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Proust, Louis Joseph

proportions compounds law composition

[proost] (1754–1826) French analytical chemist: defender of the law of constant proportions.

Proust followed his father in becoming an apothecary in Paris, but in his 30s he moved to Spain. From 1789 he taught in Madrid, but his well-equipped laboratory was pillaged by Napoleon’s troops during the siege of Madrid in 1808 and he returned to France.

Proust was a skilled and prolific analyst. He opposed view that chemical compounds could vary in composition over a wide range. Proust’s extensive work led him by 1797 to the law of constant proportions: that different samples of a pure substance contain its elementary constituents (elements) in the same proportions. Thus malachite Cu2CO3(OH)2 , whether from nature or synthesized in various ways, had the same composition. In a courteous conflict of views, Proust showed that Berthollet’s samples were in fact mixtures. By 1805 Proust’s view prevailed, and soon after, the law was seen to relate directly to atomic theory. However, 130 years later it was found that some compounds (eg some intermetallic compounds, and some sulphides) can have slightly variable compositions, and are sometimes called berthollides.

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