Proust, Joseph Louis (zhôzĕfˈ lwē prōst) [key], 1754–1826, French 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 established the law of definite proportions (sometimes known as Proust's law), which states that in any compound the elements are present in a fixed proportion by weight.
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