Proust Establishes the Law of Definite Proportions Summary

  • Last updated on November 10, 2022

Through a series of meticulous experiments, Proust proved that all chemical compounds, whether found in nature or prepared in the laboratory, consist of elements in definite ratios by weight.

Summary of Event

In the late eighteenth century, the idea that chemical compounds were composed of stable ratios of elements was not uncommon. In fact, many analytic chemists based their work on just such an idea. However, the attitude of chemists toward what came to be called definite proportions was complex, since such distinguished scientists as Claude Louis Berthollet questioned definite proportions and backed this skepticism with experiments that seemed to demonstrate varying compositions of alloys, glasses, and solutions. Although Antoine-Laurent Lavoisier, the founder of the oxygen theory of combustion, made use of compounds with fixed weight ratios, he admitted that some substances might have degrees of oxygenation. Lavoisier had helped establish that true chemical compounds possessed uniform properties, but the claim by some modern scholars that the constancy of components in these compounds was a common assumption in the eighteenth century has been refuted by the detailed historical analyses of other scholars. [kw]Proust Establishes the Law of Definite Proportions (c. 1794-1799) [kw]Proportions, Proust Establishes the Law of Definite (c. 1794-1799) [kw]Definite Proportions, Proust Establishes the Law of (c. 1794-1799) [kw]Law of Definite Proportions, Proust Establishes the (c. 1794-1799) Definite proportions law Elements;chemical Compounds, chemical Chemistry;law of definite proportions [g]Spain;c. 1794-1799: Proust Establishes the Law of Definite Proportions[3120] [c]Chemistry;c. 1794-1799: Proust Establishes the Law of Definite Proportions[3120] [c]Science and technology;c. 1794-1799: Proust Establishes the Law of Definite Proportions[3120] Proust, Joseph-Louis Berthollet, Claude Louis Lavoisier, Antoine-Laurent Dalton, John

The person who did more than any other chemist to establish and authenticate the principle of constant composition was Joseph-Louis Proust. He pursued the profession of his father, an apothecary, and he worked for several years in the pharmaceutical department of the Saltpêtrière Hospital in Paris. With this background, it is understandable why Proust tended to approach chemical problems pragmatically. With the help of Lavoisier, Proust was able, during the 1780’s, to obtain academic positions in Spain, first at Madrid, then at the Royal Artillery College in Segovia. After a stay in Salamanca, he returned to Madrid in 1791, when he began to study the issue of definite proportions.

Scholars differ on how much such early ideas as “constant saturation proportions” influenced Proust. Many chemists had established that a certain amount of acid neutralized a specific quantity of base, and some posited that such reacting substances had a unique property of combination, which was sometimes called the “saturation proportion.” Proust knew of these unique combining ratios, but these previous efforts did not result in the broad conclusion that all chemical substances combine in only a small number of fixed proportions. Proust knew that the French mineralogist René-Just Haüy had discovered a relationship between chemical composition and fixed crystal form, and this discovery influenced his thinking on definite proportions. Proust also researched specific chemical compounds of interest to pharmacists, physicians, metallurgists, and painters, and these studies deepened his knowledge of the differences between true chemical compounds and mere physical mixtures.

Precisely when Proust first formulated his famous law has been controversial. Some scholars argue for 1794, others for 1797, and still others for 1799. Proust did publish a paper in 1794 in which he clearly recognized that iron not only had two oxides but also two sulfates, and he went on to state that all metals follow the same natural law regulating their definite combinations with other elements. By 1797, Proust had shown that antimony, tin, mercury, lead, cobalt, nickel, and copper formed distinct oxides with constant proportions. The oxides of these metals had specific physical and chemical characteristics, and Proust responded to those who claimed that metal oxides had variable proportions by showing that these chemists were confusing “maximum and minimum” oxides with their mixtures. He was able to separate mixtures of these maximum and minimum oxides according to the solubilities of the compounds in alcohol or other solvents.

Proust was by now a firm believer that nature’s “invisible hand” bound together elements into real combinations. In 1799, in a series of painstaking analyses, he demonstrated that the copper carbonate he prepared in his laboratory was identical in composition to the compound found in nature. The artificial and natural copper carbonate each contained identical proportions by weight of copper, carbon, and oxygen. Proust concluded that natural laws governed these proportions, and these laws were the same in the Earth’s depths as in a chemist’s flask.

Proust explained the immutability of true compounds as a function of nature’s ordering power, which he called “election” or “affinity.” This stable rate of attraction between certain substances was responsible for the fixity of composition. On the other hand, in France, Berthollet—a Newtonian—assumed that affinity, like gravity, brought about continuous attractions between substances, and he opposed Proust’s “elective” characterization of chemical affinity. He interpreted affinity not as a determinative force but as a physical power that could be influenced by the relative concentrations of reactants. In this way, he asserted, the products of chemical reactions were conditioned to have indefinite compositions.

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. In this gentlemanly dispute, Berthollet argued that variable compositions were exhibited not only by alloys, glasses, and solutions but also by oxides, sulfates, and other salts. For example, he produced experimental evidence that mercury sulfates exhibited a continuous range of combinations between two extremes. Proust, however, refuted Berthollet’s interpretation of his observations by showing that he was actually dealing with a mixture of two distinct compounds. Similarly, Proust proved that tin had two oxides and iron two sulfides, and all of these compounds had fixed compositions.

When Proust was unable to disprove that a given complex substance had a variable composition, as in the case of alloys and solutions, he declared the substance to be a mixture, an argument that Berthollet found circular. Although Proust was not correct in all particulars, and although he demeaned Berthollet’s valid observations about the effects of “active masses” on the direction of chemical reactions, his principal conclusion that true compounds have properties that are as “invariable as is the ratio of their constituents” was not only true but also important for the future of chemistry.


Proust’s law of definite composition ultimately became a fundamental principle of modern chemistry, but it took the work of many experimenters to establish it as a law. In a way, the debate between Berthollet, an insightful theoretician, and Proust, a meticulous experimenter, continued through their disciples. Proust’s followers certainly had the early victories, when they established that the constituent elements of many specific chemical compounds always exhibited fixed weight ratios.

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. More important for Dalton was the law of multiple proportions, Multiple proportions law (Dalton) which he discovered when he showed that two elements could combine in more than one set of definite proportions. Proust, an empiricist and not an atomist, came close to finding this law himself, because he had recognized cases in which the same elements formed two combinations, each with definite compositions, but he expressed his relationships in percentages, whereas Dalton expressed them in atomic weights. Both Proust and Dalton saw the same regularities, but Dalton creatively envisioned a new way of interpreting them. Indeed, Dalton was able to answer a question that Proust could not: Why should chemical compounds have definite compositions and exist in multiple proportions? Dalton’s answer was simple: Matter is atomistic, and when atoms combine with each other, their distinctive weights naturally result in definitely composed compounds or series of compounds.

The significance of some of Berthollet’s arguments in the Proust-Berthollet debate Proust-Berthollet debate[Proust Berthollet debate] did not become obvious until late in the nineteenth century, when the new disicpline of physical chemistry was founded. Berthollet had believed that chemical reactions are influenced by the masses of the reacting substances and that these “active masses” prescribed the reaction’s speed as well as the nature and amounts of the products. Although Berthollet was wrong about mass affecting the nature of the products of chemical reactions, he was right about mass affecting the reaction rates. The law of mass action, Mass action law (Berthollet) in which physical chemists quantitatively detailed how chemical reactions are influenced by the quantities of reacting substances, became a basic principle of chemical kinetics.

Like many controversies in the history of science, the Proust-Berthollet debate was not simply an instance of truth (Proust’s law) triumphing over error (Berthollet’s variable composition). Berthollet, who emphasized how compounds were formed, grasped, albeit inchoately, the law of mass action. Proust, who emphasized the empirical study of the nature of compounds, grasped the law of definite proportions, but not the law of multiple proportions. Furthermore, neither Proust nor Berthollet fully understood the significance of Dalton’s atomic theory, which would ultimately, in the hands of future chemists, make sense not only of definite and multiple proportions but also of most of chemistry.

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Ihde, Aaron J. The Development of Modern Chemistry. New York: Dover, 1984. This edition, a corrected version of a book originally published in 1964, analyzes Proust’s life and contributions in the fourth chapter, “Chemical Combination and the Atomic Theory.” Chapter bibliographic notes, indexes of names and subjects.
  • citation-type="booksimple"

    xlink:type="simple">Nye, Mary Jo. Before Big Science: The Pursuit of Modern Chemistry and Physics, 1800-1940. New York: Twayne, 1996. This book, part of Twayne’s History of Science and Society series, analyzes Proust’s work in the second chapter, “Dalton’s Atom and Two Paths for the Study of Matter.” Chronology, bibliographical essay, name and subject indexes.
  • citation-type="booksimple"

    xlink:type="simple">Partington, J. R. A History of Chemistry. Vol. 3. London: Macmillan, 1962. Partington, in his comprehensive treatment of the history of chemistry, deals with Proust’s life and achievements in chapter 14, “Foundations of Stoichiometry.” Many references to primary and secondary sources in the footnotes. Indexes of names and subjects.
  • citation-type="booksimple"

    xlink:type="simple">Pullman, Bernard. The Atom in the History of Human Thought. New York: Oxford University Press, 1998. This intellectual history of the atom analyzes Proust’s contributions in the chapter on the nineteenth century. Extensive chapter notes and an index.

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