Joseph Gay-Lussac. Portrait of Joseph Louis Gay- Lussac (1778-1850), French chemist. In 1804 he made balloon ascents to measure changes in the earth's magnetism and the composition of the air with altitude. In 1808 he published the law of combining volumes. This states that the volumes of gases that react with one another, or are produced in chemical reaction, are in the ratios of small integers. His work gave support to Dalton's atomic theory, and formed the basis for Avogadro's law. Collaborating with Thenard, he was the first to isolate the element boron, and studied the newly- isolated elements sodium, potassium, and iodine (this last one he named).