Van der Waals, Zeeman and Daily Life in the Second Golden Age of Dutch Science1

The Dutch J.D. van der Waals (1837-1923) and his 25 years younger colleague P. Zeeman (1865-1943) were very different personalities and scientists. The mathematical physicist Van der Waals was a rigid man with an overwhelming personality, Zeeman a benevolent, friendly experimentalist. Nevertheless, they had two things in common: they were both very successful researchers and they were both professors at the University of . This article focuses on the relationship between the scientific achievements of Van der Waals and Zeeman and their position in Amsterdam. The question to be answered is to what extent the circumstances in which Van der Waals and Zeeman did their work at the determined their scientific successes. It is a question with a broader meaning. Van der Waals and Zeeman were exponents of a remarkable generation of Dutch scientists. After many decades of very poor achievements, Dutch science became suddenly very successful in the last quarter of the nineteenth century and the first decades of the twentieth. Zeeman and Van der Waals were typical for this period, for which the term Second Golden Age has been coined. In addition, the circumstances for practicing at the University of Amsterdam can be considered more or less representative for the Dutch sciences at the universities. So, this study of the daily circumstances in which Van der Waals and Zeeman conducted their research gives more insight in the circumstances in which the most successful era of Dutch science could unfold. This study begins with introducing Van der Waals and his main scientific achievements. In 1877 he became the first − and for a long time the only − physics professor of the newly created University of Amsterdam. The second section concentrates on the circumstances under which he had to do his work at the Amsterdam university. I will elaborate on the means Amsterdam physics had at its disposal and the contacts of

1 This article is based on: A.J.P. Maas, Atomisme en Individualisme. De Amsterdamse Natuurkunde tussen 1877 en 1940 (Hilversum 2001).

1 Van der Waals with the administrators of the university. The findings of this part will be confronted with the historiography on the Second Golden Age in section 3. Zeeman entered the scene in 1897 and was a colleague of Van der Waals until the latter s retirement in 1908. After introducing Zeeman (section 4), the focus will be on his daily experiences at the University of Amsterdam (sections 5). Section 6 gives a short account of Amsterdam physics in the 1920s and 1930s. This section uncovers a peculiar correlation of scientific achievements and material conditions in Amsterdam physics as well as in Dutch science in general. The outcome of this study − a new view on the Second Golden Age of Dutch science — will be put in a wider (international) context in the concluding section 7. It will be argued that the transformation of Dutch science, which is outlined in this article, is an example of the invention of science , a term that is used to denote the rise of modern science as a product of modern Western society.

Toiling to the top

Johannes Diderik van der Waals was born in the city of . As the son of a carpenter he had a long way to go before he could become a professor. Actually, the social rise of Van der Waals was almost unparalleled in the rigid Dutch nineteenth-century class society. He started as a pupil-teacher at a primary school. In his free time he studied for examinations which allowed him eventually to become a principal teacher (hoofdonderwijzer). A Spartan discipline helped Van der Waals to get through. I, I! I worked from four in the morning until eleven at night. When I was of your age, I toiled , he once snapped at a − in his view − lazy student.2 Principal teacher was not the end of the road. Van der Waals began taking courses in mathematics, physics and astronomy at the University of Leiden to become in 1865 a physics teacher at a HBS, a type of school for secondary education (see section 4 on this type of education) . His academic study in the meanwhile led to a doctoral thesis in 1873, with the title: Over de continu teit van den gas- en vloeistoftoestand (On the Continuity of the Gaseous and Liquid States). Although written in the Dutch language, this work

2 received international attention, which was remarkable in a time when Dutch scientists were not at the front of the important scientific developments. The famous Scottish physicist J.C. Maxwell called it an exceedingly ingenious thesis . Two important guiding principles underlying Van der Waals physics were already eminently present in his thesis: the idea of continuity and the presence of (realistic) molecules at the basis of thermodynamical processes. The core of the thesis lies in the equation of state Van der Waals developed:

 a   P + ()Vb−= RT,  V 2  where P is the pressure, V the volume, R the gas constant, T the temperature; a and b are constants which account respectively for the intermolecular attraction and the volumes that the molecules themselves occupy. This equation meant an enormous improvement of the well-known equation of state of the ideal gas: PV=RT, which is only appropriate for dilute gases and within small temperature-margins. Van der Waals equation also applies for higher densities and even for the transition to the liquid state. Moreover, Van der Waals equation describes the existence of the critical point, which was at that time only known as a curious phenomenon that showed up in some thermodynamical experiments. The deviation from the equation of state of the ideal gas — which assumes the gas- molecules to be point-particles and to exert no forces on each other — lies in the constants a and b. In introducing the constant a Van der Waals was led by the insight that the same forces that hold the molecules of a liquid together, in principle also have to be present in the gaseous state. The introduction of the constant b meant simply that the molecules were given the only real characteristic of substance: occupying space , like Van der Waals stated in his thesis. So, by introducing correction terms based on the simple notion that a molecule possesses extension and exerts an attractive force on other molecules,

2 On Van der Waals: A.Ya. Kipnis, B.E. Yavelov, J.S. Rowlinson, Van der Waals and Molecular Science (Oxford 1996).

3 Van der Waals greatly extended the domain for the equation of state. It gave a realistic description of the thermodynamical behaviour of the fluid states. 3 In retrospect Continuity contains Van der Waals most famous achievements. However, as an Amsterdam professor he conducted important researches in the field of molecular science and thermodynamics as well. The first of these was the law of corresponding states of 1880. In his dissertation, Van der Waals had already shown that the constants a and b, could be expressed in terms of the critical values of the substance. By introducing reduced variables :

P V T l = , n = and m = , P c V c T c

it became possible to rewrite the equation of state as:

− ()ln+−=33182 () n m,

The importance of this equation is that it is valid for all substances, since with the elimination of the constants a and b it contains no parameters characteristic of individual substances. Thermodynamic properties of all substances are equal when they are in corresponding states , when — in other words — the three reduced variables have equal values for the different substances. The law of corresponding states made it possible to calculate thermodynamical properties that could not be measured experimentally, like the critical values of the so-called permanent gasses . This proved to be of great use for cryogenic experimentalists like Van der Waals colleague and friend, the Leiden professor H. Kamerlingh Onnes. When it had been calculated that the critical temperature of hydrogen had to be about five Kelvin (and not lower, as was thought before the law of corresponding states was used), Onnes decided to dedicate al his means and efforts to the liquefaction of hydrogen, which succeeded in 1908.4

3 Ibidem, 28-67. 4 Ibidem, 102-106.

4 From the historical point of view Van der Waals theory on the thermodynamics of mixtures − published in 1890 − is of special interest, because it gave rise to a true Dutch school of thermodynamics . Van der Waals starting point was his equation of state which he restated for mixtures, the constants a en b depending on the a and b of the single substances and on the proportions of both substances in the mixture. To find the equilibrium-conditions of the mixture Van der Waals used the then not very well-known work of J.W. Gibbs. In his geometrical approach, the conditions for equilibrium were determined with the help of so-called ψ-surfaces. These curved isotherm planes of (Helmoltz) free energy as function of the arguments volume V and mole fraction x provide information of the states of (the components of) the mixture at a given temperature (figure 1). Because the dependency of ψ on V and x is known from the equation of state, it is possible to construct the ψ-surface.5 After the theory of mixtures had been made public a whole generation of Dutch , chemists and even mathematicians began to construct, explore and apply ψ- surfaces. In the first and second decades of the twentieth century, the Dutch proudly spoke of their Dutch school. In the nationalistic era before the Great War, the theory of mixtures was often considered to be Van der Waals s most important contribution to science.

5 Ibidem, 106-116.

5 Figure 1. Example of a ψ-surface. In the convex parts of the surface the system is stable, the concave parts denote unstable regions. By constructing a tangential plane and rolling it over the surface, the stable situations on the surface can be found. The mixture is in equilibrium when the plane touches the surface at two points, i.e. when there is a double tangential point. In the model the strings indicate these situations. The tangential points represent the coexisting phases of the mixture.

Source: J. de Boer, Van der Waals in his time and the present revival. Opening address , Physica 73 (1974) 11

Auxiliary science

It is doubtful whether the Dutch academic community of the 1870s recognised the great value of Van der Waals thesis. Continuity, which would later be the main reason for his , was not even awarded cum laude. Nevertheless, Van der Waals star was rising. His impressive personality and his gifts as a teacher will have contributed to this. In the spring of 1877 he became director of the HBS of The Hague and later that year he was offered a chair at the University of Amsterdam. Until 1877 Amsterdam had been a capital without a university. It only possessed an Athenaeum Illustre, an old-fashioned form of higher education. The only part of this school that flourished during the nineteenth century was the medical faculty, and the administration of Amsterdam in the 1860s en 1870s had to decide upon whether to transform the Athenaeum into a medical school altogether, or to upgrade it into a university. It opted for the latter possibility and with the law on higher education that passed the Dutch parliament in 1876 Amsterdam was permitted to do so. The University of Amsterdam, which opened its doors in 1877, was the fourth university in The Netherlands. The already existing ones were those of Leiden, Groningen en Utrecht. These latter were, however, state universities. The University of Amsterdam became a municipal university, which was both ruled and financed by the municipal administration of Amsterdam. As a fully-fledged university it needed a faculty of science. Within that faculty a chair was created for physics, and after J. Bosscha had

6 turned down an offer, the task to put Amsterdam physics on the map fell to Van der Waals. The starting-conditions for Amsterdam physics were far from ideal. Van der Waals acquired two or three rooms in the university-building and an old-fashioned physical cabinet. Lack of space and instruments made the practical work of the students difficult.6 Moreover, Van der Waals faced by himself the task of conducting the practical work as well as all other courses in physics. The number of attendants at the various courses differed greatly. Quite busy was the elementary course experimenteele physica. This course was attended by both all first- and second- year students of the faculty of science as well as the students of the medical faculty who started their study with a preparatory course (propedeuse) in the faculty of science. Especially this last group caused the high number of attendants of the classes experimenteele physica. In the first years there were about eighty auditors. Lecturing four hours a week for such a crowd was a heavy burden for Van der Waals, who felt more at ease before a smaller audience. Moreover, the demonstration-experiments for the lessons, which at that time were considered of prominent pedagogic importance, took a lot of preparation. Finally, the time-consuming (oral) examinations contributed to the weight experimenteele physica put on Van der Waals shoulders. Completely different were the classes mathematische physica . These were meant for the few advanced students in physics, mathematics and chemistry. In the early years there were only four students attending these mathematical courses, which comprised three or four hours a week. The mathematische physica course treated electricity and magnetism and Van der Waals own molecular physics. Finally, Van der Waals also had to supervise experimental work of the advanced students and the Ph.D- researches that were conducted (on average two students in three years obtained their doctoral degree under the guidance of Van der Waals). The reason that Van der Waals had to spend almost all his time in his teaching- obligations was that the administrators of the university were only concerned with the educational part of physics. They had no eye for research. More precisely, they considered physics mainly as an auxiliary science for the medical faculty, which was both

6 Gedenksteen Van der Waals , Algemeen Handelsblad, July 13th 1908.

7 the cornerstone and the figure head of the university. How the administrators perceived physics is clearly shown by the discussion about the creation of a new laboratory of physics. From the start everybody agreed that physics needed a new laboratory.7 However, when in 1879 the Amsterdam town-counsel, which eventually decided about the budget for the university, discussed about a new laboratory of physics, not everybody was very enthusiastic to spend a lot of money on it. One counsellor thought that it should be possible to accommodate the laboratory that for the time will only be used intensively by the Professors, for their own study , in an existing building. One might think of building a new laboratory when the number of students would increase. 8 Another counsellor, Teixeira de Mattos, advocated constructing a new building. He called a physics laboratory one of the first necessities of the university :

Practicing sciences is indispensable; the education of good physicians would be impossible without it. Physics, as well as chemistry are the foundations to build on. Moreover, one should not lose sight of the fact that the scientific-medical department of our university is the source, from which most fruit will come in the future ( ) If one is willing to do justice to physics, then one should give her the required educational means.9

The alderman of Education Koning agreed with Teixeira de Mattos: Especially at this University, where medical science, which is so closely related to the sciences, constitutes such an important part of the curriculum, such a laboratory is required .10 Because almost all counsellors shared the view of Koning and Teixeira de Mattos, it was decided that a new building would be erected for Van der Waals. The new laboratory came into use in 1882. Not surprisingly, the Natuurkundig Laboratorium, as it was called, was a typical teaching laboratory. Central in it was a large auditorium, which could contain 140 students. Smaller rooms for experimentation surrounded the auditorium. There were rooms for the study of , sound, heat, static electricity, galvanic electricity and magnetism. Thus every natural phenomenon got its

7 B en W (Burgomaster and Aldermen) to Curatoren (Trustees) 5-4-1879, G.A.A (Municipal Archives Amsterdam) 279, inv. nr. (inventory number) 8, nr. 56. 8 G.A.A. (note 7), Gemeenteblad 1879, 2th section, 573. 9 Ibidem, 574.

8 own chamber.11 Van der Waals from then had to take on, in addition to his teaching- activities, the management of a laboratory. His task was somewhat relieved by the appointment of an assistant. It is clear that the administrators only approved of a money-consuming expansion of physics if there were compelling reasons in connection with teaching-matters. In practice, this meant that Amsterdam physics grew in pace with the number of students. So, when in 1890 Van der Waals was permitted to appoint an extraordinary professor to assist him, the number of students had about doubled. The administrators of the university approved of Van der Waals request for an extra instructor whose presence, with an increasing number of students, is becoming more necessary day by day, for the sake of the education of coming scientists and medics . Van der Waals new colleague was the experimentalist W.H. Julius.12 Together they shared the teaching duties and the directorate of the laboratory. Thus, when Zeeman entered the stage, Amsterdam physics possessed a teaching laboratory with one ordinary and one extraordinary professor.

The Second Golden Age revised

Obviously, Van der Waals move from HBS to university enhanced his status and gave him a higher salary, but it did not improve his possibilities to conduct his research. Of course, as a mathematical physicist he did not have a lot of material needs, but his move to the University of Amsterdam did not provide him with what he required most: time. Considering moreover that he did (in retrospect) his most important research as HBS- teacher (his dissertation), it can be concluded that Van der Waals scientific achievements were not significantly related to his appointment. This is an important observation, because historians have traditionally connected the rise of the Second Golden Age with improved circumstances for scientific research at the universities.

10 Ibidem, 577. 11 For a further description: E.P. van Emmerik, Zoals het Nog is; het Laboratorium voor Van der Waals , Laboratorium Praktijk 2 (1988) 276-280. 12 Curatoren to B en W 24-6-1890, G.A.A. 279, inv. nr. 88, nr. 2476 (note 7); Van der Waals to B en W 27- 2-1890, G.A.A. 5191, 1890-523.

9 Van der Waals thesis was as it were the starting shot for the Second Golden Age. From the mid-1870s a generation of pioneers emerged, who were responsible for starting this period of exceptional scientific performance. Besides Van der Waals, the most important scientists were the mathematical physicist H.A. Lorentz, the experimental physicist H. Kamerlingh Onnes, the chemist J.H. van t Hoff, the botanist Hugo de Vries and the astronomer J.C. Kapteyn.13 In historiography, the causes of the Second Golden Age have traditionally been largely attributed to the direct and indirect results of educational reforms. Especially the law on secondary education of 1863 and the law on higher education of 1876 (the same one that brought the University of Amsterdam to life) have — in this view− greatly improved the possibilities for scientific study and research at the universities and have shaped up the research ethos in the scientists.14 The most important provision of the law of 1863 was the creation of the hoogere burger school (HBS), a type of school that, after the example of the German Realschule, was meant to prepare pupils for positions in trade and manufacturing. In contrast to the traditional preparatory school for the university, the gymnasium, the HBS paid a lot of attention to the sciences. In fact, the HBS proved to be a very adequate preparatory school for future scientists. Although HBS-students had no (direct) access to the university, many of them managed to matriculate. Besides, the HBS-schools — the number of which increased rapidly — created an expansion of the labour market for graduated scientists due to increased demand for teachers (in the sciences). In short, thanks to the HBS more and better-prepared students started a study at the faculty of science.

13 Al these men have an entry in: C.C. Gillespie e.a. (ed.), Dictionary of Scientific Biography (New York 1970-1990) 18 Vols.; see also: B. Willink, De Tweede Gouden Eeuw. Nederland en de Nobelprijzen voor Natuurwetenschappen (Amsterdam 1998) 65-128; K. van Berkel, The Legacy of Stevin. A Chronological Narrative , in: K. van Berkel, A. van Helden, L. Palm, A History of Science in the Netherlands. Survey, Themes, Reference (Leiden, Boston, K ln 1999) 140-163; A.J.P. Maas, Tachtigers in We wetenschap. Een Nieuwe Kijk op het Ontstaan van de Tweede Gouden Eeuw in de Nederlandse Natuurwetenschap , Tijdschrift voor Geschiedenis 3 (2001) 354-376. 14 This view is developed in: B. Willink, Burgerlijk Sci ntisme en Wetenschappelijk Toponderzoek. Sociale Grondslagen van Nationale Bloeiperioden in de Negentiende-Eeuwse B tawetenschappen (Rotterdam 1988); B. Willink, Origins of the Second Golden Age of Dutch Science after 1860: Intended and Unintended Consequences of Educational Reform , Social Studies of Science 21 (1991) 503-526; Willink, (note 13), 24-62; Van Berkel, Citaten uit het Boek der Natuur. Opstellen over Nederlandse Wetenschapsgeschiedenis (Amsterdam 1998) 149-187; Van Berkel, Legacy (note 13), 123-133.

10 An improved preparatory training alone is, however, not sufficient to give rise to a complete Second Golden Age.15 In addition to the foundation of the HBS historians have been pointing at the law on higher education of 1876. This law is said to have established research at the university, after the German example. Before 1876 the university was seen mainly as an institute for education. The administrators did not feel obliged to provide research facilities. Professors were teachers who could conduct their research as they wished, but there was no compulsion to do so. After the law went into effect, many new laboratories and chairs in the sciences were created to enable the university to carry out its new research task. According to one historian in the years after 1876 the university was even a scientific paradise . 16 Recently some authors have questioned whether the new law really put so much emphasis on research.17 In any case, it is clear that the situation of physics in Amsterdam does not corroborate the traditional interpretation. As previously noted, the administrators did not have an eye for research and were only willing to supply extra provisions for educational necessities. In fact, a close look at the Dutch faculties of science teaches that the case of Amsterdam physics may be representative and that the expansion after 1876 can be interpreted as aimed at improving teaching conditions instead of research. An increasing number of students and the introduction of new pedagogical methods (like practical work for the students for which educational laboratories were needed)18 required more teachers and more material means. The reason the expenditures were made precisely in the years after 1876 was that almost no new investments were done during the long time it took to prepare the new law on higher education. It took about three decades of discussions before the Dutch politicians could decide upon the new law. For the universities this meant a period of uncertainty and indecisiveness.19

15 In my view the role of the HBS is often exaggerated. See: Maas, Tachtigers (note 13), 357. 16 Willink, (note 14), 240. 17 J.C.M. Wachelder, Universiteit tussen Vorming en Opleiding. De Modernisering van de Nederlandse Universiteiten in de Negentiende Eeuw (Hilversum 1992); P. Baggen, Vorming door Wetenschap: Universitair Onderwijs in Nederland 1815-1960 (Delft 1998) 121; B. Theunissen, Nut en Nog eens Nut . Wetenschapsbeelden van Nederlandse Natuuronderzoekers, 1800-1900 (Hilversum 2000) 99-104; K. van Berkel (ed.), Het Oude Instituut en de Nieuwe Akademie. Overheid en Wetenschapsbeoefening Omtrent het Midden van de Negentiende Eeuw (Amsterdam 2000) 39-63. 18 Baggen, (note 17), 80-86. 19 J. Huizinga, De Geschiedenis der Universiteit Gedurende de Derde Eeuw van haar Bestaan, 1814-1914 , in: Verzamelde werken VIII (Haarlem 1951) 36-339; Willink, (note 13), 37-41; Maas, (note 1), 17-19.

11 Yet the only large-scale research effort that started to be built before 1900 was the famous cryogenic laboratory of Kamerlingh Onnes in Leiden. How Kamerlingh Onnes, known as a skilled organizer, managed to build up his laboratory has never been thoroughly investigated. Other germs of the later research-university were created without advanced research equipment and were fully the initiatives of the science professors themselves. Van t Hoff, for example, brought a research school of physical chemistry to life, which mainly meant that he deliberately included his students in his research-program.20 The interpretation that the law of 1876 established research at the universities seems to be a misinterpretation. This becomes even more clear when the text of the law is taken into consideration. It only speaks about teaching. Nowhere research is mentioned as a task neither for the university nor for its employees.21 Similar to Van der Waals the pioneers of the Second Golden Age often did some of their most important research before 1876 and before they were appointed at the university. Van t Hoff wrote his famous booklet, in which he proposed a spatial, three- dimensional structure for molecules in 1874, when he was a teacher at the Veterinary School in Utrecht. The physicist Lorentz laid the foundations of his theory when he was HBS teacher, likewise the botanist Hugo de Vries began his pioneering physiological study of plant cells while he was a teacher at a gymnasium. Their new position did not mean a discontinuity in their research. Like Van der Waals the other pioneers too had heavy teaching loads as professors (with the exception of Kapteyn and Kamerlingh Onnes) and in general worked under primitive conditions. In short, the Second Golden Age cannot be attributed to changing conditions at the university or a changed perception of science in society. Apart from the plain coincidence of a simultaneous appearance of several remarkably gifted scientists, for the causes of the Second Golden Age, one must look at the scientists themselves who changed their attitude towards practicing science.

20 H.A.M. Snelders, J.H. van t Hoff s Research School in Amsterdam (1877-1895) , Janus. Revue Internationale de l Histoire des Sciences, de la M dicine, de la Pharmacie et de la Technique 71 (1984) 1- 30. 21 Maas, Tachtigers (note 13), 356; Handelingen Staten Generaal, bijlagen zitting 1874-1875, nr. 30.1, Memorie van Beantwoording , 7-8, and nr. 30.2, Gewijzigd Ontwerp van de Wet , 14.

12 During the first half of the nineteenth century Dutch science professors did not identify themselves with their scientific discipline. They presented themselves first of all as a professor , not as mathematician or physicist . 22 This self-image was in line with their scientific activities, which were by no means limited to specialised ivory-tower research. Scientific activities were diverse and largely aimed at a general audience. A scientist wrote textbooks and popularisations, conducted demonstration-experiments in front of general audiences or his students and placed his immense wisdom in the service of society by becoming member of advisory counsels and the boards of charitable institutions. All these varying activities fell under the denominator of practising science . Although research meant to provide new insights in the working of was not absent, it was not the (only) core-business of the university scientist. And the (original) research that was done was often descriptive and devoid of the sophisticated use of theory. The then well-known nineteenth century physicist P.L. Rijke was occupied with observing electric sparks, magnetic phenomena, heat and sound and was an exemplary academic scientist of that time. One of his experiments was about the sound that originates in a tube which is open on both sides, when pieces of wire gauze are jammed in it, which are heated with a flame or an electric spark . Rather than as a researcher he had a reputation as a teacher. He was, more particularly, famous for his well-prepared demonstration experiments.23 While Rijke was amazing his audience with his experiments, at the University of Utrecht some innovating professors started to change the scientific practice in The Netherlands. Historians have sometimes considered the activities of these scientists − Chr.H.D. Buys Ballot, P. Harting, G.J. Mulder and F.C. Donders − as a kind of prelude to the Second Golden Age. Although these people certainly were remarkable intellectuals, only Donders made important achievements as a researcher. Donders, however, was a physician. The other Utrecht scientists did praiseworthy work by organizing Dutch meteorology to improve weather forecasts (Buys Ballot), by introducing the microscope in (organic) science education (Harting) and by introducing the educational laboratory at

22 F. van Lunteren, Van Meten tot Weten . De Opkomst der Experimentele Fysica aan de Nederlandse Universiteiten in de Negentiende Eeuw , Gewina 18 (1995) 102-138.

13 the universities (Mulder).24 Yet, their activities and attitudes towards science do not justify considering them as the precursors of the Second Golden Age (Mulder and Harting were even outspoken adversaries of specialized research).25 The mentality of the new generation of scientists in the last decades of the nineteenth century was completely different. The scientist began — in an individualizing society26− to withdraw into the ivory tower and focused his attention increasingly on scientific research which was not directed to the surrounding society, but towards colleague scientists in a for laymen incomprehensible jargon. The world of Van der Waals consisted of research on molecular physics and thermodynamics, not of textbook writing. The price of this change was that somehow the scientist lost his prestigious position in Dutch society as an educator and provider of culture. Van der Waals and the other scientists of his generation worked in relative isolation until the end of the century. Their merits were not recognised in broad circles. On the other hand, the focus on specialised research is an important element in explaining the sudden outburst of important scientific results which has impressed later observers. The next question to be answered is how society began to appreciate the new position of science and its practitioners and — in relation to this — how research gained firm ground at the university. The answer can be found in the next episode of Amsterdam physics, when Zeeman became the leading physicist.

Brahman of science

The Second Golden Age did begin, but did not end with the pioneers . Though this generation contains the most famous names, younger scientists were able to keep the level of Dutch science remarkably high in the first decades of the twentieth century.27 Zeeman was one of these talented, younger scientists. Pieter Zeeman (1865-1943) was

23 H.A.M. Snelders, Rijke, Petrus Leonardus , in: J. Charit (ed.), Biografisch Woordenboek van Nederland III ( s-Gravenhage 1979-1994) 511-512; W. Otterspeer, De Wiekslag van hun Geest. De Leidse Universiteit in de Negentiende Eeuw ( s-Gravenhage, Haarlem 1992),121. 24 On these scientists: Theunissen, (note 17), 57-124. 25 Maas, (note 1), 16. 26 More about the changing orientation of the scientists in relation to the development of Dutch society in: Maas, Tachtigers (note 13), 369-375.

14 born in the small village of Zonnemaire, close to the sea. From his earliest youth he had shown interest in investigating nature. As a teenager he observed the northern for hours on the roof of a shed. His results were mentioned in Nature. It was an early demonstration of the concentration, accuracy and perseverance, which would characterise the experimentalist Zeeman. Because of his reservedness the mathematician L.E.J. Brouwer once called him a Brahman of science . 28 His parents allowed Pieter to give free rein to his research-inclinations. From studying a book: What will my son be?, his father − who was a protestant minister − concluded that Pieter was suited to become a surveyor. This observation was not at all too bad, but happily Pieter s parents permitted him to study, first at a HBS and later at the University of Leiden. There he learned physics under the expert guidance of Kamerlingh Onnes and Lorentz.29 The experimental physics of Leiden has acquired world fame thanks to the unparalleled cryogenic facilities Kamerlingh Onnes managed to develop. In 1908 helium was liquefied as the last of the so-called permanent gasses, and in 1911 superconductivity was observed for the first time.30 These startling results, however, obscure the fact that in the eighties and nineties of the nineteenth century there was also an experimental magneto-optic line of research, which was at that time of at least equal weight. J.Z. Buchwald even speaks of a Leiden School of magneto-optics . 31 The aim of the experimental activities of this school was to give support to the theoretical work of Lorentz on the relation and interaction of matter and the electromagnetic field, eventually resulting in Lorentz electron theory . The experimental work concentrated largely on the Kerr effect, the change of the polarisation of light when it is reflected in a mirror of

27 Willink, (note 13), 129-177; Van Berkel, Legacy (note 13), 170-209. 28 Huldiging van Prof. Dr. Zeeman , Nieuwe Rotterdamsche Courant 26-5-1935. 29 Biographical information on Zeeman: W. de Groot, C.J. Bakker, Pieter Zeeman , in: T.P. Sevensma (ed.), Nederlandse Helden der Wetenschap. Levensschetsen van Negen Nobelprijswinnaars. Hoogtepunten van Wetenschappelijken Arbeid in Nederland (Amsterdam 1946) 95-123; J. B. Spencer, Zeeman, Pieter , in: Gillespie, (note 13) XIV, 597-599 ; A.J. Kox, Pieter Zeeman (1865-1943). Meester van het Experiment , in: J.C.H. Blom e.a. (ed.), Een Brandpunt van Geleerdheid in de Hoofdstad. De Universiteit van Amsterdam in Vijftien Portretten (Hilversum, Amsterdam 1992) 213-228. 30 P.F. Dahl, Kamerlingh Onnes and the Discovery of Superconductivity: the Leyden Years, 1911-1914 , Historical Studies in the Physical Sciences 15 (1985) 1-37. 31 J.Z. Buchwald, From Maxwell to Microphysics. Aspects of Electromagnetic Theory in the Last Quarter of the Nineteenth Century (Chicago 1985) 200.

15 magnetised metal. 32 Zeeman too worked on the Kerr-effect. He was however inspired by Faraday to try another way of examining the influence of magnetism on matter and light. From an obituary of Faraday he learned that the great experimentalist had tried to influence the spectrum of a flame by a . Faraday had not noticed any change, but Zeeman thought he might succeed thanks to the better instruments of that time. Especially the Rowland s grating meant an important improvement in analysis spectra. Zeeman heated a preparation of sodium with a flame (figure 2). The emitted light was analysed by a Rowland s grating so that the spectral lines became visible. Then the preparation was subjected to a magnetic field. Initially, Zeeman failed to notice any changes but in September 1896 he observed a broadening of spectral lines (i.c. the 33 characteristic D1 and D2 -lines of sodium) when the magnetic field was activated.

32 H.A. Lorentz e.a., Optische en Magneto-Optische Onderzoekingen , in: H. Haga e.a., Het Natuurkundig Laboratorium der Rijks-Universiteit te Leiden in de Jaren 1882-1904 (Leiden 1904) 179-258; H. Haga, E. van Everdingen, Het Natuurkundig Laboratorium der Rijks-Universiteit te Leiden , in: Haga e.a., (note 32), 20-27. 33 A.J. Kox, W.P. Troelstra, Uit het Zeeman-Archief: de Ontdekking van het Zeeman-Effect , Gewina 19 (1996) 155-157; P. Zeeman, Researches in Magneto-Optics. With Special Reference to the Magnetic Resolution of Spectrum Lines (London 1913) 24-30.

16 Figure 2. Zeeman s set-up Source: J. Bank, M. van Buuren, 1900. Hoogtij van Burgerlijke Cultuur (Den Haag 2000) 281

Zeeman s discovery was of major importance to Lorentz. The fact that matter interacted with the magnetic field proved that it contained electrically charged particles (subjected to a Lorentz force). It was a fundamental assumption of the electron theory that contain one charged particle (called ion by Lorentz), bound by harmonic forces to a centre. When an ion is excited (for example by heating) the harmonic force causes it to move with a certain frequency, resulting in the emission of a of the same frequency. What Zeeman observed as a broadening, had to be, according to the electron theory, in fact a symmetrical splitting of the original spectral line into three components. Due to the Lorentz force on the excited ion, it will start to move with two other frequencies next to the original one.34 Zeeman himself has elaborated this in a most illustrative way. The drawing (figure 3) shows a harmonic of a charged particle resolved into three separate movements: one rectilinear motion in the direction of the r magnetic field H and two circular in opposite directions perpendicular to it. The r rectilinear motion in the direction of H will experience no influence from the magnetic field and keeps moving with the original frequency. In contrast, the circling motions will, since they are perpendicular to the magnetic field, undergo a Lorentz force which, depending on their directions, enhances or reduces their frequency. The Lorentz forces are denoted by the double arrows in the picture. Thus, three frequencies result: the original one with frequency f, one that is ∆f smaller and one that is ∆f larger.35

34 A more elaborate (mathematical) treatment on Lorentz explanation of the : A.J. Kox, The Discovery of the Electron: II. The Zeeman Effect , European Journal of Physics 18 (1997) 139-144; 35 Zeeman, (note 33), 30-42.

17 Figure 3. Zeeman effect of a harmonic moving particle resolved in three motions Source: De Groot, Bakker, Zeeman (note 28), 103

From the magnitude of the broadening, which Zeeman was able to measure, Lorentz could calculate the ratio of the charge and mass of the ion . Zeeman and Lorentz could also show that the ion must be negatively charged. These findings in combination with other experiments (especially on cathode rays) paved the way for the identification of the electron, the name that eventually was given to Lorentz ion. Despite his great discovery, which was soon called the Zeeman effect , Zeeman could not secure tenure in Leiden. Fortunately, Van der Waals was able to manage a lectureship in Amsterdam for Zeeman as successor of Julius who left for Utrecht where he was offered a full professorship. Zeeman s first result in Amsterdam was to make the actual splitting in three visible, helped by a stronger Rowland s grating (which he had borrowed from a colleague in Groningen). In the meantime, other physicists had begun investigating the intriguing Zeeman effect as well and soon they found cases of Zeeman splittings with more than three components and with other, sometimes asymmetrical patterns. It was not possible to explain these anomalous Zeeman splittings within the framework of the electron theory. Lorentz tried it in vain by introducing more ions into his model. Zeeman on the other hand started a cooperation with the German physicist W. Voigt, who had chosen another approach. Starting from the inverse Zeeman effect, where not the emission spectrum, but the absorption spectrum of a preparation is used, Voigt considered the preparation as a medium causing optical effects. Thus it became possible to describe the Zeeman effect (i.c. the splitting of the absorption line) in

18 a phenomenological manner, ignoring the mechanism on atomic scale in which Lorentz got stuck. This pragmatic approach too failed to explain the anomalous Zeeman effect (which could only be understood after the discovery of the electron spin in 1925), but the cooperation of Voigt and Zeeman yielded some interesting insights in the behaviour of magnetic splittings.36 On the other hand, the Zeeman effect could be useful as an analytical tool. Zeeman was able to demonstrate that lithium belonged to the alkali group in the periodic table, when he found that the splitting patterns of lithium resembled those of the alkalis. Another application lay in the field of solar physics. The astronomer G.A. Hale had observed spectral lines from light coming from sunspots showing magnetic splitting. Together with Zeeman he could establish that the heavy cyclones that exist in sunspots must consist of charged particles, causing a magnetic field of about four Tesla.37 A practical reason for Zeeman to look for cooperation with Voigt was that the latter s approach did not demand too much experimental accuracy. The Natuurkundig Laboratorium suffered severely from vibrations due to outside traffic, disqualifying the lab as a research institute. When in the 1910s a lack of space became also pressing, Zeeman decided to turn away from magneto-optical research altogether. In 1914 a new laboratory was promised to him by the city s administration, which, however, only came into use in 1923. In the meantime, Zeeman performed experiments on the special theory of relativity. By measuring the speed of light in moving transparent bodies (viz. water, glass and quartz) he was able to show the existence of a relativistic effect predicted by Einstein and Lorentz. Einstein was delighted that Zeeman had filled eine bisher unangenehm f hlbare L cke with these experiments. Another series of experiments (which turned out to be less innovating) aimed at establishing the equivalence of so- called gravitational and inertial mass, which constitutes an important assumption of the general theory of relativity.38

36 Zeeman, (note 33), 43-92; P.F.A. Klinkenberg, Zeeman s Great Discovery , in: H.B. van Linden van den Heuvell, J.T.M. Walraven, M.W. Reynolds (red.), . Fifteenth International Conference on Atomic Physics, Zeeman-Effect Centenary (Singapore e.a. 1996) 224-230. 37 Zeeman (note 33), 125-135; Klinkenberg, Discovery (note 36), 229-130; De Groot, Bakker, Zeeman (note 29), 111-112. 38 A.J. Kox, Pieter Zeeman s Experiments on the Equality of Inertial and Gravitational Mass , in: J. Earman, M. Janssen, J.D. Norton (red.), The Attraction of Gravitation. New Studies in the History of General Relativity (Boston, Basel, Berlin 1993) 173-181.

19 Research institutionalised

Zeeman was not the only successor of Julius. With him R. Sissingh (1858-1927) was appointed as extraordinary professor. Though somewhat older than Zeeman, Sissingh too was an exponent of the magneto-optical school in Leiden. Until the call from Amsterdam Sissingh had been teaching at the Polytechnical School in Delft.39 It is not clear why Van der Waals was permitted to appoint two replacements for Julius. The choice for Sissingh and Zeeman was nevertheless a happy one. Though he had obtained interesting results with magneto-optical experiments in Leiden, Sissingh gradually turned away from research to become a dedicated teacher. He, for instance, took over the courses experimenteele physica from Van der Waals. Until his death in 1927 he managed, to bring more structure in the physics education. One of his innovations was the introduction about 1920 of practical work for medical students. Zeeman on the other hand only gave one course of one hour a week (in optical crystallography) and could further dedicate himself fully to his research. Later he would also teach other subjects as well, like electron theory. When Zeeman entered the Natuurkundig Laboratorium he must have made a youthful impression. His later colleague Kohnstamm, who was a student then, witnessed Zeeman s arrival in Amsterdam:

So I was talking in the spring of 97 with Nolke, who was an assistant, when a very young, very slim, and, as it seemed to me, very timid fellow came in, whom I at first sight estimated a first or second years student ( ) when I heart Nolke s voice may I introduce you to our new lecturer? 40

If not timid, Zeeman was certainly withdrawn. Another student also remembered the moment Zeemans came in, showing him all over:

39 F. de Boer, In Memoriam. Prof. Dr. R. Sissingh , Amsterdamsche Studenten-Almanak voor het Jaar 1928 (Amsterdam 1929) 67-69; N.H. Kolkmeijer, Prof. Dr. R. Sissingh 1897-1922 , Physica. Nederlandsch Tijdschrift voor Natuurkunde 2 (1922) 129-135; N.H. Kolkmeijer, ’Prof. Dr. R. Sissingh’, Physica. Nederlandsch Tijdschrift voor Natuurkunde 7 (1927) 257-258.

20 We did not notice too much of him; a room was equipped for him, where he, plunged into darkness, continued his experiments .41

Zeeman, nevertheless, knew what he wanted. From the beginning he aimed all his efforts at creating the best circumstances to perform his experiments. In practice, this meant that he systematically started to gather instruments for his magneto-optical researches and that he managed to improve his position. At the same time, he lobbied for realising his biggest ambition: to have his own laboratory.42 With these initiatives and by including his students into his magneto-optic research-program, he gave research firm ground in Amsterdam physics. Zeeman could be successful in his attempts, because they coincided with a change in the perception of science on the part of the administrators. From the first decade of the twentieth century they gradually discovered that Zeeman and Van der Waals had a huge reputation in international science. The two physicists were rewarded with a myriad of awards and honorary memberships43 and especially Zeeman received a number of prestigious offers from other universities. These marks of honour could not have escaped the attention of the administrators, who began to realise that scientists like Zeeman and Van der Waals contributed considerably to the (international) fame of the university. Thus, the importance of ivory tower-research was becoming evident for them as well as the necessity to provide means for scientific research. The new status of the scientists became visible with the retirement of Van der Waals in 1908. His proposed succession by his son J.D. van der Waals jr., who became professor of theoretical physics, provoked no discussion. Van der Waals sr., however, also tried to arrange for a new (extraordinary) chair in applied thermodynamics for his prot g Ph. Kohnstamm. The administrators went along with this extension of physics (after some discussion) because exactly in this field of thermodynamics the research of the withdrawing professor Van der Waals sr. has indicated a direction in physics and has

40 Ph. Kohnstamm, P. Zeeman , Physica. Nederlandsch Tijdschrift voor Natuurkunde 1 (1921) 225. 41 Bij Prof. Zeeman s Jubileum , Algemeen Handelsblad, 24-5-1935. 42 Some of these efforts can be found in Zeeman s diary: Rijksarchief in Noord-Holland (RANH), Papers of Pieter Zeeman (1865-1943), Physicist and Nobel Price Winner, c. 1877-1946, inv. nr. 314. 43 Those of Van der Waals: RANH, Papers of J.D. van der Waals, inv. nr. 5, 7, 61, 66; Zeeman s rewards: RANH, Pieter Zeeman Papers, inv. nr. 851-909.

21 created a specific, Amsterdam school of physicists .44 Thanks to Van der Waals reputation as researcher the staff of Amsterdam physics was expanded. Besides, he arranged a full professorate for Zeeman as he had done one year earlier for Sissingh. The retirement of Van der Waals had changed the constellation considerably. Now there were four professors of which one was fully dedicated to education and the others had a considerable amount of time and means to conduct research. The theorist Van der Waals jr. (1873-1971) was specialised in molecular and statistical physics.45 He lectured advanced students about electromagnetism and thermodynamics. Unfortunately, the younger Van der Waals lacked the ambition and perseverance to make great achievements in physics. He preferred to move in philosophic and literary circles and from about 1920 on he seems to have lost touch with current developments in theoretical physics altogether. This was the kiss of death for theoretical physics in Amsterdam, which did not recover until after the Second World War. The task of Kohnstamm (1875- 1951) was twofold.46 Firstly, he had to familiarise future physicists and chemists with Van der Waals body of thought, by teaching thermodynamics in the spirit of Van der Waals. Secondly, he was assigned to work out Van der Waals theories experimentally, particularly his theory of mixtures, which constituted the Dutch (or, from the municipality s point of view: Amsterdam ) school. Experiments were carried out to explore Ψ-surfaces of different mixtures and under variable circumstances. The construction of the necessary equipment was financed by the Van der Waals fund, which had been founded in 1898 by former students and admirers aimed at propagating Van der Waals work. However, like Van der Waals jr. Kohnstamm was more and more distracted by other activities. He had been interested in philosophy and politics from his student years and when he became occupied with pedagogy in the 1910s as well, he terminated his research-activities in thermodynamics. The experimental research in thermodynamics was taken over and further developed by A.M.J.F. Michels, who eventually would become Kohnstamm s successor.47

44 Curatoren aan Gemeenteraad (city council) 3-8-1908, G.A.A. 1020 (note 7), inv. nr. 196. 45 On Van der Waals jr.: S.R. de Groot, Waals jr., Johannes Diderik van der , in: J Charit , (note 23) I, 637-638. 46 On Kohnstamm: G.J. van der Poll, Kohnstamm, Philipp Abraham , in: Charit , (note 23) I, 306-310. 47 J.M.H Levelt Sengers, J.V. Sengers, Van der Waals Fund, Van der Waals Laboratory and Dutch High- Pressure Science , Physica A 156 (1989) 1-5.

22 It was Zeeman who made the greatest advances as a researcher, both from a scientific (as pointed out in the previous section) and an institutional point of view. Concerning the latter aspect, Zeeman was particularly successful in using the offers he got from other universities. The procedure was that with the offer in hand, Zeeman went to the burgomaster where he expressed some specific wishes to be granted in exchange for rejecting the offer. An offer to succeed Arrhenius in Stockholm yielded Zeeman a laboratory assistant and an accumulator battery, a call from Geneva raised his salary, an offer in 1909 to become director of the optical department of the Physikalisch-Technische Reichsanstalt put a new laboratory on the agenda, and so on.48 Besides these offers, Zeeman, notwithstanding that he rather stayed in the isolation of the laboratory, gradually became a well-known public figure, and someone who was listened at. Thus Zeeman could with increasing success turn to the administrators with requests and gradually managed to build up a marvellous research apparatus. Since about 1890 there had been no significant rise in the number of students visiting the physics laboratory. That a lack of space in the Natuurkundig Laboratorium was becoming pressing in the 1910s was because its tasks had changed considerably. In 1890 the lab had been a building for physics education, directed by one professor. Twenty years later there were four physics professors, and aside from the space needed for education there were two sections where Zeeman and Kohnstamm had built up their research instruments. The Natuurkundig Laboratorium had to accommodate education as well as research. The realisation of Zeeman s research lab Physica can be considered as the symbolic end point of the development that led to the institutionalisation of research. In 1917 the city administration agreed upon a proposal for what was euphemistically described as an extension of the Natuurkundig Laboratorium :

Built in a time when physics education still was charged to one professor, it cannot satisfy the needs of education anymore, ( ) since the state of science has made a division of labour among four professors necessary. Up to now, the professors, by sharing different locations, have been able to manage, but in the long run this will not do without damaging education and the practicing of science.

48 RANH, Pieter Zeeman Papers (note 42), inv. nr. 314; Kox, Meester (note 29), 222-224.

23 It is interesting to notice that there seemingly existed no discourse to formulate what was really the case. What was called an extension of the laboratory for (mainly) educational reasons was nothing less than the building of a costly research laboratory for Zeeman s optical research. The most remarkable thing about the whole issue is that not even one critical note was uttered within the city counsel. Where the 80,000 guilders for building the Natuurkundig Laboratorium had provoked lengthy discussions in 1879, now 440,000 guilders were spent without the slightest hesitation.49 Physics was no longer an auxiliary science. Physica, as the new lab was called, opened its doors in 1923. Everything symbolised that it was not an educational laboratory but a research institute. Physica was only accessible for advanced students (those who had passed their kandidaats-exam). In contrast to the Natuurkundig Laboratorium the auditorium was not placed in the centre, but somewhere in a corner at the second floor and was used mainly for meetings of small groups of physicists and advanced students. The experimental rooms were all clustered around a central hall. Two of the most important rooms were those dedicated to research of magnetic splitting. These had separately founded tables, of which the heaviest was 250,000 kg, meant to keep the grating arrangement free from disturbances. In order to provide the best possible circumstances to conduct research, temperature could be kept constant within 0.01 degrees, light traps guaranteed absolute darkness. The instruments, gathered piece by piece in twenty years, together formed a splendid optical research apparatus.50 The working places, the dark rooms, the library, the machine room, the workshop, and the glassworks are all of a perfect design, which even now still impresses experts visiting the laboratory , someone wrote in 1965. 51 And from the beginning such visitors came: Monsieur Zeeman, votre laboratoire est une perle! , one of them exclaimed.

49 G.A.A. (note 7), Gemeenteblad 1917, 1st section, 240-241. 50 Kox, Meester (note 29), 226; De Groot, Bakker, Zeeman (note 29), 113; P.F.A. Klinkenberg, Honderd Jaar natuurkunde aan de Plantage Muidergracht , in: R. Roegholt e.a., Wonen en Werken in de Plantage. De Geschiedenis van een Amsterdamse Buurt in Driehonderd Jaar (Amsterdam 1982) 125; P.F.A. Klinkenberg, Monsieur Zeeman. Votre Laboratoire est une Perle! , Stroom. Mededelingenblad Sub-Faculteit Natuur- en Sterrenkunde 1 (1986) nr. 4, 7-16. 51 W.J.A. Schouten, Pieter Zeeman. Natuurkundige (1865-1943) , AO-reeks, may 21st 1965, 13.

24 From the beginning the Amsterdam university had been for municipal administrators an institute to bring prestige to the city.52 When they started to realise that research could apparently contribute to the university s fame, they began considering it an important part of physics. The fact that research became an institutionalised part of Amsterdam physics was not a matter of expected technological and economic gain, nor of humboldtian or other intellectual and pedagogical ideals. People like Zeeman and Van der Waals contributed to the glamour of the city, and this made the investments in their occupations — whatever these were — worthwhile.

Diminishing returns in Amsterdam

The existence of a research lab and the presence of four professors to run Amsterdam physics created possibilities to conduct research which were previously unknown. These circumstances moreover created their own momentum. Once there were four professors, it was not easy to argue that this number should be diminished. And once Physica had been realised one could not refuse it the necessary operating funds. When Zeeman asked for his laboratory an annual amount of money, higher than the Natuurkundig Laboratorium received, the administration agreed upon it with the words: the wonderful equipment of the excellently led laboratory Physica , makes that the presently asked amount will also be allocated .53 Furthermore, the general public became familiar with the specialised researcher and his inaccessible activities . It yielded to the romantic lure of the lonely scientist in his laboratory. The important scientists of those days became celebrated people. In fact, there has probably never been a time in Dutch history when science and its practitioners were as popular as between the World Wars. All events of Amsterdam physics, like successions and the opening of Physica, were amply reported on in the papers. Under these circumstances the administrators granted the physicists more money than ever before, so that in the thirties Amsterdam physics could develop and sustain

52 P.J. Knegtmans, Tot Nut en Eer van de Stad , in: P.J. Knegtmans, A.J. Kox (ed.), Tot Nut en Eer van de Stad. Wetenschappelijk Onderzoek aan de Universiteit van Amsterdam (Amsterdam 2000) 7-11. 53 Curatoren aan B en W 14-2-1924, G.A.A. 5191 (note 7), 1925-3433.

25 three full research schools. The successor of Sissingh, J. Clay, founded a school for cosmic ray research, Michels developed the department of thermodynamics left by Kohnstamm into a world leading laboratory in high pressure physics, and finally there was, of course, Zeeman s Physica. All these groups were supported by municipal money (Michels moreover raised money through applied research for industrial companies) and by a large influx of students who were placed into the research groups.54 Paradoxically, once the institutionalisation of research had been established, Amsterdam physics was past its peak. Zeeman somehow lost his zest, Clay proved to be no extraordinary talented researcher. Michels became an authority in the field of experimental high-pressure physics, but never had a reputation like that of Van der Waals and Zeeman. It shows that material conditions do not fully determine scientific achievements. It is hard to establish to what extent the case of Amsterdam physics is representative for Dutch science in the first decades of the twentieth century. On the one hand, there was certainly a considerable expansion of the research facilities. On the other hand, it is not easy to make a clear assessment of the scientific achievements of this period. The level of Dutch physics and astronomy remained high with many scientists belonging to the top of their field, though this generation lacked leading figures like Van der Waals, Lorentz and Kamerlingh Onnes. Other disciplines, like chemistry and biology, were less successful. Examples of famous scientists in this period are the mathematician L.E.J. Brouwer, the astronomers A. Pannekoek and J.H. Oort and the physicist H.A. Kramers. Historians hesitate whether to put the end of the Second Golden Age in 1914 or to see it last until 1940. 55 It is all in all clear that the most remarkable scientists belonged to the generation, which initiated the Second Golden Age. So, in their levelling off, the achievements of the Amsterdam physicists reflect somehow the course of Dutch science in general.

54 On Michels: P.J. Knechtmans, Onderwijs, Wetenschap en Particulier Initiatief aan de Universiteit van Amsterdam, 1920-1950 , in: Knegtmans, Kox, (note 52), 83-90; On Clay: J.A. Prins, Clay (Claij), Jacob , in, Gillespie, (note 13) III, 312-313. 55 On this period: Willink, (note 13), 129-177; Van Berkel, Legacy (note 13), 170-209.

26 The Dutch invention of science

By using Amsterdam physics as a case study it has been shown in this article that Dutch science underwent a transition in the last quarter of the nineteenth century and the first decades of the twentieth. This transition contains similarities with what A. Cunningham and P. Williams have called the invention of science . 56 Against the traditional view in historiography, in which the birth of modern science is being found in the dramatic paradigmatic and methodological turns of the sixteenth and seventeenth centuries, they emphasise that the current scientific practice is a product of Western culture after the Age of Revolutions. Only with the transformation of society as a result of the French, the post- kantian intellectual and the industrial revolutions, the values were shaped in which science could take root. From about 1800 the term science got its actual meaning, the different scientific disciplines were either created (like biology and geology) or radically transformed (like physics), the laboratory was introduced (followed by research schools), and it became possible to have a career as an investigator of nature pure for itself. Moreover, science became a secularised practise, i.c. it ceased to be seen as an activity in service of religion. A lot of the changes Cunningham and Williams point at can be recognised in this study. It has been elaborated that the physics discipline in Amsterdam changed from an auxiliary science to a full academic discipline where research, aside from education, had become an intrinsic part and that as a corollary the research laboratory was introduced as well as the research professor, who could spend his time almost entirely as an investigator of nature pure for itself. I have tried to show — where possible — the representativeness of this case for Dutch science in general. Further, I have indicated that the values of the scientific practice as we know it today were largely lacking in the Dutch class society in the first half of the nineteenth century, were the scientific practice was directly aimed at (and in service of) society. One could indeed speak of the invention of Dutch science . Yet, it is notable that the timing of the Dutch invention of science seems to be rather late (Cunningham and Williams have placed the invention of science on the Continent in the period 1780-1848). In my view this Dutch delay must be attributed to

56 A. Cunningham, P. Williams, De-centring the Big Picture : The Origins of Modern Science and the Modern Origins of Science , British Journal for the History of Science 26 (1993) 407-432.

27 the inert character of the Netherlands, where a social transition from a class society to a more achievement oriented society, as well as the industrialisation, started only in the second half of the nineteenth century.57 What can be learned from this study is not if the provocative claim of Cunningham and Williams is justified, to prefer the Age of Revolution above the era of the traditional Scientific Revolution as the decisive period for (Western) science. This article gives, however, an example of how the nonetheless important developments after the demise of the ancien r gime in practice could occur. The changes began with a changing attitude of the scientist towards practising science, in which the research ethos became the central principle. Within their limitations, they started to use the institutions for their new purposes. Only after some decades the rest of society began appreciating the new course science had taken and decided it to be worthwhile to spend money on. The main reason for this appreciation seemed the (international) successes of the scientists, providing pride and prestige to the mother country or city. From then on scientific research could gain firm ground at the university and in society. This scheme is however largely based on just one example. The change of attitude of the scientists and the consequent appreciation of their ivory-tower research in society is something that needs to be further investigated, for the Netherlands as well as for other countries.

57 Maas, Tachtigers (note 13), 369-375.

28