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Joseph Louis Gay-Lussac (December 6, 1778 - May 9, 1850) was a French chemist and physicist. His first major investigation concerned the thermal expansion of gases. This was significant in the establishment of the Kelvin temperature scale later in the century. In 1805, he collaborated with Alexander von Humboldt in determining the proportions of hydrogen and oxygen present in water. His last great piece of pure research was on prussic acid when he described cyanogen as a compound radicle, prussic acid as a compound of that radicle with hydrogen alone, and the prusiates (cyanides) as compounds of the radicle with metals.