A small problem with the "hydrogen fire" idea.
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When mixed with oxygen across a wide range of proportions, hydrogen explodes upon ignition. Hydrogen burns violently in air. Pure hydrogen-oxygen flames are nearly invisible to the naked eye, as illustrated by the faintness of flame from the main Space Shuttle engines (as opposed to the easily visible flames from the shuttle boosters). Thus it is difficult to visually detect if a hydrogen leak is burning. The Hindenburg zeppelin flames seen in the adjacent picture are hydrogen flames colored with material from the covering skin of the zeppelin which contained carbon and pyrophoric aluminium powder.[21] (Regardless of the cause of this fire, this was clearly primarily a hydrogen fire since skin of the Zeppelin alone would have taken many hours to burn).[22] Another characteristic of hydrogen fires is that the flames tend to ascend rapidly with the gas in air, as illustrated by the Hindenberg flames, causing less damage than hydrocarbon fires. For example, two-thirds of the Hindenburg passengers survived that hydrogen fire, and many of the deaths which occurred were from falling or from gasoline burns.[23]
http://en.wikipedia.org/wiki/HydrogenSo in fact it hydrogen was the gas use for dragon fire, it would not be too bad.
and for the lifting help,.
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Hydrogen and helium are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference is inconsequential. (Both provide about 1 kilogram of lift per cubic meter of gas at room temperature and sea level pressure.) Helium is preferred because it is not combustible.
http://en.wikipedia.org/wiki/Lighter_than_airand
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Basically, as the temperature increases the volume of the gas increases.
http://en.wikipedia.org/wiki/Charles%27s_law
And seeing a dragon's body temperture will be higher than room temperture, they can not hold enough gas to help them lift.
