Joseph Louis Gay-Lussac (, , ; 6 December 1778 – 9 May 1850) was a French chemist and physicist. He is known mostly for his discovery that water is made of two parts hydrogen and one part oxygen by volume (with Alexander von Humboldt), for two laws related to gases, and for his work on alcohol–water mixtures, which led to the degrees Gay-Lussac used to measure alcoholic beverages in many countries.