Proust \prüst\ Joseph–Louis (1754–1826), French chemist. Proust spent the major part of his professional life in Spain where he taught chemistry and undertook laboratory research. He was an outstanding chemical analyst. In his use of quantitative methods he consistently gave the results of his analyses in terms of percentage weight composition and sometimes the weight of oxygen or sulfur in comparison with the constant weight of the metal under analysis. In 1808 he proved that the relative quantities of any given pure chemical compound's constituent elements remain constant, regardless of the source of the compound. He thereby provided important evidence to support John Dalton's law of definite proportions.