In 1782, the brothers discovered that heated air from a fire directed into a paper or fabric bag made the bag rise. They demonstrated this discovery in 1782 when a balloon they made rose into the air about 3,000 feet (1,000 meters), remained aloft some 10 minutes, and then settled to the ground more than a mile and a half from where it rose. In early June of 1783, in the town of Annonay, they gave a public exhibition of their discovery with a balloon made of silk and lined with paper to trap the gas. It rose to an altitude of about 6,000 feet (1,830 meters), traveled more than 1 mile (1.6 kilometers), and stayed aloft for 10 minutes. On September 19, in a demonstration before Louis XVI and Marie Antoinette, they put a sheep, duck, and rooster aboard a balloon to determine the effect of altitude on living creatures. The balloon floated for about 8 minutes and landed safely about 2 miles (3.2 kilometers) from the launch site. The brothers went on to many more experimental flights.
At the time, the Montgolfiers believed they had discovered a new gas. However, in 1785, the buoyancy was shown to be caused by heated air, which is less dense than the surrounding atmosphere.
The two brothers were honored by the French Académie des Sciences. During their careers, they published books on aeronautics, Joseph invented a calorimeter and the hydraulic ram, and Étienne developed a process for manufacturing vellum.
Joseph died on June 26, 1810. Jacques died on August 2, 1799.