Louis-Joseph Gay-Lussac entered École Polytechnique in 1797 and, after graduating, joined the Bridges and Roads Corps. One of his most significant discoveries is the equal expansion of gases and vapors, a claim that he made in 1802 at the age of 24 while still a student engineer. Moreover, the physical law on volume ratios in gas combinations bears his name. Gay-Lussac’s air travel experiments built upon the Montgolfier brothers’ invention of the hot air balloon, which had never before been involved in a true scientific investigation of the atmosphere. His valuable studies on temperature, magnetic force and air chemistry form the building blocks of modern meteorology. This discoverer of cyanogen and Prussic acid also made a decisive contribution to industrial chemistry. Gay-Lussac was a member of the Academy of Sciences and taught chemistry at École Polytechnique.