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Atomism from the 17th to the 20th Century

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Atomism from the 17th to the 20th CenturyFirst published Thu Jun 30, 2005; substantive revision Thu Oct 28, 2010

Atomism in the form in which it first emerged in Ancient Greece was ametaphysical thesis, purporting to establish claims about the ultimatenature of material reality by philosophical argument. Versions of atomismdeveloped by mechanical philosophers in the seventeenth century sharedthat characteristic. By contrast, the knowledge of atoms that is now takenfor granted in modern science is not established by a priori philosophicalargument but by appeal to quite specific experimental results interpretedand guided by a quite specific theory, quantum mechanics. If metaphysicsinvolves an attempt to give an account of the basic nature of materialreality then it is an issue about which science rather than philosophy hasmost to say. A study of the path from philosophical atomism tocontemporary scientific atomism helps to shed light on the nature ofphilosophy and science and the relationship between the two.

From the nineteenth century onwards, when serious versions of scientificatomism first emerged, the philosophical relevance of a history ofatomism becomes epistemological rather than metaphysical. Since atomslie far beyond the domain of observation, should hypotheses concerningthem form part of empirical science? There were certainly philosophersand scientists of the nineteenth century who answered that question in thenegative. Contemporary philosophers differ over the question of whetherthe debate was essentially a scientific one or a philosophical one. Wasthere a case to oppose atomism on the grounds that it was unfruitful orlacking in adequate experimental support, or did such a case stem fromsome general epistemological thesis, perhaps some brand of positivism,that ruled out of court any attempt to explain observable phenomena byinvoking unobservable atoms? Many contemporary philosophers see theultimate triumph of atomism as a victory for realism over positivism.Such claims are historical as well as philosophical, so it is important to

1

Such claims are historical as well as philosophical, so it is important toget the history straight when evaluating them. In this respect thephilosophical literature has yet to catch up with recent advances in thehistory of nineteenth-century chemistry. This entry gives an account ofthe key developments in atomism from the seventeenth century until thetime, early in the twentieth century, when the existence of atoms ceasedto be a contentious issue. The focus is on the epistemological status of thevarious versions, and on the relationship between science and philosophy.

1. Introduction2. Atomism in the Seventeenth Century

2.1 Atomism and the Mechanical Philosophy2.2 Mechanical Reductions and the Problem of Transdiction2.3 Natural Minima2.4 Seventeenth-Century Eclecticism

3. Newtonian Atomism3.1 Newton's Atomism3.2 Eighteenth-Century Developments in Newtonian Atomism

4. Chemical Atomism in the Nineteenth Century4.1 Dalton's Atomism4.2 The Status of Daltonian Chemistry4.3 Progress in Organic Chemistry Using Chemical Formulae4.4 Implications of Organic Chemistry for Atomism

5. Atomism in Nineteenth-Century Physics5.1 The Kinetic Theory of Gases5.2 The Status of the Kinetic Theory5.3 Phenomena Connected Via Atomism5.4 Thermodynamics as a Rival to Atomism

6. Brownian Motion6.1 The Density Distribution of Brownian Particles6.2 Further Dimensions of Perrin's Case

7. Concluding Remarks


2 Stanford Encyclopedia of Philosophy

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1. IntroductionVersions of atomism developed by seventeenth-century mechanicalphilosophers, referred to hereafter as mechanical atomism, were revivalsof Ancient Greek atomism, with the important difference that they werepresumed to apply only to the material world, and not to the spiritualworld of the mind, the soul, angels and so on. Mechanical atomism was atotally general theory, insofar as it offered an account of the materialworld in general as made up of nothing other than atoms in the void. Theatoms themselves were characterised in terms of just a few basicproperties, their shape, size and motion. Atoms were changeless andultimate, in the sense that they could not be broken down into anythingsmaller and had no inner structure on which their properties depended.The case made for mechanical atomism was largely prior to andindependent of empirical investigation.

There were plenty of seventeenth-century versions of atomism that werenot mechanical. These tended to be less ambitious in their scope thanmechanical atomism, and properties were attributed to atoms with an eyeto the explanatory role they were to play. For instance, chemicals wereassumed by many to have least parts, natural minima, with those minimapossessing the capability of combining with the minima of otherchemicals to form compounds.

The flexibility and explanatory potential of mechanical atomism wasincreased once Newton had made it possible to include forces in the listof their properties. However, there was no way of specifying those forcesby recourse to general philosophical argument and they were remote from

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by recourse to general philosophical argument and they were remote fromwhat could be established empirically also. Newtonian atomism was notfruitful as far as eighteenth-century experimental science is concerned.

It was only in the nineteenth century that atomism began to bearsignificant fruit in science, with the emergence of atomic chemistry andthe kinetic theory of gases. The way in which and the point at whichatomic speculations were substantiated or were fruitful is controversialbut by the end of the century the fact that the properties of chemicalcompounds are due to an atomic structure that can be represented by astructural formulae was beyond dispute. The kinetic theory of gases metwith impressive empirical success from 1860 until 1885 at least.However, it also faced difficulties. Further, there was the emergence andsuccess of phenomenological thermodynamics, which made it possible todeal with a range of thermal and chemical phenomena without resort toan underlying structure of matter. Atomism was rejected by leadingscientists and philosophers such as Wilhelm Ostwald, Pierre Duhem andErnst Mach up to the end of the nineteenth century and beyond. By thattime atomism had been extended from chemistry and the kinetic theory tooffer explanations in stereochemistry, electro-chemistry, spectroscopy andso on. Any opposition from scientists that remained was removed by JeanPerrin's experimental investigations of Brownian motion. However, thetask of explaining chemical properties in terms of atoms and theirstructure still remained as a task for twentieth century science.

Twentieth-century atomism in a sense represents the achievement of theAncient Greek ideal insofar as it is a theory of the properties of matter ingeneral in terms of basic particles, electrons, protons and neutrons,characterised in terms of a few basic properties. The major difference isthat the nature of the particles and the laws governing them were arrivedat empirically rather than by a priori philosophical argument.



Suggested Reading: Melson (1952) is a somewhat dated but stillinteresting and useful overview of the history of atomism from aphilosophical point of view.Chalmers(2009) is a history of atomism thatfocuses on the relationship between philosophical and scientific theoriesabout atoms.

2. Atomism in the Seventeenth Century2.1 Atomism and the Mechanical Philosophy

Influential versions of Greek atomism were formulated by a range ofphilosophers in the seventeenth century, notably Pierre Gassendi(Clericuzio, 2000, 6374) and Robert Boyle (Stewart, 1979 and Newman2006). Neither the content of nor the mode of argument for these variousversions were identical. Here the focus is on the version articulated anddefended by Robert Boyle. Not only was Boyle one of the clearest andablest defenders of the mechanical philosophy but he was also a leadingpioneer of the new experimental science, so his work proves to beparticularly illuminating as far as distinguishing philosophical andempirical aspects of atomism are concerned.

The mechanical philosophy differed from the atomism of the Greeksinsofar as it was intended to apply to the material world only and not tothe spiritual world. Apart from that major difference, the world-views arealike. Fundamentally there is just one kind of matter characterised by aproperty that serves to capture the tangibility of matter and distinguish itfrom void. Boyle chose absolute impenetrability as that property. Thereare insensibly small portions of matter that, whilst they are divisible inthought or by God, are indivisible as far as natural processes areconcerned. Boyle, misleadingly drawing on another tradition that will bediscussed in a later section, referred to these particles as minima naturaliaor prima naturalia. Here they are referred to as atoms, a terminology onlyvery rarely adopted by Boyle himself. Each atom has an unchanging

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very rarely adopted by Boyle himself. Each atom has an unchangingshape and size and a changeable degree of motion or rest. All propertiesof the material world are reducible to and arise as a consequence of thearrangements and motions of the underlying atoms. In particular,properties possessed by macroscopic objects, both those detectabledirectly by the senses, such as colour and taste, and those involved in theinteraction of bodies with each other, such as elasticity and degree ofheat, are to be explained in terms of the properties of atoms. Thoseproperties of atoms, their shape, size and motion, together with theimpenetrability possessed by them all, are the primary ones in terms ofwhich the properties of the complex bodies that they compose, thesecondary ones, are to be explained. Such explanations involve thefundamental laws of nature that govern the motions of atoms.

Not all of the mechanical philosophers were mechanical atomists.Descartes provides a ready example of a mechanical philosopher who wasnot an atomist insofar as he rejected the void and held that particles ofmatter could be broken down into smaller particles. The mechanicalphilosophers were divided on the question of the existence of the void,some sharing the opinion of the Greek atomists that void was a pre-requisite for motion but others, like Descartes, rejecting the void asunintelligible and hence regarding all motion as involving thesimultaneous displacement of closed loops of matter whether that matterbe continuous or particulate. Arguments at the most general level for theintelligibility of the void and its relation to the possibility of motion wereinconclusive. In addition to the question of the void, there is the questionof whether matter is particulate and whether there are indivisible particlescalled atoms. Once again, general a priori philosophical arguments werehardly able to settle the question.

Boyle, along with his fellow mechanical philosophers, argued for hisposition on the grounds that it was clear and intelligible compared to rivalsystems such as Aristotelianism and those developed in chemical and



systems such as Aristotelianism and those developed in chemical andrelated contexts by the likes of Paracelsus. The argument operated at thelevel of the fundamental ontology of the rival philosophies. Boyle insistedthat it is perfectly clear what is intended when shape, size and degree ofmotion are ascribed to an impenetrable atom and when arrangements areascribed to groups of such atoms. That much can surely be granted. ButBoyle went further to insist that it is unintelligible to ascribe to atomsproperties other than these primary ones, that is, properties other thanthose that atoms must necessarily possess by virtue of being portions ofmatter, such as the forms and qualities of the Aristotelians or theprinciples of the chemists. Nor could I ever find it intelligibly made out,wrote Boyle, what these real qualities may be, that they [the scholastics]deny to be either matter, or modes of matter, or immaterial substances(Stewart, 1979, 22). If an atom is said to possess elasticity, for example,then Boyle is saying that the ontological status of whatever it is that isadded to matter to render it elastic is mysterious, given that it cannot bematerial. This is not to claim that attributing elasticity and othersecondary properties to gross matter is unintelligible. For such propertiescan be rendered intelligible by regarding them as arising from the primaryproperties and arrangements of underlying atoms. Secondary propertiescan be ascribed to the world derivatively but not primitively. So the starkontology of the mechanical philosopher is established a priori byappealing to a notion of intelligibility.

2.2 Mechanical Reductions and the Problem of Transdiction

Explaining complex properties by reducing them to more elementary oneswas not an enterprise unique to the mechanical philosophers. After all, itwas a central Aristotelian thesis that the behaviour of materials was due tothe proportions of the four elements in them, whilst the elementsthemselves owed their properties to the interaction of the hot and the coldand the wet and the dry, the fundamental active principles in nature. Whata mechanical atomist like Boyle needed, and attempted, to do was

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a mechanical atomist like Boyle needed, and attempted, to do wasestablish that they could provide examples of successful mechanicalreductions that were clear and intelligible. It was to this end that Boylestressed how the workings of a key could be explained in terms ofnothing other than its shape and size relative to the lock and the workingsof a clock can be explained by appeal to nothing other than the propertiesof its parts.

There is a basic problem with this type of illustration of and support forthe mechanical philosophy. Firstly, whilst the examples may indeed beexamples of successful reductions, they are not strict mechanicalreductions, and they are certainly not reductions to the mechanicalproperties of atoms. The functioning of a key depends on its rigiditywhilst that of clocks and watches depend crucially on the weight ofpendulum bobs or the elasticity of springs. On a number of occasionsBoyle himself observed that explanations that appealed to such things aselasticity, gravity, acidity and the like fall short of the kind ofexplanations sought by a mechanical atomist (Chalmers, 1993).

To attempt to produce examples of reduction that conform to the ideal ofthe mechanical atomists is, in effect, to attempt to bolster the argumentsfrom intelligibility with empirical arguments. The issue of empiricalsupport for mechanical atomism, or any other version of atomism, raises afundamental problem, a problem that Maurice Mandelbaum (1964, 88112) has called the problem of transdiction. How are we to reachknowledge of unobservable atoms from knowledge of the bulk matter towhich we have observational and experimental access? Mandelbaumcredits Boyle with proposing a solution to the problem and he is endorsedby Newman (2006). Roughly speaking, the solution is that knowledge thatis confirmed at the level of observation, that is found to apply to allmatter whatsoever, and is scale invariant can be assumed to apply toatoms also. There is no doubt that an argument of this kind is to be foundin Boyle, but it is highly problematic and can hardly be regarded as the



in Boyle, but it is highly problematic and can hardly be regarded as thesolution to the epistemological problems faced by a seventeenth-centuryatomist.

There is something to be said for an appeal to scale invariance along thelines that laws that are shown to hold at the level of observation in a waythat is independent of size should be held to hold generally, and inparticular, on a scale so minute that it is beyond what can be observed.Boyle draws attention to the fact that the law of fall is obeyed by objectsindependently of their size and that the same appeal to mechanism can beapplied alike to explain the workings of a large town clock and a tinywristwatch (Stewart, 1979, 143). The question is to what extentrecognition of scale invariance of this kind can aid the atomist. There is arange of reasons for concluding that it cannot.

A key problem is that laws established at the level of observation andexperiment involve or imply properties other than the primary ones of themechanical atomist. As mentioned above, the mechanisms of clocksinvolve the elasticity of springs, the weight of pendulum bobs and therigidity of gear wheels and the law of fall presupposes a tendency forheavy objects to fall downwards. So the mechanical atomist cannotapply knowledge of this kind, scale invariant or otherwise, to atoms thatare presumed to lack such properties. If we are looking for an empiricalcase for the list of properties that can be applied to atoms then it wouldappear that we need some criteria for picking out that subset of propertiespossessed by observable objects that can be applied to atoms also. Boyleoffered a solution to this problem. He suggested that only those propertiesthat occur in all observable objects whatsoever should be transferred toatoms. Since all observable objects have some definitive shape and sizethen atoms do also. By contrast, whilst some observable objects aremagnetic, many are not, and so atoms are not magnetic. This strategydoes not give an atomist what is needed. All observable objects are elasticto some degree and are even divisible to some degree and yet mechanical

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to some degree and are even divisible to some degree and yet mechanicalatoms are denied such properties. Conversely, no observable macroscopicobject is absolutely impenetrable whereas Boyle assumes that atomsposses precisely that property. Perhaps it should not be surprising that themechanical atomists of the seventeenth century lacked the resources toforge links between their conjectured atoms and experimental findings.

2.3 Natural Minima

Many speculations about atoms in the seventeenth century came from asource quite distinct from mechanical atomism. That source was thetheory of natural minima which had its roots in Aristotle and that wastransformed into a detailed atomic theory mainly applicable to chemicalchange.

Aristotle (On Generation and Corruption, Bk 1, Ch 10) clearly identifiedwhat we would refer to as chemical change as a special categorypresenting problems peculiar to it. It differs from mere alteration, such asthe browning of an autumn leaf, where an identifiable material substratumpersists, and from generation and corruption, such as the transformation ofan olive seed into a tree or the decay of a rose into a heap of dust, whereno identifiable material substratum persists. The transformation of amixture of copper and tin into bronze, an example of what Aristotle calledcombination, is intermediate between alteration and generation andcorruption. Copper and tin do not persist as such in the bronze and toassume so would fail to make the appropriate distinction between acombination and a mixture. Nevertheless, there is some important sense inwhich the copper and tin are in the bronze because they are recoverablefrom it. Aristotle had put his finger on a central problem in chemistry, thesense in which elements combine to form compounds and yet remain inthe compounds as components of them. Aristotle did not use thisterminology, of course, and it should be recognised that he and thescholastics that followed him had few examples of combination, as



scholastics that followed him had few examples of combination, asopposed to alteration and generation and corruption, to draw on. Alloys,which provided them with their stock and just about only example, are noteven compounds from a modern point of view. The importance ofcombination for Aristotelians lay in the philosophical challenge it posed.

Many scholastics came to understand combination as the coming togetherof the least parts of the combining substances to form least parts of thecompound. These least parts were referred to as natural minima.Substances cannot be divided indefinitely, it was claimed, becausedivision will eventually result in natural minima which are eitherindivisible or are such that, if divided, no longer constitute a portion ofthe divided substance. But the theory of natural minima was developed toa stage where it involved more than that. The minima were presumed toexist as parts of a substance quite independent of any process of division.What is more, chemical combination was understood as coming about viathe combination of minima of the combining substances forming minimaof the compound. Talk of chemical combination taking place perminima became common.

Natural minima were presumed by the scholastics to owe their being bothto matter and form in standard Aristotelian fashion. A key problem theystruggled with concerned the relation of the form characteristic of theminima of combining substances and the form of the minima of theresulting compound. Natural minima of copper and tin cannot remain assuch in the minima of bronze otherwise the properties of copper and tinwould persist in bronze. On the other hand, the form of copper and tinmust persist in some way to account for the fact that those metals can berecovered. A common scholastic response was to presume that the formsof the combining minima persist in the minima of the resulting compoundbut in a way that is subservient to the form of those latter minima.Elements persist in the compound somewhat as individual notes persist ina chord.

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a chord.

Whilst Aristotle and the scholastics can be given the credit for pinpointinga fundamental problem associated with chemical change they can hardlybe credited with providing a definitive solution. It should be recognisedthat adding the assumption of natural minima does not contribute in anyway to a solution to the problem posed by chemical change. The problemof understanding how components persist in compounds simply becomestransferred to the problem of how minima of components persist inminima of compounds. So the extent to which acceptance of naturalminima became widespread cannot be explained in terms of theircontribution to a solution to the fundamental problem of chemical change.There were a number of motivations for assuming minima, all having atleast their germs in Aristotle. One idea was that a portion of a substancecan resist the corrupting influence of the surrounding medium only ifthere is a sufficient amount of it. Another stemmed from the commonrecognition that substances must come into contact if they are to combine.The particulate nature of substances facilitates such contact, as Aristotlehinted (On Generation and Corruption, 1, 10, 328a, 34). A thirdmotivation concerned the logical problems, dating back to Zeno, that wereunderstood to flow from assuming infinite divisibility.

Recognising the need to avoid problems perceived to be associated withinfinite divisibility was a point shared by proponents of natural minimaand mechanical atomists. But this one point of contact must not blind usto the crucial differences between the two traditions. Mechanical atomswere proposed as components of matter in general. They wereunchangeable and possessed a minimum of properties, shape, size and adegree of motion or rest together with the impenetrability of theircomponent matter. The motivation for ascribing just those properties toatoms was to provide an intelligible account of being and change ingeneral. By contrast, natural minima possess properties characteristic ofthe substances of which they are the minima. The minima are not



the substances of which they are the minima. The minima are notunchangeable because they are transformed into more complicatedminima via chemical combination. The minima were not basic buildingblocks for the scholastics that developed this theory because theirproperties needed to be traced back to their composition from the fourAristotelian elements. Finally, the minima theory was developed as anattempt to accommodate chemical change. It was not intended as a theoryof everything in the way that mechanical atomism was.

2.4 Seventeenth-Century Eclecticism

Atomic theories became common in the seventeenth century. Theemerging emphasis on experiment led the proponents of those theories tobecome less concerned with philosophical systems and more concernedwith the explanation of specific phenomena such as condensation andrarefaction, evaporation, the strength of materials and chemical change.There was an increasing tendency for atomists to borrow in anopportunist way from both the mechanical and natural minima traditionsas well as from the alchemical tradition which employed atomistictheories of its own as Newman (1991, 143190 and 1994, 92114) hasdocumented. Thus an Aristotelian proponent of the natural minimatradition, Daniel Sennert, whose main interest was in chemistry inmedical contexts, drew on the work of the alchemists as well as that ofthe minima theorists, employed minima in physical as well as chemicalcontexts, and insisted that his atomism had much in common with that ofDemocritus (Clericuzio, 2000, 2329 and Melsen, 1952, 8189). Boylereferred to his mechanical atoms as natural minima and his first accountof atomism involved attributing to an atom properties distinctive of thesubstance it was a least part of (Newman, 2006, 162ff, Clericuzio, 2000,166ff) and in fact borrowed heavily from Sennert (Newman, 1996). Insubsequent writings he made it clear that in his view least parts ofsubstances are composed of more elementary particles possessing onlyshape, size and a degree of motion. Whether, according to Boyle,

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shape, size and a degree of motion. Whether, according to Boyle,properties other than primary mechanical ones emerge at the level of leastparts or at the macroscopic level is an issue on which contemporarycommentators disagree (Chalmers, 2009, 155161), Chalmers, 2010, 89,Clericuzio, 2000, 103148, Newman, 2006, 179189). The theories of anumber of atomists, such as Sebastien Basso, Etienne de Clave andThomas Digges, were an eclectic mixture of ingredients drawn frommechanical atomism, minima theory and alchemy. (Clericuzio, 2000,Melsen, 1952, Newman, 2006)

The seventeenth-century certainly witnessed the growth of a range ofexperimental sciences, an occurrence of considerable epistemologicalsignificance. However, the experimental basis for seventeenth-centuryatomism remained extremely weak and none of the various versions of itcan be said to have productively informed experiment or to have beenconfirmed by it, a claim that has been documented by Meinel (1988) inhis survey of the experimental basis for atomism in the seventeenthcentury and is argued in detail in Chalmers (2009). Appeal to atoms toexplain the gradual wearing away of a stone, the evaporation of a liquid,the passage of a solution through a filter paper folded multiple times andso on dated back at least as far as Lucretius and were hardly sufficientlypowerful to convince anyone disinclined to accept the reality of atoms.Experimental knowledge of the combination and recovery of reactingchemicals, which certainly experienced marked growth in the course ofthe seventeenth century, did not of itself warrant the assumption thatatoms were involved. Evidence revealed by the microscope was new tothe seventeenth century, of course, and did reveal a microscopic worldpreviously unknown. But the properties of microscopic systems were notqualitatively distinct from macroscopic ones in a way that aided thedemonstration of the emergence of the properties of observable systems,whether microscopic or macroscopic, from the properties of atoms.



Suggested Readings: Clericuzio (2000) is a detailed survey ofseventeenth-century atomic theories. Stewart (1979) is a collection ofBoyle's philosophical papers related to his mechanical atomism. Boyle'satomism is detailed in Newman(2006) and Chalmers (2009). Debatesconcerning the nature and status of it are in Chalmers(1993), Chalmers(2002), Chalmers (2009), Chalmers (2010), Newman (2006), Newman(2010), Anstey (2002) and Pyle (2002).

3. Newtonian Atomism3.1 Newton's Atomism

The key sources of Newton's stance on atomism in his published work areQuerie 31 of his Opticks, and a short piece on acids (Cohen, 1958, 2578). Atomistic views also make their appearance in the Principia, whereNewton claimed the least parts of bodies to beall extended, and hardand impenetrable, and moveable, and endowed with their proper inertia(Cajori, 1962, 399). If we temporarily set aside Newton's introduction ofhis concept of force, then Newton's basic matter theory can be seen as aversion of mechanical atomism improved by drawing on the mechanics ofthe Principia. Whereas mechanical atomists prior to Newton had beenunclear about the nature and status of the laws governing atoms, Newtonwas able to presume that his precisely formulated three laws of motion,shown to apply in a wide variety of astronomical and terrestrial settings,applied to atoms also. Those laws provided the law of inertia governingmotion of atoms in between collisions and laws of impact governingcollisions. Newton also added his precise and technical notion of inertiaor mass, another fruit of his new mechanics, to the list of primaryproperties of atoms. These moves certainly helped to give precise contentto the fundamental tenets of mechanical atomism that they had previouslylacked.

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There is no doubt that Newton shared the assumption of the Ancient andmechanical atomists that there is just one kind of homogeneous matter ofwhich all atoms are composed. This is clear from the way in whichNewton explained differing densities of observable matter in terms of theamount of space intervening between the component atoms. Newtonargued, for instance that the ratio of space to volume occupied by atomswas seventeen times greater in water than in gold on the grounds that goldis seventeen times more dense. The fact that thin gold films transmit lightconvinced Newton that the atoms of gold already contains enough spaceto permit the transmission of light particles. The preponderance of spacebetween the atoms of matter, however bulky or solid they might appear atthe observational and experimental level, became a characteristic featureof Newtonian atomism, as Thackray (1968) has stressed.

The picture of Newton's atomism as an elaboration and improvement ofmechanical atomism becomes untenable once the role of force inNewton's theorising is taken into account. There is no doubting thatNewton's introduction of forces, especially the gravitational force, into hismechanics was a major scientific success borne out by observational andexperimental evidence. Newton famously speculated in the Preface to thePrincipia (Cajori, 1958, xviii), that if all forces operative in nature,including those acting between the smallest, unobservable, particles, wereknown, then the whole course of nature could be encompassed within hismechanics. However, the fulfilment of such a dream would not constitutethe fruition of the mechanical philosophy because of the ontologicalproblems posed by the concept of force.

Newton explicitly rejected the idea that gravitation, or any other force, beessential to matter. But the major point of mechanical atomism had beento admit as properties of atoms only those that they must, essentially, haveas pieces of matter. It was in this way that they had endeavoured to avoidintroducing Aristotelian forms and qualities, which they regarded asincomprehensible from an ontological point of view. The introduction of



incomprehensible from an ontological point of view. The introduction offorces as irreducible entities flew in the face of the major aim of themechanical philosophers for clarity and intelligibility on ontologicalmatters. Newton was unable to fashion an unambiguous view on theontological status of gravity, a force manifest at the level of observationand experiment, let alone forces operative at the atomic level. It is truethat, in the case of gravity, Newton had a plausible pragmatic response.He argued that, whatever the underlying status of the force of gravitymight be, he had given a precise specification of that force with his law ofgravitation and had employed the force to explain a range of phenomenaat the astronomical and terrestrial level, explanations that had beenconfirmed by observation and experiment. But not even a pragmaticjustification such as this could be offered for forces at the atomic level.

Mechanical atomism had faced the problem of how to introduce theappropriate kinds of activity into the world relying solely on the shapes,sizes and motions of atoms. They had struggled unsuccessfully to explainelasticity and gravity along such lines and chemistry posed problems of itsown. Newtonian forces could readily be deployed to remove theseproblems. Newton presumed that forces of characteristic strengths(affinities) operated between the least parts of chemicals. What displaceswhat in a chemical reaction is to be explained simply in terms of therelative strengths of the affinities involved. Elasticity was attributed toattractive and repulsive forces acting between particles of an elasticsubstance and so on.

Newton developed theories of optics and chemistry that were atomistic inthe weak sense that they sought to explain optical and chemical propertiesby invoking interacting particles lying beyond the range of observation.However, the particles were not ultimate. Newton's position on the leastparts of chemical substances was similar to that of Boyle and othermechanical philosophers. They were regarded as made up of a hierarchyof yet smaller particles. So long as the smallest particles were held

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of yet smaller particles. So long as the smallest particles were heldtogether by forces, the problem of the ontological status of the forcesremained. The least parts of chemicals in Newton's theory were akin tonatural minima with the added detail that their action was due to attractiveand repulsive forces. As far as the particles of light in Newton's optics areconcerned, whether they were ultimate or not, they too acted by way offorces and also suffered fits of easy reflection and easy refraction, thelatter being used to explain interference phenomena such as Newtonsrings and why a ray incident on a boundary between two refracting mediacan be partially reflected and partially transmitted.

However attractive the reduction of the material world to particlesinteracting by way of forces may have appeared, it must be recognisedthat there was scant empirical support for the idea. This point is clearestin the context of chemistry. The affinities presumed to act betweenchemical atoms were postulated solely on the basis of the observedchemical behaviour of bulk substances manipulated in the laboratory. Theassumption that the chemical behaviour of bulk substances were due tocombining atoms added nothing that made a difference to what wastestable by experiment. Observed properties of chemical substances weresimply projected onto atoms. Newtonians had not formulated a chemicalatomic theory that could be used as a basis for the prediction of chemicalphenomena at the experimental level. Newton's optics was in ananalogous situation. However, here it can be said that that optical theorywas able to accommodate a range of optical phenomena in a coherentway that rendered it superior to any rival. The result was the widespreadacceptance of the theory in the eighteenth century.

When Newton took for granted that there is just one kind of universalmatter and refused to include gravity as a primary property of matterbecause of worries about the ontological status of force, he was playingthe role of a natural philosopher in the tradition of the mechanicalphilosophy. When he offered a pragmatic justification of his specification



philosophy. When he offered a pragmatic justification of his specificationof the force of gravity independently of how that force might be explainedhe was acting as one who sought to develop an experimentally confirmedscience independent of the kinds of ultimate explanation sought by themechanical philosophers. His atomism contained elements of both ofthese tendencies. A sympathiser could say that whatever the philosophicalproblems posed by forces, Newtonian atomism was a speculation that atleast held the promise of explaining material phenomena in a way thatmechanical atomism did not and so experimental support in the futurewas a possibility. A critic, on the other hand, could argue that, from thephilosophical perspective, the introduction of force undermined the casefor the clarity and intelligibility of mechanical atomism on which itsoriginators had based their case. From a scientific point of view, therewas no significant empirical support for atomism and it was unable tooffer useful guidance to the experimental sciences that grew andprospered in the seventeenth century and beyond.

3.2 Eighteenth-Century Developments in Newtonian Atomism

Force was to prove a productive addition to experimental science in nouncertain manner in the eighteenth century. Force laws in addition to thelaw of gravitation, involving elasticity, surface tension, electric andmagnetic attractions and so on were experimentally identified and put toproductive use. In the domain of science, scruples about the ontologicalstatus of forces were forgotten and this attitude spread to philosophy.Eighteenth-century updates of mechanical atomism typically includedgravity and other forces amongst the primary properties of atoms.Acceptance of force as an ontological primitive is evident in an extremeform in the 1763 reformulation of Newtonian atomism by R. Boscovich(1966). In his philosophy of matter atoms became mere points (albeitpossessing mass) acting as centres of force, the forces varying with thedistance from the centre and oscillating between repulsive and attractiveseveral times before becoming the inverse square law of gravitation at

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several times before becoming the inverse square law of gravitation atsensible distances. The various short-range attractive and repulsive forceswere appealed to as explanations of the cohesion of atoms in bulkmaterials, chemical combination and also elasticity. Short-range repulsiveforces varying with distance enabled Boscovich to remove theinstantaneous rebounds of atoms that had been identified as anincoherency in Newton's own atomism stemming from their absolutehardness and inelasticity.

While most atomists were able to rid themselves of scruples aboutaccepting forces as ontologically primitive, the issue of the empiricalfoundation for the various unobservable forces hypothesised remained.The best arguments that could be mounted were hypothetical-deductive.Forces postulated at the atomic level were credited with some empiricalsupport if they could serve to explain observable phenomena. The form ofsuch arguments, as well as their inconclusiveness, can be illustrated byNewton's demonstration in the Principia (Bk. 2, Prop. 23) that a gasconsisting of a static array of atoms repelling each other with a forceinversely proportional to their separation would obey Boyle's law. Thefact that some of these theories did indeed reproduce the experimentallyestablished facts was certainly a point in their favour, but hardly served toestablish them. Whewell brought the point home by identifyingcompeting theories of capillarity, due to Poisson and Laplace, that wereequally able to reproduce the phenomena but which were based onincompatible atomic force laws, as Gardner (1979, 20) has pointed out.

The problem besetting those seeking experimental support for atomictheories is most evident in chemistry. Although many eighteenth-centurychemists espoused versions of Newtonian chemistry their chemicalpractice owed nothing to it (Thackray, 1970). As philosophers they payedlip-service to atomism but as experimental chemists they workedindependently of it. As early as 1718 Ettienne Geoffroy spelt out how theblossoming experimental science of chemical combination, involving



blossoming experimental science of chemical combination, involvingextensive use of mineral acids to form an array of salts, could beunderstood in terms of what substances combined with what and could berecovered from what and to what degree. His table of the degrees ofrapport of chemical substances for each other summarised experimentaldata acquired by manipulating substances in the laboratory and became anefficient device for ordering chemical experience and for guiding thesearch for novel reactions. Klein (1995) has highlighted this aspect ofGeoffroy's work and how his 1718 paper in effect shows how a largesection of the experimental chemistry of the time could be construed as apractical tradition divorced from a speculative metaphysics, atomistic orotherwise. Eighteenth-century tables of affinity, modelled on Geoffroy'sversion, became increasingly detailed as the century proceeded. Many ofthe chemists who employed them interpreted the affinities featuring inthem as representing attractions between chemical atoms, but such anassumption added nothing that could not be fully represented in terms ofcombinations of chemical substances in the laboratory.

The fact that Newtonian atomism offered little that was of practical utilityto chemistry became increasingly recognised by chemists as theeighteenth century progressed. The culmination of the experimentalprogram involving the investigation of the combination and analysis ofchemical substances was, of course, Lavoisier's system involvingchemical elements. But whatever sympathy Lavoisier may have had forNewtonian atomism of the kind championed by Laplace, he was at painsto distance his new chemistry from it. Substances provisionally classifiedas elements were those that could not be broken down into somethingsimpler in the laboratory. Progress in eighteenth-century chemistry ledaway from rather than towards atomism. It was not until Dalton that thesituation changed early in the nineteenth century.

The assessment that eighteenth-century atomism was ill-confirmed byexperiment and failed to give useful guidance to experimentalists is a

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experiment and failed to give useful guidance to experimentalists is ajudgement that is fairly insensitive to what theory of confirmation oneadopts or what one might require of an adequate scientific explanation.This situation was transformed by the emergence of Daltonian atomism, astrong candidate for the first atomic theory that had a productive link withexperiment.

Suggested Reading: Thackray (1970) is an authoritative and detailedaccount of Newton's atomism and its development in the eighteenthcentury. The relation between Newton's atomism and his mechanics isdiscussed in Chalmers (2009, Chapter 7).

4. Chemical Atomism in the Nineteenth Century4.1 Dalton's Atomism

The status of atomism underwent a transformation when John Daltonformulated his version of chemical atomism early in the nineteenthcentury. His atomic theory had implications for the way chemicalscombine by weight and, for the first time, it would seem, a direct pathwas uncovered that took scientists from experimental measurement to aproperty of atoms, namely, their relative weight. An assessment of thefruitfulness and epistemological status of Dalton's atomism can easily bedistorted if we are uncritically influenced by the recognition that Dalton'sbasic assumptions are in fact correct from a modern point of view. Thissection will involve a summary of the basic features of Dalton's chemistryas he published it in 1808 together with the way in which its content canbe usefully expressed using chemical formulae introduced by Berzeliusfive years later. The following sections will explore, first the issue of theepistemological status of this early version and then the nature and statusof subsequent elaborations of chemical atomism during the first halfcentury of its life. These latter issues very much involve developments inorganic chemistry, issues that have been highlighted by historians of



organic chemistry, issues that have been highlighted by historians ofchemistry only in the last few decades.

Dalton was able to take for granted assumptions that had become centralto chemistry since the work of Lavoisier. Chemical compounds wereunderstood as arising through the combination of chemical elements,substances that cannot be broken down into something simpler bychemical means. The weight of each element was understood to bepreserved in chemical reactions. By the time Dalton (1808) made his firstcontributions to chemistry the law of constant composition of compoundscould be added to this. Proust had done much to substantiateexperimentally the claim that the relative weights of elements making upa chemical compound remain constant independent of its mode ofpreparation, its temperature and its state.

The key assumption of Dalton's chemical atomism is that chemicalelements are composed of ultimate particles or atoms. The least part ofa chemical compound is assumed to be made up of characteristiccombinations of atoms of the component elements. Dalton called thesecompound atoms. According to Dalton, all atoms of a given substancewhether simple or compound, are alike in shape, weight and any otherparticular. This much already entails the law of constant proportions.Although Dalton himself resisted the move, Berzelius was able to showhow Dalton's theory can be conveniently portrayed by representing thecomposition of compounds in terms of elements by chemical formulae inthe way that has since become commonplace. Hereafter this device isemployed using modern conventions rather than any of the various onesused by Berzelius and his contemporaries,

As Dalton stressed, once the chemical atomic theory is accepted, thepromise is opened up of determining the relative weights of atoms bymeasuring the relative weights of elements in compounds. If an atom ofelement A combines with an atom of element B to form a compound atom

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element A combines with an atom of element B to form a compound atomof compound AB, then the relative weights of A and B in the compound asmeasured in the laboratory will be equal to the relative weights of atomsof A and B. However, there is a serious under-determination of relativeatomic weights by measurements of combining weights in the laboratory.If the compound atom in our example were A2B rather than AB then therelative atomic weight of B would be twice what it would be if theformula were AB. Dalton himself attempted to resolve this problem with asimplicity assumption. Formulae were always to take the simplest formcompatible with the empirical data. If there was only one compound of Aand B known then it was assumed to be AB, whilst if there were two thena more complicated compound, A2B or AB2 became necessary. As isillustrated by the latter example, as well as the problem of the truth of thesimplicity assumption there was the problem of its ambiguity. Chemicalatomists were to struggle for several decades with various solutions to theproblem of arriving at definitive formulae and relative atomic weights, aswe shall see.

This deficiency of Dalton's atomism aside, links were forged between itand experimentally determined combining weights that went beyond thelaw of constant proportions to include the laws of multiple and reciprocalproportions. If two elements combine together in more than one way toform compounds, as is the case with the various oxides of nitrogen andcarbon, for example, then Daltonian atomism predicts that the weights ofone of the elements in each compound, relative to a fixed weight of thesecond, will bear simple integral ratios to each other. This is the law ofmultiple proportions, predicted by Dalton and soon confirmed by a rangeof experiments. Daltonian atomism also predicts that if the weights ofelements A and B that combine with a fixed weight of element C are xand y respectively, then if A and B combine to form a compound then therelative weights of A and B in the compound will be in the ration x:y orsome simple multiple of it. This law was also confirmed by experiment.



There is a further component that needs to be added to the content of earlyatomic chemistry, although it did not originate with Dalton, who in factdid not fully embrace it. Gay Lussac discovered experimentally that whengases combine chemically they do so in volumes that bear an integralratio to each other and to the volume of the resulting compound ifgaseous, provided that all volumes are estimated at the same temperatureand pressure. For instance, one volume of oxygen combines with twovolumes of hydrogen to form two volumes of steam. If one acceptsatomism, this implies that there are some whole-number ratios betweenthe numbers per unit volume of atoms of various gaseous elements at thesame temperature. Following suggestions made by Avogadro and Ampereearly in the second decade of the nineteenth century, many chemistsassumed that equal volumes of gases contain equal numbers of atoms,with the important implication that relative weights of atoms could beestablished by comparing vapour densities. As Dalton clearly saw, thiscan only be maintained at the expense of admitting that atoms can besplit. The measured volumes involved in the formation of water, forexample, entail that, if equal volumes contain equal numbers of atomsthen a water atom must contain half of an oxygen atom. The resolutionof these problems required a clear distinction between atoms of achemical substance and molecules of a gas, the grounds for which becameavailable only later in the century. This problem aside, the empirical factthat gases combine in volumes that are in simple ratios to each otherbecame a central component of chemistry, although it should be notedthat at the time Gay Lussac proposed his law, only a small number ofgases were known to chemists. The situation was to change with thedevelopment of organic chemistry in the next few decades.

4.2 The Status of Daltonian Chemistry

If Dalton's atomism was viewed as a contribution to natural philosophy inthe tradition of mechanical atomism, designed to give a simple and

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the tradition of mechanical atomism, designed to give a simple andintelligible account of the ultimate nature of the material world, then itdid not have a lot going for it. It marked a decisive break with the ideathat there is just one kind of matter, an assumption that extended fromDemocritus to Newton and beyond. If Dalton's atoms were regarded asontologically basic, then there needed to be as many kinds of matter asthere are chemical elements. Further, atoms of each element needed toposses a range of characteristic properties to account for chemicalcombination as well as physical aggregation and other physical properties.As a philosophical theory of the ultimate nature of material reality,Daltonian atomism was not a serious contender and was not treated assuch. A more significant issue is the status of Daltonian chemistry as anexperimental science. To what extent was Daltonian chemistry borne outby and able to fruitfully guide experiment?

A basic issue concerning the empirical statues of Daltonian atomism wasalready pinpointed in an early exchange between Dalton (1814) andBerzelius (1815). Dalton was keen to present himself as the Newton ofchemistry. In his view, just as Newton had explained Keplers laws withhis new mechanics, so he, Dalton, had explained the laws of proportionwith his atomism. Without atomism the joint truth of the three laws ofproportion is a mystery. Berzelius questioned the experimental groundsfor assuming anything stronger than the laws of proportion, since, heargued, all of the chemistry could be accommodated by the latter. That is,nothing testable by the chemistry of the time follows from Dalton'satomic theory that does not follow from the laws of proportion plus theexperimental law of combining volumes for gases.

Berzelius (1814) expressed his version of Daltonian chemistry usingformulae. Dalton had pictured atoms as spheres and compound atoms ascharacteristic arrangements of spheres. Berzelius claimed that the twomethods were equivalent but that his method was superior because it wasless hypothetical. It is clear that Berzelius's version cannot be both less



less hypothetical. It is clear that Berzelius's version cannot be both lessspeculative and equivalent to Dalton's theory at the same time. But it isalso clear what Berzelius intended. His point was that the testableempirical content of the two theories were equivalent as far as thechemistry of the time was concerned, but that his version was lessspeculative because it did not require a commitment to atoms. Thesymbols in Berzelian formulae can be interpreted as representing combingweights or volumes without a commitment to atoms. A Daltonian atomistwill typically take the hydrogen atom as a standard of weight and theatomic weight of any other element will represent the weight of an atomof that element relative to the weight of the hydrogen atom. On such aninterpretation the formula H2O represents two atoms of hydrogencombined with one of oxygen. But, more in keeping with the weightdeterminations that are carried out in the laboratory, it is possible tointerpret atomic weights and formulae in a more empirical way. Anysample of hydrogen whatever can be taken as the standard, and the atomicweight of a second element will be determined by the weight of thatelement which combines with it. The formula H2O then represents thefact that water contains two atomic weights of hydrogen for every one ofoxygen. Of course, determining atomic weights and formulae requiressome decision to solve the under-determination problem, but that is thecase whether one commits to atoms or not.

Berzelius was right to point out that as far as being supported by andserving to guide the chemistry of the time was concerned, his formulationusing formulae served as well as Dalton's formulation without committingto atomism. What follows from this will depend on one's stand onconfirmation and explanation in science. A strong-minded empiricistmight conclude from Berzeliuss observation that Dalton's atomism hadno place in the chemistry of the time. Others might agree with Dalton thatthe mere fact that Dalton's theory could explain the laws of proportion ina way that no available rival theory could constituted a legitimateargument for it in spite of the lack of evidence independent of combining

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argument for it in spite of the lack of evidence independent of combiningweights and volumes. Atomism could be defended on the grounds thatattempts to articulate and improve it might well fruitfully guideexperiment in the future and lead to evidence for it that went beyondcombining weights and volumes. But such articulations would clearlyrequire properties to be ascribed to atoms in addition to their weight.

Berzelius himself took this latter option. He developed an atomic theorythat attributed the combination of atoms in compounds to electrostaticattractions. He developed a dualist theory to bring order to compoundsinvolving several types of molecules. For instance, he represented coppersulphate as (CuO + SO3). Here electropositive copper combines withelectronegative oxygen but in a way that leaves the combination slightlyelectropositive, whereas electropositive sulphur combines with oxygen ina way that leaves the combination slightly electronegative. The residualcharges of the radicals as they became known could then account fortheir combination to form copper sulphate.

Berzelius's conjectures about the electrical nature of chemicalcombination owed their plausibility to the phenomenon of electrolysis,and especially the laws governing it discovered by Faraday, which linkedthe weights of chemicals deposited in electrolysis to chemicalequivalents. But evidence for the details of his atomistic theoryindependent of the evidence for the experimental laws that the theory wasdesigned to support was still lacking. Contemporaries of Berzeliusproposed other atomic theories to explain electrical properties of matter.Ampre proposed electrical currents in atoms to explain magnetism andPoisson showed how electrostatic induction could be explained byassuming atomic dipoles. In each of these cases some new hypothesis wasadded to atomism for which there was no evidence independent of thephenomenon explained. Nevertheless, the fact that there existed this rangeof possible explanations all assuming the existence of atoms can be seenas constituting evidence for atoms by those favouring inferences to the



as constituting evidence for atoms by those favouring inferences to thebest explanation.

In the early decades of the life of Dalton's atomic chemistry variousattempts were made to solve the problem of the under-determination ofatomic weights and formulae. We have already mentioned the appeal tothe equal numbers hypothesis and vapour densities. The fact that chemistsof the time did not have the resources to make this solution work has beenexplored in detail by Brooke (1981) and Fisher (1982). A second methodwas to employ an empirical rule, proposed by Dulong and Petit, accordingto which the product of the specific heats and the atomic weights of solidsis a constant. The problem with this at the time was, firstly, that someatomic weights needed to be known independently to establish the truth ofthe rule, and, secondly, there were known counter-instances. A thirdmethod for determining atomic weights employed Mitscherlichs proposal(Rocke, 1984, 1546) that substances with similar formulae should havesimilar crystal structure. This method had limited application and, again,there were counter-examples.

Our considerations so far of the status of Daltonian atomism have not yettaken account of the area in which chemistry was to be makingspectacular progress by the middle of the nineteenth century, namely,organic chemistry. This is the topic of the next section.

4.3 Progress in Organic Chemistry Using Chemical Formulae

The period from the third to the sixth decades of the nineteenth centurywitnessed spectacular advances in the area of organic chemistry and it isuncontroversial to observe that these advances were facilitated by the useof chemical formulae. Inorganic chemistry differs from organic chemistryinsofar as the former involves simple arrangements of a large number ofelements whereas organic chemistry involves complicated arrangementsof just a few elements, mainly carbon, hydrogen, oxygen and to a lesser

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of just a few elements, mainly carbon, hydrogen, oxygen and to a lesserextent, nitrogen.

It was soon to become apparent that the specification of the proportionsof the elements in an organic compound was not sufficient to identify itor to give an adequate reflection of its properties. Progress becamepossible when the arrangements of the symbols representing the elementsin formulae were deployed to reflect chemical properties. The historicaldetails of the various ways in which chemical properties were representedby arrangements of symbols are complex. (For details see Rocke (1984)and Klein (2003)). Here we abstract from those details to illustrate thekinds of moves that were made.

The simplest formula representing the composition of acetic acid is CH2Ousing modern atomic weights. This formula cannot accommodate the factthat, in the laboratory, the hydrogen in acetic acid can be replaced bychlorine in four distinct ways yielding four distinct chemical compounds.Three of those compounds are acids that have properties very similar toacetic acid, and in which the relative weights of chlorine vary as 1:2:3.The fourth compound has the properties of a salt rather than an acid.These experimental facts can be captured in a formula by doubling thenumbers and rearranging the symbols, so that we have C2H4O2,rearranged to read C2H3O2H. The experimental facts can now readily beunderstood in terms of the substitution of one or more of the hydrogensby chlorine, with the three chloro-acetic acids represented as C2H2ClO2,C2HCl2O2H and C2Cl3O2H and the salt, acetyl chloride, as C2H3O2Cl.Such formulae came to be known as rational formulae as distinct fromthe empirical formula CH2O. Representing the replacement of oneelement in a compound by another in the laboratory by the replacement ofone symbol by another in a chemical formula became a standard andproductive device that was to eventually yield the concept of valency inthe 1860s. (Oxygen has a valency of two because two hydrogens need tobe substituted for each oxygen.)



be substituted for each oxygen.)

Other devices employed to fashion rational formulae involved the notionof a radical, a grouping of elements that persisted through a range ofchemical changes so that they play a role in organic chemistry akin to thatof elements in inorganic chemistry. Series of compounds could beunderstood in terms of additions, for example to the methyl radical, CH3,or to the ethyl radical, C2H5, and so on. Homologous series ofcompounds could be formed by repeatedly adding CH2 to the formulaefor such radicals so that the properties, and indeed the existence, ofcomplex compounds could be predicted by analogy with simpler ones.Another productive move involved the increasing recognition that theaction of acids needed to be understood in terms of the replacement ofhydrogen. Polybasic acids were recognised as producing two or moreseries of salts depending on whether one, two or more hydrogens arereplaced. Yet another important move involved the demand that rationalformulae capture certain asymmetric compounds, such as methyl ethylether, CH3C2H5O, as distinct from methyl ether, (CH3)2O, and ethylether, (C2H5)2O. By 1860, the idea of tetravalent carbon atoms that couldcombine together in chains was added. By that stage, the demand thatrational formulae reflect a wide range of chemical properties had resultedin a set of formulae that was more or less unique. The under-determination problem that had blocked the way to the establishment ofunique formulae and atomic weights had been solved by chemical means.

4.4 Implications of Organic Chemistry for Atomism

The previous section was deliberately written in a way that does notinvolve a commitment to atomism. It is possible to understand the projectof adapting rational formulae so that they adequately reflect chemicalproperties by interpreting the symbols as representing combining weightsor volumes as Berzelius had already observed in his early debates withDalton. Philosophers and historians of science have responded in a variety

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Dalton. Philosophers and historians of science have responded in a varietyof ways to this situation.

Pierre Duhem (2002), in his classic analysis of the logic of nineteenth-century chemistry at the end of that century, construed it as beingindependent of, and offering no support for, atomism. Paul Needham(2004a, 2004b) has recently supported his case. Klein (2003, 1820) notesthat many of the pioneers of the developments in organic chemistryreferred to combining volumes or portions or proportions rather thanatoms. She attributes the productivity of the use of formulae to the factthat they conveyed a building-block image of chemical proportionswithout simultaneously requiring an investment in atomic theories,together with the simplicity of their maneuverability on paper (2003, 35).

A number of chemists involved in the early advances of organic chemistrywho did adopt atomism expressed their ontological commitment tochemical atoms. In doing so they distinguished their theories from thosebrands of physical atomism that were in the tradition of mechanical orNewtonian atomism and which sought to explain phenomena in general,and chemistry in particular, by reference to a few physical properties ofatoms. Chemical atoms had more in common with natural minima insofaras they were presupposed to have properties characteristic of thesubstances they were atoms of. Chemical atomism lent itself to the ideathat it was developments in chemistry that were to indicate whichproperties were to be attributed to chemical atoms, as exemplified in thepath that led to the property valency. Alan Rocke (1984, 1015)interprets the use of formulae in organic chemistry as involving achemical atomism that is weaker than physical atomism but stronger thana commitment only to laws of proportion.

Dalton's atomism had given a line on just one property of atoms, theirrelative weight. But it is quite clear that they needed far richer propertiesto play there presumed role in chemistry. It was to be developments in



to play there presumed role in chemistry. It was to be developments inchemistry, and later physics, that were to give further clues about whatproperties to ascribe to atoms. (We have seen how chemists came toascribe the property of valency to them.) There was no viable atomistictheory of chemistry in the nineteenth century that was such that chemicalproperties could be deduced from it. The phenomenon of isomerism isoften regarded as a success for atomism. (See Bird, (1998, p. 152) for arecent example.) There are reasons to doubt this. The fact that there arechemical substances with the same proportional weights of the elementsbut with widely different chemical properties was a chemical discovery. Itcould not be predicted by any atomic theory of the nineteenth-centurybecause no theory contained within its premises a connection between thephysical arrangement of atoms and chemical properties.Isomerism couldbe accommodated to atomism but could not, and did not, predict it.

The emergence of unique atomic weights and the structural formulae thatorganic chemistry had yielded by the 1860s were to prove vitalingredients for the case for atomism that could eventually be made. Butthere are reasons to be wary of the claim that atomism was responsible forthe rise of organic chemistry and the extent to which the achievementimproved the case for atomism needs to be elaborated with more cautionthat is typically the case. Glymour (1980, 226263) offers an account ofhow Dalton's atomism was increasingly confirmed and relative atomicweights established by 1860 that conforms to his bootstrapping accountof confirmation, an account that is adopted and built on by Gardner(1979). These accounts do not take organic chemistry into account. In onesense, doing so could in fact help to improve Glymour's account byoffering a further element to the interlocking and mutually supportinghypotheses and pieces of evidence that are involved in his case. But inanother sense, the fact that organic chemistry led to unique formulae bychemical means casts doubt on Glymour's focus on the establishment ofdefinitive atomic weights as the problem for chemistry. There is a casefor claiming that correct atomic weights were the outcome of, rather than

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for claiming that correct atomic weights were the outcome of, rather thana precondition for, progress in organic chemistry prior to 1860. After all,the majority of the formulae productively involved in that dramaticprogress were the wrong formulae from a modern point of view! Forinstance, use of homologous series to project properties of lowerhydrocarbons on to higher ones are not affected if the number of carbonatoms in the correct formulae are doubled, which results from taking 6 asthe relative atomic weight of carbon, as many of the contemporaryorganic chemists did.

Suggested Readings: Rocke (1984) is a detailed study of the relevanttheories in eighteenth-century chemistry whilst Klein (2003) is ahistorical and philosophical analysis of the introduction of formulae intoorganic chemistry. The empirical status of atomism in nineteenth-centurychemistry is discussed in Chalmers (2009, Chapters 9 and 10)

5. Atomism in Nineteenth-Century Physics5.1 The Kinetic Theory of Gases

The first atomic theory that had empirical support independent of thephenomena it was designed to explain was the kinetic theory of gases.This discussion will pass over the historical detail of the emergence of thetheory and consider the mature statistical theory as developed by Maxwellfrom 1859 (Niven, (1965, Vol. 1, 377409, Vol. 2, 2678) and developedfurther by Boltzmann (1872).

The theory attributed the behaviour of gases to the motions and elasticcollisions of a large number of molecules. The motions were consideredto be randomly distributed in the gas, while the motion of each moleculewas governed by the laws of mechanics both during and in betweencollisions. It was necessary to assume that molecules acted on each otheronly during collision, that their volume was small compared with the total



only during collision, that their volume was small compared with the totalvolume of the gas and that the time spent in collision is small compared tothe time that elapses between collisions. While the molecules needed tobe assumed to be small, they needed to be sufficiently large that theycould not move uninterrupted through the gas. The irregular path of amolecule through the body of a gas from collision to collision wasnecessary to explain rates of diffusion.

The kinetic theory was able to explain the gas laws connecting volume,temperature and pressure. It also predicted Avogadros law that equalvolumes of gases contain equal numbers of molecules and so explainedGay Lussac's law also. This legitimated the use of vapour densities for thedetermination of relative molecular weights. This in turn led to definitiveatomic weights and formulae that coincided with those that organicchemistry had yielded by the 1860s. The kinetic theory of gases alsoexplained the laws of diffusion and even predicted a novel phenomenathat was quite counter-intuitive, namely, that the viscosity of a gas, theproperty that determines its ease of flow and the ease with which objectsflow through it, is independent of its density. Counter-intuitive or not, theprediction was confirmed by experiment.

It was known from experiment that the behaviour of gases diverges fromthe gas laws as pressure is increased and they approach liquefaction. Thegas laws were presumed to apply to ideal gases as opposed to real gases.The behaviour of real gases approaches that of ideal gases as theirpressure is reduced. The kinetic theory had an explanation for thisdistinction, for at high pressure the assumptions of the kinetic theory, thatthe volume of molecules is small compared with the total volume of thegas they form part of and that the time spent in collision is smallcompared to the time between collisions, become increasingly inaccurate.The theory was able to predict various ways in which a real gas willdiverge from the ideal gas laws at high pressures (Van der Waalsequation) and these were confirmed by experiments on gases approaching

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equation) and these were confirmed by experiments on gases approachingliquefaction.

The kinetic theory of gases explained a range of experimental laws andsuccessfully predicted new ones. However, there were some keydifficulties. One of them was the departure of experimentally measuredvalues of the ratio of the two specific heats of a gas, measured at constantpressure and at constant volume, from what the theory predicted. Thisprediction followed from a central tenet of the theory that energy isdistributed equally amongst the degrees of freedom of a molecule. Thedifficulty could be mitigated by assuming that molecules of monatomicgases were perfectly smooth spheres that could not be set rotating and thatdiatomic molecules were also smooth to the extent that they could not beset rotating about the axis joining the two atoms in the molecule. But, asMaxwell made clear, (Niven, 1965, Vol. 2, 433) it must be possible formolecules to vibrate in a number of modes in order to give rise to thespectra of radiation that they emit and absorb, and once this is admittedthe predictions of the theory clash unavoidably with the measured specificheats.

The second major difficulty stemmed from the time reversibility of thekinetic theory. The time inverse of any process is as allowable as theoriginal within the kinetic theory. This clashes with the time asymmetryof the second law of thermodynamics and the time-directedness of theobserved behaviour of gases. Heat flows spontaneously from hot regionsto cold regions and gases in contact spontaneously mix rather thanseparate. It is true that defenders of the kinetic theory such as Maxwelland Boltzmann were able to accommodate the difficulty by stressing thestatistical nature of the theory and attributing time asymmetries toasymmetries in initial conditions. But this meant that a fundamental tenetof thermodynamics, the second law, was in fact only statistically true.Violations were improbable rather than impossible. Defenders of thekinetic theory had no direct experimental evidence for deviations from the



kinetic theory had no direct experimental evidence for deviations from thesecond law.

5.2 The Status of the Kinetic Theory

The kinetic theory explained known experimental laws and predicted newones. That empirical success could not be accommodated by sometruncated version of the theory that avoided a commitment to atomism inthe way that use of chemical formulae could for chemistry. Insofar as thekinetic theory explained anything at all, it did so by attributing thebehaviour of gases to the motions and collisions of molecules. On theother hand, it did face apparent empirical refutations as we have seen.Those wishing to assert the truth of the kinetic theory, and hence of anatomic theory, had a case but also faced problems.

For those inclined to judge theories by the extent to which they fruitfullyguide experiment and lead to the discovery of experimental laws, we get amore qualified appraisal. For two decades or more the mature kinetictheory proved to be a fruitful guide as far as the explanation andprediction of experimental laws is concerned. But, in the view of anumber of scientists involved at the time, the kinetic theory had ceased tobear fruit for the remainder of the century, as Clarke (1976, 889) hasstressed.

It might appear that the success of the kinetic theory marked a successfulinstantiation of the kind of atomism aspired to by the mechanical orNewtonian atomists, since macroscopic phenomena are explained interms of atoms with just a few specified mechanical properties. There arereasons to resist such a view. Firstly, neither the molecules of the kinetictheory nor the atoms composing them were ultimate particles. As we havenoted, it was well appreciated that they needed an inner structure toaccommodate spectra. Secondly, it was well apparent that the mechanicalproperties attributed to molecules by the kinetic theory could not

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properties attributed to molecules by the kinetic theory could notconstitute an exhaustive list of those properties. Further properties wererequired to explain cohesion and chemical interaction for instance.Thirdly, and perhaps most fundamentally, the kinetic theory was not anattempt to give an atomic account of the ultimate structure of matter.Maxwell, for one, was quite clear of the distinction between an atomismthat made claims about the ultimate structure of matter for some verygeneral metaphysical reasons, on the one hand, and a specific scientifictheory postulating atoms on the other (Niven, 1965, Vol. 2, 3614). Thekinetic theory was an example of the latter insofar as it was proposed, notas an ultimate theory, nor as a theory of matter in general, but as a theorydesigned to explain a specified range of phenomena, in this case themacroscopic behaviour of gases and, to a less detailed extent, of liquidsand gases too. As such, it was to be judged by the extent it was able tofulfil that task and rejected or modified to the extent that it could not. Acase for the existence of atoms or molecules and for the properties to beattributed to them was to be sought in experimental science rather thanphilosophy.

5.3 Phenomena Connected Via Atomism

During the half-century that followed the emergence of unique chemicalformulae and viable versions of the kinetic theory around 1860 thecontent of atomism was clarified and extended and the case for itimproved by the development of atomic explanations of experimentaleffects that involved connections between phenomena of a variety ofkinds, the behaviour of gases, the effect of solutes on solutions, osmoticpressure, crystallography and optical rotation, properties of thin films,spectra and so on. In several of these cases atomic explanations wereoffered of experimental connections for which there were no availablealternative explanations so that the case for atomism understood as aninference to the best explanation was strengthened.



Stereo-chemistry emerged as a result of taking the structures depicted inchemical formulae of substances to be indicative of actual structures inthe molecules of those substances. Pairs of substances that had crystalstructures that were mirror images of each other but which were otherwisechemically identical were represented by formulae that were themselvesmirror images of each other. Optical rotation gave independent evidencefor the reality of these underlying structures. Some chemists werereluctant to assert that the structures were in fact depictions of thephysical arrangements of atoms in space, a stand supported by the factthat there was still no theory that connected physical arrangements ofatoms with physical and chemical properties. There were eminentscientists, notably Ostwald (1904) and Duhem (2002), who, whilstaccepting that the phenomena were indicative of some underlyingstructure, refused to make the further assumption that the formulae withtheir structures referred to arrangements of atoms at all. Two factorsprovide a rationale for their stance. Firstly, the use of formulae inchemistry could be accepted without committing to atomism, as we havediscussed above, and as both Ostwald and Duhem stressed. Secondly, ananalogy with electromagnetism indicates that structural features need notbe indicative of underlying physical arrangements accounting for thosestructures. The electric field has the symmetry of an arrow and themagnetic field the symmetry of a spinning disc, but there is no knownunderlying physical mechanism that accounts for these symmetries.Stereo-chemistry may not have provided a case for atomism that waslogically compelling, but it certainly enabled that case to be strengthened.

Another set of phenomena providing opportunities to develop atomisminvolved the effects of solutes on solutions. It was discovered that effectssuch as the depression of freezing point and vapour pressure and theelevation of boiling point of a solvent brought about by dissolving a non-electrolytic solute in it are proportional to the weight of dissolvedsubstance and, what is more, that the relative effects of differing solutes in

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substance and, what is more, that the relative effects of differing solutes ina given solvent were determined by the molecular weight of the solute.More specifically, the magnitude of the various physical effects of asolute was dependent on the number of gram molecules of the dissolvedsolute, independent of the chemical nature of the solute. This provided away of measuring the molecular weight of soluble substances thatcomplimented the method involving the measurement of the vapourpressure of volatile ones. The strong suggestion that these effectsdepended on the number of molecules per unit volume was strengthenedwhen it was discovered that the osmotic pressure of a solute in a solventobeys the gas laws. That is, the osmotic pressure exerted by a solute in adefinite volume of solvent, measurable as the pressure exerted on amembrane permeable to the solvent but not the solute, was exactly thesame as if that same amount of solute were to fill that same volume as agas.

While the above could readily be explained by atomism, an anti-atomistcould still accept the experimental correlations by interpreting molecularweights as those yielded by chemical formulae independently of anatomic interpretation. Ostwald took that course. The move became lessplausible once the phenomena were extended to include solutions of non-electrolytes. For electrolytes, physical phenomena such as modification ofboiling and freezing points and osmotic pressure could be explained interms of the concentration of ions rather than molecules, where the ionswere the charged atoms or complexes of atoms employed by the atomiststo explain electrolysis. This enabled new experimental connections to beforged between, for example, osmotic pressure, and the conductivity ofelectrolytes. What is more, the charges that needed to be attributed to ionsto explain electrolysis were themselves linked to the valencies of thechemists. The atomic interpretation of electrolysis required acorresponding atomistic interpretation of electric charge, with eachmonovalent ion carrying a single unit of charge, a bi-valent ion carryingtwo such units and so on.



two such units and so on.

Yet another breeding ground for atomism came in the wake of theelectromagnetic theory of light (1865) and the experimental production ofelectromagnetic radiation by an electric oscillator (1888). Helmholtz(1881) observed that optical dispersion could be readily explained if itwere assumed that the transmission of light through a medium involvedthe oscillation of particles that were both massive and charged. Theadsorption and emission of spectra characteristic of atoms also suggestedthat they were due to the oscillations of charged particles on the atomic orsub-atomic scale. These assumptions in conjunction with the kinetictheory of gases led to an explanation of the width of spectral lines as aDoppler shift due to the velocity of radiating molecule, making possibleestimates of the velocities of molecules that were in agreement with thosededuced from the diffusion rate of gases.

Strong evidence for the charged and massive particles assumed in anatomic explanation of electrolysis and radiation was provided by theexperiments on cathode rays performed by J. J. Thomson (1897). Theexperimental facts involving cathode rays could be explained on theassumption that they were beams of charged particles each with the samevalue for the ratio of their charge to their mass. Thomsons experimentsenabled that ratio to be measured. A range of other experiments in theensuing few years, especially by Milliken, enabled the charge on thecathode particles, electrons, to be estimated, and this led to a mass of theelectron very much smaller than that of atoms. The fact that identicalelectrons were emitted from cathodes of a range of materials under arange of conditions strongly suggested that the electron is a fundamentalconstituent of all atoms.

5.4 Thermodynamics as a Rival to Atomism

Alan Chalmers


As the considerations of the previous section indicate, there is no doubtthat those wishing to make a case for atoms were able to steadilystrengthen their case during the closing decades of the nineteenth century.However, it is important to put this in perspective by taking account ofspectacular developments in thermodynamics which were achievedindependently of atomism, and which could be, and were, used toquestion atomism, branding it as unacceptably hypothetical.

Phenomenological thermodynamics, based on the law of conservation ofenergy and the law ruling out spontaneous decreases in entropy, supportedan experimental programme that could be pursued independently of anyassumptions about a micro-structure of matter underlying properties thatwere experimentally measurable. The programme was developed withimpressive success in the second half of the nineteenth century.Especially relevant for the comparison with atomism is the extension ofthermodynamics, from the late 1870s, to include chemistry. Two of thestriking accomplishments of the programme were in areas that had proveda stumbling block for atomism, namely, thermal dissociation andchemical affinity.

Gibbs (18768) developed a theory to account for what, from the point ofview of the atomic theory, had been regarded as anomalous vapourdensities by regarding them as consisting of a mixture of vapours ofdifferent chemical constitution in thermal equilibrium. The theory wasable to predict relative densities of the component vapours as a functionof temperature in a way that was supported by experiment. It is true thatatomists could not only accommodate this result by interpreting it inatomic terms but also welcomed it as a way of removing the problems thephenomena had caused for the determination of molecular weights fromvapour densities. But it remains the fact that the thermodynamicpredictions are independent of atomic considerat

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