Joseph-Louis Proust Biography

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World of Scientific Discovery on Joseph-Louis Proust

Until he was twenty, Joseph Proust followed dutifully in his father's footsteps, learning chemistry from him and eventually becoming his apprentice in a pharmacy in Angers, France. But in 1774 Proust left for Paris, against his family's wishes and apprenticed himself to another pharmacist. By 1776 he had won a position at a Paris hospital, where he worked as a chemist and pharmacist while lecturing at the Royal Palace.

In 1778 Proust went to Spain, having obtained the post of chemistry professor. In 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. In 1784 Proust participated in a ballooning event at Versailles that was witnessed by French and Swedish royalty.

In 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). In addition to teaching chemistry, Proust worked for the Spanish government, frequently conducting geological surveys and analyzing the nation's mineral resources.

While in Spain, Proust also began studying different types of sugar. He was the first to identify the sugar that comes from grapes as glucose. In 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.

Proust based his theory on careful analysis of copper carbonate, which he prepared in various ways. He also compared his laboratory results with analyses of copper carbonate taken from mineral rocks. Proust found that all pure copper carbonate samples had the same composition--that is, the same proportion of copper, carbon, and oxygen--no matter how they were produced or where they came from. In analyses of other compounds, Proust obtained similar results.

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.