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Hydrogen balloon. Illustration of a hydrogen- filled balloon taking off at night during the 19th century. Hydrogen balloons float because the gas is less dense than air. However, hydrogen gas is highly inflammable and its use was largely discontinued after accidents in the early 20th century. This balloon and others were used by the citizens of Paris, France, to carry letters to friendly territory whilst the city was besieged during the Franco-Prussian War of 1870-71. In one dramatic incident the French minister Leon Gambetta successfully escaped from the beleaguered city by balloon.
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