Oh, that's right! It has a measurable negative weight!

That leads to all sorts of nifty concepts, doesn't it? You could create much more powerful zepplins with this stuff in gaseous form, which would be a much, much greater explosion risk than hydrogen.

It should be possible to work out the substance's specific gravity and its energy density -- so you could work out what weapons based on it would do.

See, THIS is why people like steampunk.