HISTORICAL TIMELINE OF JOSEPH PROUST

1774- Proust left for Paris, against his family's wishes and apprenticed himself to another pharmacist.

1776- He had won a position at a Paris hospital, where he worked as a chemist and pharmacist while lecturing at the Royal Palace.

1778- Proust went to Spain, having obtained the post of chemistry professor.

1780- He returned to France and stayed there for five years; during this time he taught chemistry and experimented with the new scientific sport of ballooning..

1785- Proust accepted a lucrative teaching position offered by the Spanish government. He spent the next twenty years in Spain at various posts in Madrid and Segovia, thus missing the French Revolution and the rise to power of Napoleon Bonaparte (1769-1821).

1799- When the chemical laboratories of Segovia and Madrid were merged, Proust became director of the new, lavishly equipped facility. While there, Proust published his law of constant composition, which later evolved into the law of definite proportions. At the time, most chemists agreed with Claude Berthollet, who believed the composition of a compound would vary according to the amounts of reactants used to produce it. In contrast, Proust proposed that pure reactants always combine in the same proportions to produce exactly the same compound.

For about eight years, Proust and Berthollet engaged in a friendly controversy over this issue, but, in the end, Proust was proved to be right. Berthollet had used impure reactants in his experiments, and thus he had analyzed the products inaccurately. Meanwhile, John Dalton had been formulating his atomic theory which was published in 1808. In this theory, Dalton rephrased Proust's law, calling it the law of multiple proportions. Although it is unclear whether Dalton was directly influenced by Proust, the law of constant composition provided evidence for Dalton's atomic theory, which in turn provides an explanation for Proust's observations.