Gay-Lussac, Joseph Louis (1778-1850), a French chemist and physicist. The formulation of Gay-Lussac's Law (1808) was an important step toward the discovery of fundamental properties of gases. Gay-Lussac had also independently discovered Charles' Law in 1802. In 1804 he ascended to 23,000 feet (7,000 m) in a balloon, to study the effect of magnetism in the upper atmosphere.

In 1808 Gay-Lussac, working with Louis Jacques Thenard, isolated boron. Gay-Lussac discovered cyanogen in 1809. In 1815 he proved that prussic acid contains hydrogen and not oxygen. This completed the downfall of Lavoisier's theory that all acids contain oxygen.

Gay-Lussac attended the cole Polytechnique in Paris, 1797-1800, and was made a professor of chemistry at the school in 1809. He was also a professor at the Sorbonne, 1808-32, and at the Botanical Garden after 1832. He was made a member of the French Academy in 1806 and was elected to the Chamber of Deputies in 1831.

Gay-Lussac's Law states:

When gases combine, their relative combining volumes and the volume of the product, if it is a gas, can be expressed by small, whole numbers.

This law led directly to the formulation of Avogadro's Law.