Gas That Has Replaced Hydrogen in Most Airships
Puzzle 10 of Vienna Hard Pack
Question is: Gas That Has Replaced Hydrogen in Most Airships
and the answer is: HELIUM
Word Meaning:
HELIUM — (uncountable) The second-lightest chemical element (symbol He), with an atomic number of 2 and atomic weight of 4.002602, a colorless, odorless and inert noble gas.
Definitions from Wiktionary.