The periodic table and chemical pedagogy

3From late eighteenth century onwards, textbook writers had argued over the best way to organise the “coherent pluralism of chemistry” in their publications.4 Their intended readership (mostly made up of medicine and pharmacy students) was mainly interested in the medical properties and technological uses of chemicals. Organising this information in a manageable, pedagogically sound order required a classification that dealt with tensions between the increasing number of compounds and the emerging principles concerning chemical properties and analogies. Several options were available, and the dramatic increase in the number of chemical elements and compounds fuelled controversies on “artificial” and “natural” classifications during the first half of the nineteenth century. By the 1850s, many chemistry textbooks presented “hybrid” arrangement, that is, a creative mixture of natural and artificial classifications.5

4Mid-nineteenth-century textbook writers were concerned with these pedagogical issues as they discussed and adopted different arrangements which were suitable for their intended audiences, sometimes introducing small or significant changes to previous classifications. One of these books was penned by Auguste Cahours (1813–1891) who not only adopted the hybrid classification but also included a full review of different chemical groups. The textbook was not “pedagogically handicapped”, neither was it arranged “in an almost dictionary-style fashion”, as it has been described.6 In fact, Cahours adopted the natural classification of metalloids in his first volume, so these elements were arranged in groups very similar to the current families of halogens, oxygen and nitrogen. The metals were arranged according to their reaction to oxygen by adapting Jacques Thenard’s classification and the improvements suggested by Victor-Henri Regnault in 1836. Two full chapters were devoted to the discussion of both natural and artificial classifications.7 The book went through several editions and Cahours updated the text with new data on new elements and compounds. He kept using the previous classifications because they were “convenient” and “useful” for teaching purposes, even if he acknowledged that Thenard-Regnault’s artificial classification of metals, “like any artificial classification”, should be regarded as “transitory and susceptible to constant change with the progress of science”.8

5Cahours’s textbooks were translated to different European languages, including Spanish (by the apothecary Ramón Ruiz Gómez [1804–1860]) and Russian (by Nikolai Pavlovich Ilyin [1832–1892] with the aid of Mendeleev in the second part).9 His approach to chemical classifications and textbook arrangements was shared by many textbook writers in the second half of the nineteenth century. They usually employed revised versions of Dumas’s natural classification of metalloids and Thenard-Regnault’s artificial classification of metals. Some of the resulting groups (particularly in the case of metalloids) were very similar to the natural families adopted by Mendeleev’s first versions of the periodic system. This is one of the reasons why the periodic classification was hardly regarded as a dramatic revolution in chemical pedagogy. In the last two decades of the nineteenth-century, Mendeleev’s periodic system was frequently mentioned and discussed, sometimes in sections dealing with other classifications, in other cases in chapters dealing with atomic theory.

6In Spain, the first references appeared around 1880, just after the discovery of the elements gallium and scandium. With the exception of Russia and Germany, the situation was not substantially different from other European countries: the first textbooks to mention the periodic law were published around 1880, and references to Mendeleev were rare until the 1890s. In Russia, apart from Mendeleev’s one, the first textbooks including references to the periodic system was published in 1874 by Victor von Richter, who translated his work into German one year later. Other German textbooks including references to Mendeleev were published during the late 1870s, while the first British and American ones were published around 1877, that is, almost at the same time as the Czech textbooks, just a few years before the French and Spanish ones.10

7Only a small group of Spanish authors criticised the periodic classification, but even the ones that spoke very highly of the periodic law did not use Mendeleev’s work as the organising principle in their textbooks. In Spain as elsewhere, textbook writers kept using the hybrid chemical classifications mentioned above and the most widely applied organising principle during the 1880s and 1890s was “dynamicity” (valence), which was used both by authors who praised the periodic law and by those who ignored it entirely. It was common to include chapters reviewing different classifications, sometimes including Mendeleev’s, sometimes with new proposals or slightly different arrangements. Many other classifications, including some based on Mendeleev’s periodic law and particular versions of old ones, found their way into Spanish textbooks during the early twentieth century.11

Atoms and primary matter

8The periodic classification was not an organising principle in most chemistry textbooks published before 1920. Mendeleev’s works were frequently mentioned with regard to other issues, such as primary matter, atomism and the nature of elements. These connections have already been noted by chemist-historians such as Georges Urbain. Writing in 1934, Urbain affirmed that late nineteenth-century supporters of atomism (like himself) were the most enthusiastic “propagandists” for the periodic system, whereas “equivalentists” “attempted to undermine the system of atomic weights by questioning the value of its most prominent expression: the periodic classification.”12 Further studies on nineteenth-century textbooks have shown that the relation between the periodic system and atomism is not as clear-cut as Urbain claimed: many books using atomic weights did not mention the periodic system at all, while the use of equivalent weights did not prevent some authors from including long descriptions of the periodic classifications in their works. Convinced atomists such as Adolphe Wurtz, who regarded Mendeleev’s work as a support for atomic weights, never adopted it as an organising principle for his popular textbook.13 This conclusion hints the different uses of the periodic system: it could be regarded as very useful for supporting author’s views on atoms, primary matter and elements, but fruitless as source of pedagogical sequences for organising lectures on chemistry.

9The previous analysis applies to the Spanish context, where the connections between the periodic system and atomism were also complex. Santiago Bonilla (1844–1899), who wrote one of the first Spanish textbooks including references to Mendeleev’s periodic system, was also a firm supporter of atomism and enthusiastic about the revival of the protyle hypothesis. However, other Spanish authors did not perceive periodic law as so relevant to atomism. For instance, Gabriel de la Puerta Ródenas (1839–1908), in an 1882 paper discussing the issue of the unity of matter in atomic terms, did not include any reference to the periodic law; José Ramón Luanco (1825–1905), another leading supporter of atomic weights, only mentioned the periodic law in the third edition of his textbook (1893), just as an appendix at the end of the book. Even more surprisingly, Ramón Torres Muñoz de Luna (1822–1890), who attended the famous Karlsruhe meeting in 1860, employed both equivalent and atomic weights in his popular textbooks, but never mentioned the periodic law, not even in the last editions published during the 1880s.14

10Then there were those who regarded atomism as a useless, perhaps even a harmful, hypothesis José Rodríguez Carracido was author of a very critical review of Mendeleev’s classification, in which he criticised the inconsistencies and remarked on the numerous exceptions.15 Nevertheless, he played an important role in the acceptance of the periodic law in Spain. He was part of a group of popularisers of science who pursued intermingled scientific and political agendas, in which the periodic system was a departure point to discuss very general scientific topics (such as cosmology and the evolution of matter), to convey broad philosophical ideas (for example, monistic and dynamic views of nature related to the unity of human knowledge) and to gain support for experimental science.

An evolutionary approach to the periodic kingdom

11One of the most active popularisers of these issues was José Rodríguez Mourelo (1857–1932). After teaching physics and chemistry at a secondary school, he moved to Madrid and started to give talks on popular scientific topics at the Athenaeum. During the 1880s, he travelled to several European research institutions and met leading scientists like Marcellin Berthelot, whose laboratory he visited in 1886.

12In February 1880, Rodríguez Mourelo was lecturing on physics and chemistry at the Madrid Athenaeum. This institution offered lectures on a wide range of topics from humanities to natural science. After the fall of the First Spanish Republic, became a meeting point for liberal-minded audiences who were eager to hear alternative ideas that challenged the predominant Catholic and idealistic worldviews. In tune with the new ideas of positivism, these audiences were particularly interested in new perspectives on both human and natural phenomena. Evolutionary ideas and Laplacian cosmology, as well as experimental psychology, provided excellent arguments with which to combat the religious and metaphysical currents that prevailed at the time.16

13Rodríguez Mourelo’s lectures and papers addressed many of these issues. His first lectures were on the “fourth state” of matter, an idea recently put forward by William Crookes. With the lights turned off, he performed experiments with electricity and low-density gases in vacuum-tubes, showing dramatic phenomena which, he regarded as evidence of “the fourth state of matter”. The lectures ended far into in the night “to the applause of the [Madrid Athenaeum] members and general public who had listened to him with such attention.”17

14In November 1880, Rodríguez Mourelo delivered a new group of lectures on the recent conceptions of the cosmos. He expressed his support for monistic and dynamic views which were founded on thermodynamics and evolution theory. His evolutionary approach was encouraged by the group of intellectuals in Madrid who supported Darwinism and their application to sciences other than biology18. One of them was Rodríguez Carracido who also lectured at the Athenaeum and published several papers on “inorganic Darwinism”.19 It was an attempt to explain inorganic evolution based on a reformulated version of Prout’s protyle, empirical data from stellar spectroscopy, and analogies with biological evolution. All of these issues informed the new version of the periodic system proposed by the Irish physician James Emerson Reynolds (1844–192020).

15Surprisingly, Rodríguez Mourelo first referred to the periodic law in a lecture on experimental psychology. He employed the periodic law as a good example of how experimental science could conquer academic spaces until then under the domination of idealistic philosophy and religion, an approach which appealed strongly to his audiences at the Madrid Athenaeum. He claimed that scientific methodology could be applied to psychology thanks to the continuous and dynamic character of all natural phenomena, which allowed scientists to establish series and groups. As an example, he mentioned Gerhardt’s type theory and Mendeleev’s classification of elements.21

16In his papers and talks Rodriguez Mourelo also discussed the notion of the “simple body”, highlighting its provisional character, namely, the fact that simple bodies were defined by the limits of chemical analysis. He compared the contested status of the notion of simple bodies to the idea of species in biology after the development of the theory of evolution. He also discussed the differences between the concepts of the simple body and element.22 Relying more on Berthelot’s mechanistic approach than on Mendeleev, Rodríguez Mourelo regarded elements as specified values of a mathematical function of energy, characterised by their optical, thermodynamic and electric properties and by their “power of combination”. Mourelo thought that these values were related to each other, so that chemical elements could be organised in series, the “most ingenious one in chemistry” being “Mendeleev’s periodic law”. However, he described it as “a mere conjecture”, “not well-proven in all cases”, “a sort of empirical law”. These critical remarks did not prevent him from praising the successful prediction of gallium and germanium, or from foreseeing future discoveries by means of the periodic law.23

17To sum up, Rodríguez Mourelo defended an anti-atomistic and evolutionary approach to chemistry, which emerged from his own reading of Berthelot’s and Crookes’s works. In doing so, his approach challenged “Mendeleev’s wager” on the irreducible plurality of elements, many years before the arrival of radioactive elements and the problem of isotopes.24 In doing so, Rodríguez Mourelo was not an exception in the late nineteenth-century European chemical community. Apart from Crookes, authors such as Julius Thomsen (1826–1909) fully discussed the periodic law in relation to the problems of the unity of matter, Prout’s hypothesis and inorganic Darwinism.25

18Rodríguez Mourelo was supported by other members of the late nineteenth-century Spanish scientific community. Rodríguez Carracido chose “the current idea of chemical element” as the topic for his address to the members of the Madrid Academy of Science in 1888, expressing similar views on the unity of matter and inorganic Darwinism and criticising atomic theory. Carracido’s speech was very positively received by one of the most influential mandarins of Spanish science, José Echegaray (1832–1916). Just a few days later, Rodríguez Mourelo commented very favorably on the works of his two “friends” in an essay published in a popular newspaper, summarising the shared framework in which the periodic law was accommodated:

[Rodríguez Carracido] deals in detail with the theory of evolution in chemistry, with the admirable ideas of Crookes on the genesis of simple bodies […]. He relies on the data used by Mendeleev to build his periodic law by using the numbers representing the atomic weights of the so-called chemical elements, on Lockyer’s spectroscopic works, on the existence of the proto-element “helium”, which provides a new form to Prout’s renowned hypothesis and, above all, on the admirable advances of chemical mechanics, a magnificent consequence of the determination of the mechanic equivalent of heat.26

19Both Carracido and Mourelo employed the periodic law to discuss issues such as the difference between simple bodies and elements and the evolution of elements. In doing so, they produced a discourse that appealed not only to their fellow members of the Madrid Academy of Science but also to the middle-class audiences of the Athenaeum. To interest these audiences, said Mourelo, a good popular-science paper had to combine factual data on the properties underlying the technological applications of these chemical bodies with “obscure and difficult” issues such as the origin of simple bodies. He mentioned Tyndall’s popular lectures as a model to be followed by Spanish scientists.27 The periodic law was particularly well suited to this purpose, as it offered an opportunity to talk about the properties and uses of simple bodies as well as cosmological conceptions, the genesis of elements, the unity of matter and energy and the value of the scientific method. Lacking substantial laboratory resources, Rodríguez Mourelo and Rodríguez Carracido, as well as other Spanish chemists, could hardly expect to uncover new empirical data (such as more precise atomic weights) or to isolate a new chemical element, so their contributions were on theoretical issues and epistemological concerns regarding the value and meaning of the periodic system.

From Inorganic Darwinism to Stoichiology

20Following this evolutionary approach, José Muñoz del Castillo (1850–1926) proposed the most original Spanish contribution to the periodic law at the turn of the nineteenth century. After studying at the Madrid Faculty of Science, he worked as a secondary school teacher, then as professor of physics at the University of Zaragoza and lecturer at the Madrid School of Engineering. In 1892, he reached the height of his career when he was appointed professor of inorganic chemistry at the Madrid Faculty of Science. He lectured on chemical taxonomy and suggested his own “cyclical classification”. He regarded himself as following the path initiated by Mendeleev and Meyer towards a complete understanding of the “laws of formation and definition of simple bodies” which would eventually be expressed in mathematical terms.28

21His ideas on the periodic law relied on the Laplacian nebular hypothesis updated with the new laws of thermodynamics, empirical spectroscopic data on the composition of stars and Crookes’ ideas on the genesis of elements and his three-dimensional array of “odds” and “even” elements. In fact, Muñoz del Castillo followed the discussions and concerns of his colleagues Rodríguez Mourelo and Rodríguez Carracido. The main difference is that Muñoz del Castillo was not an anti-atomist. In fact, he drew heavily on Dulong-Petit’s law, one of the bases of nineteenth-century atomism, to support his hypothesis of the evolution of the universe and the genesis of the elements. Muñoz del Castillo was more ambitious than his colleagues and he attempted to create a new discipline (which he called “chemical cosmology” or “chemical geology”), whose role he took to be similar to that played by geology in biological evolution.

22Like Rodríguez Mourelo, he regarded simple bodies as values of a continuous mathematical function, whose characteristics were unknown but whose variables were likely to be affinity, electricity and atomic condensation. According to Muñoz del Castillo, simple bodies with low atomic weights were formed in the primitive period of the universe’s evolution, characterised by high temperatures and slow nebular condensation. Higher atomic weights were stable only in the lower temperatures of planets such as the Earth, which were formed at more recent stages of the evolution of the universe. The available spectroscopic data on light emitted by stars and planets seemed to support these claims. Therefore, he affirmed that the list of simple bodies classified in an increasing order of atomic weights was similar to a classification according to age, that is, with the oldest elements (with low atomic weights) at the beginning and the most recent elements (with high atomic weights) at the end.

23Muñoz del Castillo asserted that several series of elements were formed in different periods of cosmic evolution. Two non-periodical series appeared at the beginning of the universe: noble gases, lacking chemical properties (valence, electrical charge, etc.), were the most primitive products of the universe; and rare earths (including scandium) which were “intermediate products” of the astro-chemical evolution, characterised by the non-periodicity and similarity of their properties, that is, their scarce individuality. The most recent creation of elements was the periodic series, that is, the remaining chemical elements whose properties were a periodic function of atomic weights.29

24With this classification, Muñoz del Castillo addressed two puzzling problems of periodic classification in the late nineteenth century: noble gases, and rare earths. The discovery of terrestrial helium and, most notably, argon introduced a new group of substances whose place in the periodic table was unclear, not only because of their singular properties (they were monoatomic gases lacking reactivity) but also due to the atomic weight of argon, which would have placed it between potassium and calcium. Many different options were suggested, including Mendeleev’s hypothesis on argon as “polymerised nitrogen” (which reflected the difficulties concerning its placement in his periodic table). In the case of rare earths, the similarity in chemical and physical properties made their isolation a challenge for traditional methods of chemical analysis. By the end of the 1860s, when Mendeleev introduced his periodic table, just six rare earths were known. During the following years, the advent of spectroscopic techniques offered new resources for the detection of new elements, sometimes revealing the compound nature of some substances which had been considered elements (as in the case of rare earths), but it also sparked many controversies about real or false discoveries. An additional difficulty for placing rare earths in the periodic table was their conspicuous lack of periodic variation of valence and other chemical properties. Several possible solutions were discussed in those years, from the distribution of rare earths into other groups in the periodic law (as Mendeleev at first attempted) to setting them apart in a separate family either inside or outside other groups in the periodic table.30

25Muñoz del Castillo opted for a radical version of the last option: rare earths (as well as noble gases) were regarded as a non-periodical group, lying outside the periodic kingdom. He claimed that many rare earths were, in fact, “migmoid bodies” (“cuerpos migmoides”, from the Greek word meaning “mixture”). With this neologism, Muñoz aimed to reinstate the mixtures of elements which had been incorrectly regarded as simple bodies in the previous years. A similar concept (“meta-elements” or “elementoids”) had been suggested by William Crookes in previous years. Like Crookes, Muñoz del Castillo expected that chemical analysis would add new components to the group of rare earths, as had happened in earlier decades.31

26Muñoz del Castillo thought that his approach had both pedagogical and heuristic virtues. Students could learn the properties of the groups by focusing on the “typical elements”. Students could study the properties of the group. In addition, Muñoz del Castillo established “cycles” including the elements placed in a row. The characteristic valence of the elements in a cycle continuously varied from negative (– 1, – 2, – 3) to positive (+ 3, + 2, + 1) values. His table was also intended as a tool for the search for new elements. He predicted many new elements: a silver-like element between gold and mercury; an element called “tessexium” (whose characteristic valence was 4 and 6) inside the group of silicon and zirconium; another element with valence 2, 6 and 8, inside the group of uranium, and so on. Muñoz del Castillo even claimed to have predicted the new element “radium”.32

27He defended his classification, its pedagogical virtues and predictions, in his speech on his election as a new member of the Madrid Academy of Science in February 1901. Furthermore, he suggested an ambitious programme for a new discipline called “stoichiology” or “science of the elements”. A textbook on stoichiology, he said, would include the study of chemical taxonomy (dealing with the natural classification of elements), a full section on the prediction, study, and isolation of new simple bodies, and a group of descriptive chapters on the chemical properties and the technological uses of elements. He argued that stoichiology would offer important theoretical and practical results not only in science, but in philosophy and arts as well. Claiming that it would help to “encourage experimental research on chemistry in Spain”, Muñoz del Castillo linked the study of stoichiology to the general movement of the regeneration and modernisation of Spanish science.33

28This programme was never fulfilled. In the years that followed, Muñoz del Castillo abandoned his project and moved into new areas of study. His last studies of classification were on the place of radium in his cyclic classification and a brief discussion of whether radioactivity could be regarded as a periodic property.34 Radium and radioactivity became his main research focus when he ran his own laboratory, published a specialised journal, and promoted agricultural and medical uses of radioactivity.35

29Muñoz del Castillo’s studies on periodic classification were well known among the Spanish scientific community of the early twentieth century. He presented and disseminated his cyclic classification in his lectures on inorganic chemistry at the Madrid Faculty of Science, where many prominent Spanish chemists studied. However, his classification was mentioned in a small group of early twentieth-century Spanish textbooks. One of them was written by Josep Prats i Aymerich (b. 1873), professor at an Industrial School in Terrassa (near Barcelona) and author of a doctoral thesis on argon. Writing in 1906, he claimed that he had adopted the cyclic classification because it was the “most rational and complete” and in tune with recent scientific developments:

After Mendeleev’s brilliant idea, I regard Muñoz del Castillo’s classification as the most rational and complete, and most in harmony with the trends in science. And I claim that it is rational because the study of spectra of the very luminous and hot stars shows that they only contain hydrogen and a few metallic vapours, whereas the low temperature stars, such as our Sun, attest to the existence of a multitude of metals, which, according to Lockyer, appear [in these stars] in a descending order of atomic weights. What could be more tangible proof of the parallelism between astronomical and chemical evolution?36

30After his death in 1926, Muñoz del Castillo was replaced at the Academy of Science by the spectroscopist Ángel del Campo (1881–1944), who delivered his introductory speech on “the evolution of the periodic system”, conceived as a tribute to his colleague and master. Del Campo recalled the teaching of Muñoz del Castillo at the Faculty of Science and described “his masterly lectures” on physical chemistry and the periodic system of elements. In his review of the history of the periodic system, del Campo summarised the “well-known” work of Muñoz del Castillo, which he regarded as “perhaps the most important” example of late nineteenth-century attempts to understand the meaning of the periodic system and the origins of periodicity and to explain the cause of exceptions to periodic law.37 But perhaps due to this excess of ambition, Muñoz del Castillo’s cosmic hypothesis is neglected today: the advent of the new quantum explanations of the periodic system condemned to oblivion all previous attempts to explain its regularities and exceptions, including inorganic Darwinism.

many layers of meaning

31The above discussion reveals the many layers of meaning associated with the periodic system which were highlighted (or sometimes neglected) by the protagonists: a pedagogical tool for organising the factual data included in many chapters of chemistry textbooks; a classification of elements which could shed new light on the natural order of chemical substances; a natural law which could be used to review atomic weights or to predict the properties of unknown elements; a point of departure for discussions on the nature of chemical elements and atomic theory; a positivistic tool for expanding experimental science into the realms of cosmology or psychology; a resource for liberal-minded audiences who looked for alternative ideas to the predominant Catholic and idealistic worldviews; or even an argument for increasing resources for experimental science with the promise of dramatic technological breakthroughs in the near future. The scientific, pedagogical and political agendas of Spanish chemists shaped their selective and creative appropriation of these different meanings of the periodic system.

32The association of the periodic law and theories of matter was not exceptional at this time, but it was encouraged by Spain’s peripheral situation: the lack of significant laboratory facilities made any substantial experimental contributions impossible and obliged a focus on theoretical aspects. The middle-class audiences of institutions such as the Athenaeum were receptive to Muñoz del Castillo’s mixture of cosmological evolution and matter theory, which provided an alternative to the predominant religious conceptions and idealistic philosophy. Moreover, in the hands of creative teachers and popularisers (like Rodríguez Mourelo and Muñoz del Castillo), the periodic law provided rhetorical resources for essays and lectures in which theoretical problems on the grand scale (such as the origins of the universe or the nature of chemical elements) could be debated, and the promise of technological breakthroughs could be exploited to increase economic and social support for Spanish science. Finally, the background in physics and chemistry of many Spanish authors, who taught both disciplines in secondary schools and universities, encouraged a joint physico-chemical approach, in which the periodic law was linked to astronomy, cosmology and matter theory. In Spain, as elsewhere, the local context broadly shaped the changing meanings and unforeseen uses of the periodic law in the late nineteenth-century, while creating a fertile ground for ingenious constructs such as Muñoz del Castillo’s cyclic classification. In their own terms and addressing the interests of their audiences, late nineteen-century Spanish chemists critically discussed the periodic system not so much in regard to its virtues for chemical pedagogy, but rather in connection to philosophical issues such as the problems of reductionism in chemistry, the precarious ontology of chemical elements and the challenges of the coherent pluralism of chemistry.