(1778-1850) French physicist and chemist who investigated the physical properties of gases, and discovered new methods of producing sulfuric and oxalic acids. In 1802 he discovered the approximate rule for the expansion of gases now known as Charles's law; see also gas laws.
Gay-Lussac was born near Limoges and studied at the Ecole Polytechnique. He became assistant to chemist Claude Berthollet 1801, made balloon ascents to study the weather in 1804, and accompanied Alexander von Humboldt on an expedition 1805–06 to measure terrestrial magnetism. In 1809 Gay-Lussac became professor of chemistry at the Ecole and professor of physics at the Sorbonne. He held various government appointments, including that of superintendent of a gunpowder factory (1818) and chief assayer to the Mint (1829). He was a member of the chamber of deputies for a short time in the 1830s.
With Humboldt he accurately determined the proportions of hydrogen and oxygen in water, showing the volume ratio to be 2:1; they also established the existence of explosive limits in mixtures of the two gases.
In 1808 he formulated Gay-Lussac’s law of combining volumes, which states that gases combine in simple proportions by volume and that the volumes of the products are related to the original volumes.