Hot air filled the envelope of the Montgolfier balloon used for the first free human flight in 1783.
The Montgolfier brothers believed that smoke helped balloons rise, so their early demonstrations used burning straw, wool and other materials beneath the envelope. The actual lifting force came from heated air: warm air is less dense than the cooler surrounding atmosphere.
This made the Montgolfier design fundamentally different from contemporary gas balloons, which used hydrogen. A hot-air balloon does not need a lifting gas stored in tanks; its pilot continually heats the air inside the envelope with burners.
A common mix-up is calling all early balloons “hydrogen balloons.” Jacques Charles and the Robert brothers developed a hydrogen balloon in 1783, but the Montgolfier brothers’ famous human flight used hot air.