ATOMISM

Atomism is a doctrine that has a long history in both philosophy and science. For this reason it is not easy to define its content in such a way as to comprehend all the historical variations and especially the historical development of the doctrine. In a very general sense, however, atomism may be defined as the doctrine that material reality is composed of simple and unchangeable minute particles, called atoms. It holds that all observable changes must be reduced to changes in the configuration of these particles. The multiplicity of visible forms in nature must likewise be based upon differences of configuration. The best way to discuss the variations of this general idea of atomism is to follow the historical development, which shows a gradual shift of emphasis from philosophical to scientific considerations. Consequently, the first part of this article, covering the period from the sixth century BCE to the seventeenth century, will be of a philosophical nature because in this period atomism was considered preponderantly from a philosophic point of view. The second part is concerned primarily with science, for it was in the period after the seventeenth century that atomism evolved in a scientific theory.

The Philosophical Period

In Greek philosophy we are already confronted with several types of atomism. Atomism in the strict sense, propounded by Leucippus and Democritus (fifth century BCE), should be looked upon as an attempt to reconcile the data of sense experience with Parmenides' thesis that matter is unchangeable. Parmenides rejected the possibility of change on rational grounds; change seemed to be unintelligible. He was convinced that reality must be one, that it must possess unity, and that, being one reality, it could not change. It may be remarked that this thesis of Parmenides is a presupposition for all rational science. Without fundamental unity, no universal laws are possible; without fundamental immutability, no laws covering past, present, and future can be valid. Yet, it is clear that Parmenides' approach is one-sided. Science may presuppose unity and immutability, but it also presupposes change. Only by studying changes is science able to discover the immutable laws of nature.

Democritus agreed with Parmenides on the unintelligibility and impossibility of qualitative change. He did not agree on the unintelligibility and impossibility of quantitative change. This type of change is subject to mathematical reasoning and therefore is possible. By the same token, Democritus denied qualitative multiplicity, but accepted multiplicity based on purely quantitative differences. Consequently, he accepted a numeric multitude of original beings, the atoms. These atoms did not differ qualitatively; only their sizes and figures differed. The infinite variety of observable things could be explained by the different shapes and sizes of the atoms that constituted them and by the different ways in which the atoms were combined. Observable changes were based upon a change in combinations of the atoms. During such changes, however, the atoms themselves remained intrinsically unchanged. They did not change their nature, or even their size or figure; they were indivisible (hence their name 'άτομος or indivisible).

Other forms of Greek atomism differed from that conceived by Democritus mainly in two points. First, they did not restrict the differences between the atoms to purely quantitative ones, but also accepted differences in quality. There was even a system that assumed as many qualitatively different atoms as there are different observable substances (Anaxagoras, fifth century BCE). Usually, however, only a few kinds of atoms were assumed, based upon the famous doctrine of the four elements: earth, water, air, and fire (Empedocles, fifth century BCE).

The second point of difference concerned the indivisibility of atoms. It is evident that a system that does not accept the indivisibility of atoms cannot properly be called atomism, but since such systems have played an important role in the history of atomism, we must mention them. For Democritus, the indivisibility of atoms was an absolute indivisibility, being the consequence of an absolute immutability. There were systems, however, that considered the indivisibility and immutability as only relative. The "atoms" could be divided, but they then became "atoms" of another substance; they changed their nature. (Here again an exception must be made for atoms as conceived of by Anaxagoras. These could be divided, but remained of the same kind. Hence they received the name of homoiomerics, possessing similar parts.) From the historical viewpoint, the most important system with qualitatively different atoms is that developed by the commentators on Aristotle—Alexander of Aphrodisias (second century CE), Themistius (fourth century) and John Philoponus (sixth century). In their system the atoms are called elachista (very small or smallest), the Greek equivalent of the Latin minima, which in medieval Latin writings indicates the smallest particles.

That these commentators on Aristotle combined the existence of "atoms" with the possibility of their changing their nature is not surprising. Aristotle was not satisfied by Democritus' atomism and was of the opinion that Democritus went only halfway. Atomism certainly opened up the possibility of explaining some changes that occur in nature, but not all. Nor did it account for all variety. Thus, the first task imposed upon Aristotle was a careful and critical reexamination of Parmenides' thesis. The result was his matter-form doctrine, stating that every material being is composed of primary matter and form of being. This composition, however, is not chemical or physical; it goes deeper. The possibility of change presupposes a certain fundamental nonsimplicity, for otherwise it is not possible to account for both aspects that are present in change: the aspect of a certain permanence (matter) and the aspect of something that is really new (form). Matter in the Aristotelian sense is not a substance, but the capacity to receive "forms."

To a certain extent, Democritus followed the same line of thought. Democritus, however, "substantialized" the permanent aspect (the atoms), thus narrowing the possibility of change. For Aristotle the "atoms" too should be subject to change and therefore "composed." Aristotle, however, did not propound a corpuscular theory of his own. Only a few remarks that could have been the starting point are found in a passus (Physics I 4, 187B18–34) in which he criticizes Anaxagoras' theory about the infinite divisibility of material things. Somewhere there must be a limit to divisibility. This limit depends on the specific nature of a thing. It was left to Aristotle's Hellenistic, Arabian, and medieval commentators to develop the casual remarks of their master into the minima naturalia theory, stating that each kind of substance has its specific minima naturalia.

In Greek philosophy there were also transitional theories between qualitative and quantitative forms of atomism. Plato (427–347 BCE), for example, adhered to the doctrine of the four elements; but the differences between the atoms of the respective elements were quantitative. An atom of fire had the form of a tetrahedron; that of air, an octahedron; that of water, an icosahedron; and that of earth, a cube.

When evaluating the importance of Greek atomism in the light of modern atomic theories, it should be borne in mind that in Greek thought philosophy and science still formed a unity. Greek atomism, therefore, was as much inspired by the desire to find a solution to the problem of mutability and plurality in general as by the desire to provide scientific explanations for specific phenomena. Although we meet with some ideas that can rightly be considered as precursors of classical physics and chemistry, the main importance of the old atomistic doctrines to modern science does not lie in these rather primitive scientific anticipations. The greatest achievement of Greek atomism was its general view of nature. The multitude of phenomena must be based upon some unity, and the ever-changing aspects of the phenomena are nevertheless aspects of a fundamentally unchanging world. To this view both the quantitative and the qualitative atomism have contributed—the latter by drawing attention to empirical aspects; the former, to the mathematical.

The history of the two forms of philosophical atomism until the birth of a scientific atomic theory has been rather different. This can easily be explained. Owing to the influence of Plato and Aristotle, Democritus' atomism did not gain preeminence in Greek, Arabian, and medieval thought. Yet that is not the only reason. Much more important is the fact that Democritus' atomism was more or less complete; and his followers, such as Epicurus (341–270 BCE) and the Latin poet Lucretius Carus (96–55 BCE), could confine themselves simply to taking over Democritus' doctrine.

The Aristotelian minima theory, however, existed only in an embryonic state. To Aristotle and his Hellenistic commentators the minima naturalia did not mean much more than a theoretical limit of divisibility; they were potentialities rather than actualities. With Averroes, however, we find an important development. According to him, the minima play an important role during chemical reactions. The Latin Averroists followed up this line of thought. Whereas most of the Latin commentators on Aristotle restricted themselves to a more or less systematic treatment of the minima as theoretical limits of divisibility, such Averroists as Agostino Nifo (1473–1538) attributed to the minima a kind of independent actual existence. The minima were considered as actual building stones of reality. The increase or decrease of a quantity of a substance amounts to the addition or subtraction of a certain number of minima. A chemical reaction takes place among the minima.

The fundamental importance of this view to science will be clear. Because the minima had acquired more physical reality, it became necessary to examine how their properties could be reconciled with the specific sensible properties of different substances. A first attempt to do so is found in Julius Caesar Scaliger (1484–1558). Some properties of matter, such as fineness and coarseness, depend on the minima themselves, while others depend on the manner in which the minima configurated. Rain, snow, and hail are composed of the same minima; but their densities are different because the minima of these three substances are at smaller or greater distances from one another. As to the chemical reaction, Scaliger remarked: "Chemical composition is the motion of the minima towards mutual contact so that union is effected" (Exercitationes, p. 345). Like Aristotle, he was convinced that Democritus was wrong. In a chemical compound the particles are not just lying close together; they form a real unity. Scaliger, however, was also convinced that the minima play a role in effecting the composition; and for that reason he was not satisfied with the Aristotelian definition of chemical composition as "the union of the reagents," in which the minima are not mentioned.

To sum up our survey of the development of the minima doctrine, and to prove that the opinions of Nifo and Scaliger were no exceptions, we may quote Francis Toletus (1532–1596), one of the best-known sixteenth-century commentators on Aristotle: "Concerning the manner of chemical composition, the opinions of authors vary, but they all agree in this: the reagent substances are divided into minima. In this division the separated minima of one substance come alongside the minima of the other and act upon each other till a third substance, having the substantial form of the compound is generated" (De Generatione et corruptione I, 10, 19).

The Scientific Period

The seventeenth century is an important period in the history of atomism. Not only did atomism come to occupy a central position in philosophical discussion, but it also became an inspiring idea for the spiritual fathers of modern science. The philosophic differences between the atomic systems were soon pushed into the background, while the more scientific aspects that were held in common came to the foreground. Daniel Sennert (1572–1657) offers a clear example of this tendency. Basically, his corpuscular theories were derived from the doctrine of minima naturalia, but they also contain typically Democritean ideas. In a sense the same could be said of Scaliger; but the difference is that Scaliger discussed the philosophical controversies between Aristotle and Democritus, whereas Sennert showed a pronounced eclectic tendency. He was interested mainly in a chemical theory, and he found that from a chemical point of view the two theories really amount to the same thing. In order to support this opinion, Sennert refused to accept the interpretation that Democritus meant to deny the qualitative differences of atoms. As a chemist, Sennert was convinced that elementary atoms differ qualitatively. His main contribution to the corpuscular theory lies in the clear distinction that he made between elementary atoms and atoms of compounds (prima mista ). This distinction forced itself upon Sennert through chemical experience. Each chemical substance, elementary or compound, must have its own atoms.

Contrary to Sennert, Pierre Gassendi (1592–1655) faithfully copied Epicurus and therefore Democritus as well. His own contribution consisted of a number of annotations designed to make the original atomic doctrine acceptable to his contemporaries. In order to effect this purpose, two things were necessary. First of all, the atomic system had to be divested of the materialistic interpretation with which it was hereditarily connected. Second, Gassendi had to "adapt" the original atomic theory to the science of his time. Science had reached the stage at which certain definite physical and chemical properties were attributed to the atoms—i.e., the atoms must possess definite natures; they could not be qualitatively equal. For this reason Gassendi stated that from the original atoms certain molecules were formed first; these differed from each other and were the seeds of different things.

While Gassendi's system is basically without any trace of originality, the corpuscular theory of René Descartes (1596–1650) is original in outline and execution. According to Descartes, matter and extension are identical. This thesis of course excludes the idea of indivisible atoms, but not of smallest particles. To the question of how such particles are separate and distinct from each other, Descartes answered that when a quantity of matter moves together, that quantity forms a unit, distinct from other units that have different motions. Along these lines, Descartes succeeded in devising a corpuscular theory in which the corpuscles were characterized by differences in mass, in amount of motion, and other properties that could be expressed in physical terms and treated mathematically. Descartes's corpuscles were endowed with exactly those properties that could be used in contemporaneous mechanics. As we have seen with Sennert, the seventeenth century was less interested in philosophical considerations than in scientifically fruitful ideas. Therefore, a corpuscular theory was judged, first of all, by this standard; and underlying philosophical discrepancies did not much interest the scientist. This explains why, to their contemporaries, Gassendi and Descartes could stand fraternally united as the renovators of the atomic theory.

Robert Boyle (1627–1691), for example, repeatedly confessed how much both Descartes and Gassendi had inspired him. On the other hand, Boyle was too much a chemist to be satisfied with a general idea of atoms or even with atoms endowed only with mechanical properties. Boyle looked for specific chemical properties. In contrast with mechanics, however, chemistry was not yet sufficiently developed to provide the theoretical framework necessary for a satisfactory chemical atomic theory. Boyle was keenly aware of this situation, as his The Sceptical Chymist (Oxford, 1661) proves. Neither the traditional theory of four elements nor the three-principle theory current among chemists could be of any use to him. Yet he was convinced that the distinction between elements and compounds was a sound one. This distinction therefore governed his own atomic theory. Theoretically, he adhered to the atoms of Democritus; practically, he did not use them. He was convinced that atoms were associated into so-called primary concretions, "which were not easily dissipable into such particles as composed them." Thus the primary concretions were corpuscles with definite qualities; they corresponded to the smallest particles of elements, and consequently Boyle treated them as such. The primary concretions could combine to form compounds of a higher order that may be compared with Sennert's prima mista. Although Sennert's corpuscular theory was based more on the minima theory and Boyle's theory more on the ideas of Gassendi and Descartes, in practice their theories were not very different. Both theories recognized atoms of compounds that are composed of atoms of elements. For Sennert the latter were elements, both theoretically and practically. For Boyle, theoretically they were not elements, but practically they were, because in chemical and physical processes primary concretions are not dissolved.

By combining the relative merits of the minima theory (qualitative atoms) and of Democritus' atomism (open to quantitative treatment), the seventeenth century laid the foundations for the scientific atomic theory of the nineteenth century. The further development of the seventeenth-century atomic theory, however, required better chemical insights, and especially a method of distinguishing elementary from compound substances. This method was found by Antoine Lavoisier (1743–1794), who postulated the conservation of weight as the guiding principle in chemical analysis. For the first time in history, a list of chemical elements could be given, based upon the results of chemical analysis.

The outstanding achievement of John Dalton (1766–1844) was that he connected these chemical results with the atomic theory. His atoms were no longer smallest particles with some general and rather vague physical properties, but atoms endowed with the properties of chemical elements. Dalton himself in A New System of Chemical Philosophy stressed the great importance of "ascertaining the relative weights of the ultimate particles, both of simple and compound bodies, the number of simple elementary particles which constitute one compound particle, and the number of less compound particles which enter into the formation of one more compound particle" (2nd ed., p. 213).

The fact that Dalton's theory is primarily a chemical theory does not mean that it has no philosophical implications. It is interesting to note that Dalton conceived the union of atoms in a compound as their simple juxtaposition without their undergoing any internal change. On this point the founder of the chemical atomic theory did not differ from the Democritean tradition. On another point, however, he followed the minima tradition. Dalton's atoms were specifically different for every kind of substance. He did not even think of building these atoms from particles without qualities, as Gassendi and Boyle had done.

After Dalton, the development of the atomic theory was very rapid. Jöns Jakob Berzelius (1779–1848) determined the relative atomic weights with surprising accuracy, guided by the hypothesis that under the same pressure and at the same temperature the number of atoms in all gaseous substances is the same. Since hydrogen and oxygen combine in the constant volume proportion of two to one, Berzelius concluded correctly that two atoms of hydrogen combine with one atom of oxygen. Berzelius also gave to chemistry its modern symbols. Amedeo Avogadro (1776–1856) completed the atomic theory by assuming that compound atoms, or molecules, do not necessarily have to be formed out of atoms of different elements; molecules of elements (H₂; O₂) also exist. According to Avogadro, the law that postulated an equal number of atoms in equal volumes of gas had to be understood as applying to an equal number of molecules. In a short time, the framework for classical chemistry was completed on the basis of Dalton's atomic theory. Chemical reactions were conceived of as a reshuffling of atoms and described by such chemical equations as 2 H₂ + O₂ → 2 H₂O.

An important contribution to the development of the atomic-molecular theory came from physics in the form of the kinetic theory of gases. With the aid of the calculus of probability, James Maxwell and Ludwig Boltzmann succeeded in deriving the behavior of gases, as described in the empirical laws of Boyle and Joseph-Louis Gay-Lussac, from the motions of the molecules.

The discovery of the electron, the electric atom, paved the way for a new theory about the nature of chemical compounds and chemical reactions. According to the new theory, a molecule such as NaCl did not consist of an Na atom and a Cl atom, but of an Na ion and a Cl ion; the Na ion was an Na atom minus an electron, and the Cl ion was a Cl atom plus an electron. Thus the so-called ionic theory revealed the nature of the forces of attraction between the various atoms of a molecule. The Na ion with its positive electric charge was attracted by the Cl ion with its negative charge. As a result of the connection that the theory of electricity established between physics and chemistry, theoretical and experimental materials were available at the beginning of the twentieth century. They led to a new development of the atomic theory that would endeavor to penetrate into the interior of Dalton's atoms.

The atomic model of Niels Bohr (1913) considered every atom as built of a positively charged nucleus around which circled, in fixed orbits as many electrons as were indicated by the charge of the nucleus. This charge corresponded to the place of the element in the periodic system. Bohr's model could explain not only the fundamental chemical properties of the elements, but also such physical properties as the spectrum that is characteristic of each element when it is emitting or absorbing light. Nevertheless, there were also serious difficulties with this model. According to electrodynamics, the moving electrons would ceaselessly emit electromagnetic waves. The atom would not be stable, but would always be losing energy. Hence, the motion of the electrons would gradually decrease and finally cease entirely. In order to save his model, Bohr postulated that emission of energy occurs only when an electron "jumps" from one orbit to another. In other words, the emission of energy is discontinuous. The emitted energy could be only a whole multiple of an elementary quantity of energy.

Thus, following the work of Max Planck, the idea of minima of energy was added to the idea of minima of matter, the traditional basis of atomism. Even light seemed to show an atomistic structure (photon theory). This would have meant a complete victory for the atomistic view if there had not been a complication. This complication was that the reasons which had formerly settled the dispute about the nature of light in favor of Christian Huygens's wave theory against Isaac Newton's corpuscular theory still retained their value. Light showed a dual character. In 1924, it occurred to Louis de Broglie that the same dualism might very well apply to the particles of matter. On the basis of this hypothesis, he could readily explain Bohr's postulate. This resulted in quantum mechanics, a new theory propounded by Erwin Schrödinger and Werner C. Heisenberg, which showed that both the atomic theory and the wave theory were only approximate models and not adequate representations of material reality.

The evolution of the atomic theory in the twentieth century was not limited to these rather startling new theoretical developments; it also gave rise to a new branch of physical science, nuclear physics, which studies the changes that the atomic nucleus is subject to. The first work in this area was in connection with the study of natural radioactivity. It had been observed that through radiation the nucleus of one element changes in charge and mass and thus becomes the nucleus of another element. In 1919 Ernest Rutherford succeeded in effecting an "artificial" transmutation; many others followed. The atoms of chemical elements appeared to be composed like the molecules of chemical compounds. Through nuclear processes a confusingly great number of new elementary particles has been discovered, all of which are subject to transformation under certain conditions. Particles can be changed into other particles and even into radiation. With such transmutations enormous amounts of energy are released.

Thus, twentieth-century science revolutionized many fundamental ideas of the nineteenth century; the atom is not only much more complex than Dalton thought; it is also much more dynamic. Yet Dalton is far from antiquated. Modern chemistry still works along the lines drawn by Dalton and his contemporaries. Can the same be said in relation to his forerunners in the philosophical period of atomism? The answer to this question can be found in the fact that the main mistake of Dalton and other advocates of essentially mechanistic theories lay in the conviction that atoms did not undergo any internal change. Science showed that this assumption was erroneous, but this should not be a de facto statement only. For if we think of the nature of science as experimental, then it is clear that unchangeable atoms would not offer any possibility of being investigated by experimental means. Without change, matter could not respond to experimental questions. Classical science could overlook this simple truth by assuming that it already knew all the relevant features of atoms. This assumption followed from the mechanistic doctrine that, from the seventeenth century onward, formed the philosophical background of the atomic theory and of classical science in general. The mechanistic doctrine points up the fact that classical science originated in a rationalistic climate. The idea of an unchangeable atom endowed with mechanical properties seemed to be in accordance with what an element should be. It satisfied both the imagination and the intellect. The program of science seemed to consist in explaining the forms of nature on the basis of component elements that were already known.

With the development of science, however, increasing knowledge of chemical compounds affected our understanding of elements. The elements, too, became the object of experimental investigation. From this it may be concluded that the mechanistic doctrine was not a real presupposition of the scientific method. In using the experimental method, science presupposed a much more fundamental mutability in nature than traditional mechanism could account for, and the scientific method implied a much more refined view of material reality than the mechanistic interpretations of science suggested. For this reason, the less orthodox forms of atomism were as important to the origin of the scientific atomic theory as were the orthodox. From the point of view of twentieth- and twenty-first-century science, the Greek philosophical discussions about the nature of change remain amazingly modern.

See also Alexander of Aphrodisias; Anaxagoras of Clazomenae; Aristotle; Averroes; Bohr, Niels; Boltzmann, Ludwig; Boyle, Robert; Chemistry, Philosophy of; Descartes, René; Empedocles; Epicurus; Gassendi, Pierre; Heisenberg, Werner; Lavoisier, Antoine; Leucippus and Democritus; Lucretius; Maxwell, James Clerk; Newton, Isaac; Parmenides of Elea; Philosophy of Science, History of; Planck, Max; Plato; Schrödinger, Erwin; Themistius; Toletus, Francis.

Bibliography

Dalton, John. A New System of Chemical Philosophy. London: Bickerstaff, 1808; 2nd ed., 1842.

Dijksterhuis, E. J. The Mechanization of the World Picture. Oxford: Clarendon Press, 1961. Excellent history of science from antiquity to the seventeenth century.

Hooykaas, R. "Elementenlehre und Atomistik im 17. Jahrhundert." In his Die Entfaltung der Wissenschaft, pp. 47–65. Hamburg: Augustin, 1957.

Lasswitz, K. Geschichte der Atomistik vom Mittelalter bis Newton, 2 vols. 2nd ed. Leipzig, 1926. A nineteenth-century classic.

Melsen, A. G. M. van. From Atomos to Atom, the History of the Concept Atom, 2nd ed. New York: Harper and Row, 1960. Includes references for the primary sources.

Nash, Leonard K. The Atomic-Molecular Theory. Cambridge, MA: Harvard University Press, 1950. Discusses the classical chemical theories.

Scaliger, J. C. Exotericarum Exercitationum Libri XV de Subtilitate ad Hier. Frankfurt, 1557; 2nd ed., 1607.

Whittaker, E. History of the Theories of Aether and Electricity, 2 vols. New York: Philosophical Library, 1951 and 1954. For readers with a good background in science.

Whyte, L. L. Essay on Atomism: From Democritus to 1960. Middletown, CT: Wesleyan University Press, 1961. A brief introduction to the idea of atomism and its history.

Yang, Chen Ning. Elementary Particles, a Short History of Some Discoveries in Atomic Physics. Princeton, NJ: Princeton University Press, 1962. Gives a general outline of the research done since 1900.

Andrew G. M. van Melsen. (1967)