Assertion: balloon | /r/HasProperty | filled with hydrogen
Facets:
value: due to lower cost
type: CAUSE
value: usually
type: TEMPORAL
Source sentences:
text: In December of that year the physicist Jacques Charles, accompanied by Nicolas-Louis Robert, flew a balloon filled with hydrogen on a two-hour flight.
text: The balloon is usually filled with hydrogen due to lower cost, though helium can also be used.
text: Following Henry Cavendish's 1766 work on hydrogen, Joseph Black proposed that a balloon filled with hydrogen would be able to rise in the air.