Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
HELGE KRAGH THE FIRST SUBATOMIC EXPLANATIONS OF THE PERIODIC SYSTEM ABSTRACT. Attempts to explain the periodic system as a manifestation of regularities in the structure of the atoms of the elements are as old as the system itself. The paper analyses some of the most important of these attempts, in particular such works that are historically connected with the recognition of the electron as a fundamental building block of all matter. The history of the periodic system, the discovery of the electron, and ideas of early atomic structure are closely interwoven and transcend the physics–chemistry boundary. It is pointed out that J. J. Thomson’s discovery of the electron in 1897 included a first version of his electron atomic model and that it was used to suggest how the periodic system could be understood microphysically. Thomson’s theory did not hold what it promised, but elements of it were included in Niels Bohr’s first atomic model. In both cases, Thomson’s and Bohr’s, the periodic system played an important role, heuristically as well as justificatory. 1. INTRODUCTION From an early date, many chemists saw in the periodic system of the elements an indication that the atoms are complex bodies with a constitution that somehow is reflected macroscopically in the properties of the elements that the system summarises.1 Not content with accepting the periodic system as merely an empirical classification, they wanted to explain it in terms of atomic constitution and the idea of the unity of matter. Among the chemists who saw a profound connection between the periodic law and atomic structure were Lothar Meyer in Germany, Gustavus Hinrichs in the United States, William Crookes in England, Julius Thomsen in Denmark, and Thomas Carnelley in Ireland, to mention but a few. The conviction resulted in numerous numerological proposals, more or less fanciful periodic tables, and vague speculations concerning the constitution of atoms (Rudorf, 1904; van Spronsen, 1969). However, these Foundations of Chemistry 3: 129–143, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 130 HELGE KRAGH attempts of explanation were all hypothetical and speculative in the sense that they built on the assumption of some (usually unspecified) kind of subatomic unit for which there was no empirical evidence. We know that the periodic system does, in fact, reflect the internal constitution of the atoms, namely, the configuration of electrons in various shells or quantum states. This is how the system is presently explained in textbooks of chemistry. Whether the system can also be reduced to atomic physics, or completely accounted for in this way, is a different question that I shall not consider here. (For this question, and a brief history of electron explanations of the periodic table, see Scerri, 1997.) Perhaps the first ‘modern’ and successful explanation along this line was Niels Bohr’s theory of 1922, at the time widely acclaimed as a breakthrough in the understanding of the periodic system in terms of atomic structure (Kragh, 1979). However, although Bohr’s theory was the first quantitative (or semi-quantitative) quantum theory of the periodic system, it built in important respects on earlier models that anticipated the presently accepted theory. In this essay I focus on the first explanations of the system that were framed in terms of a known subatomic particle and, in this sense, were realistic explanations. In 1895, the German chemist Victor Meyer expressed the view of many scientists when he wrote that “The knowledge that all chemical elements belong to a common series (which, since the discovery of the periodic law, practically no chemist denies) has established . . . that a common something is present in all the elements” (Meyer, 1895: p. 96). Two years later, J. J. Thomson identified the “common something” with the electrons he had identified in cathode rays and thereby laid the foundation for the modern explanation of the periodic system. 2. FROM VORTEX THEORY TO THE ELECTRON J. J. Thomson’s ‘plum pudding model’ of the atom is usually dated 1904, the year when he gave a detailed and mathematically elaborate version of it. However, the essence of the model can be found, if only qualitatively, in his famous paper of 1897 in which he announced the discovery of the ‘corpuscle’ – Thomson’s name for what would soon be called the electron, a name introduced by George Johnstone Stoney in 1891 to denote an elementary quantity of electricity. By THE PERIODIC SYSTEM 131 and large, the difference between the 1904 conception of the atom and the original one of 1897 was that in his first version Thomson did not introduce the positive, spherical distribution of positive electrification as an electrical container of the electrons. It is also in the 1897 paper that we find the first electronic explanation sketch of the periodic system. Today it is generally recognised among historians that Thomson’s model of the atom, and his conception of the electron (as I shall call the corpuscle), was greatly indebted to an earlier tradition in theoretical Victorian chemistry that has been called ‘vortex chemistry’ or ‘knot chemistry’ (Chayut, 1991). The idea that matter and forces can be understood as manifestations of the vortical motions of an underlying, continuous quasisubstance, the ether, was introduced in 1867 by William Thomson (the later Lord Kelvin), who built on an earlier hydrodynamical theory of Hermann von Helmholtz. In the 1870s and 1880s the vortex atomic theory attracted much attention among British mathematical physicists, who were impressed by the theory’s unitary features and fascinated by its mathematical challenges (Siegel, 1981; Epple, 1998). For more than two decades it was highly regarded and developed by physicists such as J. Clerk Maxwell, Peter G. Tait, John Tyndall, Augustus Love, William Hicks, and J. J. Thomson. It is less well known that Kelvin’s dynamical atomism in the shape of the mathematically abstruse vortex theory was sought applied also to chemical problems. The theory was favourably received by a few theoretically inclined chemists who believed that it might furnish a foundation of a theoretical, indeed a deductive chemistry. This kind of vortex chemistry was advocated by Tait in 1876 and taken up a few years later by young J. J. Thomson, one of Tait’s former students. The topological approach to chemistry was developed by the Edinburgh chemist Alexander Crum Brown, who used it in his graphic representations of chemical compounds and thereby contributed to the early conception of valency. The fascinating, but short-lived tradition of British vortex chemistry was mainly concerned with structural formulae and chemical combination, and did not explicitly address the periodic system of the elements. Nonetheless, indirectly it provided an important source not only for the first electron atomic theory, but 132 HELGE KRAGH also for an explanation of the periodic system in terms of subatomic particles. J. J. Thomson’s early work on the vortex atomic theory provided him with a conceptual framework that would prove essential to his later interpretation of cathode rays as streams of electrons liberated from matter (Kragh, 2000). In his Adams Prize essay of 1882, 25year-old Thomson greatly developed Kelvin’s theory and applied it to a number of chemical problems, including affinity and chemical combination. Although he did not mention the periodic system by name, he did suggest that “the atoms of the elements called by the chemists monads, dyads, triads, tetrads, and so on, consist of one, two, three, four, &c., vortex rings linked together”. Furthermore: “Atomicity corresponds to complexity of atomic arrangement; and the elements of high atomicity consist of more vortex rings than those whose atomicity is low; thus high atomicity corresponds to complicated atomic arrangement” (Thomson, 1883: pp. 120 and 124). For a small number of interacting vortex rings, Thomson determined the stability of the atomic system by means of elaborate perturbation calculations, but for more than six rings he relied on the analogy with an interesting experiment with floating magnets that the American physicist Alfred Mayer had published in 1878 (Snelders, 1976). Subjecting equally magnetised needles floating in water to the attractive force of a central electromagnet, Mayer noticed that the needles took up equilibrium positions on concentric circles. His purpose with the experiment was primarily didactical, and he used it to illustrate phenomena of molecular structure such as allotropy and isomerism. Mayer’s experiment is a beautiful illustration of the role of analogies in scientific thinking. Whereas Mayer did not relate his findings to atomic structure, Kelvin was quick to point out that the experimentally determined configurations of equilibrium would correspond to those of an atom made up of columnar vortices. As he wrote, “Mr. Mayer’s beautiful experiment brings us very near an experimental solution of a problem which has for years been before me unsolved – of vital importance in the theory of vortex atoms – to find the greatest number of bars which a vortex mouse-mill can have” (Kelvin, 1878: p. 14). J. J. Thomson carried the analogy further and elaborated it greatly in his Adams Prize essay. We should THE PERIODIC SYSTEM 133 not be surprised that neither Mayer, Kelvin nor Thomson pointed out the similarity between the patterns of magnetised needles and the structure of the periodic system. Explicit references to the periodic system were rare in the 1870s, even in the chemistry literature, and only by the late 1880s had Mendeleev’s system become widely accepted and incorporated into most textbooks (Brush, 1996). In 1892 Thomson pointed out, for the first time, the suggestive similarity between Mayer’s configurations of magnetised needles, the arrangement of interacting vortices, and the periodicity of the properties of the chemical elements: “If we imagine the molecules [atoms] of all elements to be made up of the same primordial atom, and interpret increasing atomic weight to indicate an increase in the number of such atoms, then, on this view, as the number of [primordial] atoms is continually increased, certain peculiarities will recur” (Thomson, 1892: p. 410). At the time, Thomson no longer identified the primordial particle with an ether vortex, but his thinking about atomic constitution continued to be guided by the vortex atom theory even after the announcement of the true primordial atom, the electron or corpuscle. “Personally I generally endeavour (often without success) to picture to myself some kind of vortex-ring mechanism”, he wrote in a letter of 1898; “I regard . . . the vortexatom explanation as the goal at which to aim” (Holman, 1898: p. 226). The conceptual framework of the vortex theory was clearly exhibited in his 1897 work, where he pictured the electron in the shape of the vortex theory that he held in such esteem. Thomson’s corpuscle of 1897 differed in certain respects from the ‘electron’ that Joseph Larmor had introduced three years earlier as a singularity in the electromagnetic ether. However, the two versions of the electron had in common their indebtedness to the theory of vortex atoms, at the time abandoned by most physicists. Moreover, Larmor’s electron had in common with Thomson’s that it was seen as a building block of all chemical elements. In 1895, two years before the celebrated discovery of the electron, Larmor suggested “a molecule [atom] to be made up of, or to involve, a steady configuration of revolving electrons” (Larmor, 1927: p. 595). However, Larmor was uninterested in chemistry and did not consider the periodic system or other chemical phenomena. By contrast, Thomson was well informed about chemistry, a field that 134 HELGE KRAGH greatly interested him and acted as a source of inspiration for his atomic theory.2 Conversely, from 1897 onward he sought eagerly to apply his new atomic theory to chemical phenomena, of which he considered the periodic system to be particularly important. 3. THE THOMSON ATOM AND THE PERIODIC SYSTEM In his first announcement of the identification of cathode rays as streams of electrons, in a Royal Institution lecture of April 1897, Thomson did not mention the periodic system. But he did introduce the basic features of a new conception of the atom, as a vast conglomeration of electrons; and he explicitly connected the picture with the Proutean tradition of composite elements, as “has been put forward from time to time by various chemists” (Davis and Falconer, 1997: p. 151). As Thomson was well aware, advocates of this tradition considered the periodic system to be a strong argument in favour of their cause. Thomson made the connection explicit half a year later, in his paper in Philosophical Magazine of October 1897 where he presented a better argued and more detailed version of his electron hypothesis. As far as the arrangement of intra-atomic electrons was concerned, Thomson made use of Mayer’s experiment, just as he had done fifteen years earlier in the context of the vortex atomic theory. “A study of the forms taken by these magnets seems to me to be suggestive in relation to the periodic law”, he wrote. Assuming that a certain chemical property was associated with a group of magnets, that the magnets corresponded to electrons, and that the atomic weight was expressed by the number of electrons, he suggested: Now suppose that a certain property is associated with two magnets forming a group by themselves; we should have this property with 2 magnets, again with 8 and 9, again with 19 and 20, and again with 34, 35, and so on. If we regard the system of magnets as a model of an atom, the number of magnets being proportional to the atomic weight, we should have this property occurring in elements of atomic weight 2, (8,9), 19, 20, (34,35). . . . [I]n fact, we should have something quite analogous to the periodic law . . . (Thomson, 1897: p. 314). Here we have the essential features of Thomson’s explanation of the periodic system, features that did not change much over the next decade. Although by 1904 a sphere of positive electrification THE PERIODIC SYSTEM 135 had been added in order to keep the electrons together, this did not change the equilibrium configurations and the validity of the analogy to Mayer’s experiment. In his works from 1904 to about 1912, Thomson repeatedly referred to his explanation of the periodic system as a major triumph of his atomic model. For example, in a Royal Institution lecture of 1905 he mentioned that atoms with the same inner groups of electrons would “have much in common, and we might therefore expect the physical as well as the chemical properties of the atoms to have a general resemblance to each other”. This property, he added, was “analogous to that indicated by the periodic law in chemistry” (Davis and Falconer, 1997: p. 218). As an example, consider the electron configurations in select Thomson atoms with three to six rings. The configurations are denoted with the symbol n(x1 , x2 , . . . ), meaning a total of n electrons distributed with x1 electrons in the innermost ring, x2 electrons in the second ring, etc.: 21(1,8,12) 37(1,8,12,16) 56(1,8,12,16,19) 78(1,8,12,16,19,22) 22(2,8,12) 38(2,8,12,16) 57(2,8,12,16,19) 79(2,8,12,16,19,22) 23(2,8,13) 39(2,8,13,16) 58(2,8,13,16,19) 80(2,8,13,16,19,22) Here, the atoms in the same columns will correspond to elements of the same chemical group because they have electron groups in common. Contrary to the later explanation of the periodic system, in Thomson’s view the similarity between elements was the result of common internal groups of electrons. Thomson’s account of the periodic system was a suggestion or an explanation sketch, not a quantitative explanation. His model was unable to associate real elements with definite electron configurations. Not only was his model a two-dimensional representation of the three-dimensional atom, the number of electrons in real atoms was also unknown. Therefore, although Thomson’s atomic model furnished a striking analogy to the periodic system, it was no more than an analogy. Thomson believed originally that the entire mass of the atom was made up of electrons, which meant that even the lightest atoms would consist of thousands of electrons. For this reason alone, realistic atomic models could not be constructed. 136 HELGE KRAGH However, in 1906 he concluded from various experimental evidence that the number of electrons was probably of the same order as the atomic weight A; and four years later he was inclined to believe that the number was between A and A/2. The new insight changed the status of Thomson’s model, for now it would be possible to construct a realistic model of, say, helium (with 2, 3 or 4 electrons) and confront the predictions of the model with the known physical and chemical properties of helium. It should be possible to explain, for example, the similarity between helium and neon. Although Thomson himself did not make such (three-dimensional) calculations and compare the results with the empirical data, a few other physicists did (Haas, 1911; Föppl, 1912). The result was not encouraging: It turned out that the Thomson model was unable to account for the real structure of atoms and hence also of the periodic system.3 The failure of Thomson’s model to reproduce the properties of real atoms was not much noted and did not seriously damage the reputation of the model; for a period, its suggestive explanation of the periodic system was broadly accepted or at least considered interesting. According to the chemist Ida Freund, Thomson’s explanation “supply an explanation for the empirical relations between atomic weight and atomic properties embodied in the periodic law” (Freund, 1904: p. 624). The Cambridge chemist Matthew M. Pattison Muir similarly concluded that “[Thomson’s] electrocorpuscular theory of the structure of atoms leads to the recognition of a periodic connexion between the atomic weights and the properties of the elements” (Pattison Muir, 1907: p. 374). Also Harry Clary Jones, an American physical chemist, was impressed by the chemical merits of the Thomson model and praised its explanation of the periodic system (Jones, 1907). Although no one believed that Thomson had actually explained the periodic system, many believed that his approach was a step in the right direction and that it might one day lead to a complete understanding of the system in terms of electron physics. As early as 1901, the German physicist Walter Kaufmann, himself a pioneer of electron research, wrote (1901: p. 97): We might assume, for instance, that among the innumerable possible groupings of electrons only a relatively limited number would be sufficiently stable to occur THE PERIODIC SYSTEM 137 in larger quantities. These stable groupings would then be the elements as known to us. Perhaps a mathematical treatment of this question will one day succeed in presenting the relative frequency of the elements as a function of their atomic weights and in solving many another problem of the periodic system. It was such a mathematical treatment that Thomson offered in 1904, but without solving the problems that Kaufmann alluded to and without being able to reproduce models of “the elements as known to us”. Although Thomson’s model was the only electron-based theory of the periodic system that attracted wide attention, it was not the only one.4 Thus, Gilbert N. Lewis’s model of the cubic atom had its origin in an attempt to explain the periodic system and was used for this purpose as early as 1902, although it was published only in 1916 (Lewis, 1966: p. 29; Kohler, 1971). Another example is provided by H. Strache, a German chemist who in 1908 claimed to have explained the periodic system in terms of a speculative conception of the atom as made up of electrons (which he conceived as the elementary particles of the world ether). Contrary to Thomson, Strache undertook to use his explanation to predict (or postulate) new chemical elements, namely, of atomic weights 99, 176, 233 and 235 (Strache, 1908). It is unclear how he arrived at these predictions. 4. THE PERIODIC SYSTEM IN BOHR’S 1913 THEORY Niels Bohr’s famous atomic theory of 1913 was based on Rutherford’s new nuclear atom, but in many respects also indebted to J. J. Thomson’s rival model which furnished the natural starting point for Bohr’s treatment of many-electron atoms. In fact, there is a striking similarity between Bohr’s calculations in the second (and less well known) part of the 1913 trilogy and those performed almost a decade earlier by Thomson (Heilbron, 1977). Bohr was well aware of Thomson’s theory of the periodic system, but believed that the theory was incorrect. In the so-called Manchester memorandum from the summer of 1912, a draft version of his theory that he sent to Rutherford, he mentioned in a footnote that “it [is] impossible, to give a satisfactory explanation of the periodic law from the last mentioned [Thomson’s] atom-model” (Bohr, 1981: p. 136). Bohr considered an explanation of the periodic system to be an important 138 HELGE KRAGH part of his theory. In February 1913, he wrote to the chemist George de Hevesy that his still unpublished theory would include “a very suggestive indication of an understanding of the periodic system of the elements” (ibid.: p. 530). In building up atomic models, Bohr had the advantage over Thomson that he could make use of the recent introduction of the atomic number as the ordinal number of the periodic system, equal to the number of electrons in a neutral atom. Hence he was able to offer realistic models of the atoms. Bohr had developed the idea of the atomic number independently of the Dutch scientist Antonius J. van den Broek. In early 1913, Bohr wrote to the Swedish physicist Carl Oseen that he had come across the work of van den Broek, “who empirically had arrived at conceptions of the periodic system similar to those I have arrived at deductively from my theoretical speculations” (ibid.: p. 552). I shall not go into the details of Bohr’s theory, which has been treated elsewhere (Kragh, 1977; Heilbron, 1977). As far as the reconstruction of the periodic system is concerned, Bohr summarised his results in a table including electron configurations for the first 24 elements: 1(1) 2(2) 3(2,1) 4(2,2) ... 8(4,2,2) 9(4,4,1) 10(8,2) 11(8,2,1) 12(8,2,2) 17(8,4,4,1) 18(8,8,2) 19(8,8,2,1) 20(8,8,2,2) 16(8,4,2,2) 24(8,8,4,2,2) The cautious Bohr did not explicitly identify the electron structures with definite elements but merely noted that “it seems not unlikely that this constitution of the atoms will correspond to properties of the elements similar with those observed” (ibid.: p. 209). On the other hand, it would be quite clear to his readers that the structure 6(2,4) referred to carbon, 11(8,2,1) to sodium, and so on. Comparing Bohr’s explanation with that offered by Thomson, there are similarities as well as dissimilarities to be noticed. The general features of Bohr’s theory were clearly indebted to Thomson’s reasoning, but according to Bohr the chemical similarity between elements in the same group was associated with the same THE PERIODIC SYSTEM 139 number of electrons in the outermost ring. As mentioned, whereas Thomson’s structures did not claim to represent real atoms, those proposed by Bohr were realistic models. Also with regard to method and approach, the two physicists differed. Thomson’s theory was, or can be regarded as, an attempt to establish a deductive or ‘mathematical chemistry’, at least in principle.5 His electron structures were the result of calculations, and empirical considerations of the properties of elements did not enter. Bohr, on the other hand, was well aware that he could not do without empirical input. In spite of what he told in his letter to Oseen, his theory of the periodic system was not obtained “deductively”. As he admitted, physical principles, whether classical or based on the new quantum theory, “[are] in most cases not sufficient to determine completely the constitution of the system” (ibid.: p. 198). He therefore followed an eclectic methodology, freely mixing mathematical calculations and general principles with knowledge of the physical and chemical properties of the elements. The latter considerations were no less important than the first, and if the two approaches collided he usually gave priority to the results suggested by empirical knowledge. This particular eclecticism was not only a characteristic methodological feature in Bohr’s 1913 theory, but also in his later theory of 1922 (Kragh, 1979). 5. DISCUSSION AND CONCLUSIONS The historical roots of the modern theory of atomic structure are to be found not only in the development of physics, but also in the development of chemistry. Among the chemical phenomena that have influenced atomic theory, the periodic system of the elements is perhaps the most important. The existence of regularities in the properties of the elements, such as summarised in the periodic table, was a source of puzzle as well as inspiration to many scientists who wanted to know the underlying mechanisms and causes they assumed to exist. Mendeleev’s classification scheme played a heuristic role in early views of atomic structure: a good atomic theory was expected to give some kind of explanation of the periodic system, or, at least, be consistent with it. Of course, an atomic model could not be derived from the periodic system alone and then be 140 HELGE KRAGH used (circularly) to explain the system. The early atomic models of Thomson, Bohr and others rested on many sources, theoretical as well as empirical, and the periodicity of the elements was only one source among several others. Between 1897 and 1913 the new electron-based atomic models were used, with more or less success, to explain the periodic system. The period can be seen as leading up to the much more satisfactory explanations that were formulated in the 1920s on the basis of principles of quantum theory. According to Dudley Shapere (1977: p. 538), the expectation of an underlying, deeper explanation of the periodic system in terms of atomic contituents reached the status of a demand as more and more phenomena came under the domain of atomistic explanation. Considering that many eminent chemists (including Mendeleev) denied the value of such explanations, this is probably an exaggeration. On the other hand, it is undoubtedly true that expectations of what Shapere calls a ‘compositional theory’ increased during the two last decades of the nineteenth century. These expectations were not easily shaken, nor were they abandoned simply because the proposed theories were not very satisfactory. In this respect, they had the character of a research programme in the sense of Imre Lakatos. Shapere (1977: p. 540) writes about the atomistic expectations: Nor were they shaken by the failure of successive atomistic theories to account successfully for the features of the periodic table, . . . Considerations [of the validity of atomistic explanations in other domains] . . . were strong enough to constitute reasons in favor of a search for such a theory. That the reasons were not logically conclusive did not make them any the less reasons, and good ones, relative to the state of science at the time, nor did it make action in accordance with them any the less rational. This assessment seems to agree well with the first phase of electronbased explanations of the periodic system, from Thomson to Bohr. The theories were suggestive, perhaps even promising, but can hardly be called successful. Yet, with the general success of electron theories in other domains of both physics and chemistry it was natural to expect that the periodic system, too, would find its explanation in terms of the electrons of which the atoms were composed. Indeed, by the mid-1920s, the expectation turned out to be justified. THE PERIODIC SYSTEM 141 NOTES 1. I shall mostly refer to the periodic ‘system’ rather than ‘table’ or ‘law’, although ‘periodic law’ was frequently used at the time and can still be met in the literature. For a philosophically oriented discussion of the various terms, see Shapere (1977: pp. 535–536). Whereas the periodic table is one form of representation of the concept of the periodic system, neither the table nor the system qualify (or, in the past, qualified) as a causal and fundamental law of nature. Yet the periodic system of the elements can be ascribed the status of a law. Like several other statements of regularities (such as the Dulong-Petit law), it can be described as a semi-quantitative, functional and phenomenological law of nature (Weinert, 1995). 2. Thomson’s continual interest in chemistry, and the continuity of his ideas about atomic structure from the 1880s to the electron atom of 1897 and beyond, is richly documented in Sinclair (1987) and Chayut (1991). 3. After lengthy calculations, Ludwig Föppl, a German physicist, concluded that the periods of the periodic system would correspond to either 24 or 48 electrons, a result that did not harmonise with the properties of the chemical elements (Föppl, 1912: p. 300). 4. Among the many ideas of atomic structure in the period 1900–1912 were proposals by James Jeans, Rayleigh, Jean Perrin, Philipp Lenard, Hantaro Nagaoka, Ernest Rutherford, and John Nicholson. Except for Nicholson’s system of 1911, the periodic system played no great role in these atomic hypotheses. 5. “It will be the task of the future”, wrote Arnold Sommerfeld in 1922, “to sketch a complete topology of the interior of the atom and, in addition, a mathematical chemistry, that is, to show the exact position of the electrons in the shell of the atom” (Sommerfeld, 1922: p. 88). The ultimate aim of Thomson’s programme was a mathematical chemistry in this reductionist sense. REFERENCES N. Bohr. In: U. Hoyer (Ed.), Niels Bohr. Collected Works, Vol. 2. North-Holland, Amsterdam, 1981. S.G. Brush. The Reception of Mendeleev’s Periodic Law in America and Britain. Isis 87: 595–628, 1996. M. Chayut. J. J. Thomson: The Discovery of the Electron and the Chemists. Annals of Science 48: 527–544, 1991. E.A. Davis and I.J. Falconer. J. J. Thomson and the Discovery of the Electron. Taylor & Francis, London, 1997. M. Epple. Topology, Matter, and Space, I: Topological Notions in 19th-Century Natural Philosophy. Archive for History of Exact Sciences 52: 297–392, 1998. 142 HELGE KRAGH L. Föppl. Stabile Anordnungen von Elektronen im Atom. Journal für reine und angewandte Mathematik 141: 251–302, 1912. I. Freund. The Study of Chemical Composition: An Account of Its Method and Historical Development. Cambridge University Press, Cambridge, 1904. A.E. Haas. Über Gleichgewichtlagen von Elektronengruppen in einer äquivalenten Kugel von homogener positiver Elektrizität. Sitzungsberichte der kaiserliche Akademie der Wissenschaften (Wien) IIa(120): 1111–1171, 1911. J.L. Heilbron. J. J. Thomson and the Bohr Atom. Physics Today (April) 30: 23–30, 1977. S.W. Holman. Matter, Energy, Force and Work. Macmillan, New York, NY, 1898. H.C. Jones. Elements of Physical Chemistry. Macmillan, New York, NY, 1907. W. Kaufmann. The Development of the Electron Idea. The Electrician 48: 95–97, 1901. Kelvin [W. Thomson]. Floating Magnets. Nature 18: 13–14, 1878. R.E. Kohler. The Origin of G. N. Lewis’s Theory of the Shared Pair Bond. Historical Studies in the Physical Sciences 3: 343–376, 1971. H. Kragh. Chemical Aspects of Bohr’s 1913 Theory. Journal of Chemical Education 54: 208–210, 1977. H. Kragh. Niels Bohr’s Second Atomic Theory. Historical Studies in the Physical Sciences 10: 123–186, 1979. H. Kragh. The Electron, the Protyle, and the Unity of Matter. In: J. Buchwald and A. Warwick (Eds.), Histories of the Electron: the Birth of Microphysics. MIT Press, Cambridge, MA, 2001. J. Larmor. Mathematical and Physical Papers, Vol. 1. Cambridge University Press, Cambridge, 1927. G.N. Lewis. Valence and the Structure of Atoms and Molecules. Dover Publications, New York, NY, 1966. V. Meyer. Probleme der Atomistik. Verhandlungen der Gesellschaft deutscher Naturforscher und Ärzte 67: 95–110, 1895. M.M. Pattison Muir. A History of Chemical Theories and Laws. John Wiley & Sons, New York, NY, 1907. G. Rudorf. Das periodische System: Seine Geschichte und Bedeutung für die chemische Systematik. L. Voss, Hamburg, 1904. E.R. Scerri. The Periodic Table and the Electron. American Scientist 85: 547–553, 1997. D.M. Siegel. Thomson, Maxwell, and the Universal Ether in Victorian Physics. In: G.N. Cantor and M.J.S. Hodge (Eds.), Conceptions of Ether. Cambridge University Press, Cambridge, pp. 239–268, 1981. D. Shapere. Scientific Theories and their Domains. In: F. Suppe (Ed.), The Structure of Scientific Theories. University of Illinois Press, Urbana, IL, pp. 518–599. S.B. Sinclair. J. J. Thomson and the Chemical Atom: From Ether Vortex to Atomic Decay. Ambix 34: 89–116, 1987. H. Snelders. A. M. Mayer’s Experiments with Floating Magnets and their Use in the Atomic Theories of Matter. Annals of Science 33: 67–80, 1976. THE PERIODIC SYSTEM 143 H. Strache. Die Erklärung des periodischen Systems der Elemente mit Hilfe der Elektronentheorie. Verhandlungen der deutschen physikalischen Gesellschaft 10: 798–803, 1908. J.J. Thomson. A Treatise on the Motion of Vortex Rings. Macmillan, London, 1883. J.J. Thomson. Molecular Constitution of Bodies, Theory of. In: H.F. Morley and M.M. Pattison Muir (Eds.), Watt’s Dictionary of Chemistry, Vol. 3.Macmillan, London, pp. 410–417, 1892. J.J. Thomson. Cathode Rays. Philosophical Magazine 44: 293–316, 1897. J.W. van Spronsen. The Periodic System of Chemical Elements. A History of the First Hundred Years. Elsevier, Amsterdam, 1969. F. Weinert (Ed.). Laws of Nature: Essays on the Philosophical, Scientific and Historical Dimensions. Walter de Gruyter, Berlin, 1995. Aarhus University Ny Munkegade, Aarhus C 8000, Denmark E-mail: [email protected]