen can be let off in one-way check valves that require a certain pressure difference to open, so that the density can be allowed to decrease.