Rarified air balloon. Historical artwork of the inflation of a balloon by burning a substance to create hot air and smoke. Hot air and gases are less dense than cold air therefore the balloon would rise. The Montgolfier brothers, who invented the hot air balloon, experimented with burning different substances. They found that damp wool and chopped straw worked best, probably because it produced a lot of smoke and some oil to seal the fabric of their balloon. Published in A system of aeronautics, comprehending its earliest investigations, and modern practice and art, Philadelphia, 1850.