Karlsruhe United

The scales fell from my eyes and my doubts disappeared and they were replaced by a feeling of quiet certainty.

Lothar Meyer, co-discoverer of the periodic law, recalling the Karlsruhe Congress
(as quoted in: Periodic Table: Its Story and Its Significance, Eric R. Scerri, Oxford University Press, 2006)

Have you ever experienced the same eureka moment while getting back home from a meeting? Personally, I have often had sudden brainwaves or flashing ideas while listening to someone’s presentation, but, honestly, never have I felt so close to universal enlightenment as Lothar Meyer did in Karlsruhe in 1860. I am not as brilliant, and, besides that, the scope of my research is not as universal as the quest for a comprehensive periodic law for all the elements.

This month marks the 155th anniversary of a milestone in the history of chemistry: the International Meeting which took place at Karlsruhe from 3rd to 5th September (yes, I know, my post is a few days late…I have been awfully busy as of late). Not only is this congress extraordinary because it is the first international scientific conference ever1: this is also an example of an apparently inconclusive meeting that instead sparked an electrifying sequence of (philosophical and scientific) breakthroughs in the research of a periodic system of the elements. Like a stone thrown in a calm lake, the ripples of the Karlsruhe congress reached the shore only after a while. Scientific ideas migrate slowly: it takes time for them to come home to roost.

No formula one

Say acetic acid, and you know what it is, well, at least what it smells and tastes like. What about its formula? Nowadays it is hard even to imagine that there was a time in chemistry when a textbook could list as many as nineteen different formulae:

By A. Kekulé (edit NobbiP (Lehrbuch der Organischen Chemie, 1861) [Public domain], via Wikimedia Commons, from https://commons.wikimedia.org/wiki/File:Kekule_acetic_acid_formulae.png
To make matters worse, there were even more fundamental problems affecting mid-19th century chemistry:

  • Atomic and/or equivalent weight: Different laboratories used one or the other quantity, which led to an astounding variability in the chemical formulae of chemical substances. In other words, oxygen could weigh 8 or 16 (with respect to hydrogen). The former is the equivalent weight of oxygen in water, the latter its atomic weight. The problem partly stemmed from the fact that equivalent weights have a more immediate significance when doing experiments: they immediately show the mutual proportions of the reactants of a chemical reaction. Atomic numbers, on the other hand, depend on the determination of an empirical formula. In addition, the available values of atomic weights were often significantly inaccurate.
  • Nomenclature: ‘Atom’, ‘molecule’, ‘radical’, ‘equivalent’: there was no common guideline for the usage, and the meaning, of these terms.
  • Theories of the structure of matter. This is where the fundamental problems lay. The remarkable expansion of organic chemistry outpaced the reflection on the structure of matter. Alternative theories of matter coexisted but they were not universally applicable, being mostly suitable for a single class of compounds. For example, Berzeliuselectrochemical theory, which interpreted all chemical bonding in terms of the attraction of fragments (‘atoms’) of opposing charges, stemmed from Berzelius’ titanic work on oxides and salts to draw up an accurate table of atomic weights. It was the prestige of Berzelius’ 1818 textbook Essay on the Theory of Definite Proportions and the Chemical Influence of Electricity that thwarted the acceptance of the so-called Avogadro’s Hypothesis (1811): “equal volumes of different gases at the same temperature and pressure contain the same amount of molecules”. In fact, only apparently is this a purely stoichiometric statement: it implies the existence of diatomic gases like N2, H2, or, in other words, the possibility for an element to combine with itself. This was pure heresy for most of the chemists of the first half of the 19th century: Avogadro’s hypothesis simply did not fit in the theoretical framework of Berzelius’ theory: fragments of equal charge should simply repel.
    From the point of view of organic chemistry, the years immediately preceding the Karlsruhe Congress saw the birth of a structural theory of organic compounds based on the tetravalency of the carbon atom. It has been shown2 that the scientific milieu of chemistry was ripe for the development of this concept in the late 1850s, and that scientific research at Paris exerted a major influence on the three chemists that worked on the structural theory of organic compounds. August Kekulé, a veritable chemistry polymath with an impressively extensive background, had been a research assistant at Paris in 1851-1852; Archibald Scott Couper was working with Wurtz (yes, the same of the Wurtz reaction, an Alsatian chemist who advocated atomicity, the combination of atomic theory and valency2) in the same city when he published a paper on tetravalence in the following year, while Alexander Butlerov, the Russian chemist who, in Markovnikov’s own words “gave full value to this [Kekulé’s and Couper’s] hypothesis and developed it into the whole structural system3 also worked for five months with Wurtz at Paris in 1858.

Going back to the Karlsruhe Congress, one can say that it coincided with the transition from the generation of Liebig and Dumas to that of Meyer (he was 30), Mendeleev (26) and Kekulé (31). Although it would be incorrect to depict the congress as an all-out rebellion of the youngsters against the old guard, the junior delegates would definitely turn out to be more receptive to the new ideas being discussed at the congress. Reportedly4, Meyer approached the congress with significant skepticism, predicting that the “idiotic church-council in Karlsruhe” would end up in the “the election of an infallible [molecular] formula-pope”. On the other hand, it is impossible to understate the role played by Kekulé (then full professor at Gent, Belgium), who, despite his relatively young age (sorry for the ‘relatively’, dear thirty-something reader like me!) , had been one of the main catalysts for the organisation of the Karlsruhe Congress5. Kekulé and Weltzien (a chemist based in Karlsruhe who would be the ‘business manager’ of the conference) went to Paris in March 1860 to meet the third organiser, Wurtz, and draw up plans for the conference together. Looking at the dates, one realises that the conference was actually called on quite a short notice, and this is all the more impressive if one considers that information and people travelled with 19th century means of transportation, the handwritten letter and the steam train.

The invitation letter was sent out in three versions, German, French and English6

Carlsruhe [sic], den 10. Juli 1860


Die Chemie ist auf einem Standpunkte angelangt, wo es den Unterzeichneten zweckmäßig erscheint, durch Zusammentritt einer möglichst großen Anzahl von Chemikern, welche in der Wissenschaft thätig und diese zu lehren berufen sind, eine Vereinigung über einzelne wichtige Punkte anzubahnen […]7

Chemistry has reached a state of development when to the undersigned, it seems necessary that a meeting of a great number of chemists, active in this science, who are called upon to do research and teach, be held so that a unification of a few important points shall be approached[…]8

Good old Lothar Meyer, summarises it once again for us in a note he wrote thirty years after the Karlsruhe Congress:

We now easily recognize that the argument was mainly about three things: electrochemical dualism (i.e. Berzelius’ theory), Avogadro’s Hypothesis, and the relative atomic weights of the elements. However, at the time, this was not so obvious; the most common arguments were about the formulas used to represent how chemical compounds were formed. . . . As a result, there was much confusion, every substance, even the simplest, had a series of formulas, e.g., water: H2O or HO or H2O2, mine gas (methane): CH4, C2H4 . . . . Even a simple compound such as vinegar could have enough proposed formulas to fill an entire printed page9

Meyer’s remark is important in reminding us the role of perspective when looking at the past. From the vantage point of the scientific consensus of the 21st century, it is all too easy to sneer at our chemical forefathers and point out the fallacies of some of the older theories, and their blindly stubborn rejection of Avogadro’s Hypothesis; yet, we should try and imagine how philosophically challenging it was for a chemist of that era to reconcile Berzelius’ and Avogadro’s theories. At the same time, let us not forget that even well-earned prestige, such as in Berzelius’ case, should never convince a researcher to abandon his/her skepticism, one of the four inspiring principles of scientific research, as defined by Robert Merton10:

  • Communism
  • Universalism
  • Disinterestedness
  • Organised skepticism

Bin it or read it

It is thanks to the Stanislao Cannizzaro’s mercurial speech that the Congress reaches its climax just before its end. As the conference is drawing to a close, time is running out and the delegates try to reach an agreement on chemical notation. In particular, they discuss if chemists are to revert back to the principles of Berzelius’ venerable system11, which in Cannizzaro’s view had a major shortcoming: it could not accommodate Avogadro’s hypothesis and the most recent development in chemical theory. It is at this point that, as Wurtz’s minutes of the Congress reports12, “Mr. Cannizzaro takes the floor in order to oppose the second proposition. It scarcely appears fitting or logical to him to move science back to the time of Berzelius”. Cannizzaro’s speech goes on for quite a while, or at least we are inclined to think so because of the extensive notes in Wurtz’s minutes. Moreover, we can only imagine what Cannizzaro’s stage persona looked like (Mendeleev remarks that Cannizzaro’s speeches were “heated” and “animated”8): was he waving hands extensively as Italians are often supposed to be doing all the time when they are talking? Could one hear a thick Italian accent in his French, or his German? Did he make direct eye contact with the audience? Were the listeners engaged or, rather, bored to death and already thinking about “rushing to catch that train back home”, their ongoing experiments, a doctoral thesis to correct? We will never know.

Cannizzaro’s take-home message was simple: the chemical sciences have significantly moved on since Berzelius’ times, and recent, compelling evidence supporting Avogadro’s hypothesis has been published by several groups using different experimental techniques. Therefore, the value of some of the atomic weights in Berzelius’ table must be doubled and these doubled atomic weights should be clearly rendered in notation with crossed-through symbols. Also, the simultaneous use of old and new symbolism must be discouraged to avoid all misunderstanding. Significantly, he concluded by saying: “And if we are unable to reach a complete agreement upon which to accept the basis for the new system, let us at least avoid issuing a contrary opinion that would serve no purpose, you can be sure. In effect, we can only obstruct Gerhardt’s [the newer] system from gaining advocates every day. It is already accepted by the majority of young chemists today who take the most active part in advances in science“. Reflect on this statement. The deliberations of a congress cannot stop the spreading of a novel outlook on chemical facts which is more in line with available experimental evidence. After all, as another French delegate (Boussingault) wittily remarked, “It is not chemistry that grows old, but chemists“; we should praise Boussingault for his objectivity in spite of his own age: in 1860 he was just 59.

Yet, Cannizzaro’s speech alone would probably have been much less convincing if the second Italian delegate at Karlsruhe, Angelo Pavesi, professor at the University of Pavia, had not stepped in. As recalled by Meyer1,8, Pavesi supported his friend’s speech by handing out copies of what I would call Cannizzaro’s educational pamphlet, entitled Sunto di un corso di filosofia chimica (“Sketch of a course of chemical philosophy13) and first published in Italy in 1858. Once more, how did Pavesi actually pull it off? Had he agreed with Cannizzaro to join forces and stage this combined coup de théâtre at the end of the conference? How did Pavesi stop delegates who, probably, were hurrying to Karlsruhe train station? Again, we will never know. Lots of copies must have ended up in the bin, or slipped into briefcases and forgotten there forever. However, Lothar Meyer (and certainly Dmitri Mendeleev) read the Sketch as he was travelling back to Wrocław 8 (then known as Breslau), and he was deeply impressed, hence the quote that opens this post.

Sketch of a course of chemical philosophy, it’s all in the name, those two words that immediately stir up my attention, chemistry and philosophy. But there is more: look at these lines from the first page of this pamphlet: “In order to lead my students to the conviction which I have reached myself, I wish to place them on the same path as that by which I have arrived at it –the path, that is, of the historical examination of chemical theories14. Condensed in a few words there is an entire pedagogical approach to the teaching of chemistry, and remember that Cannizzaro was lecturing on the then frontiers of chemistry, not some commonly accepted “textbook” knowledge. Here is -you will forgive the misuse of this term- a peripatetic approach: retrace your own scientific journey together with your students and let them come to the same conclusions.

No paradigm for social constructivism

A final remark. I can already hear some voices in the background saying that scientific conferences like the one at Karlsruhe are compelling evidence that “science is just social construction”: after all, these chemists actually assembled at Karlsruhe in order to decide what ‘atoms’ and ‘molecules’ are, didn’t they? Well, unfortunately I do not have time to elaborate on this very interesting topic any further, a topic that should be addressed in a post of its own. That said, let me just add a quote from the fourth Reith lecture 2010 by Martin Rees15, already cited in a previous post:

The physicist Steven Weinberg has given an apt metaphor for scientific breakthroughs. He says: “A party of mountain climbers may argue over the best path to the peak, and these arguments may be conditioned by the history and social structure of the expedition, but in the end either they find a good path to the summit or they do not, and when they get there they know it“.

Deciding on the correct use of scientific terms is one thing (by the way, nomenclature has played a paramount role in chemistry since Lavoisier’s times), discovering a law which is valid for all known (and yet to be discovered) elements is quite another cup of tea. Call it tungsten or wolfram as you wish, its place in the periodic system will remain always the same. After all, the Karlsruhe congress failed to achieve consensus, and, despite this, the shock waves generated by Cannizzaro’s paper (and speech) would shake the foundations of chemistry to rebuild this discipline on firmer ground: the periodic law.

A handshake is not a chemical bond.


  1. See also The Karlsruhe Congress: a Centennial Retrospect, Aaron J. Ihde, Journal of Chemical Education, 1961, 38, 83
  2. The interested reader could refer to A History of Chemistry, Bernadette Bensaude-Vincent and Isabelle Stengers, Harvard University Press, 1996
  3. As quoted in Alexander Mikhaĭlovich Butlerov, Henry M. Leicester, Journal of Chemical Education, 1940, 17, 203, doi:10.1021/ed017p203
  4. As reported in When Science Went International, Sarah Everts, Chemical & Engineering News
  5. “L’idée de provoquer une réunion internationale des chimistes appartient à M. Kekulé”, in Compte rendu des séances du congrès international des chimistes réuni à Carlsruhe, les 3, 4 et 5 septembre 1860, Charles-Adolphe Wurtz.
  6. Not surprisingly, these three languages are still today’s working languages of the European Commission, as in http://ec.europa.eu/stages/information/faq_en.htm.
  7. As reported in Der internationale Chemiker-Kongreß Karlsruhe 3.-5. September 1860 vor und hinter den Kulissen, Alfred Stock, Verlag Chemie, 1933
  8. Translation included in The Congress at Karlsruhe, Clara de Milt, Journal of Chemical Education, 195128, 421–425, doi:10.1021/ed028p421
  9. Retrieved on Thriving for Unity in Chemistry: The First International Gathering of Chemists, Michael W. Mönnich, Chemistry International, 2010, 32 . Meyer wrote this quote in the foreword to the 1891 translation of Cannizzaro’s textbook, Abriss eines Lehrganges der theoretischen Chemie. Translation by A. Miolati and edited by L. Meyer. pp. 52–58, Engelmann, Leipzig (1891).
  10. Robert K. Merton, The Normative Structure of Science, 1973. Recently, originality has been added as fifth principle before skepticism to obtain a nice acronym, CUDOS…
  11. Berzelius’ chemical notation had a major impact on chemistry: the symbols of the elements still follow Berzelius’ approach (the first letter of the Latin name of the element, or a capital letter and a lower case letter to avoid confusion), and the use of subscript numbers to indicate the proportions of elements in a certain compound (Berzelius himself used superscripts). The Swedish chemist was certainly influenced by Linnaeus’ nomenclature. Berzelius’ own symbolism incorporated conventions, now no longer used, to shorten the formulae, such as dots above the symbol of an element to indicate oxygen and a horizontal bar to indicate two atoms of a certain element. Berzelius developed his system, which eventually replaced Dalton’s notation, while he was writing a chemistry textbook in 1808. Similarly, both Lothar Meyer and Dmitri Mendeleev would include their own periodic systems in their chemistry textbooks as learning tools. For more information see A History of Chemistry, Bernadette Bensaude-Vincent and Isabelle Stengers, Harvard University Press, 1996
  12. I have not been able to retrieve the original French language version. An English translation is available online: https://web.lemoyne.edu/giunta/karlsruhe.html
  13. Available online.
  14. The original reads: “Per condurre i miei allievi al medesimo convincimento che io ho, gli ho voluto porre sulla medesima strada per la quale io ci son giunto, cioè per l’esame storico delle teorie chimiche
  15. Transcripts available online.

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