Joseph Proust

Joseph Louis Proust (26 September 1754 – 5 July 1826) was a French chemist. He was best known for his discovery of the law of definite proportions in 1797, stating that chemical compounds always combine in constant proportions.

Life Joseph-Louis Proust was born on 26 September 1754 in Angers, France. His father served as an apothecary in Angers. Joseph studied chemistry in his father's shop and later went to Paris where he gained the appointment of apothecary in chief to the Salpêtrière. He also taught chemistry with Pilâtre de Rozier, a famous aeronaut. Between the two types of the other compounds, Proust showed that no intermediate compounds exist between them. Proust published this paper in 1797, but the law was not accepted until 1812, when the Swedish chemist Jöns Jacob Berzelius gave him credit for it.

There are, however, exceptions to the Law of Definite Proportions. An entire class of substances does not follow this rule. The compounds are called non-stoichiometric compounds, or Berthollides, after Berthollet. The ratio of the elements present in the compound can fluctuate within certain limits, such as for example ferrous oxide. The ideal formula is FeO, but due to crystallographic vacancies it is reduced to about Fe0.95O.

Proust was also interested in studying the sugars that are present in sweet vegetables and fruits. In 1799, Proust demonstrated, to his class in Madrid, how the sugar in grapes is identical to that found in honey.

Works

Translated to German in Journal für die Chemie und Physik vol. 1 (1806) p. 249-270