molecular representations

Science & Education 10: 423451, 2001. 2001 Kluwer Academic Publishers. Printed in the Netherlands. 423

Molecular Representations: Building TentativeLinks Between the History of Science and theStudy of Cognition

MARIA YAMALIDOUMax Planck Institute for the History of Science, Wilhelmstr. 44, D-10117 Berlin, Germany

Abstract. This paper addresses questions concerning the cognitive character of nineteenth-centuryBritish molecular discourse. At a time when no proof of the existence or the intimate structure ofthe material particle was yet available or even possible, scientists were free to suggest and discusspossible, alternative, or even incompatible, molecular pictures of the unseen level of the materialsubstratum, leaving aside all realistic considerations. The role of these molecular representationswas to provide the necessary causal links between physical phenomena and underlying mechanisms,thus infusing intelligibility into scientific explanations. Focusing on processes of thinking ratherthan on formal theories, the analysis in this paper will suggest that, precisely because of its fluidcharacter, molecular discourse produced a common universe of meanings which sustained an on-going thought experiment regarding the intimate structure of matter, and that, by so doing, it initiateda process of familiarisation of scientists with the unobservable realm. Beyond realism and scepticism,the attitude of nineteenth-century molecularists, which can be adequately described as suspensionof judgement, may prove highly suggestive in science education.

1. Introduction

The language and point of view of psychology can find a prominent position inthe expanding field of the history of science. Peter Gays recognition of the fact,that the writing of history often involves questions that cannot be settled unlesswe talk, in one way or another, about human nature or behaviour, brought to theforeground what seems to have been a hidden agenda for historians.1 WilliamMcKinley Runyans assertion that psychology is unquestionably relevant in thewriting of biographies2 resonates with the initial interest of psychologists to un-derstand the achievements of eminent men and, less often, women of sciencein psychological terms.3 According to Thomas Kohut, the relevance of psychologyfor history is almost self-evident because there is a psychological dimension in thepast, which cannot be ignored by historians.4

Although a considerable amount of evidence has been accumulated by diversestudies, which indicate that the process of evaluation and interpretation of sci-entific data are greatly determined by personal attitudes and dispositions which,in turn, can be affected by educational background, social conditions, political

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attitudes, and gender,5 these early attempts to fertilise historical investigation withpsychological considerations did not exhaust the suggestive possibilities of thisinterconnection. Since R. Fischs review of the field of psychology of science,6which diagnosed a series of methodological and philosophical problems in theearlier attempts of psychologists to explicate the experience of scientists in theproduction of knowledge including lack of integration, fragmentation of the re-search, and absence of cohesive concepts the joint efforts of cognitive scientistsand historians of science have been intensified and tend to acquire, in recent years,a programmatic form.

The central desideratum of this synthetic approach concerns its unavoidableinterdisciplinary character. From the point of view of cognitive science, this in-terdisciplinarity has been established in the very definition of the field, whichcomprises cognitive psychology, studies on artificial intelligence, and cognitiveneuroscience.7 And although the recognition of the fact that science is primarilya cognitive activity makes the involvement of cognitive scientists self-evident,their own understanding of this involvement encourages the interaction betweendifferent approaches. Hence, Ronald Giere points out that cognitive science is oneof the potentially most powerful resources for studying science,8 and William R.Shadish Jr. and Robert A. Neimeyer, argue that this interdisciplinarity expresses acommitment to a metascience in which psychology, although an important force,represents only one of many valid approaches.9

For historians of science, the prerequisite of interdisciplinarity was arrived at,mainly, through a critique of the constraints that the traditional, prescriptive philo-sophy of science imposed on the understanding of scientific developments. Tryingto loosen the strong embracement of theoretical explanations which focus on thelogical structure of scientific theories and on the notions of truth and progress, inrecent years, historians of science shifted their attention from the finalised productof scientific investigation to the specific intellectual, religious, socio- economical,and political conditions that affect the production of knowledge. However, thisintense preoccupation with the contextual dependency of scientific developmentsin recent historiography should not be equated with a regression to empiricism.On the contrary, the consideration of the complex interactions between the per-sonal and the collective in the negotiation of the meaning of scientific concepts,between the technical and the rhetorical manipulation of facts and ideas, betweenthe prerequisites for internal coherence of the content of science and the socialjustification of its results, invites a multiplicity of theoretical frameworks for itsunderstanding and interpretation.

At each of these steps which lead from private contemplations to shared systemsof knowledge, an understanding of the cognitive dimension of science is becomingincreasingly relevant. The potential of certain great thinkers to produce ideas whichtranscend the constraints of the current understanding of the world, and whosevalidity is tested through a complex process of justification, was among the firstquestions which attracted the attention of cognitive psychologists and creativity

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quickly became one of the favourite topics of research in this field.10 Valuable asthis approach has been in providing an understanding of scientific innovation, itperpetuates and, somehow, magnifies the distinction that Reichenbach introducedbetween the context of discovery and that of justification, a distinction which hasbeen under critical examination in recent years. Nancy Nersessian negates thevalidity of this distinction arguing that discovery is both creative and reasoned,a fact which becomes more apparent if we redefine creativity and understand itas a process rather than an act.11 In a similar tone, David Gooding suggests thatthe basic question that should be addressed in relation to scientific innovationconcerns the way in which the unfamiliar can be represented yet still retains itspotential to change the structure that represents and explains it.12 His analysis ofMichael Faradays experimental work as well as Nersessians analysis of JamesClerk Maxwells electrodynamical models exemplify this choice in practice.

The main thrust for such studies has been provided by questions concerningconceptual change. Although catalytic for the change of outlook of the history ofscience, Thomas Kuhns suggestion of an abrupt change between pre-paradigmaticand normal science did not leave much space for the consideration of suchquestions.13 According to Ronald Giere, it was the choice of gestalt psychologyon behalf of Kuhn which restricted his understanding of the gradual character ofconceptual change,14 or, as Nancy Nersessian puts it, his scheme does not providean understanding of the mechanism which affects this change.15 Similar concernshave been expressed by Michelene Chi who insists on the distinction between theoutcome of conceptual change and the process which affects it. She suggests a wayto understand conceptual change, as a process, in terms of ontological categor-ies and of the migration of concepts within and across these categories.16 RonaldGiere, on the other hand, chooses models as the units of his analysis of science17and argues that scientific theories are structured on the basis of certain families ofmodels and that the progressive understanding of physics is exemplified by a shiftfrom central to peripheral models.18

The notion of mental models has become central to cognitive psychology espe-cially through the work of Johnson-Laird.19 Nancy Nersessian brings his analysisto bear upon concrete historical episodes in order to understand the representationalresources that are available to scientists and the way in which these resourcesare utilised during the process of problem-solving through which new conceptsemerge. She argues that the cognitive dimension of analogies, metaphors, andthought experiments is central, in the sense that it is these tactics which suggest theinferential reasoning which actually generates new representations form existingones.20

The move from the private understanding of individual scientists to the sharedunderstanding of the scientific community has been put within the scope of cog-nitive science most prominently by David Gooding. Far from denying the role ofindividual imagination, which plays a central role in the process of abstractionform actual observations to constructed visual and linguistic representations of the

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observed phenomena, Goodings approach seeks to understand the process throughwhich a newly discovered phenomenon is made easy-to-see, that is easily ac-cessible to others. According to his analysis, the modelling of the world is basic,and the mastering of observational and experimental techniques enables scient-ists to stabilise the meaning of the observed phenomena. Hence, the symbiosis ofthought and action results in the merging of representable cognitive processes andunrepresentable skills.21

Finally, the explanatory possibilities of a combined examination of the socialand the cognitive dimensions of science, although not unquestionable,22 have beenexemplified by Terry Shinns detailed study of everyday work in a modern laborat-ory. His interpretation of science as a network of activities where, at certain nodes,cerebral and social events merge and function symbiotically,23 thus producinga convergence of cognitive operations and social conduct, attempts to resolve theearlier rivalry between social hegemony and epistemological determinism.

The analysis presented in this paper will attempt to bring a specific aspect ofnineteenth-century British science, that addressing the general idea that matter isparticulate at the unobservable level, within this synthetic framework of analysis. Itwill be suggested that the consideration of the way in which scientists were think-ing and talking about material particles, and more specifically, in physics, aboutmolecules, may prove highly suggestive in our attempt to understand the intricatecognitive processes which turns a body of initial speculations and contradictoryobservations into that widely accepted framework of knowledge which graduallybecomes the authoritative science. The extended examination of instances of mo-lecular discourse, in Section 1, will attempt to highlight the fact that, in the periodwhich preceded the establishment of formal theories concerning molecular interac-tions, scientists were free to explore alternative molecular models. This pluralismin the expression of the basic idea of material molecularity did not give rise toscientific debates; on the contrary, it provided a rich repertoire of ideas, whoseproductive character has not passed unnoticed by Stephen Toulmin, who points outthat the major triumphs of the atomic theory were achieved at a time when eventhe greatest scientists could regard the idea of atoms as hardly more than a usefulfiction.24

That the construction of alternative possibilities constitutes a productive way ofthinking was made explicit, in 1889, by Thomas Chamberlin, who argued that thetrue explanation is . . . necessarily multiple, while the interpretative effort of scient-ists consists in the organisation of a complete set of multiple hypotheses.25 Thisview is not irrelevant to certain suggestions that are being put forward within cog-nitive science. For example, Shadish Jr. and Neimeyer point out that [s]cietists usemultiple cognitive strategies in their work,26 and Nersessian argues that Maxwellused multiple knowledge domains and informational formats.27 Howard Gruberargues that the division of the creative life of a person into separate, distinctiveenterprises is constructive; on the one hand, their relative autonomy enables con-structive work in one without affecting the other, while on the other hand, chancy


interactions between them enhances creativity.28 From a different perspective, Jo-han de Kleers study of expert problem-solvers led him to recognise the fact thatan expert problem-solver should be able to employ multiple representations forthe same problem in order to be effective.29

In Section 2, a parallel examination of the similarities between nineteenth-century molecular discourse and the intuitive explanations of ordinary people willprovide a further opportunity to discuss processes of thinking. It seems that the im-portance of molecular explanations, for both groups, is related to the establishmentof causal relations between observable phenomena and underlying mechanism.The aim of these explanations was to make these phenomena intelligible ratherthan to express any final view about the nature of the individual particle. That theprerequisite of intelligibility is important for the selection of meaningful conceptshas been highlighted by Goodings analysis of Faradays treatment of the notion oflines of force.30 Moreover, for both scientists and ordinary people, the process ofunderstanding resulted in a distinct, three-level categorisation of molecular mod-els. This evidence can be understood in relation to the views expressed by RyanTweney, Michael Doherty and Clifford Mynatt that the establishment of taxonomicsystems creates meaning for both unity and diversity, thus facilitating scientificcommunication.31 Lastly, both groups were producing molecular models of the un-seen level of physical reality by pasting together familiar pictures and this processwas not restricted by any strict criterion of coherence of the suggested models.The cognitive dimension of this tactic has been highlighted by Nersessian whoargues that [b]y clustering connected information and making visual a chain ofinterconnected inferences the imagistic representations support a large number ofimmediate perceptual inferences.32

The aim of Section 3 is to investigate the conditions of intelligibility ofnineteenth-century molecular discourse. It will be suggested that, precisely becauseof its highly flexible nature, this discourse created cognitive loops through which,on the one hand, physical phenomena became intelligible because they could bevisualised in terms of molecular interactions, and, on the other hand, molecularreality became a familiar and, hence, intelligible framework of analysis of physicalphenomena. The production of mental images of the unseen level of physical real-ity, which were sometimes highly complex and did not correspond to any specificphysical system, creates the opportunity for a discussion which centres on the roleof metaphors and analogies in scientific thinking. Arthur Millers investigation ofthe role of mental imagery in the formulation of scientific theories in the nineteenthand twentieth century,33 and Michael J. Webbs argument that analogies cannotbe constructed in situations beyond the limit of our pictorial representations,34suggests a meaningful way to relate the discussions on analogical thinking to thebroader context of visual, or imagery thinking.35

In the concluding section, the main points of this understanding of molecularscience will be used in order to disentangle certain problematic aspects of grammarschool curricula. Far from being exhaustive, these concluding remarks will attempt

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to highlight the fact that the way of thinking suggested by nineteenth-century mo-lecularists may be further explored as a possible pattern of molecular thinking foryoung children.

The historical understanding of science has become, in recent years, increas-ingly relevant to science education.36 The factor which mostly contributed to thisturn, must have been the phenomenal persistence of childrens erroneous ideasabout physical world, which problematised the traditional methods of instructionand urged for a more detailed examination of what learning involves. Hence, ithas been suggested that the question of instruction cannot be resolve unless weunderstand the dynamical interaction between the partial pieces of informationpresented to children and their own knowledge structures.37 The assimilation of theformer into the latter is not a simple and straightforward process, because childrensmental representations do not comprise only concepts, that have to be somehowchanged, but also beliefs about the truth-value of those concepts which are mean-ingful for children.38 The realisation that scientific and non-scientific thinkers shareimportant characteristics concerning not only the process of cognition but also theirmethodological choices,39 highlighted the possibility that the study of the historicalprocess may reveal a model of the learning activity itself.40

The main argument of this paper, namely that the consideration of alternativepossibilities plays an important role in the process of understanding, repeats itselfin the implicit assumption which permeates the whole paper and culminates in thesuggestion that an understanding of the history of science may assist educationiststo understand the children as thinking acotrs and to design more effective strategiesof instruction, exactly because the knowledge of history provides us a repertoire ofpossibilities.

2. Molecular Discourse: A Fluid Dialogue About Alternative Possibilities

Around the middle of the nineteenth century, molecular science was not yet anestablished field of scientific investigation. Although an increasing amount of evid-ence indicated that it was possible to understand macroscopic phenomena in termsof molecular activity at the unobservable level of physical reality, neither the math-ematical tools nor the art of experimentation were so advanced as to provide thenecessary framework which could deal with such immensely complex interactions.The alternative approach, namely the formulation of macroscopic theories whichcould be both heuristically suggestive and mathematically sound proved productivein diverse scientific fields as for example in hydrodynamics, thermodynamics,and the methodologically positivistic chemistry and liberated many scientistsfrom the obligation to formulate sub-microscopic models about the unobservablerealm. Maxwells suggestion, in the 1860s, of a statistical framework, which coulddescribe molecular complexity, translating various experimental results into math-ematical language, gave a tremendous impetus to the investigation of the ultimatestructure of matter.


In a large part of the historical literature, this distinction, between the pre-history of molecular speculations and the genesis of a new scientific field, hasbeen put into a narrative of pivotal moves: from the pre-paradigmatic to normalscience, from inconclusive evidence to mathematical theories, from speculativeideas to testable truths. Especially because the history of nineteenth-century Brit-ish science has been, largely, understood, until recently, within an interpretativeframework which stressed the view of a supposedly widespread empiricism,41 theintense preoccupation of historians with the establishment of Maxwells kinetictheory of gases, put into a prominent position the role of authority for the accept-ance and establishment of scientific innovation.42 However, this reading of historypasses over in silence a remarkable fact: that molecular explanations of physicalphenomena were frequently suggested and openly discussed even by scientists oflesser importance, even before the establishment of the Maxwellian framework,at a time when such terms as molecules and atoms had not yet acquired theirfinal position in scientific semantics. And in so doing, this narrative of distinct-ive and pivotal moves minimises the potential significance that diverse molecularconceptualisations might have had for the development of science, promulgatingthe view that pre-Maxwellian molecular models were nothing more than barrenspeculations.43

The extended use of a vocabulary of material particles is not difficult to prove.Numerous instances have been preserved in the historical record which show thatthe notion of molecularity created a shared discourse which cut across diversescientific fields, referred to all three states of matter, and suggested meaningfulexplanations for scientists of different educational backgrounds.44 Even when mo-lecules could not claim any higher position in the hierarchy of scientific notionsthan that of speculations, they became equally important for great mathemat-ical physicists like William Thomson,45 as for self-educated experimentalists likeGeorge Gore,46 and engineers like John Scott Russell.47 And if the widely ac-cepted mobility of gases could be easily justified through the mobility of smallerparts,48 the gap between the macroscopic cohesion of liquids or the rigidity ofsolid masses and the supposition of unobservable molecular mobility49 signifiedan intellectual transformation which was not at all self-evident in the context ofnineteenth-century science. Neither did the state of knowledge of such fields asgeology50 and physiology51 require the introduction of sub-microscopic consider-ations for their further development at that period. And yet, molecules were there,almost visible to scientists, even when these same scientists did not know withabsolute certainty what they were referring to. The frequent and free negotiation ofmolecular representations of the unobservable level of physical reality, in parallelwith the existence of a strong rhetorical framework of empiricism throughout thenineteenth century,52 poses an urgent and interesting question to the analyst ofscientific knowledge: if molecules were mere hypothetical constructions and if,moreover, they were totally unnecessary for the development of formal theories,why were they so frequently evoked by scientists? In other words, why did George

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Gabriel Stokes, just to mention the most provocative case, feel obliged to discussmolecular interactions in a scientific paper which introduced and established asuccessful macroscopic hydrodynamical theory, a theory which could be expressedin mathematical equations for which no molecular view of matter was required?53

Historians and philosophers of science often discuss the importance of modelsin the development of science in relation to the distinction between their realisticand heuristic role. The former presupposes an ontological commitment on behalfof the scientists as regards the ultimate level of physical reality, whereas the latterstresses the fact that no such commitment was necessary and that these modelswere, merely, tools for the composition of theoretical hypotheses which couldbe experimentally tested.54 The discussion of the heuristic function of scientificmodels has been especially important to historians who subscribe to the view ofthe so-called British empiricism, because it provided an explanation for the, other-wise unintelligible, persistence of theoretical models of unobservable particles.55According to this view, the subsequent experimental verification, at the level ofmacroscopical phenomena, of theoretical views based on sub-microscopical mod-els, validated, in the minds of the empirically-minded Britons, this brief diversionfrom their own philosophical dicta.

However, this distinction cannot provide a sufficient understanding of the situ-ation of molecular discourse as described above. As regards realism, apart fromthe philosophical lacuna that Sign Goldstein has pointed out, namely that there isno final way of checking any claim that scientists might make about the reality oftheir models,56 there is no historical evidence, to my knowledge, that a discourse,which attempted to validate or negate the ontological status of the material particle,was of any significance to physicists, at least until the late nineteenth century. It isvery hard to find instances of explicit reference to the reality of molecules in thescientific literature of that period, and hence, I find it highly restrictive to arguethat British scientists were either realists or anti-realists. On the contrary, one hasto realise the historicity of the discourse of realism itself.

As regards the heuristic value of molecular models, it must be understood thatthis argument presupposes a conscious design of a two-step procedure, involvingthe formulation of testable theories based on these models and the subsequent rig-orous testing of these theories. This schema does not explain the historical materialexamined in this paper. If we consider, for example, Thomas Tates experimentalwork on the absorption of liquids by porous substances, we shall see that theconcept of molecularity was introduced, without further justification, as an ex-planation of the deviation between the observed and the expected values of certainparameters. Tates assertion that, during the process of filtration, the filters undergoa progressive molecular change57 was not accompanied either by a formal theoryor by a parallel formulation of testable predictions that his molecular hypothesissuggested.

Tates attitude was not atypical. Quite the contrary: for the larger part of thenineteenth century, discussions about the conditions of material molecularity were


very rarely tied to formal theories. They constituted, rather, a parallel discoursewhich cut across the main line of scientific arguments at certain very delicatepoints, in order to explain away difficulties that the formal theories could not evenaddress. Stokes use of molecular conceptualisations, which has been mentionedabove, signifies one such instance: the development of the formal, mathematicaltheory of hydrodynamics in the nineteenth century excluded, explicitly, a wholerange of observable phenomena, those concerning internal friction and viscosity.These phenomena, Stokes argued, could not be understood without reference to themolecular conditions of liquid matter.58

According to the view that scientific hypotheses nurture scientific imaginationand out of which theoretical views are being crystallised, one expects such mo-lecular hypotheses to be part of the private contemplations of scientists. But theappearance of molecular conceptualisations in the published papers, even whenthese conceptualisation did not participate either in the mathematical formalismor in the construction of testable predictions, produces, not infrequently, an em-barrassment to certain historians. An even greater embarrassment is caused by thefact that scientists were suggesting significantly different and, even, incompatiblemolecular models, and that, for a long period of time, no attempt was made tohomogenise molecular terminology. For Foster molecules were spherical59 forRichard Potter they were small cubes60 for Robert Mallet they were fibrous.61Particles were considered as simple portions of mass implicitly taken as homogen-eous by William Rankine,62 but they were complex systems of internal structurefor O. Richter.63 Thomas Graham believed that the crucial factor which affectedthe rate of liquid transpiration was molecular magnitude,64 whereas John Tyndallsexperiments with magnetic and diamagnetic substances showed that the phenom-ena in question are not due to the shape of the molecules, but to their mannerof arrangement.65 James Challis envisaged a molecular theory of liquids, whichwould take into account molecular forces,66 while for John Scott Russell the oper-ative concept was molecular motion.67 This pluralism in the conceptualisations ofmaterial molecularity was the most persistent characteristic of molecular discourse.

The degree to which this plurality of meanings causes a problem to a significantpart of the traditional historiography of science is apparent in the fact that certainhistorians were led to assume that this was due to lack of communication amongscientists.68 Additionally, the fact that, for a long period of time there was neithera consensus nor any attempt to arrive at a consensus regarding the inner structureof particles cannot be given any intelligible explanation within the framework ofanalysis which concentrates upon the procedure which aims at the stabilisation ofmeaning of scientific concepts.69 Instead of attempting to elucidate and finalisetheir views of the material particle, nineteenth-century molecularists were engagedin a discourse, which was sufficiently fluid so as to encourage a multiplicity ofalternative conceptualisations.

The fact that this pluralistic discourse was not merely tolerated but activelyencouraged by scientists can be made more obvious if we consider Maxwells own

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views on the subject. In both his seminar papers, which are taken to be the mostformal articulation of molecular science in the nineteenth century, Maxwell insistedon a multiple representation of the individual molecule. Hence, in 1860 he arguedthat [i]nstead of saying that the particles are hard, spherical and elastic, we may ifwe please say that the particles are centres of force,70 while in 1867 he repeatedthat [t]hese molecules may be mere points, or pure centres of force, they may besystems of several such centres of force bound together . . . if necessary, we maysuppose them to be small solid bodies.71 Even in the late 1880s William Thomsonwould insist that he had put forward various suggestions . . . towards a Theory ofMatter but he has never settled any in his own mind.72

2.1. HISTORICAL RECORD AND EXPERIMENTS CONCERNING COGNITIVEPROCESSES: A ONE-TO-ONE CORRESPONDENCE

In order to understand why leading scientists of the nineteenth century, like WilliamThomson, could not settle their minds as to such serious matters, I shall now turnto Allan Collins and Dedre Gentners experiments concerning ordinary peoplesunderstanding of certain molecular phenomena. In a preliminary study, Collins andGentner presented four subjects with eight questions concerning evaporation andarrived at two very important results: first, that one subjects explanation involvedmolecular models constructed by pasting together his models of how familiar ob-jects behave,73 and second, that this subject could deal with given questions evenwhen his model was not correct. Collins and Gentner attempted to formalise thedifferent kinds of mental models that people use, and produced a classification ofmacroscopic models, aggregate models and molecular models. At the secondphase of their research, they arrived, through an extended experiment, at two fairlygeneral conclusions: first, that by mapping down to the aggregate level people canunderstand a macroscopic dependency in terms of a set of dependencies at theaggregate level,74 and second, that people have many different kinds of models ateach level of analysis.75

The results of this investigation bear a striking similarity with the historicalsituation of molecular discourse. First, the process of pasting together familiaraspects of the world in order to envisage molecular interactions was frequentlyencountered in the nineteenth century. Take, for example, the molecular modelwhich presupposed point masses interacting through their inherent forces to certaindistances, which were given the name spheres of action. This model is an intel-lectual construct which synthesises a perception of basic characteristic of physicalspace (matter and empty space), elements of the established dynamical frameworkof nineteenth-century science (the relation between motion and force), commonexperience (repulsion internalised through observations on magnetic phenomena),and previously established metaphysical beliefs (the belief in the existence of in-herent forces in matter). In a similar way, Thomas Grahams explanation of liquidtranspiration: [w]hen one of these definite hydrates . . . is being forced through


the capillary, it may be imagined that a small portion of the hydrate compound ismolecularly decomposed by friction,76 denotes actually a way of thinking whichsynthesises knowledge about chemical decomposition and fluid friction into asingle model.

Second, the fact that the subjects understanding of given phenomena was nothindered by lack of true knowledge of the ultimate structure of the individualmolecule also resonates with nineteenth-century evidence. William Thomsons in-genious conception of aetherial vortices in the 1860s had all the characteristics ofa successful theoretical construct: it could provide explanations both for physicaland for chemical phenomena, it could be put in mathematical language throughthe framework of the hydrodynamics of perfect fluids, it resolved the philosophicalproblem of circularity in the explanation of the elasticity of matter, and it couldbe made experimentally visible.77 It was not a true representation of the individualmolecule after all, and it was eventually disproved. But, in the meantime, it encour-aged a productive way of thinking about the dynamical interactions of vortices andfoster original research on the subject.78

Third, the fact that the same person may use many different models bears astriking similarity with Maxwells pluralistic representation of the material particlewhich was discussed earlier, an attitude which was, by no means, unique or idio-syncratic. Similar attitudes can be found, for example, in the case of James PrescottJoule, who argued, in 1848, that we conceive the particles to be revolving roundone another, according to the hypothesis of Davy, or flying about in every dir-ection according to Herapaths view,79 and also of John Scott Russell, whosediscussion of the sound wave, in 1844, introduced a series of possible, alternativemolecular transformations which could provide an intelligible explanation of thisphenomenon: a change in the form of the molecules of the liquid, or a changeof their density if they were supposed to be in contact, or even a change in theintermolecular distances if they were not in contact.80

Fourth, Collins and Gentners categorisation of three kinds of models mac-roscopic, aggregate and molecular corresponds, largely, to the three levels ofphysical reality that British scientists defined, namely, the level of phenomena, thelevel of molecular interactions, and the level of the individual particle. The initialcoarse distinction between macroscopic and microscopic phenomena has beenan elegant but insufficient dualism for the analysis of the development of molecularscience.81 Major nineteenth-century texts and discussions aimed at a translation ofscientific results in molecular terms while, at the same time, they left untouchedall questions regarding the specificities of the structure of the individual particle.We must now leave these speculations about the nature of molecules . . . and con-template the material universe as made up of molecules, declared Maxwell in hisarticle on Atom, which can be safely regarded as the manifesto of nineteenth-century molecular science.82 William Thomsons attempt to clarify the relationbetween Maxwells molecular theory of gases and the much needed and difficultto arrive at ultimate theory of matter, made it clear that the former constitutes just

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a first step towards the latter, and that the discussion concerning molecular condi-tions does not lie at the same level with the discussion concerning the molecule perse.83

Lastly, Collins and Gentners observation that the establishment of some cor-respondence between dependencies at the macroscopic and the aggregate levelfulfilled some basic condition of understanding, resonates with one of the mostpronounced characteristics of nineteenth-century molecular discourse, namely theestablishment of causal links between what was observed and what those obser-vations presupposed at the level of submicroscopic reality. Hence, liquid frictionwas attributed to the sliding of molecules as one moves relatively to the other,84electrolysis was understood in terms of the motion of ionised particles within themass of a solvent,85 the strain of a metallic bar was connected to the state of tensionor relaxation of the constituent particles,86 the structure of crystalline bodies wasenvisaged as a spatial symmetrical arrangement of the individual molecules,87 thedirectionality of certain phenomena was explained through the directionality ofmolecular behaviour,88 expansion or contraction of masses presupposed molecularmobility.89

It may seem paradoxical that nineteenth-century molecularists, as well as thesubjects of Collins and Gertners experiments, were not embarrassed by the in-compatibility of their diverse molecular explanations. I believe that any attemptto understand this attitude should take into consideration the fact that the questionposed to both groups was not a question about the individual molecule. If such werethe case, than, most probably, members of both groups would have felt obliged toexpress a clear opinion true or false about the nature of the material particle. Thequestion posed to both groups concerned the explanation of certain macroscopicphenomena.

This line of thought may shed some light, if further pursued, on the importantquestion concerning the role of coherence in a given discourse. Evidence fromthe history of science suggests that coherence and the deduction of reasonableconclusions operate within a given framework of analysis which determines centralquestions that should be answered and basic restrictions that should be observed.What is a molecule? the anonymous writer who set for himself this questionin the Popular Science Monthly, in 1881, left it unanswered, because molecularscience itself left it unanswered. Instead of turning to this question, the writerchose to begin his presentation with the single, secure proposition of modern sci-ence, namely that every substance consists of an aggregation of extremely smallparticles, which are called molecules,90 and then presented a significant numberof phenomena which were explicable in molecular terms, without ever attemptingto clarify the exact meaning of this concept.

Molecules could be anything for mid-nineteenth-century science. Even in themost formal expression of molecular theorising, that of Maxwells kinetic theoryof gases, the nature and structure of the material particle was not directly addresses.On the contrary, as Peter Harman accurately points out, the gradual elaboration of


Maxwells theory was such that left open the question of the physical nature ofthe molecules.91 As late as 1873, basic characteristics of the individual particles,like absolute mass and diameter of molecules, as well as their number in a givenvolume, were still undecided, as Maxwell clarified in his review of the field, in thearticle on Molecules.92

This attitude did not make molecular discourse incoherent. Quite the opposite:the determination of a minimal set of constraints ensured that molecular explana-tions did not disturb the established framework of dynamics. Notice that Maxwellsadvocacy of molecular explanations was based on a two-step argument: first thatOur definition of a molecule is purely dynamical and second that [t]his kind ofreasoning [i.e. dynamical reasoning] . . . has a high degree of cogency.93,94 Yearslater, talking at the British Association for the Advancement of Science, HoraceLamb would say that what characterised the modern school of English physicistswas this attempt to make out . . . how much can be recognised as a manifestationof general dynamical principles.95

3. Molecular Pictures Conditions of Intelligibility

As we have discussed above, molecular representations were adequately diversifiedso as to encourage and sustain a pluralistic discourse about physical phenomena.At the level of epistemic content, the requirements that this discourse should fulfilwere minimal: acceptance of the particulate nature of gross matter, establishmentof causal links between physical phenomena and underlying mechanisms, and sub-mission of all molecular models to the requirements of the framework of dynamics.By keeping the prerequisites at a minimum number, molecular discourse couldplay an important role in the establishment of a broad consensus among scientists.Genevive Paichelers review of studies in the psychology of group interactionmakes it obvious that the establishment of a consensus presupposes a sharing of[the] same representations by both influencer and influenced . . . and their referenceto the same universe of meanings.96

No such broad consensus could be established for any specific molecular modelat the period under consideration, because it was obvious to scientists that at thepresent state of knowledge a phrase which was often repeated during the nine-teenth century and expressed the measured optimism of the scientific communityfor the development of science any final suggestion concerning the structure ofthe individual molecule would violate basic philosophical requirements. For, ex-ample, the fact that Stokes never attempted to work out a coherent molecular theoryof fluid motion, in spite of the fact that he frequently referred to the unobservablelevel of reality, is interpreted, by David Wilson, as a sign of Stokes characteristicattitude, a blend of caution and realism.97 According to this interpretation, itwas impossible for Stokes to subscribe to any final molecular model since it wasobvious, to him and to his peers, that a multiplicity of molecular hypotheses mightbe true, in so far as none could be finally proved or disproved.

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Leaving aside, temporarily, all questions concerning the realistic content of theirmodels, scientists often linked molecular concepts with mental states, and in sodoing they established the notion of intelligibility as a central aim of scientificinvestigation. William Grove, for example, in his Friday lecture at the Royal Insti-tution on January 25, 1858, connected explicitly the conception of molecules witha line of reasoning which leads the mind to regard the so-called imponderables as amode of motion with the ultimate goal of comprehending natural phenomena.98In 1871, Peter Guthrie Tait expressed his conviction that this splendid suggestionof Vortex-atoms, if it be correct, will enable us thoroughly to understand matter.99William Thomson believed that Stokes elastic solids must when we understandthem properly, be recognised as properly packed crowds of vortices.100 GeorgeGore stated that a scientific investigation of electrolysis should aim at a moreclear understanding of the circuit and the circulation of the electric forces.101 In1863, A. W. Williamson argued in a reverse form, saying that chemical formulae[contrary to atomistic conceptions of chemical reactions] give no physical imageof the process by which the reaction is brought about.102 Scott Russell believedthat it is only by supposing some kind of molecular change that the existence ofsuch a wave [i.e. a sound wave] can be conceived to be possible.103

The suggestion that molecular discourse constituted, for many years, just away of thinking which infused intelligibility into scientific observations may seemparadoxical. The notion of theory has been so catalytic in the interpretationsof scientific development that it is difficult to believe that the fluid and unstruc-tured molecular discourse of the nineteenth century was of any significance toscience. And the emphasis put on mathematical results and on testable predictionssometimes conceals that fact that, as Norwood Russell Hanson puts it, scientificendeavour aims towards a conceptual pattern in terms of which . . . data will fitintelligibly.104 The establishment of a closure which presupposes, among otherthings, a broad consensus among the members of the scientific community aboutthe meaning of scientific terms, is indeed the final product of the process of doingscience. However, in the absence of conclusive evidence about the existence anddetailed nature of the individual molecule, no such closure could be produced fora long time. Molecular discourse became an extended thought experiment of thefollowing form: what can we understand about physical phenomena if moleculesare billiard balls, elastic springs, aetherial vortices, etc.? What would the world belike if molecules are in motion, if they rotate, if they vibrate, if they occupy stablepositions? Concepts like the incubation of ideas, the mental completion of no-tions referring to unobservable entities, or even what Crovits called the tinkeringwith a model are more apt for understanding this situation.

In its etymological definition molecule was, merely, a small mass, a simplenotion that scientists could manipulate in their thought so as to subject it to a pro-cess of mental completion, utilising the evidence they had gathered from diverseinvestigations. According to John Tyndall, who believed that imagination was afaculty inherent in human mind, scientific investigation could reveal the unseen by


prolonging the mental vision beyond the boundaries of sense, [until] we see [the]atoms.105 The creation of pictures is frequently encountered in nineteenth-centurymolecular discourse. One could, indeed, say that it was the essence of moleculardiscourse, because it participated in the structure of the arguments: unless his audi-ence could see that the particles of water describe certain curves, and not others, inflowing past a solid immersed in water, William Rankine could not explain why hepreferred to construct his ships in accordance with the plane water-lines.106 Theconstruction of large and fast-moving vessels required an understanding of fluidresistence which, in turn, required, among other things, that the engineer couldsee how water particles slide over the surface of the ship and even to estimate,approximately, what was their velocity relatively to the velocity of the ship.107

These mental pictures had a distinctive dynamical character: once introduced,they gained autonomy and could be mentally modified in accordance to specific anddiverse requirements. During this process, scientists were synthesising differentaspects of reality into a single picture and set this picture in motion, so to speak, inorder to see what kind of meaningful extrapolations could be produced that wouldmake sense in the light of relevant observations. It was perfectly clear to Rankinethat the motion of a steady current in relation to a moving ship could not be actuallyseen, but it could be represented to the eye and to the mind by means of a groupof stream-lines.108

The phrase the minds eye has been largely used by those thinkers who wishedto establish the significance of visual imagery in cognition,109 and has been ri-diculed by those who believed that all knowledge is ultimately prepositional.110Within psychology, these two approaches gave rise to a furious debate with strongarguments and experimental support in both sides.111 The central issue in this de-bate concerns the format in which information is encoded in the brain. Imagistsargue that information is represented in the brain as a spatial arrangement; non-imagists argue that all information is encoded in a propositional format while thecreation of mental images is merely an epiphenomenon. However important thequestion is in relation to the neurological basis of mental processes, for the purposeof the present analysis it is sufficient to recognise the significance of visualisationin the process of thinking, at a level where both sides agree images are indeedcreated.112 Cases from the history of science113 and technology114 have establishedthe significance of visual thinking.

The problmatique of this area of study is obviously relevant to molecular dis-course. Before mathematical formalism, before the emergence of testable theories,before the establishment of an orthodoxy, molecules were mere pictures in theminds of scientists in the dreams of scientists, William Crookes said.115 Thecreation of such pictures was not antithetical to verbal thinking; molecular concep-tualisations were not drawn but verbally described in scientific papers. Althoughvaluable, the distinction which some analysts attempt to establish between theverbal and the visual is meaningless for the material presented in this paper.116The creation of mental images within molecular science signified a moment of

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concentration, a conscious effort to focus attention on what was observed, actuallyto go beyond the level of physical vision, to reason and to articulate what could beunder the surface of phenomena, what could act under the surface of phenomena,in order for these phenomena to be intelligible.

For major figures of the nineteenth century the intelligibility of molecular rep-resentations was almost self-evident. Maxwell, for example, argued that molecularexplanations were complete because they reduced phenomena to ideas, like con-figuration, motion, mass and force [which] . . . are so elementary that they cannotbe explained by means of anything else.117 This view resonates with WilliamWhewells philosophy of fundamental ideas which were supposed to be inherentin the mind and reflected the existing harmony between the mind of God and themind of man.118 However, one should not underestimate the fact that the frameworkof analysis which made the basic ideas of molecular discourse intelligible had along history in scientific thought and shaped in very specific ways the scientistsperception of the world: on the one hand, the unchallenged authority of Newtonensured the validity of the dynamical framework of analysis; on the other hand, theancient idea of material particles came to the foreground and gained visibility, onceagain, through the intense preoccupation of Victorians with Greek antiquity.119

Scientific ideas do not operate into a cultural vacuum. The pre-existing pat-terns of thought play an important role in the establishment of the self-evidentelements of science. The utility of molecular discourse can be understood alsoin this light: during this process of mental manipulation of molecules, thesehypothetical constructs became familiar entities for scientists. According to Her-bert Simon anything can become a symbol through repeated exposure to it, orfamiliarisation.120 Gerald Holton argues that when a certain picture recurs inmany situations, it becomes an ordering element, a concept.121 Also, certain ideas,which have emerged in the context of anthropological studies, can be applied inthe situation of molecular discourse. Hence, Mary Douglas approach which fo-cuses on the role of exemplars, whose properties are collectively agreed and whoselearning actually constitutes a community,122 and Godfrey Lienhardts model ofcognition based on repeated enactement of exemplars, are crucially significant atthis point.123

According to such views it is not difficult to understand that molecular discourseconstituted a process of familiarisation to which scientists were exposed and whichestablished molecule as a symbol in scientific semantics. Given the situation asdescribed in this paper, given the absence of any conclusive evidence about theinner structure of the individual particle, the frequent appearance of molecularexplanations prepared the scientific community to accept, later, an ontology ofelectrons. From this perspective Erwin Hieberts conclusion that [t]he fundamentalsignificance of the corpuscular theory of matter for physics came about only afterthe discovery of the electric atom (the electron)124 is also understandable.


4. Conclusion

In this paper I attempted to understand the cognitive dimension of nineteenth-century molecular discourse. In the light of the view that science is not confined inmathematical equations and rational deductions, but it has something to do with theconstruction of intelligible accounts of phenomena and the production of meaningfor the physical world, it has been argued that, for a long period of time, the basicoperation of molecular representations was to infuse intelligibility into scientificexplanations by introducing causal links between the level of phenomena and thatof underlying mechanisms, by generating possible pictures of the unseen level ofphysical reality, and by creating the conditions of an extended thought experimentwhich examined alternative possibilities. For the investigation of such issues theavailable historical narratives are not sufficient. Beyond the question of rationalchoices, beyond strict categories established in the philosophy of science, beyondeven the sociologists attempt to connect dogmatic adherence to certain scientificideas with the promotion of specific social interests, the relatively new field ofthe psychology of science encourages the historian to seek something meaningfulin the unstructured but tolerant consideration of molecularity, in the free dialogueof scientists which lay outside the formal theoretical-mathematical framework ofscience.

Expressing a strong belief that this kind of synthetic approach to the investiga-tion of processes of thinking which, potentially, could be of great value to scienceeducation, I shall attempt in this concluding section, to sketch some possible routeswhich will bring the above analysis to bear upon the situation that pupils face whenthey are first introduced to basic concepts of physics. And in order to do so, I shalluse the curriculum of physics in Greek grammar school as a case study.

For molecular science, the extrapolation from the historical situation into con-temporary Greek classroom is, I believe, permissible, not so much because of thecorrespondence that some analysts attempt to establish between the unmediatedunderstanding of pupils and some obsolete scientific beliefs, but because the in-troduction of molecular ideas in the Greek curriculum of physics have specificsimilarities with nineteenth-century molecular discourse. First, the presentation ofvarious aspects of molecular activity in the grammar-school curriculum is equallysporadic and fluid as it was in the nineteenth century. Second, molecular represent-ations are evoked both in teaching and in molecular discourse precisely when somemeasure of physical understanding of the examined phenomena is required. Third,the ultimate structure of the individual molecule is not a prerequisite of molecularexplanations of physical phenomena in either setting.125

Given the requirement for some basic understanding of the submicroscopicbehaviour of matter, and given the fact that the investigation of the ultimate struc-ture of matter, as it is now understood by physicists, is beyond the capacity ofyoung children, grammar-school textbooks limited the scope of teaching mostlyto that level of molecular activity which was addresses by nineteenth-century mo-

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lecular discourse: electrolysis,126 diffusion,127 cohesion,128 adhesion,129 chemicalcomposition,130 motion of liquid currents.131 This presentation is indeed highlyunstructured: various pieces of information are scattered in various chapters; noattempt is ever made to unite these partial evidence into a coherent picture.

According to Konstantinos Tzanakis meticulous analysis of the Greekgrammar-school curriculum of physics, this unstructured presentation of the sub-microscopic realm is highly problematic mainly because no experiment whichcould provide evidence for the existence of molecules can be performed at the levelwe are referring to [i.e. 5th and 6th grade of grammar school].132 Gaining supportfrom various studies which highlight the fact that pupils do not internalise easily thebasics of the particulate nature of matter, Tzanakis argues that it is not at all obvi-ous that such a reductionist presentation of physical phenomena is paedagogicallysound. However, it is significant to notice, at this point, that the paedagogical roleof molecular concepts was the most powerful argument of molecularists againstthe scepticists attacks, which took a most combative form toward the end of thenineteenth century.133

We may resolve this apparent paradox if we consider the philosophical pre-suppositions and the kind of historical interpretations which inform the viewthat molecular concepts are difficult for the pupils to grasp. First, both ArnoldB. Arons conclusion that students should be allowed to doubt with the earlyparticipants, and to articulate uneasiness about interpretation of some of theevidence concerning the atomic-molecular picture,134 and Tzanakis assertionthat the atomic structure of matter . . . constituted a central point of intense oreven extravagant scientific and philosophical debates135 seem to be based uponearlier interpretations of what has been called the anti-atomic feelings of nine-teenth century.136 Based upon partial evidence regarding great anti-atomists ofthe nineteenth century, these interpretations underestimated the significance of alarge corpus of nineteenth-century studies which actually presupposed the exist-ence of material particles. More recent historical interpretations have qualified thesignificance and extent of the doubts expressed about the existence of materialparticles.137

Second, Joseph Nussbaums observation, that it is important for teachers torealise that molecular concepts are theoretical constructs based upon hypotheseswhich lay beyond direct perception,138 is significant but misplaced, in so far asmost basic physical concepts, which are taught in grammar school, cannot be dir-ectly perceived. In spite of the fact that physics curricula often adopt the rhetoricof empiricism, putting into a prominent position experimental results and directmeasurements, the conceptual content of science, even at the grammar-school level,is highly theoretical. Energy, electrical charge, field are equally unobservableand equally important for the teaching of physics as molecules are. On the otherhand, the most tangible thing of all, mass cannot be given any proper scientificexplanation,139 and it is left to be understood gradually through the examination ofvarious phenomena.140


At some very basic level all science has to do with the unobservable and,instead of demonising certain concepts, it may be more productive to seek a methodof presentation which could make the unobservable intelligible. John Tyndall putthis view in the most elegant form when he argued that a central goal of science isto enable us by means of the tangible . . . to apprehend the intangible.141 And hisimaginative lectures were so exciting exactly because he promised to his audiencea journey towards the unknown: I wished . . . to take you beyond the boundaryof mere observation, into a region where things are intellectually discerned, hesaid.142

The distinction between realism and scepticism which is, frequently, put for-ward, in order to understand scientific attitudes towards the hidden aspects ofthe world, is one of those familiar dualisms which haunts various intellectualdiscourses. Beyond realism and scepticism, the attitude of nineteenth-century mo-lecularists can assist us in understanding what it means to suspend judgement.When Tyndall presented his views about the nature of heat he made it clear thatthe great point, at present, is to regard it as motion of some kind, leaving its moreprecise character to be dealt with in the future.143 The same view echoes in thewritings of Maxwell who admitted that [e]very hot body, therefore, is in motionjust to add, immediately afterwards, that [w]e have next to inquire into the natureof this motion.144 This perception of a dynamic transformation of the matrix ofscience, which was very clear to the participants of the rapidly evolving nineteenth-century science, should be made explicit to young pupils for various reasons: first,it will assist them to understand the complex reality of scientific practice; second,it will provide some easily understood notion of the ethics of scientific activitywhich involves, among other things, openness to future re-examination or evenfalsification of established views; and third, it will make their basic experiencein science education, namely the gradually expanding area of investigation inconsecutive levels of study, intelligible. Such a presentation of science, not as afinal and rigid body of irrefutable knowledge, but as a process which enhance ourawareness both of physical phenomena and of our own limitations to comprehendthem, will resolve a serious problem in the teaching of molecular phenomena whichhas been identified by Tzanakis, namely the identification, on behalf of pupils, ofthe provisional molecular pictures, which are introduced as a means to understandmacroscopic phenomena, with the ultimate structure of matter.145

I believe that once this basic understanding is achieved, educationists may wishto look deeper into the way of thinking suggested by nineteenth-century molecu-larists in order to investigate what it means for something to be easily understoodand what are the specific conditions which make it easily understood. For ex-ample, Joseph Nussbaums investigation showed that 14-years-old pupils cannotinternalise, to the same degree, the 5 specific propositions of the particulate modelof gases; the best understood proposition concerns the existence of unobservablemolecules.146 If we turn, now, to the historical record, we shall realise that this isnot a peculiar phenomenon. Maxwell, the scientist who established this model, as-

442 MARIA YAMALIDOU

serted that the sole proposition which constitutes the molecular theory of matter is[t]he doctrine that visible bodies consist of a determinate number of molecules.147The acceptance of this doctrine created a language and a way of thinking, in otherwords it created the intellectual conditions which enabled scientists to translatemacroscopic phenomena into molecular terms. According to Maxwell, through thisbasic molecular understanding of matter [w]e have now arrived at a conceptionof a body,148 and not at any final description of the specific structure of matter.This would be the result of a long process which involves mental manipulation ofthe available data, instruments, and techniques, negotiation of different meanings,active thinking, and the production of meaningful and testable deductions.

If the above analysis of parallelisms and similarities between the historicalsituation of nineteenth-century molecular discourse and the teaching of the basicconcepts of physics in grammar school is meaningful, a set of questions shouldbe properly investigated. To what extent, for example, should grammar-schoolpupils be exposed to molecular discourse in order to be able to become familiarwith these difficult unobservable interactions? A number of phenomena whichencourage easy and elegant molecular conceptualisations temperature, expansionand contraction of gases, friction, energy conservation are put into an entirelymacroscopic framework, even though they are introduced after the discussion ofthe molecular structure of matter in the textbook for the 5th grade of the Greekgrammar school. Would it not be, perhaps, more fruitful to let grammar-schoolpupils play for some time with certain basic ideas of the molecular frameworkbefore we present to them the true picture of a molecule?149 Is it not, perhaps,more significant for pupils to be allowed to explore possible causal links betweenphenomena and underlying mechanisms by giving them the time to realise the lim-itations of their erroneous suggestions? Lastly, since no final experimental proofconcerning the ultimate structure of matter can be presented to grammar-schoolpupils, would it not be meaningful, from an educational perspective, to explorethe significance of the specific mode of justification established by nineteenth-century molecular discourse, a mode of justification which brought together piecesof evidence from diverse fields of research in order to make obvious that molecularnotions can produce coherent explanations?

Given the fact that pupils bring into their explanations some commonsensical oreven idiosyncratic modes of thinking,150 given the fact that these improper explan-ations are persistent and cannot be easily overthrown through a mere expositionof true, scientific conclusions, it may be worth examining whether the creation,in the classroom, of a situation which resembles the fluid discourse of nineteenth-century molecularists facilitates the process of intellectual transformation which isthe aim of science education.


Acknowledgement

I would like to thank the Max Planck Institute for the History of Science, Berlinfor the post-doctoral research grant which enabled me to complete this work.

Notes1 Peter Gay, Freud for Historians (Oxford, 1985).2 William McKinley Runyan, in Life Histories and Psychobiography. Explorations in Theory andMethod (Oxford, 1984).3 See, for example, J. Cattell, A Statistical Study of Eminent Men, Popular Science Monthly (Feb-ruary 1953): 359377; R.B. Cattell and I.E. Drevdahl, Comparison of the Personality Profiles ofEminent Researchers with Those of Eminent Teachers and Administrators and the General Pop-ulation, British Journal of Psychology 46 (1955): 248261; A. Roe, A Psychological Study ofEminent Biologists, Psychological Monographs 65 (1951): 168; idem., A Psychological Study ofEminent Psychologists and Anthropologists, and a Comparison with Biologists and Physical Scient-ists, Psychological Monographs 67 (1953); L.M. Bachtold, Women, Eminence and Carrer-ValueRelationships, Journal of Social Psychology 95 (1975): 187192; Doris B. Wallace, Giftednessand the Construction of a Creative Life, in F.D. Horowitz and M. OBrien (eds.), The gifted andtalented: Developmental perspectives (Washington, 1985); D.N. Jackson and J.P. Rushton (eds.),Scientific Excellence: Origins and assessment (Newbury Park, 1987).4 See Thomas A. Kohut, Psychohistory as History, The American Historical Review 91 (1986):336354, on p. 337.5 J.L. Chambers, Relating Personality and Biographical Factors to Scientific Creativity, Psycholo-gical Monographs 78 (1964): 120; I.I. Mitroff, The Subjective Side of Science (Amsterdam, 1974),R.K. Merton, Behavior Patterns of Scientists, American Scholar 38 (1969): 197225; M.B. Parloff& L. Datta, Personality Characteristics of the Potentially Creative Scientists, in J.H. Masserman(ed.), Science and Psychoanalysis, Vol. 7, (New York, 1965), in pp. 91106; C.S. Fisher, SomeSocial Characteristics of Mathematicians and Their Work, American Journal of Sociology 78 (1973):10941118; L.E. Datta, Family Religious Background and Early Scientific Creativity, AmericanSociological Review 32 (1967): 626635; L.V. Blankenship, The Scientist as Apolitical Man ,British Journal of Psychology 24 (1973): 269287; R. Helson, Sex Differences in Creative Style,Journal of Personality 35 (1967): 214233; J. Joesting, The Influence of Sex Roles on Creativity inWomen, Gifted Child Quarterly 19 (1975): 336339.6 R. Fisch, Psychology of Science, in K.D. Knorr-Certina (ed.), Science, Technology and Society(Dordrecht, 1980), pp. 277318.7 See, Ronald N. Giere, Introduction: Cognitive Models of Science, in Ronald N. Giere (ed.),Cognitive Models of Science (University of Minnesota Press, Minneapolis, 1992), pp. xvxxviii.8 Ronald N. Giere, Explaining Science; A cognitive approach (The University of Chicago press,Chicago and London, 1988), on p. 1.9 William R. Shadish Jr. & Robert A. Neimeyer, Contribution of Psycology to an Integrative ScienceStudies: The Shape of Things to Come, in The Cognitive Turn; Sociological and PsychologicalPerspectives on Science (Dordrecht, 1989), pp. 1338, on p. 13.10 Howard E. Gruber, G. Terrell & M. Wertheimer (eds.), Contemporary approaces to creativethinking (New York, 1962); Howard E. Gruber, Crativit et fonction constructiive de la rpti-tion, Bulletin de Psychologie de la Sorbonne 30 (1976): 235; R.J. Sternberg (ed.), The nature ofcreativity: Contemporary psychological perspectives (Cambridge, Mass., 1988); Doris B. Wallace &Howard E. Gruber, Creative People at Work (Oxford, 1989).11 Nancy Nersessian, Constructing and Instructing: The Role of Abstraction techniques in Cre-ating and Learning Physics, in Richard A. Duschl & Richard J. Hamilton (eds.), Philosophy of

444 MARIA YAMALIDOU

Science, Cognitive psychology, and Educational Theory and Practice (New York, 1992), pp. 4868,on p. 53.12 David Gooding, Experiment and the Making of meaning; Human Agency in Scientific Observationand Experiment (Dordrecht, 1990), on p. 25.13 Thomas H. Kuhn, The Structure of Scientific Revolutions (Chicago, 1961).14 Ronald N. Giere, Introduction: Cognitive Models of Science, in Ronald N. Giere (ed.), CognitiveModels of Science (Minneapolis, 1992), pp. xvxxviii.15 Nancy Nersessian, Constructing and Instructing: The Role of Abstraction techniques in Creat-ing and Learning Physics, in Richard A. Duschl & Richard J. Hamilton (eds.) Philosophy of Science,Cognitive psychology, and Educational Theory and Practice (State University of New York Press.1992), pp. 4868.16 Michelene, T. H. Chi, Conceptual Change within and across Ontological Categories: Examplesfrom Learning and Discovery in Science, in Ronald N. Giere (ed.), Cognitive Models of Science(University of Minnesota Press, Minneapolis, 1992), pp. 129186.17 Ronald N. Giere, The Units of Analysis of Science Studies, in The Cognitive Turn; Sociologicaland Psychological Perspectives on Science (Dordrecht, 1989), pp. 311.18 Ronald N. Giere, The Cognitive Strucutre of Scientific Theories, Philosophy of Science 61(1994): 276296.19 P.N. Johnson-Laird, Mental Models (Cambridge, 1983).20 See Nancy Nersessian, How Do Scientists Think? Capturing the Dynamics of Conceptual Changein Science, in Ronald N. Giere (ed.), Cognitive Models of Science (University of Minnesota Press,Minneapolis, 1992), pp. 344; also Nancy J. Nersessian, Should Physicists Preach What TheyPractice?, Science & Education 4 (1995): 203226.21 David Gooding, Experiment and the Making of meaning; Human Agency in Scientific Observationand (Dordrecht, 1990).22 Doubts about the usefulness of such an approach have been expressed, for example, by SteveWoolgar, Representation, cognition and self: What hope for an integration of psychology and soci-ology, in The Cognitive Turn; Sociological and Psychological Perspectives on Science (Dordrecht,1989), pp. 201224.23 Terry Shinn, Cognitive Process and Social practices: Experimental macroscopic Physics, in TheCognitive Turn; Sociological and Psychological Perspectives on Science (Dordrecht, 1989), pp. 119150, on p. 119.24 Stephen Toulmin, The Philosophy of Science; An Introduction (London, 1962), on p. 138.25 T. C. Chamberlin, The Method of Multiple Working Hypotheses, Science 15 (1890): 9296.26 William R. Shadish Jr. & Robert A. Neimeyer, Contribution of Psycology to an Integrative Sci-ence Studies: The Shape of Things to Come, in The Cognitive Turn; Sociological and PsychologicalPerspectives on Science (Dordrecht, 1989), pp. 1338, on p. 17.27 Nancy J. Nersessian, Should Physicists Preach What They Practice?, Science & Education 4(1995): 203226, on p. 208.28 Howard E. Gruber & P.H. Barrett, Darwin on Man; A Psychological Study of Scientific Creativity;excerpt published in Ryan D. Tweney, Michael E. Doherty, Clifford Mynatt, On Scientific Thinking(New York, 1981), pp. 340354, on p. 342.29 Johan de Kleer, Multiple Representations of Knowledge in a Mechanics Problem-solver, Pro-ceedings of the 5th international Joint Conference on scientific Intelligence (1977): 299304, on p.299.30 See David Gooding, Experiment and the Making of Meaning; Human Agency in ScientificObservation and Experiment (Dordrecht, 1990), on page 108.31 Ryan D. Tweney, Michael E. Doherty & Clifford Mynatt, On Scientific Thinking (New York,1981), on p. 285.


32 Nancy Nersessian, How Do Scientists Think? Capturing the Dynamics of Conceptual Change inScience, in Ronald N. Giere (ed.), Cognitive Models of Science (Minneapolis, 1992), pp. 344, onp. 24.33 Arthur I. Miller, Imagery and Meaning, The Cognitive Science Connection, International Stud-ies in the Philosophy of Science 5 (1991): 3548; Arthur I. Miller, Imagery in Scientific Thought:Creating 20th-century Physics (Cambridge, Mass., 1986).34 Michael J. Webb, Analogies and their Limitations, School Science and Mathematics 85 (1985):645650.35 Nancy Nersessian suggests a reverse hierarchy arguing that imagistic reasoning is a species ofanalogical reasoning; see Nancy Nersessian, How Do Scientists Think? Capturing the Dynamics ofConceptual Change in Science, in Ronald N. Giere (ed.), Cognitive Models of Science (Minneapolis,1992).36 Michael Matthews provides a review of the historical developments which brought about thischange and provides a valuable and comprehensive bibliography: Michael R. Matthews, His-tory, Philosophy, and Science Teaching, Synthese 80 (1989): 17; Michael R. Matthews, History,Philosophy and Science Teaching: A Bibliography, Synthese 80 (1989): 185195.37 See, for example, Michelene, T.H. Chi, Conceptual Change Within and Across Ontological Cat-egories: Examples from Learning and Discovery in Science, in Ronald N. Giere (ed.), CognitiveModels of Science (Minneapolis, 1992), pp. 129186.38 See for example, Susan Carey, The Origin and Evolution of Everyday Concepts, in Ronald N.Giere (ed.), Cognitive Models of Science (Minneapolis, 1992), pp. 89128.39 For the implications of this phenomenon in the interconnection of history of science and cog-nitive science see William R. Shadish Jr. & Robert A. Neimeyer, Contribution of Psycology to anIntegrative Science Studies: The Shape of Things to Come, in The Cognitive Turn; Sociological andPsychological Perspectives on Science (Dordrecht, 1989), pp. 1338.40 Nancy Nersessian, How Do Scientists Think? Capturing the Dynamics of Conceptual Change inScience, in Ronald N. Giere (ed.), Cognitive Models of Science (Minneapolis, 1992), pp. 344, onp. 40.41 See for example Stephen G. Brush, Statistical Physics and the Atomic Theory of Matter, fromBoyle and Newton to Landau and Onsager (New Jersey, 1983).42 See, for example, Robert Loqueneux, Bernard Maitte & Bernard Pourprix, Les statuts epistemo-logiques des modeles de la theorie des gas dans les uvres de Maxwell et Boltzmann, FundamentaScienti 4 (1983): 2954; Stephen G. Brush, The Development of the Kinetic Theory of GasesIII. Clausius, Annals of Science 14 (1958): 185196; Eric Mendoza, The Kinetic theory of matter18451855, Archives International dHistoire des Sciences 32 (1982): 184220; Elizabeth Garber,Clausiuss and Maxwells Kinetic Theory of Gases, Historical Studies in the Physical Sciences 2(1970): 299312.43 Donald Franklin Moyer, Continuum Mechanics and Field Theory: Thomson and Maxwell, Stud-ies in the History and Philosophy of Science 9 (1978), 3550; idem., Energy, Dynamics, HiddenMachinery: Rankine, Thomson and Tait, Maxwell, Studies in History and Philosophy of Science 8(1977), 251268.44 On the extent and significance of molecular discourse for nineteenth-century British scientists seeMaria Yamalidou, Thinking in molecular terms; British science around the middle of the nineteenth-century (Unpublished Ph.D. Thesis; University of Lancaster, September 1996).45 William Thomson referred to the essential conditions of any molecular theory of matter in Noteon Gravity and Cohesion [1862], Proceedings of the Royal Society of Edinburgh 4 (18571862):604606.46 George Gore, On the Molecular Properties of Antimony [abstract], Proceedings of the RoyalSociety of London 9 (1857): 707; idem., On a Momentary Molecular Change in Iron Wire,Proceedings of the Royal Society of London 17 (18681869): 260265; idem., On the MolecularMovements and Magnetic Changes in Iron etc. at different Temperatures, Philosophical Magazine

446 MARIA YAMALIDOU

(4th series), 40 (1870): 170177; idem., Mechanical Energy of Molecules of Gases, PhilosophicalMagazine (5th Series), 37 (1895): 340 and 508.47 John Scott Russells work on the behaviour of liquids is permeated by diverse molecular concep-tualisations. See, for example, John Scott Russell, Report on Waves, Report BAAS 14th Meeting(York, September 1844), pt. 1, pp. 311390; idem., The Modern System of Naval Architecture, Vol.1 (London, 1865), especially the paragraph entitled Molecular effect of wave motion, on p. 175;John Scott Russell & John Robinson, Report of the Committee on Waves, Report BAAS 7th Meeting(Liverpool, September 1837), pt. 1, pp. 417496.48 See, for example, Graham, Thomas, On the Molecular Mobility of Gases, PhilosophicalTransactions of the Royal Society of London (1863): 385405.49 On the internal mobility of liquids and solids see, for example, George Gabriel Stokes, On theTheories of the Internal Friction of Fluids in Motion, and of the Equilibrium and Motion of ElasticSolids [1845], Transactions of Cambridge Philosophical Society 8 (1849): 287319 and RobertWarrington, On a Re-arrangement of the Molecules of a Body after solidification, PhilosophicalMagazine (3rd Series), 20 (1842): 537539.50 Robert Mallet, On the Relation of Molecular Forces to Geology, Journal of the GeologicalSociety of Dublin 3 (1844): 2346.51 See, for example, John Hughes Bennett, On the Molecular Theory of Organization [1861],Proceedings of the Royal Society of Edinburgh 4 (18571862]: 436446.52 On the rhetorical function of empiricist language see Richard Yeo, An Idol of the Market-place:Baconianism in 19th-century Britain, History of Science 23 (1985): 251298.53 George Gabriel Stokes, On the Theories of the Internal Friction of Fluids in Motion, and of theEquilibrium and Motion of Elastic Solids [1845], Transactions of Cambridge Philosophical Society8 (1849): 287319.54 On the heuristic role of models in science see also: Mary Hesse, The Structure of Scientific Infer-ence (London; 1974); Mary Hesse, Models and Analogies in Science (Notre Dame, 1970); MichaelRedhead, Models in Physics, The British Journal for the Philosophy of Science 31 (1980): 145163; R.B. Braithwaite, Models in the Empirical Sciences, in Ernest Nagel, Patrick Suppes & AlfredTarski (eds.), Logic, Methodology and Philosophy of Science (Stanford California, 1962); WilfridSellars, Philosophical Perspectives (Springfield Illinois, 1967).55 See Kostas Gavroglu, Reaction of the British Physicists and Chemists to van der Waalss EarlyWork, Historical Studies in the Physical Sciences 20 (1990): 239; see also Kostas Gavroglu,Differences in Style as a Way of Probing the Context of Discovery, Philosophica 45 (1990): 5375.56 Sign B.B. Goldstein, The Concept and the Significance of the Model in Physics (UnpublishedPh.D Thesis; Columbia University, 1969), on p. 137.57 Thomas Tate, Experimental Researches on the Laws of Absorption of Liquids by PorousSubstances. II. On the Filtration of Liquids through different Porous Substances, PhilosophicalMagazine (4th Series), 21 (1861): 5765 and 115120, on p. 117.58 On the significance of molecular conceptualisations in hydrodynamics, see Maria Yamalidou,Molecular Ideas in Hydrodynamics, Annals of Science 552 (1998): 369400.59 Foster, On Molecular Constitution of Crystals, Philosophical Magazine (4th Series), 1 (1851):10811560 See Samuel Haughton, Remarks on Professor Potters Theory of Sound, Philosophical Magazine(4th series), 1 (1851): 332334, on p. 334; emphasis added. On Poissons cuboidal molecule see IvorGrattan-Guinness, Convolutions in French Mathematics, 18001840, Vol. 1 (Basel, 1990), on pp.462465.61 Robert Mallet, On the Molecular Constitution of the Metals of Ordnance, as Affecting its Con-struction and Its Wear in Service, Journal of the Royal United Service Institution (1858): 167206,on p. 185.


62 William J. M. Rankine, On the Mathematical Theory of Stream- Lines, Especially Those withFour Ffoci and Upwards, Proceedings of the Royal Society of London 18 (1870): 207209, on p.207.63 O. Richter, On the Chemical and Physical Principles in Connexion with the Specific Gravity ofLiquid and Solid Substances, Report BAAS 33rd Meeting (August and September 1863), pt. 2, pp.5455, on p. 54.64 Thomas Graham, On Liquid Transpiration in Relation to Chemical Composition, PhilosophicalTransactions of the Royal Society of London (1861): 372386, on p. 384.65 John Tyndall, On Poissons Theoretic Anticipation of magnecrystallic Action, Report BAAS22nd meeting (Belfast, September 1852), pt. 2, pp. 20-21, on p. 21; emphasis added.66 James Challis, Report on the Present State of the Analytical Theory of Hydrostatics andHydrodynamics, Report BAAS 3rd Meeting (Cambridge, 1833), pt. 1, pp. 131151, on p. 131.67 John Scott Russell, The Modern System of Naval Architecture, Vol. 1 (London, 1865), on p. 176.68 See Eric Mendoza, The Kinetic Theory of Matter 1845-1855, Archives International dHistoiredes Sciences 32 (1982): 184220.69 On the process of stabilization of meaning in Michael Faradays work see David Gooding, Ex-periment and the Making of Meaning; Human Agency in Scientific Observation and Experiment(Dordrecht, Kluwer, 1990).70 James Clerk Maxwell, Illustrations of the Dynamical Theory of Gases, reprinted in W.D. Niven(ed.), The Scientific papers of James Clerk Maxwell, Vol. 1 (Cambridge, 1890), pp. 377409, on p.378.71 James Clerk Maxwell, On the Dynamical Theory of Gases, reprinted in W. . Niven (ed.), TheScientific papers of James Clerk Maxwell, Vol. 2 (Cambridge, 1890), pp. 2678, on p. 33.72 In a letter of William Thomson quoted in Silvanus P. Thompson, The Life of William ThomsonBaron Kelvin of Largs, 2 vols. (London, 1910), Vol. 2, on p. 1104; emphasis added.73 Allan Collins & Dedre Gentner, Constructing Runnable Mental Models, Proceedings of the 4thAnnual Cognitive Science Society (1982): 8689, on p. 88.74 Allan Collins & Dedre Gentner, Multiple Models and Evaporation Processes, Proceedings of the5th Annual Cognitive Science Society (1983): 15, on p. 3.75 Ibid., on p. 5.76 Thomas Graham, On Liquid Transpiration in Relation to Chemical Composition, PhilosophicalTransactions of the Royal Society of London (1861): 372386, on p. 384.77 On the reception of Thomsons vortex atom see Robert H. Silliman, William Thomson: SmokeRings and Nineteenth-century Atomism, ISIS 54 (1981): 461474.78 In 1882, J.J. Thomson attempted to provide a vortex theory of chemical combination in an essayfor which he was awarded the Adams Prize. J.J. Thomson, A Treatise on the Motion of Vortex Rings(London, 1883).79 James Prescott Joule, On the Mechanical Equivalent of Heat and on the Constitution of Fluids,Report BAAS 18th Meeting (Swansea, August 1848), pt. 2, pp. 21-22.80 John Scott Russell, Report on Waves, Report BAAS 14th Meeting (York, September 1844), pt. 1,pp. 311390.81 Nineteenth-century scientists were aware of the fact that molecules lie far beyond the reach of themicroscope see John Tyndall, Optical Deportment of the Atmosphere in Relation to Putrefactionand Infection [1876], in John Tyndall, Essays on the Floating-Matter of the Air in Relation toPutrefaction and Infection (2nd edition; London, 1883), pp. 45129, on p. 76.82 James Clerk Maxwell, Atom [1875], reprinted in W.D. Niven, The Scientific Papers of JamesClerk Maxwell, Vol. 2 (Cambridge, 1890), pp. 445484, on p. 477.83 William Thomson, Steps Towards a Kinetic Theory of Matter, Report BAAS 54th Meeting(1884), pt. 2, pp. 613622, on p. 616.

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84 George Gabriel Stokes, On the Theories of the Internal Friction of Fluids in Motion, and of theEquilibrium and Motion of Elastic Solids [1845], Transactions of Cambridge Philosophical Society8 (1849): 287319.85 George Gore, A Deductive View of Electro-metallic Deposition, Pharmaceutical Journal 13(1854): 471478.86 James Thomson, On the Strength of Materials, as Influenced by the Existence or Non-existence of Certain Mutual Strains Among the Particles Composing Them, Cambridge and DublinMathematical Journal 3 (1848): 252258, on p. 252.87 James D. Dana, On Molecular Constitution of Crystals, Philosophical Magazine (4th series), 10(1855): 329; T. Foster, On Molecular Constitution of Crystals, Philosophical Magazine (4th series),10 (1855): 108115; Nevil Story Maskelyne, On the Insight Hitherto Obtained into the Nature ofthe Crystal Molecule by the Instrumentality of Light [1859], in William Lawrence Bragg & GeorgePorter (eds.), The Royal Institution Library of Science, Vol. 1 (Barking-Essex, 1970), pp. 293304.88 John Tyndall, On Molecular Influences. Part I. Transmission of Heat through Organic Structures,Philosophical Magazine (4th series), 6 (1853): 121138.89 M.D. Norris, On Certain Molecular Changes Which Occur in Iron and Steel During the SeparateActs of Heating and Cooling, Proceedings of the Royal Society of London 26 (1877): 127133.90

What is a Molecule?, Popular Science Monthly (1881): 688693. The paper was reproduced forChambers Journal 58 (1881): 298300.91 Peter M. Harman, The Natural Philosophy of James Clerk Maxwell (Cambridge, 1998), on p. 178.92 Ibid., on p. 180.93 James Clerk Maxwell, Atom [1875], reprinted in W.D. Niven, The Scientific Papers of JamesClerk Maxwell, Vol. 2 (Cambridge, 1890), pp. 445484, on p. 456.94 See Brush, Stephen G., The Kind of Motion We Call Heat, 2 vols. (Amsterdam, 1976); idem.,Statistical Physics and the Atomic Theory of Matter, from Boyle and Newton to Landau and Onsager(New Jersey, 1983); idem.95 Horace Lamb, Address to Section A. Mathematical and Physical Science, in Report BAAS 74thMeeting (Cambridge, August 1904), pt. 2, pp. 421432, on p. 423.96 Genevive Paicheler, The Psychology of Social Influence (English translation; Cambridge, 1988),on p. 208.97 David B. Wilson, Kelvin & Stokes. A Comparative Study in Victorian Physics (Bristol, 1987), onp. 110.98 W.R. Grove, Inferences from the Negation of Perpetual Motion, in William Lawrence Bragg& George Porter (eds.), The Royal Institution of Science; Being the Friday Evening Discourses inPhysical Sciences held at the Royal Institution: 18511939, Vol. 1 (Barking Essex, 1970), pp. 180187, on p. 187.99 Peter Guthrie Tait, Address to the Mathematical and Physical Section of the British Association,Report BAAS 41st Meeting (Edinburgh, August 1871), pt. 2, 18; emphasis added.100 Thomson to Stokes, 19 December 1872, in David B. Wilson (ed.), The Correspondence betweenSir George Gabriel Stokes and Sir William Thomson, Baron Kelvin of Largs, 2 vols. (Cambridge,1990), Vol. 2, on p. 379; emphasis added.101 George Gore, Inductive View of Electro-metallic Deposition, Pharmaceutical Journal 13(1854): 2128, on p. 27; emphasis added.102 See for example A.W. Williamson, Address by the President of the Chemical Section, ReportBAAS 33rd Meeting (Newcastle-upon-Tyne, AugustSeptember, 1863), pt. 2, pp. 2832, on p. 28;emphasis added.103 John Scott Russell, Report on Waves, Report BAAS 14th Meeting (York, September 1844), pt.1, pp. 311390, on p. 382; emphasis added.104 Norwood Russell Hanson, Patterns of Discovery: An Inquiry into the Conceptual Foundations ofScience (Cambridge, 1958); excerpts reprinted in Ryan D. Tweney, Michael E. Doherty & CliffordMynatt, On Scientific Thinking (New York, 1981), pp. 305312, on p. 307.


105 John Tyndall, Comparative View of the Cleavage of Crystals and State Rocks, PhilosophicalMagazine (4th Series), 12 (1856): 3548, on p. 35; emphasis added. On the significance of mo-lecular representations for Tyndalls work see Maria Yamalidou, John Tyndall, the Rhetorician ofMolecularity, Notes and Records of the Royal Society of London 53 (1999): 231242 and 319331.106 William J. M. Rankine, On Plane Water-lines in Two Dimensions, Philosophical Transactionsof the Royal Society of London 154 (1864): 369391.107 William J. M. Rankine, On the Resistance of Ships, Report BAAS 32nd Meeting (1862): pt. 2,pp. 263264, on p. 263.108 William J. M. Rankine, On the Mathematical Theory of Stream-lines, Especially Those withFour Foci and Upwards, Proceedings of the

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