Mass balance and the conservation principle

1789 Fire and matter method

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The rule that in a chemical reaction everything entering, gases included, is weighed before and after, and the sums must agree. The foundation of quantitative chemistry.

It was made a rule by the Frenchman Antoine Lavoisier in the Traité élémentaire de chimie (1789), stated as an axiom: nothing is created, either in the operations of art or in those of nature, and in every operation the quantity of matter is the same before and after.

Historians (Hooykaas, Martins 2021) stress that Lavoisier did not derive the law of conservation of mass from weighings but took it in advance; the balance served to apply the principle. It is this method that made chemistry the quantitative science of the nineteenth century.

Why this date

exact for the formulation in the Traité (1789); Lavoisier kept weight balance sheets from 1765 and ran the oxygen programme from 1772; the principle itself is older (Van Helmont, Rey 1630)

What it grew from

• Copper smelting from ore

  influenced · Confidence: medium

  The metallurgists' fire assay (cupellation, precise weighing of small samples) was the main point of intersection between metallurgy and chemistry and in practice implied conservation of mass and constant composition long before Lavoisier.

  Dispute: The sources speak of Renaissance fire assay (Agricola, Ercker), not of copper smelting in the 5th millennium BCE; the continuity runs through the millennia-long tradition of metallurgical practice that began with smelting. The gap in time is large, and the nearest link to the weight method is 16th-century fire assay.

  Link sources

  • Martinón-Torres M., Rehren Th. «Alchemy, chemistry and metallurgy in Renaissance Europe: a wider context for fire-assay remains», Historical Metallurgy 39(1) (2005) Verified 25 Sep 2026

    Fire assay, as a technique for quantitative metallurgical analysis, was the most important point of intersection and, as such, it was essential for the development of modern metallurgy, but also of chemistry and science in general.

  • Martinón-Torres M., Rehren Th., Historical Metallurgy 39(1) (2005)

    Even if it was not formulated explicitly, in practice the assay hinted at the idea of conservation of mass and constant combining proportions.

  • Newman W. R. «What Have We Learned from the Recent Historiography of Alchemy?», Isis 102(2) (2011)

    Although mass balance had been implicitly assumed by metallurgists and alchemists for many centuries, it had existed in a subterranean and uneasy relationship with the dominant Aristotelian physics of the Middle Ages.

  "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.

• Distillation and laboratory apparatus

  influenced · Confidence: medium

  The laboratory practice of alchemy (analysis and synthesis, weighing of ingredients and products, with an explicit mass-balance principle already in Van Helmont) forms a continuous line to Lavoisier's quantitative chemistry.

  Dispute: Distillation is counted from Zosimos (c. 300), whereas the historians on whom this continuity rests speak of Geber (13th c.), Van Helmont (17th c.) and Starkey: what is at issue is the alchemical laboratory tradition as a whole, not Zosimos himself. Newman stresses that mass balance among alchemists coexisted with Aristotelian physics in a "subterranean and uneasy" position.

  Link sources

  • Newman W. R. «What Have We Learned from the Recent Historiography of Alchemy?», Isis 102(2) (2011)

    employing laboratory-based analysis and synthesis, alchemists were among the first explicitly to enunciate the principle of mass balance

  • Newman W. R., Isis 102(2) (2011)

    In short, Van Helmont weighed the ingredients that went into the glass and then reweighed the final products.

  • Newman W. R., Principe L. M. «Alchemy Tried in the Fire: Starkey, Boyle, and the Fate of Helmontian Chymistry», University of Chicago Press (2002) Verified 20 Sep 2026

    A common emphasis on quantification, material production, and analysis/synthesis, the authors argue, illustrates a continuity of goals and practices from late medieval alchemy down to and beyond the Chemical Revolution.

  "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.

• Van Helmont's weight balance and the willow experiment (Ortus medicinae)

  influenced · Confidence: medium

  Van Helmont was the first to state explicitly the equality of input and output weights, and Lavoisier, his admirer, began his gravimetric method in 1768-1769 precisely by testing the Helmontian doctrine of the transmutation of water into earth, weighing the vessel before and after 101 days of heating.

  Dispute: The Linda Hall Library notes that apart from Boyle nobody attached importance to the willow experiment before the 20th century, so the influence ran not through the experiment itself but through the Helmontian tradition (Starkey, Boyle) and through the polemic against the doctrine of the transmutation of water. No historian states directly that Lavoisier's mass-balance principle was taken from Van Helmont: Newman and Principe speak of a continuity of goals and practices, Guerlac of Lavoisier testing the Helmontian thesis. Hooykaas and Martins (2021) hold that Lavoisier started from a metaphysical principle, not from other people's experiments.

  Link sources

  • Newman W. R. «What Have We Learned from the Recent Historiography of Alchemy?», Isis 102(2) (2011)

    It was only in the mid-seventeenth century that Van Helmont explicitly enunciated the identity of input and output weights and used this as a tool for undermining the traditional theory of the four elements.

  • Guerlac H. «Lavoisier, Antoine-Laurent», Dictionary of Scientific Biography (via Encyclopedia.com) (1973) Verified 20 Sep 2026

    The doctrine of transmutation had the authority of J. B. Van Helmont and of Robert Boyle, each of whom believed he had shown by experiment that the nutrition and growth of plants can only be attributed to water.

  • Guerlac H. «Lavoisier, Antoine-Laurent», Dictionary of Scientific Biography (via Encyclopedia.com) (1973) Verified 20 Sep 2026

    This well-known experiment demonstrates Lavoisier's experimental ingenuity and those gravimetric procedures that were to characterize his later work.

  • Newman W. R., Principe L. M. «Alchemy Tried in the Fire: Starkey, Boyle, and the Fate of Helmontian Chymistry», University of Chicago Press (2002) Verified 20 Sep 2026

    A common emphasis on quantification, material production, and analysis/synthesis, the authors argue, illustrates a continuity of goals and practices from late medieval alchemy down to and beyond the Chemical Revolution.

  • Burns W. E., Newman, Principe «Alchemy Tried in the Fire», Albion 36(1) (2004)

    an examination of eighteenth-century French chemistry and the roots of the quantitative chemistry of Antoine-Laurent Lavoisier (an admirer of Van Helmont)

  "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.

What grew out of it

• Law of definite proportions (Proust)

  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.

Links under review

• Oxygen theory of combustion and "Réflexions sur le phlogistique"

  prerequisite for · Confidence: medium

  The only source so far is a Wikipedia article (citing Best 2015 and Ihde 1964); a verbatim quotation from Best, Found. Chem. 17, 137-151, or from Ihde 1964 is needed.

  Without the oxygen theory (accounting for the weight of the absorbed gas) the weight balance of reactions did not close, so the Traité of 1789 rests on it.

  Link sources

  • Wikipedia «Phlogiston theory» Best 2015 Ihde 1964 (The Development of Modern Chemistry) (2024) Verified 20 Sep 2026, quotation assembled from fragments

    combustion requires a gas that has mass (specifically, oxygen) and could be measured by means of weighing closed vessels ... culminated in the principle of mass conservation.

  "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. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.

• Chemical atomic theory

  prerequisite for · Confidence: medium

  Both sources are summary sentences from Wikipedia articles. The footnote on the first leads to Roscoe H. E., Harden A., «A New View of the Origin of Dalton's Atomic Theory», 1896, and not to the notebook study the sentence reports. Verbatim quotations are needed from Roscoe and Harden and from Dalton J., «A New System of Chemical Philosophy», 1808, ch. II.

  Lavoisier's quantitative chemistry and Proust's law of definite proportions (1797) supplied the weight ratios that Dalton's atomic theory explained through atoms of definite weight; according to Grossman (2021) the theory of 1803 itself arose while reconciling the analytical data of Lavoisier and Cavendish.

  Dispute: The origin of the theory is disputed. Roscoe and Harden (1896), from Dalton's notebooks, and Thackray (Isis 1966) hold that atoms came to Dalton from the physics of gases, not from a search to explain multiple proportions; Grossman (Ambix 2021), from the same notebooks, traces the theory to the reconciliation of the weight data of Lavoisier and Cavendish. Fujii (BJHS 1986) notes that Kapoor denied Proust's influence on Dalton but does not himself find that conclusion decisive. That atomic weights depend on the weight method is uncontested, but no historian has been found who states that Dalton drew on Lavoisier in particular: the chain runs through Proust's law of definite proportions.

  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.

  • «John Dalton Proposes His Atomic Theory and Lays the Foundation of Modern Chemistry», Science and Its Times (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.

  • Wikipedia «John Dalton» (2024) Verified 20 Sep 2026

    a recent study of Dalton's laboratory notebook entries concludes he developed the chemical atomic theory in 1803 to reconcile Henry Cavendish's and Antoine Lavoisier's analytical data on the composition of nitric acid, not to explain the solubility of gases in water.

  • 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.

Node sources

• Lavoisier A.-L. «Elements of Chemistry, in a New Systematic Order», R. Kerr, Edinburgh: W. Creech, 1790, I, XIII «Of the Decomposition of Vegetable Oxyds by the Vinous Fermentation» (1790), 130-131

  We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment

• Martins R. de A. «A priori components of science: Lavoisier and the law of conservation of mass in chemical reactions», Studies in History and Philosophy of Science (2021)

  nothing is created, neither in the operations of art, nor in those of nature, and it may be advanced in principle that in every operation there is an equal quantity of matter before and after the operation

• Hooykaas R., Martins 2021 (2021)

  Lavoisier did not find the law of conservation of weight as a result of experiments; on the contrary he started from the metaphysical belief that nothing takes rise on its own account

• Beretta M. «Lavoisier, Antoine-Laurent», Complete Dictionary of Scientific Biography (New DSB, via Encyclopedia.com) (2008) Verified 20 Sep 2026

  in the gypsum experiments at the beginning of 1765, Lavoisier began to use balance sheets to compare the weights of reactants before and after distillation.

• Stanford Encyclopedia of Philosophy, «Philosophy of Chemistry», §3.2 (2023) Verified 20 Sep 2026, quotation assembled from fragments

  The metaphysical principle of the conservation of matter ... called upon here is at least as old as Aristotle

"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. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.

Further reading

Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.

• Lavoisier A.-L. «Traité élémentaire de chimie», t. 1, Paris (1789)
• ACS National Historic Chemical Landmark «The Chemical Revolution of Antoine-Laurent Lavoisier» (1999)

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