Unified system of atomic weights (Cannizzaro, Karlsruhe)

1858/1860 Fire and matter method

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A single system of atomic weights of the elements, obtained from Avogadro's hypothesis (equal volumes of gases contain equal numbers of molecules) and vapour densities. The atomic weight of an element is the smallest quantity of it entering the molecule of any of its compounds; for elements without volatile compounds the law of Dulong and Petit is brought in.

The procedure was set out by the Italian Stanislao Cannizzaro. It removed the confusion of the half century after Dalton, when some wrote water as HO with O=8 and others as H2O with O=16, and the formulae of organic substances differed by a factor of two between schools.

The pamphlet handed out to participants at the Karlsruhe congress in September 1860 brought chemists to uniform atomic weights within a few years; by Lothar Meyer's own admission, on reading it the scales fell from his eyes. The systems of the elements of Meyer (1864) and Mendeleev (1869) are built on precisely these weights.

Why this date

exact: the Sunto di un corso di filosofia chimica was printed in Il Nuovo Cimento vol. 7 (1858), pp. 321-366, and as a separate pamphlet in Pisa in 1859; the axis value is the first publication, 1858. The international congress of chemists at Karlsruhe ran for three days from 3 September 1860, and Angelo Pavesi handed out the pamphlet after the close; the system was adopted by the community only after 1860, hence 1860 in the id and label.

What it grew from

• Chemical atomic theory

  influenced · Confidence: medium

  Cannizzaro kept Dalton's atomic theory and the concept of atomic weight, but replaced the arbitrary rule of simplicity (water as HO) with the derivation of atomic weights from Avogadro's hypothesis and vapour densities, removing half a century of confusion over weights and formulae.

  Dispute: Cannizzaro's immediate support is not Dalton but Avogadro (1811), Ampere, Dumas, Gerhardt and Regnault. No direct assertion of this connection could be found in the historians: Leicester says only that Dalton considered and rejected the equal-volumes hypothesis, and Britannica speaks of the revival of Avogadro's ideas. The well-known passage in the Sunto about the application of Dalton's atomic theory refers not to Cannizzaro himself but to Berzelius, whose atomic weights Cannizzaro was correcting.

  Link sources

  • Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026

    When Dalton published the first volume of the book explaining his atomic theory in 1808, he considered but rejected the idea that equal volumes of gases under the same conditions contained equal numbers of particles.

  • Cannizzaro S. «Sunto di un corso di filosofia chimica», Il Nuovo Cimento 7, 321-366; Alembic Club Reprint No. 18 (1858) Verified 20 Sep 2026

    influenced by the atomic theory of Dalton (which had been confirmed by the experiments of Wollaston), he applied this theory

  • Britannica, «Atom», «The beginnings of modern atomic theory» (2024) Verified 20 Sep 2026

    Sicilian chemist Stanislao Cannizzaro revived Avogadro's ideas in 1858 and expounded them at the First International Chemical Congress, which met in Karlsruhe, Germany, in 1860.

  • Mendeleev D. «The Periodic Law of the Chemical Elements» (Faraday Lecture 1889), J. Chem. Soc. 55, 634-656 (1889) Verified 20 Sep 2026

    Only such real atomic weights--not conventional ones--could afford a basis for generalisation.

  "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

• Periodic law

  prerequisite for · Confidence: high

  Mendeleev arranged the elements by Cannizzaro's atomic weights, which he received at Karlsruhe in 1860, and himself called the establishment of true atomic weights the first condition for the discovery of the periodic law.

  Link sources

  • Mendeleev D. «The Periodic Law of the Chemical Elements» (Faraday Lecture 1889), J. Chem. Soc. 55, 634-656 (1889) Verified 20 Sep 2026

    In the first place, it was at that time that the numerical value of atomic weights became definitely known. Ten years earlier such knowledge did not exist, as may be gathered from the fact that in 1860 chemists from all parts of the world met at Karlsruhe in order to come to some agreement, if not with respect to views relating to atoms, at any rate as regards their definite representation. Many of those present probably remember how vain were the hopes of coming to an understanding, and how much ground was gained at that Congress by the followers of the unitary theory so brilliantly represented by Cannizzaro. I vividly remember the impression produced by his speeches, which admitted of no compromise, and seemed to advocate truth itself, based on the conceptions of Avogadro, Gerhardt and Regnault, which at that time were far from being generally recognised.

  • Kaji M. «D. I. Mendeleev's Concept of Chemical Elements and The Principles of Chemistry», Bull. Hist. Chem. 27 (1), 4-16 (2002), 5

    Along with everyone else in attendance, Mendeleev received a copy of the famous paper on the new atomic-weight system by Stanislao Cannizzaro (1826-1910), who distributed it at the meeting.

  • Kaji M. «D. I. Mendeleev's Concept of Chemical Elements and The Principles of Chemistry», Bull. Hist. Chem. (ACS HIST OPA paper) (2005)

    In pointing out the inconsistency of Gerhardt's atomic weights of metals and arguing that Cannizzaro corrected them with the 'multiatomicity of metals,' Mendeleev clearly recognized Cannizzaro's successful system of atomic weights.

  • Science History Institute, «Setting the Table» (2019) Verified 20 Sep 2026

    Mendeleev, only 26 years old when he attended the conference in Karlsruhe, made no attempt to hide the fact that his table, based on a common set of atomic masses, was inspired by Cannizzaro's work.

  • Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026

    Mendeleev also attended the congress and later wrote of the defense that Cannizzaro had presented for Avogadro's hypothesis.

  "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.

  • «Химический конгресс в Карлсруэ», 1860 (1860)

• Lothar Meyer's system of the elements

  prerequisite for · Confidence: high

  Meyer read Cannizzaro's pamphlet on the way back from Karlsruhe, in his words the scales fell from his eyes, and built his 1864 textbook with its table of 28 elements on Cannizzaro's weights.

  Link sources

  • Benfey O. T. «Meyer, Julius Lothar», Dictionary of Scientific Biography (via Encyclopedia.com) (1974) Verified 20 Sep 2026

    Meyer had attended the 1860 Karlsruhe Congress, where he heard Cannizzaro and read his paper on the use of Avogadro's hypothesis and the law of Dulong and Petit in establishing atomic weights and formulas.

  • Benfey O. T. «Meyer, Julius Lothar», Dictionary of Scientific Biography (via Encyclopedia.com) (1974) Verified 20 Sep 2026

    Meyer edited Cannizzaro's paper for Oslwald's Klassiker der Exacten Wissenschaften and describes in that work how 'the scales fell from my eyes and my doubts disappeared and were replaced by a feeling of quiet certainty.'

  • Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026

    His book Die modernen Theorien der Chemie, published in 1864, utilized Cannizzaro's ideas throughout and exerted a strong influence on the chemical world.

  • Science History Institute, «Setting the Table» (2019) Verified 20 Sep 2026

    In 1864, Congress attendee Julius Lothar Meyer incorporated Cannizzaro's ideas into his textbook Die Modernen Theorien der Chemie.

  "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.

  • Cannizzaro S. «Abriss eines Lehrganges der theoretischen Chemie», A. Miolati, L. Meyer, Ostwalds Klassiker der exakten Wissenschaften 30, Leipzig: Engelmann (1891)

Other links

• Kinetic theory of gases and statistical mechanics

  parallel to · Confidence: medium

  Avogadro's hypothesis was reached by two independent routes in 1859-1860: Cannizzaro made it the basis of the chemical scale of atomic weights, while Maxwell derived it from the mechanics of moving molecules.

  Dispute: The agreement did not come at once or by itself: according to Brush the early versions of the kinetic theory flatly contradicted Avogadro's hypothesis, and the agreement appeared only with Maxwell in 1859. There is no direct statement by a historian that Cannizzaro and Maxwell proceeded independently: that is an inference from the fact that both reached the same result in the same year by different means. The direction of influence is also the reverse of what one expects: chemistry did not supply physics, rather the kinetic theory retrospectively justified the chemical method of determining weights from vapour densities.

  Link sources

  • Stephen G. Brush, History of the Kinetic Theory of Gases (Storia della Scienza, vol. 7, ch. 44) (2004) Verified 20 Sep 2026

    Avogadro's hypothesis is favorable to the kinetic theory of of gases insofar as it implies that the volume occupied by a certain number of molecules is independent of their size and shape, which suggests they are not ordinarily in contact. But it was not until 1859 that Maxwell showed that the hypothesis could be derived from the kinetic theory; the earlier version of kinetic theory we are now doing to consider is actually inconsistent with it.

  • Klodian Coko (rev. of A. Chalmers), Atomism from the 17th to the 20th Century, Stanford Encyclopedia of Philosophy (2026) Verified 20 Sep 2026

    It also predicted Avogadro's law that equal volumes of gases contain equal numbers of molecules and thus also explained Gay-Lussac's law.

  "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

• Cannizzaro S. «Lettera del Prof. Stanislao Cannizzaro al Prof. S. de Luca; Sunto di un corso di filosofia chimica fatto nella Reale Università di Genova», Il Nuovo Cimento 7, 321-366; Alembic Club Reprint No. 18 «Sketch of a Course of Chemical Philosophy» (1858) Verified 20 Sep 2026

  I BELIEVE that the progress of science made in these last years has confirmed the hypothesis of Avogadro, of Ampère, and of Dumas on the similar constitution of substances in the gaseous state

• Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026

  'Sunto di un corso di filosofia chimica fatto nella Reale Università di Genova,' published in the journal Nuovo cimento

• Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026

  and was republished as a pamphlet (Pisa, 1859).

• Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026

  the first international chemical congress, which met at Karlsruhe for three days, beginning on 3 September 1860.

• Leicester H. M. «Cannizzaro, Stanislao», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026

  When such compounds were not known for a given element, Cannizzaro used analogies or depended on the relation between atomic weight and specific heat discovered by Dulong and Petit.

• Britannica, «Atom», «The beginnings of modern atomic theory» (2024) Verified 20 Sep 2026

  Sicilian chemist Stanislao Cannizzaro revived Avogadro's ideas in 1858 and expounded them at the First International Chemical Congress, which met in Karlsruhe, Germany, in 1860.

• Mendeleev D. «The Periodic Law of the Chemical Elements» (Faraday Lecture 1889), J. Chem. Soc. 55, 634-656 (1889) Verified 20 Sep 2026

  Ten years earlier such knowledge did not exist, as may be gathered from the fact that in 1860 chemists from all parts of the world met at Karlsruhe in order to come to some agreement, if not with respect to views relating to atoms, at any rate as regards their definite representation.

"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.

• Ihde A. J. «The Karlsruhe Congress: A Centennial Retrospective», Journal of Chemical Education 38(2), 83-86 (1961)
• de Milt C. «The Congress at Karlsruhe», Journal of Chemical Education 28(8), 421-425 (1951)

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