Joseph Gay-Lussac (1778-1850). 1867 illustration of the French chemist Joseph Louis Gay-Lussac. In 1804, Gay-Lussac made balloon ascents to measure changes in air composition and magnetism with altitude. In 1808 he published the law of combining volumes on the ratio of gas volumes produced in a chemical reaction. This supported Dalton's atomic theory, and formed the basis for Avogadro's law. With Thenard, he was the first to isolate boron, and studied newly-isolated elements sodium, potassium, and iodine.