Law of definite proportions (Proust)
The law of constant composition: a true chemical compound has a constant composition by weight, no matter how it was prepared.
The French chemist Joseph Louis Proust, working in Madrid, showed this by analyses of the oxides and sulfides of iron, tin and copper, and then of copper carbonate. A hundred pounds of copper precipitated by soda or potash invariably give 180 pounds of green carbonate, whatever the origin of the copper; iron combines with oxygen in only two constant proportions, with no intermediate degrees.
His countryman Claude Berthollet in Essai de statique chimique (1803) objected that composition depends on the masses of the reacting substances; the dispute in the journals lasted until 1808 and was decided in Proust's favour, although non-stoichiometric compounds later confirmed part of Berthollet's observations too. The law gave Dalton's atomic theory a fact which that theory explained, and the Swede Jöns Jakob Berzelius in 1811 connected Proust's work with Daltonian atoms and secured his priority.
Why this date
exact for the first formulation: Recherches sur le bleu de Prusse, Journal de physique vol. 45, pp. 334-341; the volume is dated 1794 (and the DSB cites it so), but it was actually printed in 1797 (Wikipedia, from the article on p. 314 of the same issue; Britannica: beginning in 1797). The extended proof on copper carbonate is in Annales de chimie vol. 32 (1799); the dispute with Berthollet ran 1801-1808. The axis value is 1797.
What it grew from
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Mass balance and the conservation principle
prerequisite for · Confidence: medium
The law of definite proportions is a generalisation of quantitative analyses expressed in percentages by weight and is impossible without Lavoisier's weight method; in the 1797 paper on the two oxides of iron Proust relied on Lavoisier's published analysis of iron oxides.
Dispute: Mauskopf (DSB) notes precursors of the law before Lavoisier (Dossie 1759 and others), and Proust himself had been an analyst since the 1770s. The dependence shows in the law being formulated in weight percentages, but no historian states directly that without Lavoisier's weight method there would have been no law of Proust.
Link sources
- Mauskopf S. H. «Proust, Joseph Louis», Dictionary of Scientific Biography (via Encyclopedia.com) (1975) Verified 20 Sep 2026
Lavoisier had published two papers on the analyses of iron oxides (Proust cited data from the second).
- Mauskopf S. H. «Proust, Joseph Louis», Dictionary of Scientific Biography (via Encyclopedia.com) (1975) Verified 20 Sep 2026
consistently giving the results of his analyses in terms of percentage weight composition
- «John Dalton Proposes His Atomic Theory and Lays the Foundation of Modern Chemistry», Science and Its Times (via Encyclopedia.com) (2001) Verified 20 Sep 2026
The chemical revolution due to the work of Antoine Lavoisier (1743-1794) and his circle that had occurred in the last two decades of the eighteenth century had clarified the concept of what elements are, developed a comprehensive and consistent vocabulary of chemistry, and led to the introduction of quantitative methods in chemical investigations.
- Proust J. L. «Researches on Copper», Annales de chimie 32, Leicester, Klickstein (1799) Verified 20 Sep 2026
One hundred pounds of copper, dissolved in sulfuric or nitric acids and precipitated by the carbonates of soda or potash, invariably gives 180 pounds of green copper carbonate.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Mauskopf S. H. «Proust, Joseph Louis», Dictionary of Scientific Biography (via Encyclopedia.com) (1975) Verified 20 Sep 2026
Links under review
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Chemical atomic theory
influenced · Confidence: medium
The claim rests on a summary sentence in the Wikipedia article. A verbatim quotation is needed from the work it cites: Dalton J., «A New System of Chemical Philosophy», 1808, ch. II.
The law of definite proportions gave Dalton's atomic theory a fact which it explained by atoms of definite weight, and according to Britannica Dalton extended Proust's work.
Dispute: Historians disagree on whether Dalton knew and used Proust's work. Kapoor (1965) and Mauskopf (DSB, 1975): Dalton was in a position to know of Proust and of the controversy with Berthollet, but there is no evidence of Proust's role in the genesis of the theory, and Dalton reached his law from different premises (the physics of gases, per Thackray 1966; the reconciliation of Lavoisier's and Cavendish's data, per Grossman 2021). Fujii (BJHS 1986) finds Kapoor's conclusion not decisive and insists on the interrelation of the two events; Britannica and the textbook tradition say Dalton extended Proust's work. The logical connection between Proust's law and Daltonian atomism was established by Berzelius (1811), not by Dalton himself.
Link sources
- Britannica, «Atom», «The beginnings of modern atomic theory» (2024) Verified 20 Sep 2026
English chemist and physicist John Dalton extended Proust's work and converted the atomic philosophy of the Greeks into a scientific theory between 1803 and 1808.
- Mauskopf S. H. «Proust, Joseph Louis», Dictionary of Scientific Biography (via Encyclopedia.com) (1975) Verified 20 Sep 2026
there is no evidence that Proust's work played any role in the genesis of Dalton's chemical atomic theory
- Fujii K. «The Berthollet-Proust Controversy and Dalton's Chemical Atomic Theory 1800-1820», British Journal for the History of Science 19(2), 177-200 (1986) Verified 20 Sep 2026
Dalton discovered his law from completely different premises, and does not seem to have got his ideas from Proust.
- Paradowski R. J. «Proust Establishes the Law of Definite Proportions», EBSCO Research Starters (2023) Verified 20 Sep 2026
Proust's law was also important in helping to establish the modern atomic theory of John Dalton, even though Dalton himself made only cursory references to Proust in his publications.
- Wikipedia «Law of definite proportions» (2024) Verified 20 Sep 2026
The law of definite proportions contributed to the atomic theory that John Dalton promoted beginning in 1805
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Britannica, «Atom», «The beginnings of modern atomic theory» (2024) Verified 20 Sep 2026
The dating of Proust's memoir was settled from a footnote in the Wikipedia article. It needs checking against the issue itself: Journal de physique, de chimie et d'histoire naturelle, vol. 45, pp. 334-341.
Node sources
- Proust J. L. «Extrait d'un mémoire intitulé: Recherches sur le bleu de Prusse», Journal de physique, de chimie et d'histoire naturelle 45, 334-341 (1797) Verified 20 Sep 2026
iron, like many other metals, is subject to the law of nature which presides at every true combination, that is to say, that it unites with two constant proportions of oxygen.
- Proust J. L. «Researches on Copper», Annales de chimie 32, 26-54, H. M. Leicester, H. S. Klickstein (1799) Verified 20 Sep 2026
One hundred pounds of copper, dissolved in sulfuric or nitric acids and precipitated by the carbonates of soda or potash, invariably gives 180 pounds of green copper carbonate.
- Mauskopf S. H. «Proust, Joseph Louis», Dictionary of Scientific Biography (via Encyclopedia.com) (1975) Verified 20 Sep 2026
"Recherches sur le bleu de Prusse," in Journal de physique, 45 (1794), 334-335, 341.
- Mauskopf S. H. «Proust, Joseph Louis», Dictionary of Scientific Biography (via Encyclopedia.com) (1975) Verified 20 Sep 2026
consistently giving the results of his analyses in terms of percentage weight composition
- Mauskopf S. H. «Proust, Joseph Louis», Dictionary of Scientific Biography (via Encyclopedia.com) (1975) Verified 20 Sep 2026
It was apparently left to Berzelius (1811) to give Proust his due by establishing the logical relationship between Proust's work (and the controversy with Berthollet) and Daltonian atomism, as well as to give Proust proper credit for the law of definite proportions.
- Wikipedia «Joseph Proust», 1794 (2024) Verified 20 Sep 2026
This work was actually published in 1797, as can be seen from the article published in the same issue on page 314.
- Paradowski R. J. «Proust Establishes the Law of Definite Proportions», EBSCO Research Starters (2023) Verified 20 Sep 2026
During the first decade of the nineteenth century, Proust and Berthollet, in a series of journal articles, debated whether compounds had fixed or variable compositions.
- Fujii K. «The Berthollet-Proust Controversy and Dalton's Chemical Atomic Theory 1800-1820», British Journal for the History of Science 19(2), 177-200 (1986) Verified 20 Sep 2026
The Berthollet-Proust controversy and Dalton's atomic theory are two important historical landmarks which appeared almost simultaneously at the very beginning of the nineteenth century.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.