Okay, here's something else I've been wondering about for a long time now. Hydrogen and Helium are used in balloons because they're low in density, but (I'm presuming here) they provide some component of structure to inflate the balloon. If you had a very solid type of balloon, made of thick aluminum or something, and then sucked all the air, everything, out of it, would it float? I guess you could argue that it would collapse under the weight of the atmosphere, but if you took it very high, would it float up there?
Don't they use this principle in weather balloons? Near sea-level, they're practically droopy because they have a minimal amount of gas inside, but by the time they reach the limits of the atmosphere, the relative lower pressure outside the balloon makes the gas inside expand and fill the balloon to capacity (and then eventually boil?).