Joseph Louis Proust, French chemist, c 1795.
© Science Museum / Science & Society Picture Library
Engraving by Ambroise Tardieu after a portrait by Manuel Albuerne of Joseph Louis Proust (1754-1826). Proust was born in Angers, France, but spent much of his life in Madrid, Spain where he was director of the royal laboratory (1789-1808). In 1797 he developed the Law of Definite Proportions, or Proust's Law, which states that a compound is composed of exact proportions of elements by mas regardles of how the compound was created. His views opposed Berthollet's, who believed that compunds could vary in composition. Proust's Law prevailed, although over a century later it was found that intermetalic compounds and some sulphides could have variable compounds, and these are sometimes called berthollides.