The balloon may be filled with helium or hydrogen.

气球中可能会充满氦气或者氢气。

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Balloon: Large airtight bag filled with hot air or a lighter-than-air gas such as helium or hydrogen that can rise and float in the atmosphere.

气球：一种大型气密球囊，内充热空气或轻于空气的气体（如氦或氢），以产生浮力，可在大气中上升和飘浮。

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The balloon was distended because of filling of hydrogen.

气球因充满了氢气而膨胀。

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The two men only survived because Coxwell, whose limbs were also paralysed, used his teeth to pull the rope to vent hydrogen from the balloon.

ECONOMIST: The science of survival

Unlike a conventional hot air balloon where the height is controlled by a burner, the gas balloons are filled with hydrogen and controlled by releasing gas to go down and throwing out ballast to go up.

BBC: Wiltshire