Gay-Lussac (1778-1850)
He published Charles Law and made ballon ascents (7000 meters).
Gay-Lussac and Biot (1804)
Gay-Lussac is best known for his study of combining volumes of reacting gases; at constant temperature and pressure, the ratios of the volumes were always small whole numbers.
For example, 2 volumes of hydrogen gas would react with one volume of oxygen gas to produce 2 volumes of water vapor (gas).