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Why “plum pudding”?

  1. Top of page
  2. Why “plum pudding”?
  3. Thomson's atom model and its impact
  4. The “plum pudding” in popularizations
  5. The making of a myth
  6. Acknowledgment
  7. References

Joseph J. Thomson (1856–1940) was one of the leading physicists at the turn of the last century. In 1906 he was awarded the Nobel Prize in physics for discovering the electron and for his work on the conduction of electricity in gases. Thomson's discoveries raised questions concerning the nature of the atom. He demonstrated that the atom is not the simplest unit of matter; rather, it has a structure. Thomson's atomic theory has informally been called the “plum-pudding” model, but the early history of this expression has not been elucidated. The expression is not in Thomson [1], the seminal paper in which Thomson developed his atomic theory, nor is it in any of his subsequent publications. Although the historian Heilbron [2, 3] has argued persuasively that this expression misrepresents Thomson's theory, it is still mentioned in respectable professional texts (sometimes labeled “inappropriate”): see, e.g., [4-8]. As evidence for its occurrence in popular accounts, we found that a Google search for “plum pudding atomic model” yields over 100,000 hits. In other words, the expression is a commonplace and, as we show, it is not simply an artifact of recent popularization. So, what were the circumstances that led to calling Thomson's theory the “plum-pudding” model?

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Figure 1. Joseph J. Thomson giving a lecture demonstration in the 1890s (Photo: courtesy of Niels Bohr Archive, Copenhagen).

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Thomson's atom model and its impact

  1. Top of page
  2. Why “plum pudding”?
  3. Thomson's atom model and its impact
  4. The “plum pudding” in popularizations
  5. The making of a myth
  6. Acknowledgment
  7. References

As Kragh1 has noted, between 1904 and about 1910 Thomson's model was generally accepted as the best available atomic theory.2 What distinguishes Thomson's theory is his assignment of a specific inner structure to the atom as well as a set of dynamical assumptions.

In 1899, in a paper addressed to the British Association for the Advancement of Science, Thomson introduced the essential ingredients of his model without any mathematical formulas:

  • I regard the atom as containing a large number of smaller bodies which I will call corpuscles;…. In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpuscles behave like negative ions, yet when they are assembled in a neutral atom the negative effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges on the corpuscles [10], p. 565.

These are preliminary remarks in which Thomson was exploring possibilities: he posited something which behaves as if it had a positive charge. But Thomson was certain about the overall system, namely, the atom is a spatial domain filled with numerous negative corpuscles whose total charge is neutralized somehow by the spatial positive electrification.3

In May 1903 Thomson gave a series of lectures at Yale University in which he discussed the consequences of recent advances in physics, including his own substantial contributions, remarking that “we must face the problem of the constitution of the atom, and see if we can imagine a model which has in it the potentiality of explaining the remarkable properties shown by radio-active substances” [11]. Thomson then addressed the question, how do the corpuscles arrange themselves in a sphere? He began by considering one corpuscle and appealing to equilibrium as a guiding constraint [11] (pp. 111–117). The resulting theory was based on “the atoms of the chemical elements [which] are built up of large numbers of negatively electrified corpuscles revolving around the centre of a sphere filled with uniform positive electrification” [12]. His conception of the atom now included corpuscles whose motions take place in orbits and, due to the forces exerted on the orbits, each orbit “may be constrained to occupy an invariable position with respect to the aggregate—as if, to take a rough analogy, the orbit was a tube bored through the aggregate, so that the orbit and the aggregate move like a rigid body…” [12] (p. 690). Although Thomson appeals to “a rough analogy”, what he presents is an atomic model. As far as we can determine, in his subsequent publications he invoked neither “analogy” nor “aggregation”—an indication that Thomson was groping for a way to address atomic phenomena. Both his methodology and terminology were in transition.

A theory was then required to account for the atomic structure. The full title of Thomson's paper of 1904 is “On the structure of the atom: an investigation of the stability and periods of oscillation of a number of corpuscles arranged at equal intervals around the circumference of a circle; with application of the results to the theory of atomic structure.” He began as follows:

  • The view that the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, suggests, among other interesting mathematical problems, the one discussed in this paper, that of the motion of a ring of n negatively electrified particles placed inside a uniformly electrified sphere [1].

Thomson constructed his atomic theory so that it would be conducive to mathematical analysis where stability, as the title indicates, plays a critical role. The atom in equilibrium has n corpuscles and they are arranged at equal angular intervals on rings inside a positively electrified sphere, each corpuscle carrying a charge of negative electricity. The task was then to find a stable structure which comprises corpuscles bearing negative electric charges (i.e., electrons) immersed in a sphere of positive electrification to render the atom electrically neutral and yet capable of giving an account of its chemical properties. In Thomson's view his calculations showed that the proposed structure does indeed account for the chemical properties of the atom [1] p.258.

In 1907 Thomson published a book, The corpuscular theory of matter, based on a course of lectures he had given at the Royal Institution in the spring of 1906. In these lectures Thomson discussed the theory of the constitution of matter which, for Thomson, supposes that the various properties of matter may be regarded as arising from electrical effects. The later chapters of Thomson's book contain

  • a discussion of the properties of an atom built up of corpuscles and of positive electricity, the positive electricity being supposed to occupy a much larger volume than the corpuscles…. I think that a theory which enables us to picture a kind of model atom and to interpret chemical and physics results in terms of such model may be useful even though the models are crude… [13] p. v-vi.

For Thomson the model served as a heuristic device exhibiting two principal features, namely, the stability and unity of atomic phenomena, both chemical and electrical.

Thomson's theory was part of the background for the various atomic theories that replaced his model, notably those of Ernest Rutherford (1871–1937) [14] and Niels Bohr (1885–1962) [15]. In particular, Bohr was very much influenced by the work of his mentors, Thomson and Rutherford, whom he cited extensively throughout his Trilogy of 1913, in which he introduced the quantum theory of the atom.

In his address to the British Association for the Advancement of Science, August 18, 1914, Rutherford took stock of the current state of the theory of the atom:

  • The idea that the atom is an electrical structure received a great impetus by the detection of the electron by J. J. Thomson…. In J. J. Thomson's model of the atom the positive electricity was supposed (for mathematical reasons) to be distributed throughout a large sphere with the negative corpuscles moving inside it. This hypothesis has played a useful part in indicating possible lines of advance; but it does not fit in with more recent discoveries, which point to a concentrated positive nucleus [16].

Rutherford was well positioned to assess Thomson's contribution and he noted that, while Thomson's model was not consistent with newly found experimental results of deflections of particles, his theory “played a useful part” in pointing to fruitful lines of research. One such line was taken up by Bohr in his construction of the quantum theory of the atom in 1913.

Bohr described Thomson's theory and went on to claim that the configurations allowed in Thomson's atom-model “apparently do not exist for the second [i.e., Rutherford's] atom-model” [15] (p. 2). As is well known, in his quantum theory of the atom Bohr took as his point of departure the atomic structure which Rutherford had proposed and explicitly discarded that of Thomson, inaugurating a new era in physics exactly 100 years ago.4

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Figure 2. A plum pudding (Source: http://www.thatslife.com.au, photo: Richard Whitbread).

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The “plum pudding” in popularizations

  1. Top of page
  2. Why “plum pudding”?
  3. Thomson's atom model and its impact
  4. The “plum pudding” in popularizations
  5. The making of a myth
  6. Acknowledgment
  7. References

Merck's Report was a practical journal of pharmacy as a profession and a business. In an essay published in December 1906, the question “What is matter?” was addressed, in which a report was given on a series of lectures delivered by Thomson earlier in 1906:

  • The most favored theory [for the constitution of matter] of the present time is that which Prof. J. J. Thomson … has expounded … at the Royal Institution … in a series of lectures entitled the “Corpuscular Theory of Matter.” The lectures were delivered … in March and April [1906], and these notes … are the gist of what our representative gathered from lecture to lecture [18].

Here we find, as far as we can tell, the first occurrence in print of the expression “plum pudding” with respect to Thomson's theory.5 The anonymous representative who attended Thomson's series of lectures reported among other things that

  • Professor Thomson suggests [that] … while the negative electricity is concentrated on the extremely small corpuscle, the positive electricity is distributed throughout a considerable volume. An atom would thus consist of minute specks, the negative corpuscles, swimming about in a sphere of positive electrification, like raisins in a parsimonious plum pudding, units of negative electricity being attracted toward the center, while at the same time repelling each other [18].

Now a plum pudding is a suet pudding with raisins, traditionally served at Christmas. Despite its name, this pudding contains raisins rather than plums. Clearly, the anonymous reporter knows his pudding: he describes the plum pudding correctly. But there is no correspondence between the embedded raisins and the revolving corpuscles for, according to Thomson's theory of 1906, the electrons revolve on rings about the center of the atom, and they are not distributed throughout the “pudding” as “raisins swimming about”. Merck's report tells us that the plum-pudding image was originally based on a misunderstanding of Thomson's theory.

We have also found the expression “like plums in a pudding” in an anonymous report of a discussion that took place on August 2, 1907 as part of the annual meeting of the British Association in Leicester, England [20]. The relevant passage reads:

  • Whether [the atom] consists of a central body surrounded by a ring of electrons, or whether the electrons are immersed in the central body, like plums in a pudding, are alternatives of which anyone may, at present, take his choice, for the truth is that the materials are not sufficient for a decision [21].

The invocation of “plum pudding” (here without naming Thomson) is misleading in two respects, namely, the plums in fact are raisins and, more importantly, it is misleading, as we have seen, to liken Thomson's model to a plum pudding.6

The making of a myth

  1. Top of page
  2. Why “plum pudding”?
  3. Thomson's atom model and its impact
  4. The “plum pudding” in popularizations
  5. The making of a myth
  6. Acknowledgment
  7. References

On all accounts plum pudding is a curious image for the atom, and one wonders what could have been the source for these anonymous reporters. One possibility is that they depended on a passage in the physics textbook, Properties of matter (1885) by P. G. Tait (1831–1901), which includes, inter alia, a survey of several hypotheses concerning the ultimate structure of matter.7 Against the limitations of two conceptions, the discrete hard atom on the one hand, and continuous mathematical points on the other, Tait presents what he considers a more plausible theory: “Matter is continuous (i.e. not made up of particles situated at a distance from one another) and compressible, but intensely heterogeneous; like a plum-pudding, for instance, or a mass of brick-work” [23]. Tait's plum-pudding image was offered as an attempt to reconcile the continuous with the discrete; this was a problem of accommodation within the classical domain. The imagery is designed to depict matter in general; the atom did not yet have a structure.

Given these historical data, we notice a curious fact, namely, a misleading popular image entered scientific and scholarly discussions. As we have seen, physicists like Rutherford and Bohr who responded professionally to Thomson's model atom conceived it in his own terms and did not present any pictures or introduce any analogies to facilitate comprehension. Indeed, there was no need for such imagery. However, contemporary popularizers, who sought to make the emerging modern physics accessible to the general public, wished to avoid technical aspects of the theory, offering instead a vivid image. It is unfortunately a frequent occurrence that popularizations misrepresent scientific theories; however, what is surprising in the “plum-pudding” case, is the fact that the scholarly literature adopted this misleading characterization of Thomson's model atom. At the outset Thomson considered the atom a structural unity within which numerous negatively charged corpuscles transverse a positively charged space. The model changed over the years, eventually becoming a positively electrified space within which relatively few electrons set on rings circulate about a mathematical center. The scene is energetic, clearly dynamical, and nowhere are raisins/plums stuck in a pudding.

  1. 1

    See [4], p. 20.

  2. 2

    For a brief account of contemporary alternative atomic theories with references to the original papers, see [9].

  3. 3

    Cf. [4] p. 8.

  4. 4

    For a detailed analysis of Bohr's contribution to atomic physics in 1913 in the context of the work of his predecessors (including Thomson) see, e.g., [4, 17]. An upcoming contribution to this section by Martin Jähnert will deal with Bohr's theory.

  5. 5

    For an earlier unsuccessful attempt to locate the first occurrence of “plum pudding” as a characterization of Thomson's model, see [19] pp. 148–149, 291.

  6. 6

    A detailed report on this meeting appeared in [22]. There is no “plum pudding” in this “official” account.

  7. 7

    Tait's Properties of matter appeared in four editions in his lifetime (1885, 1890, 1894, and 1899) and a fifth edition appeared in 1907, all of which are indications of its popularity.

References

  1. Top of page
  2. Why “plum pudding”?
  3. Thomson's atom model and its impact
  4. The “plum pudding” in popularizations
  5. The making of a myth
  6. Acknowledgment
  7. References
  • 1
    J. J. Thomson, Phil. Mag. 7, 237 (1904).
  • 2
    J. L. Heilbron, Lectures on the history of atomic physics 1900–1922, in C. Weiner, ed., History of twentieth century physics (Academic Press, New York and London, 1977), p. 52, 54.
  • 3
    J. L. Heilbron, Nature 498, 27 (June 6, 2013).
  • 4
    H. Kragh, Niels Bohr and the quantum atom: The Bohr model of atomic structure 1913–1925 (University Press, Oxford, 2012), p. 8, 17.
  • 5
    M. J. Nye, From chemical philosophy to theoretical chemistry (University of California Press, Berkeley, 1993), p. 133.
  • 6
    J. Navarro, Hist. Sci. 44, 470 (2006).
  • 7
    O. Bueno and S. French, Brit. J. Phil. Sci. 62, 873 (2011).
  • 8
    H. De Cruz, and J. De Smedt, Phil. Stud. 157, 421 (2012).
  • 9
  • 10
    J. J. Thomson, Phil. Mag. 48, 547 (1899).
  • 11
    J. J. Thomson, Electricity and matter (Yale University Press, New Haven, 1904), p. 92.
  • 12
    J. J. Thomson, Phil. Mag. 36, 673 (1903).
  • 13
    J. J. Thomson, The corpuscular theory of matter (Archibald Constable, London, 1907).
  • 14
    E. Rutherford, Phil. Mag. 21, 669 (1911).
  • 15
    N. Bohr, Phil. Mag. 26 1 [I], 476 [II], and 854 [III] (1913).
  • 16
    E. Rutherford, Discussion of the structure of atoms and molecules, in Report of the eighty-fourth meeting of the British Association, Australia, July 28 – August 31, 1914 (John Murray, London, 1915), p. 293.
  • 17
    J. L. Heilbron, Phys. Today, 30, 23 (1977).
  • 18
    Anonymous, Merck's Report, 15, 359 (December 1906).
  • 19
    A. A. Martínez, Science Secrets (University of Pittsburgh Press, Pittsburgh 2011).
  • 20
    E. Rutherford, Discussion on the constitution of the atom. Report of the Seventy-Seventh Meeting of the British Association for the Advancement of Science, Leicester, 31 July–7 August 1907 (John Murray, London, 1908), p. 439.
  • 21
    Anonymous, The Outlook (London), 20, 176 (August 10, 1907).
  • 22
    Anonymous, Nature 76, 457 (August 29, 1907).
  • 23
    P. G. Tait, Properties of matter (Black, Edinburgh, 1885), p. 19.