Joseph Louis Proust, born September 26, 1754 in Angers in the Maine-et-Loire, departement of France - died July 5, 1826, was a chemist. He was professor of chemistry at the artillery school in Segovia, Spain, and director of the laboratory of Charles IV at Madrid from 1789. He returned to France c.1806. He discovered grape sugar and first published his law of definite proportions (or Law of Constant Composition) in 1794.