Introduction

On the afternoon of Wednesday 21 June 1922, Professor Ernst Julius Cohen (1869–1944) opened the International Chemical Reunion Conference at Utrecht in the lingua franca of early modern science. Speaking in the Van ’t Hoff laboratory in front of about forty prominent chemists from all over the world, he effortlessly continued in French, German and English to welcome all his guests.² Bilingual by upbringing and a disciple of the first Dutch Nobel prize winner, Jacobus van ’t Hoff, Cohen was international through and through: ‘as many languages one speaks, as many times one is human’!³ Cohen’s ‘curious convention’ aspired to ‘make the chasm disappear’ that the First World War had created between chemists from the Central Powers and the Entente nations.⁴ In the following decade, in which the definite downfall of scientific internationalism became manifest, the attempts of Utrecht chemists at restoration of an international chemical community would ultimately succeed in a unique fashion.

The large scale mobilization of chemists and their consecutive intensive involvement in warfare atrocities has made historians speak of the ‘chemists’ war’.⁵ The ubiquitous use of chemical agents at the front poisoned and killed thousands. It also generated a ‘poisoned atmosphere’ in the relatively small international chemistry community, both during and after the war.⁶ The distortion of the international scientific world in the wake of the war was most clearly seen through the statutory exclusion of the Central Powers by the Entente nations in the newly founded International Research Council (IRC). This boycott also affected all disciplinary unions and would last until 1926.⁷ Before the war, German science, and chemistry in particular, was leading in research and education. The German language and reference publications were dominant in the world of chemistry and a majority of chemists had received a part of their education in the German Empire.⁸ The spirit of internationalism was strong in chemistry, especially because international agreement on values like atomic weight was essential to science and trade.⁹ The comprehensive boycott of German science in the 1920s was thus catastrophic, as international scientific bureaus were relocated to Entente nations, German was expelled as the language of science and Central Power scientists were not invited to 75% of international conferences abroad.¹⁰

As there was a harsh ostracism on the level of international scientific organizations, it has been claimed that reconstruction of international relations proceeded most importantly through informal contacts and private meetings.¹¹ The 1922 conference at Cohen’s laboratory is often presented as a pioneering unofficial effort, but has not been thoroughly studied up to now.¹² Zooming in on the mediating efforts of Cohen and his right-hand man Hugo Kruyt will self-evidently bring into play the tension between their informal attempts and the ‘de facto non-international’ organization of chemistry in the International Union for Pure and Applied Chemistry (IUPAC).¹³ The main objective here is to make the motivation and reception of their informal efforts visible for the first time. With regard to the change the Utrecht chemists achieved, historiography for long relied on (auto-)biographical displays of the actors involved.¹⁴ Recently, scepticism was raised by Geert Somsen about their deemed ‘enormous impact’, who instead suggested that the Dutch mediators fostered a ‘lack of empathy’.¹⁵ Brigitte Schroeder-Gudehus further criticizes the idea of informal reconstruction as it seems to have been invoked to challenge the unwelcome pessimistic view on science and its internationalist ideals.¹⁶ This detailed study of concrete practices is the only way to evaluate to what extent the International Reunion of Chemists can indeed be considered a ‘nucleus in a supersaturated solution’ that set in motion the crystallization of international chemistry.¹⁷

Immiscible international science

Before the end of the war, still livid over the manifesto An die Kulturwelt! (‘To the Civilized World’) and the alleged German initiation of chemical warfare, scientists from the Entente nations designed plans for post-war international organizations that explicitly ostracized scientists from Central Powers for the following decade at least.¹⁸ The Koninklijke Nederlandse Akademie der Wetenschappen (Royal Netherlands Academy of Arts and Sciences – KNAW) took a waiting attitude over these developments, and was wary not to insult either side as to maintain its neutrality.¹⁹ It led to heated discussions in Amsterdam, where the Groningen chemist Jaeger advocated collective action together with other neutral academies. Cohen, exemplary for the Dutch cautious attitude, specifically turned down a Swiss proposal for an exploratory meeting amongst neutrals for being ‘not fair’ as negotiations for entry into the International Research Council had already started. Eventually, the KNAW, like organizations from other neutral nations, entered the IRC under the condition that it would not abolish contact with the excluded academies. Cohen, chairing the newly founded Chemische Raad van Nederland (Chemical council of the Netherlands), joined the IUPAC under similar conditions.²⁰ The circumspect and conditional entry by the Dutch into the formal international organizations was not unwarranted, as neutral nations were mistrusted by the Entente nations.²¹ The inclusion of neutral nations was postponed until after the constitutive assembly of the IRC in 1919, as it was feared especially by Belgian and French scientists that they might attempt to loosen up the exclusionary politics.²² A comparison with early Swedish efforts further points at insightful judgment by the Dutch academy. Already in 1917 explicit strategies were designed in Sweden to present itself as the new centre of international science as soon as the war would end. But in 1919 they had to conclude that they had met no understanding from foreign colleagues as the ‘meddling of neutrals’ was surrounded by sensitivity, and they resorted to the same conditional entry into the IRC.²³

While the fog of war had not yet cleared, Cohen visited England in the spring of 1921 for a lecture tour.²⁴ During his stay in London, Cohen’s lifelong friend Frederick G. Donnan (1870–1956) expressed his grief at the enduring disruption of contact between scientists of the world.²⁵ He saw Cohen as the right man to break the deadlock of ‘hatred and misunderstanding’, by organizing ‘a rapprochement amongst the chemists of the world’.²⁶ There was much to say for Cohen as the man for the job as he was praised for his linguistic skills, worldly wisdom, patience and tactful humanity.²⁷ He maintained friendly relationships with many prominent foreign chemists that he met via Van ’t Hoff in Amsterdam and Berlin, and working abroad with Henry Moissan in Paris and Svante Arrhenius in Stockholm.²⁸ His international reputation was established by the 1904 foundation of his own modern physical chemical laboratory in Utrecht, while he maintained Van ’t Hoff’s legacy by baptizing it with his name.

Enthusiastically, Cohen returned to Utrecht where he, Hugo Rudolph Kruyt and their organic chemistry colleague Pieter van Romburgh swiftly organized a small meeting with chemists from the different nations (see fig. 1).²⁹ Only the Belgian chemist Jean Timmermans, director of the IUPAC’s ‘International Bureau for Physico-Chemical Standards’, refused the invitation, writing that at this moment it was ‘impossible’ for Belgian chemists ‘to renew personal relationships with the scientists across the Rhine’.³⁰ Strong cultural anti-German sentiments, caused by war atrocities like the raid of Leuven and a country torn open by trenches, overshadowed enduring high appraisal of German chemical science and industry.³¹ The memory of the war was still painfully manifest in the Belgian lives and landscape, so that the scepticism regarding international cooperation is understandable. German chemists on the contrary were delighted with the initiative as ‘reestablishment can only be successful from human to human, possibly by using old relationships’.³² The short notice of the meeting indicates that old ties were revived. Several of the men were associated with the predecessor of the IUPAC, the ‘Association Internationale des Sociétés Chimiques’ (AISC), and their academic family trees shared many intellectual fathers like Van ’t Hoff and Ostwald.³³ The meeting took place at the home and garden of Cohen, and at the closing dinner the informality of the event merged with its rationale of internationalism as each course was named after famous nineteenth century chemists from different nations: Bouillon à la Rumford, Paté de Poisson, Langue de veau à Liebig, Petits poid Landolt and Bombe Berthelot.³⁴

The meeting, held completely in German, was more than just sociable.³⁵ Situated right before the yearly IUPAC conference, Cohen wanted to see ‘what can be done’.³⁶ As the official route proved unlikely at that point, even though some English and Americans already were willing to think about IUPAC reforms, two actions were taken on an informal level.³⁷ A circular in English, French and German was distributed internationally in which was stated that the Van ‘t Hoff laboratory would reprint and send any chemical publication that could not be obtained otherwise. Because of the ‘world situation’ the normal dispersion of scientific communications had been almost completely abolished. The Journal of the Russian Physical-Chemical Society for example had not reached Paris since 1914, and the resources were lacking in many central and eastern European nations to acquire foreign periodicals.³⁸ Coincidentally, one day after the meeting in Cohen’s garden, a committee of the KNAW decided to make possible the loan of British and American publications to libraries of the Central Powers.³⁹ It was through personal correspondence and initiatives like those of the Van ’t Hoff laboratory that scientific publications found their way around Europe again.⁴⁰ The other outcome was the organization of a bigger, purely scientific meeting the following year. What Cohen would later call the ‘Pacification of Utrecht’ had to clear the poisonous clouds that were still obscuring the international chemical community by its focus on friendly intercourse about chemical ideas.⁴¹ The chemists present in 1921 anticipated success in different gradations: the depictions ‘successful experiment’ and ‘solemn occasion for the rebirth of peace’ were surpassed by the ‘dithyrambic words’ of the Russian-German Walden who placed the meeting in the context of the Dutch conquest over sea.⁴² This internationalist initiative by a neutral nation was presented as a model for the moral role of science in society and, ambiguously, as national virtue. The multidirectional plan that the Dutch chemists had now delineated attended the personal network of chemistry and emphasized the importance of the intellectual exchange of ideas for international science.

The intricacy of inviting appropriate ‘nuclei’

In the loaded atmosphere of the early 1920s Cohen proceeded carefully by first approaching closely befriended chemists in all different nations to return to him lists of ‘appropriate persons’ – leading scientists who could function as nuclei of internationalist ideas.⁴³ The Dutch invitation, that contained a complete list of invitees, was sent to about a hundred colleagues around the world. This careful and transparent approach enabled many to recognize old friends, but also raised various problems and criticisms that shed light on a distorted international community of chemists. When Cohen visited the USA at the end of 1921 for another lecture tour, the head of the University of Illinois chemistry department, William Albert Noyes, told him that many American chemists raised objections.⁴⁴ Above all, they understood the Reunion to be a challenge of the IUPAC conference in Lyon that same month, or even as an attempt at constituting a new organizational body that would include the Central Powers. Such responses reflect the continued mistrust of formerly neutral nations and emphasize that neutrality was not an unambiguously positive characteristic. Cohen sent Noyes a telegram stating the nature of his conference: ‘Only purpose meeting is bringing scientific men together for scientific work. No talk on legislation allowed. Every rivalry with Lyons excluded.’⁴⁵

Max Bodenstein, who published the German atomic weight tables, tuned the responses of all German chemists before responding.⁴⁶ He asked the invitees for their personal view on this ‘not at all simple’ situation, after which four Berlin chemists would formulate a mean standpoint if necessary.⁴⁷ Bodenstein himself thought the pleasure of the meeting to be rather ambiguous –‘manche Kollegen wird man gern wieder begrüssen, manche ganz und gar nicht’ – but he was willing to give it a try.⁴⁸ Remarkably, it was two German chemists who were not invited who roused emotions. Arrhenius reported from Berlin to Cohen that ‘the Germans’ were ‘a little bit astonished’ by the prominent absence on the invitation list of Noble prize winners Fritz Haber and Walther Nernst.⁴⁹ Although both had been leading in the German chemical war initiatives, chairing committees in the Kaiser-Wilhelm-Stiftung für Kriegtechnische Wissenschaften (Kaiser Wilhelm Foundation for Military-Technical Science), it is unlikely that this was a moral appeal by the organization.⁵⁰ Many key figures in chemical warfare from all former belligerents were invited.⁵¹ Both men were also well known in Utrecht, and their scientific work remained highly appreciated.⁵² But the Nobel Prize awarded to Haber in 1919 had aroused a huge controversy in Entente nations, as it was believed that the discovery of ammonia synthesis had actually prolonged the German war effort, and Haber was also seen as initiator of the use of poison gas. That the antipathy resounded down to the level of the actual science can be seen in an address by the notable French chemist Victor Grignard, in which he ascribed the discovery of the fundamentals of the Haber process to two French chemists.⁵³ And although mobilization of a nation’s chemical know-how was ubiquitous in the war, the active and rhetorical promotion of one’s side was not. Both Haber and Nernst signed the 1914 Aufruf an die Kulturwelt and spread it in their international networks. The immense symbolical meaning of this action can be understood by the fact that French and Belgian scientists would demand retraction of the manifesto far into the 1920s.⁵⁴ The absence of Haber and Nernst is ascribed by the Germans themselves to the persevering public image of these men as ‘war criminals’.⁵⁵ The protean public image of these two chemists indeed carried such destructive symbolic power that their presence would surely have offended French and Belgian chemists severely. This silent concession was thus necessary in the Dutch attempt to uphold neutrality and avoid getting caught in post-war rhetoric. But the collective response received from France demonstrated that the first was extremely difficult and the second inevitable. The twelve French invitees represented by Charles Moureu (1863–1929) apologized for having to decline the invitation because of existing ‘difficulties’.⁵⁶ Cohen, with rock-solid trust in transparency and his good intentions, attempted to ‘take away unfortunate misunderstandings’ by recollecting the rationale of the Reunion and Timmerman’s negative response of 1921. The French chemists were not at all familiar with ‘Cohen’s project’, but maintained that the event would ‘do more harm than good’. The Belgian refusal, that was unknown so far, was dubbed ‘critical’ and out of solidarity with their Northern neighbours the French kept to their decision.⁵⁷

The wounds of the war had clearly not yet healed in the countries most affected. For the Weimar Republic the Versailles treaty and the ensuing war debt were particularly painful, so that industry and government had to foot the bill for the attending chemists.⁵⁸ In Soviet Russia, the deplorable financial circumstances were not the only cause for concern. While N.A. Schilov had his expenses covered by the Dutch organization, three others were denied passports to travel out of the country.⁵⁹ This fitted the Communist isolationist policy, as part of which Russia did not join IRC or IUPAC, and the purchase of chemical supplies from abroad was prohibited for several years. This situation caused Germany and Russia to fall back on each other as long as scientific Europe remained divided.⁶⁰ The absence of Vladimir N. Ipatieff, chairman of the ‘Chemical Committee’ of the ‘Supreme Council of National Economy’, is to be understood in the context of the immense agricultural and industrial shortages. During his travels around Europe visits for trade and industry purposes simply had priority over his scientific work. Even though he met the initiator of the Reunion, Donnan, in London in 1922, he surprisingly did not mention his invitation for the Utrecht conference.⁶¹ From Sweden, Cohen received a cancellation from his close friend and prominent chemist Svante Arrhenius. It has been suggested that the Scandinavian countries were seen as competitors on the market of mediation in international science, but in this case the organization insisted on the ‘absolutely essential’ presence of this Swedish ‘genial and friendly mediator’ that would add ‘luster to the movement’.⁶² But Arrhenius had exhausted all his holidays in his spring travels to Copenhagen, Haber and Nernst in Berlin, Paris and the Solvay conference in Brussels. It is a further indication of the informal nature of the meeting that he could not be officially dispatched to Utrecht.⁶³ From the intricacy of invitations, a small but instable international chemistry community appears that was still disturbed by the fresh memory of the war and its materially destructive consequences.

Primary Nucleation: The Reunion

The spring of 1922 was full of international conferences on chemistry. In between the first Solvay conference on chemistry late April in Brussels and the third IUPAC meeting at the end of June in Lyon, the International Chemical Reunion in Utrecht was exclusive in the composition of its attendants (fig. 2). It was the first meeting of chemists from Germany, Austria, England and the USA since the beginning of the war. And where the Belgian industrial Solvay and the Paris based IUPAC proceeded in either French or English, the scientific discussions and speeches at the Reunion were trilingual by rule. The official report of the Utrecht meeting was purposely written in French, as to make it ‘readable to foreign chemists’, an attempt of the Dutch organization to reach the Belgian and French scientists that declined the invitation.⁶⁴ As the IUPAC meetings were primarily concerned with nomenclature, patents and values of constants and atomic weights, the Reunion differed in its focus on scientific content.⁶⁵ It is telling that Cohen directly cuts short the discussion after Mieczysław Centnerszwer’s proposal for a new mass unit.⁶⁶ The keynote lecture that opened the Reunion was given by Edward Charles Baly from Liverpool, on ‘Photochemical Catalysis’. It was a very topical lecture about the potential importance of Max Planck’s quantum theory for chemical dynamics, including the work of several chemists present in the lecture room.⁶⁷ Its scientific relevance is evident from the fact that the topic was also one of the pressing issues discussed at the Solvay conference.⁶⁸ Baly’s main contribution is the principle of photochemical catalysts; molecules that are not only able to absorb energy, but also to radiate it again, stimulating other molecules to react. The initiators of the meeting of course hoped that all chemists present would absorb internationalist energy to stimulate, like catalysts, less willing colleagues upon their return home.

On Thursday morning the first lecture by Paul Walden concerned itself with free radicals, a ‘textbook example’ of the development of chemical knowledge.⁶⁹ Part of such a development, R. Schenck later notes, is the insight that chemical issues can only properly be approached ‘without attention to national borders’.⁷⁰ The rest of that day and the next morning thirteen lectures were presented on recent findings and experiments on topics varying from Germanium (Dennis and Schenck) to the coupling of carbon monoxide with chlorine (Bodenstein). That the majority communicated not so much finished high end papers, but rather open-ended research results indicates that there was no serious streamlining of the scientific content by the organization: good fellowship was the primary aim. Furthermore it indicates that the ‘poverty and gravity of the time’ hindered proper scientific work.⁷¹ Demobilization caused a sharp growth in student numbers which immensely increased the education obligations for researchers and, especially east of the Rhine, absurdly high prices for all laboratory requirements further obstructed the normal practice of scientific research.⁷²

The objective of the organization was twofold, so that besides this exchange of ideas, work was also made of strengthening ties with old and new colleagues. They enjoyed all their meals together in a selection of restaurants in Utrecht and Baarn, and in the afternoons and evenings several excursions touched upon the technological and scientific highlights of the surroundings: a trip with the electrical tram to Zeist, a visit to the ‘modern installations’ of the Rijksmunt (State Mint) and on the final day a stroll during the twilight hours in the botanical garden.⁷³ A drink at Hotel de Pays-Bas, offered to the international guests by the Nederlandsche Chemische Vereeniging (the Dutch Chemical Society – NCV), was seized to explicitly underline the moral meaning of the Reunion. In the presence of prominent representatives of the Dutch government, Chemical Council and KNAW, three monumental speeches were delivered.⁷⁴ Gerardus Leonardus Voerman, the president of the NCV, officially welcomed all guests and stressed the importance of scientific internationalism for a peaceful world, and he hoped that ‘this conference may be the nucleus in a super-saturated solution, round which the crystallization of good international relations will take place.’ Went, both as president of the KNAW and as representative of the minister of Education, Arts and Sciences, delivered his speech alternately in French, English and German. He expressed the conviction that it was science that would break the current trend of ‘nationalism and jingoism’ because ‘Wissenschaft kennt keine Grenzen’ (‘Science knows no borders’).⁷⁵ Went added a chauvinistic flavor towards the end of his speech as he proudly described the tache sacrée (‘holy task’) of Holland ‘to open its frontiers to scientists of all nations and to serve as a link between these scientists, in order to promote science’.⁷⁶ Noyes, acting as the spokesperson of the international guests, also mingled international ideals and national pride. This ‘tortuous relationship’ between neutrality, nationalism and internationalism was common rhetoric among intellectuals in neutral countries.⁷⁷After recollecting a statement by Cohen about the unique and impressive Dutch Nobel Prize ratio, Noyes prophesized about a peaceful future for Europe in which he saw an important role for science and this conference.⁷⁸ All three lectures grounded their optimism in Comtean ideals of science that did not correspond to the then-current disturbed situation in international chemistry.⁷⁹ It had to become clear in the following years whether science could indeed take a leading role pacifying the world and if the Dutch mediators had set this ‘primary nucleation’ in motion.

Secondary Nucleation: Reactions to the Reunion

The accounts of the conference that appeared in international journals were furnished as proof of the appreciation of the Reunion as a ‘successful event’.⁸⁰ The main objective of the meeting, to restore friendly international intercourse, was thought to be attained ‘signally’ and the ‘lavish’ entertainment and hospitality that accompanied it was celebrated extensively in several German, Austrian, English and American articles.⁸¹ Where some highlighted contributions of fellow countrymen, Schenck recommended other disciplines to copy this format of ‘activating old personal ties for the idea’ which gave the Reunion its ‘very special meaning’. The Foreign Office of the Weimar Republic considered Schenck’s report so important that it was dispatched to all relevant ministries and embassies.⁸² This optimistic image was anticipated in the press releases that appeared in Dutch national newspapers during the conference.⁸³ The content of the newspaper articles varied from modest portrayals, ‘good fellowship and hearty discussion’, to high expectations, ‘the uniting of more colleagues from all over the world will soon follow’. Again Comtean prospects were not lacking: ‘international cooperation is vital to enable science to fulfill her task of elevating society intellectually and materially’.⁸⁴

The image of a successful scientific conference offering hope for worldwide peace that was spread publicly was the product of organizers and attendants of the conference, who authored the above mentioned articles. In private correspondence such optimism was not absent, but the Reunion was placed back in ‘the most unfavourable current circumstances’.⁸⁵ Georg Bredig, a foreign member of the KNAW from Germany, praised the ‘admirable tact’ of the Dutch hosts and applauded the scientific component, albeit with a sneer to the French and Belgian chemists ‘who isolated themselves … and missed the joy of hearing interesting lectures about free radicals’.⁸⁶ The principal aim of the meeting for him was ‘to forget in science the war and hate’, which proved to be hard as he was confronted upon his return with the assassination of Walter Rathenau and the iron grip of the treaty of Versailles on Germany.⁸⁷ This inner conflict manifested itself as ‘a mournful return from an after all hopeful Utrecht conference’.⁸⁸ Thus Bredig’s upright praise was tempered as he grew cautious about fostering high hopes over the effects the meeting would have in the direct future.

The hopeful perspectives of participants offer only a limited view on the Reunion. At the subsequent IUPAC meeting Cohen and Kruyt learned in personal conversations that their ‘beautiful June days [were] not put to the credit’ of themselves and the Dutch Chemical Council.⁸⁹ French and Belgian colleagues showed their discontent as they understood the Reunion as an improper expression of the Dutch effort to bring the Central Powers into the IUPAC. The manifestation of the Dutch internationalist point of view, plus the previous conditional entry into the Union, caused a certain level of ‘mistrust’, expressed in particular by the chairman Moureu. Apparently the French and Belgian fear of becoming isolated from the international scene themselves was realistic, so that the Reunion was perceived as a step in that direction. Cohen went out of his way to make a stand for Moureu, ‘a broad-minded man’, who only held on to this opinion ‘because of the necessity of the situation’. By separating the person from his political views and historical context Cohen seems to suggest that a self-evident correct moral standpoint existed, namely his own internationalism, while at the same time showing empathic insight into the post-war French perspective. Did Cohen combine stubborn pacifism with well-intended understanding? The way the Dutch mediating mission played out in the following years can provide insight. It will also show whether the Reunion and the partly positive reactions to it affected secondary nuclei in the different nations that set in motion further crystallization of international chemistry.

Crystal growth

The Utrecht chemists were certainly in the thick of things, as Cohen was successively elected vice-president and president of the IUPAC between 1923 and 1925. Not long after their informal attempt at restoration of international chemistry, the Utrecht chemists shifted their activities completely to the formal platform. The main goal of their mission was now to make the official organizations truly international by working from the inside. A proposal in 1925 by Schenck, to organize a second informal Reunion on German soil, was even declined by an optimistic Cohen.⁹⁰ This change of strategy by the Dutch chemists distinguished them from the mediation efforts by Swedish chemists, who abstained from joining the IUPAC in 1923 because that would not be ‘very neutral’.⁹¹ The fact that this Swedish abstention was explained by foreign scientists as German-sidedness once again stresses the highly politicized atmosphere the neutral mediators were working in, and that the Dutch chemists had to work hard, through official and unofficial channels, to keep things moving on both sides of the Rhine. But Cohen’s optimism about a nearing formal entry of Germany into international science took a serious knock when in 1925 a Dutch-Danish proposal for reform of the IRC failed, and German scientists were considering a counter-boycott.⁹²

Haber, now the main international spokesman for German natural science, concluded that the ‘friendly Dutch opinion has not been of decisive importance’ and that German willingness for international cooperation had become low.⁹³ All the energy invested in the mediating mission seemed futile, as opinions in France and Belgium remained unforgiving and the previous German hope was overshadowed by the enduring burden and humiliation of the Versailles treaty. Ultimately, the unremitting exclusion of Central Power scientists from the IUPAC and IRC was more a failure of science in general to break through the impasse than a sign of the limited leverage of neutral mediators. Now science had to follow politics, as the Locarno treaties, that normalized diplomatic relations in Europe, and the subsequent admission of Germany to the League of Nations paved the way.⁹⁴ Cohen had to admit that change would not easily occur from within the Union, and Haber feared the power of ‘nationalists’ in German academic circles who now boycotted the IRC. This made Haber resort to a solution on the level of the French and German governments, only with optional neutral mediation.⁹⁵ In the end public opinion, and political conflict between the former belligerents first had to calm down, and no foreigner could force that.⁹⁶ For many scientists in these nations war propaganda for a long time proved to be a stronger rhetorical instrument than the internationalist ideal of neutral mediators, as ‘time is the only healer in these matters’.⁹⁷

Still, in many ways contact was resumed through correspondence, exchange of publications and foreign visits before the official organs of science followed suit.⁹⁸ The ‘Cohen Festband’ of the Zeitschrift für Physikalische Chemie, for the 25 year professorship of this ‘inexhaustible advocate’ for friendly relations between the former belligerents, shows for example that the international situation in science was improving.⁹⁹ When scientific work necessitated comprehensive practical arrangements that included Germany, the formal pathways of IRC and IUPAC were sometimes circumvented.¹⁰⁰ Already in 1921 a committee on bibliographic classification invited Central Powers to their meetings and the 1927 opening of the International Bureau for Chemistry in Paris was attended by chemists from the Central Powers.¹⁰¹ Also historical festivities, like the Benjamin Franklin and Marcellin Berthelot centenaries, were seized to reunite chemists.¹⁰² Haber’s attendance at both, and Nernst’s at the latter show that the negative associations attached to them had subsided, so that the splendour of their Nobel Prizes remained.

The complete restoration of international relations was brought a step closer in 1928 in The Hague when Cohen and Kruyt invited German, Austrian and Russian chemists to visit the IUPAC meeting as guests.¹⁰³ That still not every German, or even everyone who had attended the Utrecht Reunion, was open to the idea of entering the IUPAC, is evident from Wieland’s reaction who judged the meeting in Holland to be a ‘summery trip to Golgotha’.¹⁰⁴ As he was not the only one with objections, the Verband deutscher Chemischer Vereine (Association of German Chemical Societies) urged the Dutch Chemical Council to organize another meeting with prominent colleagues from all nations to take away these last difficulties. This meeting would take place thirty years after the first The Hague peace convention that had forbidden the use of poison in combat, precisely an important cause for the severe breach in the international chemical community after the First World War. The atmosphere in the Palace Hotel on these two days in late June 1929 was tense, and chairman Kruyt had to tactically adjourn the meeting several times to enable people to hold private discussions in the hallways. Characteristic for the resumption of internationalism in the 1920s, the decisive contact proceeded informally outside the conference room. Eventually representatives of the Danish, English, French, Italian, Dutch and American chemical councils, in the presence of many members of the German association, distilled the ‘protocol of Scheveningen’, in which they agreed to argue for three changes within the Union: to approve the new IRC statutes, that allowed Central Powers to join, to reconsider Paris as the Union’s secretarial centre and to establish a name change into Union Internationale de Chimie, to wash away the nasty taste that the old name still carried.¹⁰⁵ It was after the continuous effort, starting in 1921 in Utrecht and ending in 1929 in Scheveningen, that the practical and emotional objections over German inclusion in the international community of chemists were finally relieved. It was the two symbolic gestures of the protocol that made it possible for German chemists to join the Union, in 1930, with their heads held high. As science remained divided much longer than politics, the ideal of science as moral leader had lost its power. This did not restrain chemists to work around the official routes, nor did it discourage the Utrecht chemists to keep pressing for an inclusive international chemical community. In the end, the community of chemists was the only one that achieved full recovery of international cooperation in a formal scientific organization.¹⁰⁶ That the two chemists from Utrecht, Cohen and Kruyt, played a central role in this process, in many ways and with varying success, cannot be denied.

A nucleus for crystallization?

In this concluding section the impact of the Reunion of 1922 and the empathic capacities of its two main organizers, Cohen and Kruyt, are evaluated. Although the two Dutch chemists first encountered both opposition and praise, eventually they constructed an image of moral righteousness for their internationalist endeavour.¹⁰⁷ Where Cohen was elected IUPAC vice-president in 1923 ‘even though he was primarily held responsible for the organization of the Reunion’, his successive election to president in 1925 was the confirmation that opposition against his internationalist ideas had begun to subside: ‘history has proven us right’.¹⁰⁸ The nuanced tone disappeared completely in later accounts. Before any real success, Kruyt deemed their impact as ‘enormous’ and Cohen was described as a ‘born mediator’.¹⁰⁹ When Kruyt wrote an obituary notice for Cohen, who fell victim to the terror of the Nazi regime in 1944, he slightly rewrote history: ‘so great was the effect of this successful attempt [the Reunion]’ that Cohen was voted chairman of the IUPAC.¹¹⁰ And Cohen’s explicit request to the Chemiker Zeitung to display his election as a consequence of the Reunion, makes one cautious about taking these (auto)biographical accounts as criteria for the impact of the Reunion.¹¹¹

Neither does it mean that the Dutch mediators were not deeply aware of the complex feelings and thoughts in the other nations. Although the ‘nineteenth century positivist’ Cohen and the Christian-socialist Kruyt indeed never abandoned their internationalist ideal of ‘as complete a cooperation of all nations in science’, they always located this, albeit euphemistically, in the ‘difficult times’.¹¹² Kruyt was well aware of the dark shadow ‘the problems of the Ruhr area’ cast over the 1923 IUPAC meeting in Cambridge, and advised his fellow Dutch delegates in the ‘Wetenschappelijke Internationale Samenwerkingscommissie’ (WIS, Commission for Scientific International Cooperation) of the KNAW not to restrict themselves to their own perspective: ‘reality is simply not the way the Neutrals would wish it to be’.¹¹³ It would be a serious misnomer to characterize either Kruyt or Cohen as a naive pacifist. Cohen was an experimenter through and through, so that he did not blindly cling on to ideas. This ‘animal disputax’ was open to different viewpoints and put a lot of trust in his experiences.¹¹⁴ From his main scientific work on allotropes, Cohen learned that the transition temperature of two forms of a material depends on the previous history of this material: an insight he seemed to have endorsed actively in his international undertakings.¹¹⁵ It is no coincidence that Schenck turned to him as the expert on the ‘mood abroad’.¹¹⁶ As scientists on both sides of the Rhine had little idea of what was going on in the minds of their former enemies, the neutral mediators had a role to play in informing both French and Germans about the situation abroad. After the failed 1925 IRC meeting, Kruyt and Went visited Haber in Berlin and Cohen wrote ‘a tactful treatment’ in the Chemiker Zeitung to provide insight in the international situation.¹¹⁷

Ultimately, the Utrecht Reunion is best understood as an experiment, to see whether a positive contribution to the relations in the international chemical community was already possible by a focus on personal ties and scientific ideas. To obtain results, they attempted to exclude political factors from their experimental set-up. This conscious choice was no neutral naiveté. Noyes, inspired by the Reunion, explicitly tried to attend to the war-inflicted political problems that separated French, Belgian and German chemists. A glance at the politicized trouble he got himself into substantiates the Dutch choice to try to forget the loaded political present for three happy June days.¹¹⁸ In the following years this first experiment became part of a larger research program, in which the organizers of the Utrecht Reunion were fully aware of, and worked on, the difficulties that had to be overcome on both sides of the Rhine.

The First World War had a disruptive impact on Europe and as the ‘chemists’ war’ in particular on the international community of chemistry. Interchange of ideas and persons was abolished for almost a decade, the work of many chemists in Europe was obstructed by deplorable practical circumstances, and the pre-war dominance of German chemistry was gradually replaced by American authority. Ultimately it was science as a whole that failed to take the lead in overcoming this war-inflicted chaos. Where the formal organization of chemistry followed international politics, only turning truly international in 1930, practical necessity and personal contact had already brought chemists progressively closer together. For the Dutch chemical community the First World War was of course not as intrusive as it was in neighbouring nations. The combination of a relatively stable national context and an international complex situation proved beneficial for the Utrecht chemists to obtain the aspired position of neutral mediator, an ambition originating from before the war. A necessary condition for this position was Cohen’s extensive pre-war personal network. While the negativity of the Dutch political neutrality was still apparent at the constitution of the ‘International Research Council’, in the following years mediating activities on both sides slowly transformed the perception of Dutch neutrality into a positive, cultural trait, even though the fiasco of 1925 and the following counter-boycott created a crisis of confidence in the meddling of neutrals.¹¹⁹ The Reunion, and the subsequent subtle efforts of Cohen and Kruyt in the IUPAC provided them credibility and a respected international reputation. Although we do not have to follow the ecstatic praise of Kruyt the biographer, and political developments preceded scientific reconciliation, it is unfair to characterize, like Somsen, the two Utrecht chemists as lacking both impact and empathy. Also the criticism of Schroeder-Gudehus is nuanced, as it is without chauvinism or scientism that the material presented here indicates that the informal and formal activities in the chemical community eventually led to a unique success.

At the turn of the century chemists from all countries had mixed, but the war inflicted harsh separations. As the Utrecht ‘nucleus’ remained relatively unharmed in these years, Cohen and Kruyt were able to initiate the process of crystallization of the international chemical community. As ‘external’ factors strongly influenced this process, and many other ‘internal’ efforts took place, the Reunion and later undertakings of the Utrecht chemists are not a sufficient explanation of the eventual success. Even so, Cohen and Kruyt can be considered decisive nuclei in bringing about the only complete crystallization of an international community in a scientific discipline after the First World War. Before more could follow, the rise of the Nazi regime in Germany obstructed further collaboration.¹²⁰ The position of neutral mediator was always precarious but by cautious manoeuvring the Utrecht chemists had managed it. After the Second World War Otto Hahn wrote Kruyt, who he had met for the first time at the 1922 Reunion, that ‘the gratitude for the effort, to reconnect Germany with foreign countries, was the death of Cohen in Auschwitz’.¹²¹