Historic Dispute : Are atoms real

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PHYSICAL SCIENCE

Historic Dispute : Are atoms real?

Viewpoint: Yes, atoms are real, and science has developed to the point that atoms can not only be seen, but can also be individually manipulated.

Viewpoint: No, many pre-twentieth-century scientists, lacking any direct evidence of the existence of atoms, concluded that atoms are not real.

At the start of his Lectures in Physics, the 1965 Nobel Laureate in Physics Richard Feynman asks what one piece of scientific knowledge the human race ought to try to preserve for future generations if all the other knowledge were to be destroyed in some inevitable cataclysm. His answer that the single most important scientific fact is that all matter is composed of atoms, now seems completely reasonable. How is it then, that less than 100 years before, the very existence of atoms could be disputed with some vehemence?

Although the notion of atoms has been around for a long time—over 2,500 years—it is important to note that it has meant different things in different epochs and to different thinkers. It meant one thing to the ancient Greek matter theorists, another to the Epicurean philosophers, something else to early modern scientific thinkers, yet another things to nineteenth century chemists, and means something a bit different again to contemporary atomic physicists.

The atomic hypothesis, that all matter is composed of tiny indestructible particles, is generally attributed to Democritus (c. 460-370 b.c.), a Greek philosopher writing in the fifth century b.c., although the idea was not entirely new with him. A century later, another Greek, Epicurus (341-270 b.c.), adopted the idea to his philosophical system, which argued against an active role for God or gods in determining the course of events in the world and denied the possibility of life after death. Plato (c. 428-348 b.c.) accepted the existence of atoms and tried to explain the properties of the four classical elements—air, earth, fire, and water—in terms of the shapes of their atoms. His student Aristotle (384-322 b.c.) dismissed this idea in favor of a metaphysics in which the form of objects was imposed on an underlying continuous substance.

Although the ideas of Aristotle were at first regarded with suspicion by church authorities, they were eventually embraced as consistent with Christian belief. In the twelfth century the Italian Saint Thomas Aquinas (1225-1274) adopted the metaphysics of substance and form to explain the sacraments of the Catholic Church, and theologians introduced the term transubstantiation to describe the transformation of the substance, but not the form or appearance, of the bread and wine used in the Mass. To advocate that matter was an aggregate of unchanging atoms became heretical and therefore dangerous, at least in Christian Europe.

Scientific and philosophical interest in the atomic hypothesis revived in the Renaissance. The Italian mathematician Galileo Galilei (1564-1642), the English physicist Isaac Newton (1642-1727), and the Anglo-Irish physicist Robert Boyle (1627-1691) all advocated the existence of atoms. Real progress toward the modern concept of the atom could not occur without the modern notion of chemical element. In the Skeptical Chymist, published in 1661, Boyle argued that there were many more elements than the four accepted in antiquity and that the list of elements could only be established by experiment. Two centuries later, in his Elementary Treatise in Chemistry, the French chemist Antoine-Laurent Lavoisier (1743-1794) published what is considered to be the first modern list of elements—"modern" in that it includes oxygen rather than the problematic phlogiston, but still included light and caloric (heat) as elements.

As the nineteenth century began, an English school teacher and tutor, John Dalton (1766-1844), began to consider the quantitative consequences of the existence of atoms in chemical analysis. According to Dalton's Law of Definite Proportions, the ratios of the weights of the elements that formed any particular compound was fixed and represented the ratio of the weights of the atoms involved. A second law, the Law of Multiple Proportions dealt with the case in which two elements formed more than one compound. In this case the weights of one element that combined with a fixed weight of another would always be in the ratio of small whole numbers. For example, the weight of oxygen combined with one gram of nitrogen in the compound NO would be half that which combined with one gram of nitrogen in the compound NO².

Dalton's understanding of compound formation underlies the discussion of the difference between physical and chemical change with which most modern chemistry texts begin. Chemical changes are more drastic and involve more energy. They also typically yield compounds with qualitatively different properties than those of the original substances. Expose a piece of soft, shiny sodium metal in an atmosphere of the irritating, green chlorine gas and one obtains the common salt that adds flavor to food. Dissolving sugar in water, in contrast, yields a solution that tastes sweet like sugar and is transparent like water. Chemical changes produce new compounds that obey Dalton's laws, physical changes do not.

But the textbooks oversimplify the reality. The weight of sodium that combines with one gram of chlorine will vary slightly depending on the exact conditions of preparation. The range of deviation from the ideal, or stoichiometric, weight ratio will be small for most ionic solids, but is measurable by the careful analytical chemist. We now understand that the deviation from the ideal ratio of atoms arises from the presence of defects that all solid structures tolerate to some extent. However, in the early days of chemical analysis these small exceptions were enough to call into question the assumption that atoms combine in definite small-number ratios. A further complicating factor is that the distinction between mixtures and compounds breaks down in some metal alloys. The forces between atoms in some metallic mixtures are as strong as those in the pure metals and the components of the alloy are not readily separated, even though the chemical composition is quite variable. The existence of deviations in stoichiometry and the development of a thermodynamic formalism which accounted for the stability of these materials without invoking the existence of atoms caused a number of the most eminent physical chemists of the nineteenth century, including the French chemist Pierre-Eugéne Marcellin Berthelot (1827-1907) and the German physical chemist Friedrich Wilhelm Ostwald (1853-1932), to remain skeptical about the existence of atoms.

The observation of the French chemist Joseph-Louis Gay-Lussac (1778-1850) in 1808 that chemical reactions between gases involve volumes combining in small number rations, and the explanation provided in 1811 by the Italian physicist Amedeo Avogadro (1776-1856) that equal volumes of gas contained equal numbers of molecules, strengthened the case for belief in atoms appreciably. It nonetheless left open the possibility that the liquid and solid states might be continuous in nature, with atoms and molecules only forming on evaporation. The fact that atoms could not be directly observed was still troublesome to the Austrian physicist and philosopher Ernst Mach (1838-1916) and his disciples, who cautioned against attributing reality to entities never observed. Mach's position is not unreasonable, given the prior history of science in attributing reality to such "unreal" concepts as phlogiston, caloric, and the luminiferous aether. Mach's stature as a highly regarded physicist, however, delayed general acceptance of the existence of atoms until early in the twentieth century.

In the end, atoms became accepted not because they were eventually observed but because they provided such a powerful and coherent explanation of the phenomena of physics and chemistry. In contrast to the original notion of indivisible particles, a detailed picture of the atom as composed of more elementary particles emerged. Further, transformations of the atoms of one element into those of another were found to occur in radioactive elements. The development, in the later twentieth century, of techniques that could form images of the atoms on a solid surface only confirmed the existence of those atomic particles that explained so much about the behavior of matter.

—DONALD R. FRANCESCHETTI

Viewpoint: Yes, atoms are real, and science has developed to the point that atoms can not only be seen, but can also be individually manipulated.

The idea that matter was not continuous but consisted of discrete particles was first proposed by the Greek philosopher Anaxagoras (c. 500-428 b.c.). He claimed that matter consisted of infinitely small particles which he called omiomeres. He believed that these small particles contained the quality of all things, and had developed a theory for the creation of matter from omoiomeres. It was, however, another Greek philosopher, Leucippus (5th centuryb.c.), who actually used the term atom. He applied the term to describe a particle that was indivisible, compact, without parts, and that had a homogeneous composition. These atoms differed by their qualities, such as size and shape. Leucippus also maintained that there were an infinite number of atoms in constant, random motion. If a collision occurred, the atoms could be scattered, or they could coalesce to form an aggregate. In addition to the existence of atoms, Leucippus postulated the existence of a void that allowed the atoms uninhibited random movement and was central to the theory of atomism, as Leucippus's hypothesis came to be called. The successor of Leucippus, another Greek philosopher named Democritus (c. 460-370 b.c.), refined the concept of atomism to the extent that it is difficult to distinguish his thoughts from those of Leucippus. Epicurus (341-270 b.c.), also a Greek, added weight as yet another defining characteristic of an atom. He modified the hypotheses of Leucippus and Democritus, saying that all atoms moved at the same speed, irrespective of their weight or volume. Based on the physics of the time, this conclusion led Epicurus to believe that all atoms moved in a slow, but definite, downward direction. This view of atomic movement made collision between atoms difficult to imagine. Epicurus, realizing this, applied a modification stating that occasionally, an atom would "swerve," allowing it to collide with another.

Unfortunately, this modification to the atomism of Leucippus and Democritus gave their detractors enough ammunition to ignore its many merits. As a result, the Greek philosopher Aristotle (384-322 b.c.) was able to propagate his antiatomistic theory. Primarily, Aristotle denied the existence of a void and affirmed the continuous nature of matter. He refused to accept any limits on the divisibility of matter, saying that matter had inherent qualities, such as color, smell, and warmth. He subscribed to the theory of four elements, namely, earth, air, fire and water, and introduced a fifth element, ether, which governed celestial qualities. In effect, Aristotle's ideas resulted in the separation of terrestrial and celestial laws. The church accepted his views, and the combined effect was that Aristotle's influence essentially halted the development of atomic theory until after the Renaissance. Atomism developed separately in India during the same time period and appeared in the Middle East during medieval times.

It may seem strange to mention religion in the discussion of the atom, but the development of atomic theory was often frustrated by the inability of its developers to separate religion and science. Many of the odd ideas and wrong turns in the history of science were made in the name of unifying science and religion.

The Development of Atomic Theory

The Seventeenth Century.

Experimental work performed by the Italian mathematician and physicist Evangelista Torricelli (1608-1647) and the French scientist and philosopher Blaise Pascal (1623-1662) on air pressure was a driving force behind the renewal of atomism. Pierre Gassendi (1592-1655), a French scientist, was a leader in refuting Aristotelian theory and relied on the work of Torricelli and Pascal to return to the atomic concepts of Democritus and Epicurus. Gassendi was the first to use the term molecule, to describe a group of atoms acting as a unit. He also returned to the concept of random atomic motion described by Democritus, rather than the downward movement of Epicurus.

Many other scientists and philosophers contributed to the slow redevelopment of atomic theory, but the next gigantic leap was made by the English physicist Isaac Newton (1642-1727). Of his laws of motion, the gravitational law had the effect of once again unifying terrestrial and celestial science. This central development overrode Aristotle's influence, and atomic theory began to develop in earnest. It was proposed at this time that Newton's gravitational law could be applied to describe the attraction between atoms, but Newton himself did not believe this. He suspected that electrical and magnetic forces were important at such small scales, but he had no idea how true his thoughts would prove to be.

It must be remembered that while the particulate nature of matter was beginning to gain popularity, all of the atomic theory proposed by Democritus was still not accepted. For example, the Anglo-Irish physicist Robert Boyle (1627-1691) supported atomism, but not the concept of random motion of atoms. He believed that atoms could not move without reason, and that it was God who decided how and where atoms moved.

The Eighteenth and Nineteenth Centuries.

The eighteenth and nineteenth centuries saw great advances in atomic theory. These theories are all the more convincing because the scientists who made them did not do so blindly. They debated endlessly among themselves about the truth of their own hypotheses, and tried often to disprove their own developments!

The French chemist Antoine-Laurent Lavoisier (1743-1794) firmly defined an element as a substance that had not yet been decomposed by any means. He also clearly stated the law of conservation of matter—during a chemical reaction, matter is neither created nor destroyed. Another French chemist, Joseph-Louis Proust (1754-1826), stated the law of constant proportion in 1806—irrespective of its source, a substance is composed of the same elements in the same proportions by mass. These two laws enabled the English chemist John Dalton (1766-1844) to propose his atomic theory and state the law of multiple proportions. Dalton's atomic theory contained four statements: 1) All matter is composed of atoms, very tiny, indivisible particles of an element that cannot be created nor destroyed. 2) Atoms of one element cannot be converted to atoms of another element. 3) Atoms of one element are identical in mass and other properties and are different from atoms of other elements. 4) Compounds are formed when specific ratios of different elements chemically combine. In spite of Dalton's insight in developing the atomic theory, he incorrectly assumed that elements such as hydrogen and oxygen were monatomic. It was the work on gas volumes of two chemists, Joseph-Louis Gay-Lussac (1778-1850) from France, and Amedeo Avogadro (1776-1856) from Italy, that led to the hypothesis that gases such as oxygen and hydrogen were formed from two atoms of the same element combined and were thus diatomic, not monatomic.

In 1869, Dmitry Ivanovich Mendeleyev (1834-1907), a Russian chemist, published his periodic table of the elements, in which he ordered the known elements based on their masses and their chemical properties. In fact, he was bold enough to switch certain elements where he thought the properties belonged in a different column, and history proved his thinking correct. Mendeleyev's greatness lies in the fact that he used his table to predict the properties of elements that had not yet been discovered.

With the introduction of the concept of valency—the property of an element that determines the number of other atoms with which an atom of the element can combine—the French chemist Joseph-Achille Le Bel (1847-1930) and Dutch physical chemist Jacobus Hendricus van't Hoff (1852-1911) were able to imagine the concept of molecules with three-dimensional structures. Even at this date, detractors of the atomic theory existed. Adolf Wilhelm Hermann Kolbe (1818-1884), a German organic chemist described as one of the greatest of that time, issued scathing comments regarding van't Hoff's vision of three-dimensional molecules.

Several schools of thought existed at this time. Some were enthusiastically in support of atomism. Others weakly supported or remained neutral on the thought, and some supported conflicting thoughts. Of the latter category, two major groups, called the equivalentists and the energeticists, were particularly vocal. The French chemist Pierre-Eugéne Marcellin Berthelot (1827-1907), an equivalentist, exerted his considerable power as a government official to prohibit the teaching of atomic theory. In fact, the mention of atoms was avoided and many texts contained the idea of atomic theory merely as an appendix, if at all. French physicist Pierre-Maurice-Marie Duhem (1861-1916), Austrian physicist Ernst Mach (1838-1916), and German physical chemist Friedrich Wilhelm Ostwald (1853-1932), all energeticists, preferred the consideration of perceived data to that of hypothetical atoms. Ostwald was said to have denied the existence of matter! Physicist Albert Einstein (1879-1955) was extremely critical of Mach's ideas. The Austrian physicist Ludwig Boltzmann (1844-1906), whose development of the kinetic molecular theory of gases relies extensively on the existence of atoms, supported Einstein. While German physicist Max Planck (1858-1947) initially accepted Mach's ideas, he later changed his mind and refuted his theories. Of Duhem, Mach and Ostwald, only Ostwald later openly accepted atomic theory. The others staunchly denied it until the end.

The Twentieth Century.

The combined efforts of English physicist Sir Joseph John (J.J.) Thomson (1856-1940), American physicist Robert Andrews Millikan (1868-1953), German physicist Wilhelm Conrad Roentgen (1845-1923), Dutch physicist Antonius van den Broek (1870-1926), British physicist Lord Ernest Rutherford (1871-1937), and English physicist Henry Moseley (1887-1915) led to the discovery of the electron and the realization that it was a component of all atoms. Further, it was realized that the number of electrons was proportional to the atomic mass, although the electron itself was much smaller than an atom. The first model of the atom, proposed by British physicists J.J. Thomson and Lord William Thompson Kelvin (1824-1907), was of a positively charged cloud containing the negatively charged electrons, much as a plum pudding contains raisins.

Rutherford's famous gold foil experiment provided the first real model of the atom. In this experiment, Rutherford found that positively charged particles directed at a thin piece of gold foil were sometimes deflected rather than going straight through consistently. At times, the particles were deflected straight back to the source. He proposed that the atom contained a dense central portion that he called the nucleus. The nucleus was much smaller than the atom, but contained the majority of the mass and had a strong positive charge. Rutherford also envisioned the electrons orbiting the nucleus in the way that the planets orbit the sun. Since the atom is electrically neutral, the number of electrons was such that the negative charge of the electrons would balance the positive charge of the nucleus. The problem with this model was that according to classical physics, the orbiting electron would constantly emit electromagnetic radiation, lose energy, and eventually spiral into the positive nucleus. If this were so, all matter would eventually self-destruct. Also, this model did not explain known spectroscopic observations. If the model were correct, the electron would pass through all different energy levels, thus emitting spectral lines at all frequencies and causing a continuous spectrum to be observed. Instead, a distinct set of spectral lines was observed for each different element.

Max Planck essentially solved the second problem when he proposed that energy was emitted not continuously, but in small packets, which he called quanta. Rather than being able to assume any value on a continuous scale—like being able to assume any position on a ramp—energy was limited to certain values—like being able to stand on one step or another, but not between steps. This concept was so radically different that Planck himself barely believed it.

Einstein immediately found an application of Planck's quantum in his explanation of the photoelectric effect. He suggested that light consisted of photons, each having the energy of a quantum. The Danish physicist Niels Bohr (1885-1962) combined the observations of both Planck and Einstein to propose a new model for the atom. He stated that instead of circling the nucleus emitting energy randomly, the electrons could only assume certain discrete energy values (i.e, quantized) that were at specific distances from the nucleus. In applying the concept of quantization, to the electron, he effectively removed the problem of the electron spiraling into the nucleus. Further, Bohr postulated that spectral lines resulted from the movement of the electron from one energy level to another. This explained the presence of discrete lines rather than a continuous band in atomic spectra. Bohr revolutionized atomic theory with his model and with the fact that it gave the correct values for the observed spectra of hydrogen. However, Bohr's model was not successful with other elements.

Further refinements were made when the English physicist Sir James Chadwick (1891-1974) discovered the neutron, thus accounting for the entire mass of the atom and explaining the existence of isotopes. French physicist Louis-Victor de Broglie (1892-1987) took the next step when he inverted Einstein's observation that light behaved like particles. He stated that particles could behave like light and exhibit wave properties. While this is true for large objects like footballs, their wavelength is insignificant due to their large mass. However, for tiny particles such as electrons, the wavelength was no longer insignificant. This revelation established the concept of wave-particle duality, the fact that matter could behave as a wave and vice versa.

Erwin Schrödinger (1887-1961), an Austrian theoretical physicist who had been a vocal critic of Bohr's theory of movement between levels, or electrons "jumping," developed the model of the atom that is with us today. His wave mechanical model of the atom was made possible by de Broglie's work. It is mathematically complex, and yet extremely elegant. Essentially, Schrödinger described certain mathematical functions, which he called wavefunctions, and upon which he placed the normal mathematical restrictions of continuity, consistency, uniformity, and finite nature. Under these conditions, only certain values of energy would be possible for the energy of the electron, thus creating a natural path for quantization, unlike Bohr's imposed quantization. The wavefunctions, also called orbitals, described the electrons in that particular energy state. The values that determine these wavefunctions are known as quantum numbers. The first is n, the principal quantum number that determines the energy level of the electron. The second is l, the azimuthal quantum number that determines the shape of the orbital. The third is ml, the magnetic quantum number that determines the multiplicity of the orbital. All these numbers are integers. Several interesting conclusions resulted from Schrödinger's wave mechanical model of the atom. The first was that little doubt was left that the atom had to be described in three dimensions. The second was that the orbital described a region in space rather than a specific path. The third was that the square of the wave-function described the region of highest probability, of finding the electron. Once again, rather than being definite, the interpretation had reverted to randomness, as described by Democritus. The work of two Dutch-born American physicists, Samuel Goudsmit (1902-1978), and George Uhlenbeck (1900-1988), added yet another quantum number, ms, the spin quantum number, the only one of the four quantum numbers that is nonintegral. This fourth quantum number allowed the Austrian-born physicist Wolfgang Pauli (1900-1958) to clarify the electronic structure of atoms using his exclusion principle, stating that no two electrons in an atom could have the same four quantum numbers. This principle led to the concepts of spin coupling and pairing of electrons and completely explained the valence structures of atoms. The valence structures were instrumental in determining periodicity of the elements, the observation that elements in certain families had very similar physical and chemical properties.

Just when it was believed that the nature of the atom had been resolved, German physicist Werner Heisenberg (1901-1976), using a different method, declared that, mathematically, a limitation was inherent in the extent to which we could determine information about the atom. His uncertainty principle stated that the product of the uncertainty in the position and the uncertainty in the velocity (or momentum) of the particle had to be greater than or equal to a constant, h/2 [.pi]. This constant is very, very small. This would not normally be considered a problem, since it is possible to know exactly how fast a football is flying and also exactly where it is. However, the mass of a football is so large that its momentum is large. The mass of an electron is so small, however, that the product of the uncertainties in its position and momentum come very close to this constant. Essentially, Heisenberg maintained that if we knew one value (such as the position), we could not be certain of the other (such as its momentum). Further, this result indicated that by the very act of observing the position of an atom, we interfere in its behavior. With this statement, Heisenberg brought atomic theory back to the region of philosophy! Einstein never accepted this limitation.

The determination of whether atoms are real thus centers on the clarification of what "real" is. If reality is determined by perception, then a perception that is universal is truly real. A color described objectively using a wavelength is more real than one described by eye since no two people see the same way. The universe is made of matter. We can touch and see it and subjectively, we know it exists. The fact that certain types of matter behave identically has been shown above. The experimental evidence clearly demonstrates that matter can be divided into the distinct classifications of pure substances (elements and compounds) and mixtures (physical combinations of elements and/or compounds). Further, the above scientists categorically determined that compounds are formed from combinations of elements whose relative proportions can be measured, and that elements are composed of atoms. Later experiments have demonstrated that the atom itself is composed of a number of subatomic particles, the major three being the proton, the neutron, and the electron. The fact that irrespective of the element, all these constituents are present, indicates that they are objectively real.

Today, it is possible to see atoms, not with our eyes, but using sophisticated technology called scanning tunneling microscopy, or STM. Not only can we resolve surfaces to the extent that the individual atoms can be seen, but we can manipulate the atoms on the surface, pluck an atom from one place, and place it elsewhere. It is possible to construct a circuit using one molecule connected to an electrode consisting of one atom! It is also possible to design and construct molecules that have very specific properties and structures. None of this would be possible if atoms were not real.

Are atoms real? For those of us who do not have access to a scanning tunneling microscope, we have only to look at all around us to say yes.

—RASHMI VENKATESWARAN

Viewpoint: No, many pre-twentieth-century scientists, lacking any direct evidence of the existence of atoms, concluded that atoms are not real.

Today the reality of the atom is taken for granted. Pictures taken by tunnelling electron microscopes can even "show" individual atoms. However, while now the reality of the atom is accepted as commonplace, it was not always so. Only at the turn of the twentieth century were experiments conducted that gave any direct evidence of atoms. Before that, the atomic hypothesis was a "best guess," and was opposed by many scientists, since atoms could not be seen, felt, or sensed in any manner or form. Given the state of evidence at the time, the scepticism shown towards atoms was completely justified, and helped provide the impetus for the theoretical and experimental innovations that led to the existence of atoms being proved.

Philosophical Atoms

The atom as we understand it today is a recent invention, a product of experimental evidence and quantum theory. Yet the general idea of atoms, small invisible particles as the building blocks of the universe, is a very old one. The Greek philosopher Democritus (c. 460-370 b.c.) expanded on earlier ideas to give an atomic theory of matter, reasoning that it was impossible to divide an object forever, there must be a smallest size. Other Greek philosophers developed the atomic theory into an all-encompassing idea that even explained the soul, which was supposed to consist of globular atoms of fire. However, the atomic hypothesis had powerful opponents in the Greek philosopher Aristotle (384-322 b.c.) and his followers, who strongly denied such entities for their own philosophical reasons. Aristotle's ideas were to become the dominant school of thought, becoming entrenched in the Middle Ages when Aristotle's teachings were linked to the Bible, and atomism disappeared from intellectual thought.

The idea of the atom was rediscovered in the Renaissance during the sixteenth and seventeenth centuries. Like the Greeks, the supporters of seventeenth-century atomism were more concerned with ideas than experiments, and the indivisibility of matter was the key philosophical argument for the existence of atoms. Two of the biggest names in seventeenth-century science, the French mathemetician René Descartes (1596-1650), and the English physicist Sir Isaac Newton (1642-1727), both endorsed atomism, and with such authority behind the idea, it soon became a given. However, as was typical with the ideas of the two great men, each formulated a version of atomism that contradicted the other's. Various followers of Newton and Descartes debated the finer points of atomism and atomic collisions for over two centuries, until a compromise was finally reached. Although experiments played a part in the development of such theories, much of the debate was based on philosophical, and even nationalist lines, with many French scientists supporting Descartes's ideas, and the majority of British scientists slavishly following Newton's. The authority of the two men was considered greater than some experimental evidence by their supporters, and many false paths were followed in the cause of championing one over the other.

Dalton's Chemical Atom

It was the field of chemistry, not physics, that mounted the strongest scientific campaign for the existence of atoms. The English chemist John Dalton (1766-1844) observed that, in some chemical reactions, there are no fractions when the chemicals combine, and concluded that atoms are bonding in set integer ratios according to the compound produced. Importantly, Dalton's model allowed for predictions to be made, and he proposed some general rules for chemical combinations. However, Dalton did not give any reason for the validity of his rules, and many of his conclusions appeared arbitrary to most early readers. Also, his work on specific heats was by no means convincing, and seemed to contradict his own rules. While hindsight has proved Dalton mainly right, the strength of his arguments, and those of his few supporters, did not convince the majority of his contemporaries of the reality of atoms.

Antiatomism

That there were strong objections raised against the atomic hypothesis should not be surprising. There were many different hypotheses about atoms circulating towards the end of the nineteenth century, a number of them contradictory, and often supporters of atomism seemed to invoke philosophy and authority over experiment. Perhaps the most surprising thing is that such opposition to atoms was not taken seriously until the 1890s. One of the foremost voices in opposition was Ernst Mach (1838-1916), an Austrian physicist who also dabbled in psychology and physiology and had a strong interest in the philosophy and history of science. Mach argued that while the idea of atoms explained many concepts, this did not mean they were to be considered real. Mach's scientific philosophy owed a great deal to that of the Scottish philosopher David Hume (1711-1776), and the German philosopher Immanuel Kant (1724-1804). Mach placed observation at the forefront of the scientific process, and demanded that scientific assertions about nature be limited to what could be experienced. Mach rejected causes in favor of laws, but he did not see these laws as true; rather, they were to be thought of only as an economical way of summarizing nature. Such ideas brought Mach into furious debate with German physicist Max Planck (1858-1947), for whom laws such as the Conservation of Energy were to be considered real, not just a convenient fiction.

Mach's view has been called a phenomenological philosophy of science, as he stressed observations of actual phenomena, and claimed if something could not be sensed, it could not be called real. Mach claimed that science should not proceed from objects, as they are only derived concepts. The only thing that can be known directly is experience, and all experience consists in sensations or sense impressions. Hence Mach denied the existence of atoms simply because they could in no way be sensed. However, he did allow the notion of atoms to serve as a useful and economical method of explaining certain observations. To Mach atoms were a mathematical shortcut, much like the symbols used in algebra. However, to claim they were real was, Mach argued, empty theorizing, since there was no way of experiencing them.

Such positivist, or antimaterialist, views were popular toward the end of the nineteenth century, and Mach's ideas were shared by many others. Energeticists such as the German physical chemist Friedrich Wilhelm Ostwald (1853-1932), and the French physicist Pierre-Maurice-Marie Duhem (1861-1916), who unlike Mach considered energy to be real, shared Mach's antiatomism, and mounted strong, sustained, and successful attacks on those supporting the reality of atoms. Energeticists argued that there was no need to reduce thermodynamics to the statistical motion of theoretical atoms, when all could be explained in terms of energy. Ostwald wrote that the "atomic hypothesis had proved to be an exceedingly useful aid…. One must not, however, be led astray by this agreement between picture and reality and combine the two."

While denying the existence of atoms may seem wrong today since we "know" that atoms exist, the sceptical scientific approach of Mach, Ostwald, Duhem, the French mathematician Jules Henri Poincaré (1854-1912), and many others, proved correct when applied to other constructions in science such as the notion of absolute space and the concept of the ether. In the nineteenth century it was assumed that light waves travelled through a medium, like other waves, and this was dubbed the ether. Mach argued that while the notion of a substance for light to move through was useful, this did not mean the ether was real, as it could not be detected in any way. As it turned out, he was completely correct, as the Michelson-Morley experiment was to show, and Einstein's theory of relativity was to explain. Mach's interest in the history of science led him to attack the mystical elements he saw as leftovers from past giants such as Newton and Descartes. Newtonian concepts such as action at a distance could not be experienced, and Mach showed that mechanics could have been developed into just as reliable a science without Newton's assumption of absolute space.

The Statistical Atoms of Boltzmann and Maxwell

At the same time that Mach and others were insisting that atoms could not be said to be real, the work of the Austrian physicist Ludwig Boltzmann (1844-1906), and Scottish physicist James Clerk Maxwell (1831-1879), attempted to show that atoms had certain specific characteristics and obeyed Newton's laws. Boltzmann used statistical mechanics to predict the visible properties of matter. His work described things such characteristics as viscosity and thermal conductivity in terms of the statistical analysis of atomic properties. Maxwell, better known for his work on electricity, also formulated a statistical kinetic theory of gases independently of Boltzmann. However, while the mathematics and physics in their work is now recognized as outstanding, the Boltzmann-Maxwell atom hypothesis failed to convince opponents. Their work was strongly attacked, and was often misunderstood, partly because neither was a clear writer nor an accomplished self-publicist. There were also a number of problems that arose from Boltzmann and Maxwell's work. For example, the form of entropy that Boltzmann derived contradicted the Newtonian notion of reversibility in mechanics. Some critics argued that Boltzmann's work was incompatible with the second law of thermodynamics, and although he tried to defend his theories against these charges, they remained serious defects for atomism.

Boltzmann attempted to write and teach philosophy to counter the views of Mach, Ostwald, and other antiatomists, but failed to formulate a convincing philosophical explanation. He became embroiled in private debates with Mach, through an exchange of letters, which show his frustration and confusion with many of the developments of late nineteenth-century physics. Boltzmann also had some very public debates with Ostwald, which were seen by many as too vicious in nature, and which often disguised the close friendship of the two men. Indeed, Mach was so worried that the arguments were getting out of control that he proposed a compromise theory in an attempt to cool the situation. However, Boltzmann became tormented by his failures to convince the majority of scientists of the reality of atoms, and began to feel that the antiatomists were winning. Indeed, he became something of a lone voice in the wilderness, the last staunch atomist when the majority saw atoms as an arcane notion, a hangover from the mysticism of the Greeks. Boltzmann suffered from bouts of depression, and eventually committed suicide, sadly only a few short years before the final victory of the atomic hypothesis.

Near the end of his life Boltzmann wrote: "In my opinion it would be a great tragedy for science if the [atomic] theory of gases were temporarily thrown into oblivion because of a momentary hostile attitude toward it, as was for example the wave theory because of Newton's authority." Yet it was precisely because of the weight of authority of Newton and Descartes and others that the atomic hypothesis had been so accepted, and it was the inability of experimental and theoretical science to show the effects of atoms that had led to such damning criticism.

The Atom Is Victorious

Experimental work that finally showed direct evidence of atoms began to emerge at the turn of the twentieth century. Work on Brownian motion, where small particles in a dilute solution "dance" in constant irregular motion, showed it to be an observable effect of atomic collisions. Theoretical calculations by Albert Einstein (1879-1955) were supported by experimental work by the French physicist Jean Perrin (1870-1942), and led many antiatomists to concede defeat. New research in the field of radioactivity was also providing strong evidence for small particles that were even smaller than atoms, such as English physicist Sir Joseph John (J.J.) Thomson (1856-1940) discovery of the electron in 1897 (which took some time to be fully accepted).

In 1908 Ostwald became convinced that experiments had finally given proofs of the discrete or particulate nature of matter. In 1912 Poincaré declared that "[A]toms are no longer a useful fiction … The atom of the chemist is now a reality." Mach never seems to have been totally convinced—after all atoms could still not be experienced directly—but he ceased to pursue the antiatomist case with any vigor.

Although the antiatomists were shown to be wrong, their stand against atoms was still an important one for science in general. The ideas of Mach in particular were to have a lasting influence on physics, and his attack on Newtonian concepts such as the absolute character of time and space were important to development of relativity theory. Einstein acknowledged his debt to Mach in 1916, saying "I even believe that those who consider themselves to be adversaries of Mach scarcely know how much of Mach's outlook they have, so to speak, absorbed with their mother's milk," and noted that Mach's writings had a profound early influence on him, and were a part of the puzzle that led to the theory of relativity. Mach also had a founding influence on the Vienna Circle of logical positivists, a group of philosophers, scientists, and mathematicians formed in the 1920s that met regularly in Vienna to investigate the language and organizing principles of science.

The antiatomists were correct to question the existence of atoms, and it must be remembered that their views held sway at the end of the nineteenth century, and for good scientific and philosophical reasons. At the very least, the scepticism of the antiatomists pushed others to search for strong experimental proof of atoms, and thereby put the whole of physics and chemistry on a much stronger base.

—DAVID TULLOCH

KEY TERMS

ATOM:

Originally believed to be the smallest indivisible particle of which matter was composed, it is now known to consist of protons, neutrons, and electrons.

BROWNIAN MOTION:

Phenomenon discovered by the Scottish botanist Robert Brown (1773-1858) in 1828 where tiny particles in a dilute solution constantly move in random motion. Brown first used pollen, and so thought that some spark of life was making the particles move, but later work showed than inanimate substances also had the same effect. In the early twentieth century Brownian motion was shown to be a product of atomic collisions. The slides used by the French physicist Jean Perrin in his 1905 investigations into Brownian motion contain moving particles to this day.

ELECTRON:

Negatively charged subatomic particle found in an area around the nucleus determined by the orbital it occupies. Its mass is approximately one two-thousandth that of the proton. In a neutral atom, the number of electrons equals the number of protons.

NEUTRON:

Subatomic particle that has no charge; a component of the nucleus, its mass approximately equals that of the proton. A variation in the number of neutrons in a particular element leads to the formation of isotopes.

NUCLEUS:

Dense core of the atom containing almost all the mass of the atom and consisting of protons and neutrons.

MICHELSON-MORLEY EXPERIMENT:

Attempt to detect to try to detect a difference in the speed of light in two different directions: parallel to, and perpendicular to, the motion of Earth around the Sun. First performed in Berlin in 1881 by the physicist Albert A. Michelson (1852-1931); the test was later refined in 1887 by Michelson and Edward W. Morley (1838-1923) in the United States. Michelson and Morley expected to see their light beams shifted by the swift motion of Earth in space, but to their surprise, could not detect any change.

POSITIVISM:

Philosophy, most popular in the nineteenth century, that denies the validity of speculation or metaphysics, and stresses scientific knowledge. The English philosopher Francis Bacon (1561-1626) and the Scottish philosopher David Hume (1711-1776) were early positivists, but it was the French philosopher Augustus Comte (1798-1857) who developed positivism into a coherent philosophy.

PROTON:

Positively charged subatomic particle; a component of the nucleus. The number of protons in an element determines its atomic number, and each element has a unique number of protons.

QUANTUM:

Small packet of energy. Its energy is in multiples of h [.nu], where h is Planck's constant and [.nu] is the frequency of the radiation being described.

QUANTUM MECHANICS:

Most current model of the atom. Quantum mechanics uses wave functions to describe the region of greatest probability of finding the electrons in an atom.

SPECIFIC HEAT:

The amount of heat needed to raise the temperature of a unit mass by one temperature unit (degree).

THERMODYNAMICS:

Branch of physics concerned with the nature of heat and its conversion to other forms such as mechanical, chemical, and electrical energy.

WAVEFUNCTION:

Mathematical solution to the wave equation developed by Erwin Schrödinger (1887-1961). The wave-function mathematically contains the limitations originally set out by Danish physicist Niels Bohr (1885-1962) to describe the energy states of electrons in an atom.

Science in Dispute