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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
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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
Atomism from the 17th to the 20th Century
2 Stanford Encyclopedia of Philosophy
BibliographyOther Internet ResourcesRelated Entries
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
Alan Chalmers
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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.
Atomism from the 17th to the 20th Century
4 Stanford Encyclopedia of Philosophy
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
Alan Chalmers
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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
Atomism from the 17th to the 20th Century
6 Stanford Encyclopedia of Philosophy
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
Alan Chalmers
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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
Atomism from the 17th to the 20th Century
8 Stanford Encyclopedia of Philosophy
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
Alan Chalmers
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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
Atomism from the 17th to the 20th Century
10 Stanford Encyclopedia of Philosophy
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.
Alan Chalmers
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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
Atomism from the 17th to the 20th Century
12 Stanford Encyclopedia of Philosophy
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,
Alan Chalmers
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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.
Atomism from the 17th to the 20th Century
14 Stanford Encyclopedia of Philosophy
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
Atomism from the 17th to the 20th Century
16 Stanford Encyclopedia of Philosophy
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
Alan Chalmers
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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
Atomism from the 17th to the 20th Century
18 Stanford Encyclopedia of Philosophy
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
Atomism from the 17th to the 20th Century
20 Stanford Encyclopedia of Philosophy
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
Atomism from the 17th to the 20th Century
22 Stanford Encyclopedia of Philosophy
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.
Atomism from the 17th to the 20th Century
24 Stanford Encyclopedia of Philosophy
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
Atomism from the 17th to the 20th Century
26 Stanford Encyclopedia of Philosophy
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
Atomism from the 17th to the 20th Century
28 Stanford Encyclopedia of Philosophy
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
Alan Chalmers
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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.)
Atomism from the 17th to the 20th Century
30 Stanford Encyclopedia of Philosophy
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
Atomism from the 17th to the 20th Century
32 Stanford Encyclopedia of Philosophy
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
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34 Stanford Encyclopedia of Philosophy
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
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36 Stanford Encyclopedia of Philosophy
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.
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38 Stanford Encyclopedia of Philosophy
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.
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40 Stanford Encyclopedia of Philosophy
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
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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