Wiktionary
n. A gas that is used to produce buoyancy for e.g. balloons or airships (such as helium or hydrogen.)
Wikipedia
Usage examples of "lifting gas".
The fact that he backed away a safe distance before lighting his tobacco showed the dirigible was charged with explosive hydrogen lifting gas.
The airship was in perfect working order, it was well stocked with food, arms, articles and trinkets with which to trade among the natives, spare parts for the machinery, special tools and a good supply of the chemicals needed to manufacture the lifting gas.
Tests had shown that the lifting gas provided by the djinn would not burn, but the problem of arrow-proofing the bags had not been entirely solved.
Construction of the craft was the acme in lightness, and the lifting gas used in the ballonets was the result of a great deal of experimenting in the bronze man’.
They only had to walk through the jungles for about three weeks after the lifting gas gave out.
It was, very clear that my conjecture was right, and that these vast clear bladders were distended with some lifting gas, for in an instant the huge cloud−.