Class: Friday November 3, 1995
11:15-12:05, 212 Boucke Bldg.
Holton, G. and Brush, S. G., 1973, Introduction to Concepts and Theories in Physical Science, 2nd ed., Reading, Massachusetts: Addison-Wesley Publishing Company,
Levenson, T., 1994, Measure for measure: a musical history of science, New York: Simon & Schuster.
The History of Chemistry: the Hunt for the Atom
Chemistry has had only a short history on its own. The early figures of the science that called itself chemistry -- Lavoisier, Dalton Berzelius, and Avogadro -- all lived in the period roughly from the second half of the 18th century till the first half of the 19th century. The questions that were answered were, however, from a much older date.
From the early Greek period on, there had been speculations about the internal structure of matter. There had been roughly two answers, as Levenson points out:
- The one says that the world is all of a piece, formed from the same stuff, a single kind of matter than can be rearranged to build all the variety that we see.
- The other holds that the world is made up from many different stuffs, matter that in its simplest state retains the distinct qualities that produce all the varied substances we encounter in the ordinary world of experience.
Before Berzelius made in his "Transaction of Physics, Chemistry & Mineralogy" of 1818 the choice for the latter, a long history of speculation and research on the matter had shaped many intuitions and conflicting concepts.
The First Atom
As early as the 6th century BD, Thales of Milete had answered the question about the inner structure of matter according to the first option: Everything is Water. The choice was not random or absurd, and one could give several reason why he made this choice. The distinction between form (eidos, idea, essence!) and matter (body, Christian contempt for the body as the inessential) is the distinction between that what is essential and that what is accidental. It is the form that really matters. Now if the form or the shape of a thing is that what really matters and we want to think of what that form actually is, then we could think of a substance that is flexible and able to take all possible shapes: Water!
However, this theory was soon challenged, and one century later Democritus formulated another theory, which can be called the first atomic theory. He also wanted to answer the question of the inner structure of matter. He reasoned that it is impossible to divide a physical object infinitely, because infinite division would create "cosmic mush, formless and ultimately nonexistent." The infinite divided stuff, being formless, would be unable to reconstitute itself. Infinite division is taking away the form, i.e., the essence of the physical object, which is impossible. Therefore, division has to stop somewhere on something indivisible -- a-tomos.
According to the atomists, atoms were invisible and made of the same stuff, but they differed in size and shape. But it was still a miracle how the atoms could be responsible for the diversity of the world, if they were all made of the same stuff. The atomists used a beautiful metaphor to explain that mystery. There exist another area, in which limited number of units -- only differing in size and shape -- construct a diverse reality: letters put together create a wor(l)d with meaning! The atomist argued that the atoms were like letters: if they were put together in a specific way, they would make a molecule (Epicures) which was another substance -- This suggests that they might have know something about chemical reactions.
Lucretius, a Roman writer, summarized the atomist theory in his "De rerum natura":
Obviously it makes a great difference in these verses of mine in what context and order the letters are arranged. If they are not all alike, yet the most are so; but differences in their position marks the difference in what results [the words]. So it is when we turn to real things; when the combination, motion, order, position, shapes of matter [the atoms] change, so does the thing composed [out of these atoms].
Democritus’ reasoning came in certain instances very close to certain positions that we hold today:
- Brownian motion, discovered in 19th century, later explained by Einstein.
- Democritus' account of certain macroscopic properties of certain physical objects (hardness of certain metals, gas-like property of gases, etc.) on the basis of the microscopic structure.
However, the Greek atomist were philosophers and not experimental scientists. They never tried to find experimental evidence for their conjectures.
Nonetheless, in the Greek concept of the atom lay the seeds of a renewed atomic theory more than 2,000 years later. The Greek theory had made several important assumptions:
- the possibility of simple particles beyond the reach of the human senses,
- the possibility to reason from what we can see to what we cannot, to build an inferential chain to bridge the gap between experience and the hidden essence of matter,
- the existence of a void between the atoms,
- the conservation law of atoms: no atom can be created or destroyed.
The Greek atomic theory was quickly pushed aside and in the Medieval times the 4-elements theory (water, fire, earth and air) was adopted. It was not until the end of the 16th century that Bruno took up the atomic theory again -- for which (and other things) he paid with his life: burned in 1600 by the Inquisition.
Re-emergence of the Atom
In retrospect it is clear that the model of the atom re-emerged out of several distinct questions:
- What is the physical structure of matter? What is the nature of heat?
- What is the nature of fire? (The Phlogiston Theory)
- What is the basis of chemical phenomena?
ad 1. What is the physical structure of matter? What is the nature of heat?
In Newton's theory of physics the universal law of gravitation postulates that particles of matter are the agencies of mutual attraction. Newton speaks of point-masses and his theory only works for those ideal entities. This methodological requirement could, however, be translated into a certain view of matter. Rather than seeing matter as a kind of homogeneous tissue, which was not uncommon at that time, Newton suggested that on a sub-human, microscopic level matter consisted of "solid, massy, hard, impenetrable moveable particles" (1704, Opticks) This conception of matter is beautifully consistent with his theoretical "point-masses."
In the 17th century people like Torricelli and Boyle developed a Gas Theory. They found that there existed a certain relationship between Pressure P, Temperature T (in K) and Volume V of a gas, the Gas Law:
P*V/ T = constant
When Boyle tried to explain this relationship, he suggested two different atomic gas-models:
- static model
- kinetic model
The Static Model
A gas consists of particles at rest, and, therefore, they must be comprised out of compressible material to explain the fact that a gas can expand. Some suggested things like little springs or, as Torricelli had proposed, like little pieces of wool. This explained part of the expandable nature of a gas, but, according to the theory, it is possible for a gas to expand infinitely -- a thing for which the idea of a spring did not seem to fit. Newton had shown that bodies could act on a distance, but that was an attractive force. Therefore, some suggested that on the atomic level there existed a repulsive force -- N.B. to be able to explain expansion. It was Newton himself who showed that if we assumed that the repulsive force was inversely proportional with the distance between the two centers, it explained the pressure-increase, when the volume would decrease.
In this model heat was viewed as a special kind of substance, caloric, surrounding the atom. When we heat something, we would in fact add more caloric to the object. This static atomic theory pictured the atoms as balls with some caloric stuff around them, and very often the atoms were contiguously positioned in the material -- depending on whether or not one accepted the repulsive force working at a distance. The static theory was widely accepted until mid-nineteenth century.
The Kinetic Model
Already in 1738 the Swiss physicist/statistician Daniel Bernoulli worked out several implications that Boyle had called the kinetic theory. Many of his notions were extremely revolutionary; so revolutionary, that his work on the atomic theory was soon forgotten. Boyle had thought of the kinetic model of gases as describing the atoms in constant agitation, moving around in some kind of "imponderable fluid." This fluid, also called ether, has been one of the most recalcitrant notions in the history of science. For a long time people have tried to prove that there exist something like ether, but all endeavors have been in vain. Bernoulli, on the other hand, discarded this notion and followed the original Greek idea: atoms are moving around in a void. The elements are perfectly elastic and in the void they moved according to Newton's laws of motion. His theory was a form of Impact-Theory, which is still accepted until this day. The pressure exerted by the gas is nothing but the result of the impact of the collisions of the many, fully elastic particles. This would explain the Gas Law: if we decrease V, then the particles have lesser space to move around, and, therefore, more impacts occur per square inch: i.e., increase of P.
Bernoulli had also a substantially different answer to the question of the nature of heat. Instead of considering it to be another element, it was his hypothesis that heat is nothing but the motion of particles. This is the contemporary notion of heat.
Whereas retrospectively we see that Bernoulli had a much more advanced notion of the atom, his theory was difficult to accept for his contemporaries because of two of his hypotheses:
- The equivalence of heat and internal molecular motion. This notion of heat completely ignored the very fashionable concepts of ether and caloric.
- The idea that a well-defined numerical relationship, such as the above described Gas Law, could be deduced from a chaotic picture of randomly moving particles. Is was an old idea that no order could result from chaotic events. It was only with the development of statistics in the 19th century (the invention of "averages" and stochastic laws) that people started to understand that some order could result from chaos: for instance, social science discovered that the number of crimes and suicides exhibited ordered yearly averages.
ad 2. What is the nature of fire? (The Phlogiston Theory)
Besides the concepts of ether, the concept of the phlogiston hunted the history of science for centuries and let to many fruitless research. It was finally Lavoisier who after 250 years solved the mystery, by which he triggered the first developments of what can now be called chemistry. In 1772 the French Academy of Sciences put together a committee to investigate how it could be possible that diamonds are consumed by fire. In that time fire was still a mysterious phenomena. For a long time, during the Middle Ages, fire had been considered a separate element, but since the development of the sciences in the 16th and 17th century it became clear that such an explanation was not tenable. Thus, instead, the phlogiston theory had been presented. A phlogiston was thought as an insensible, undetectable fluid that left the material, when the material was burning. Fire was nothing but phlogistons leaving the object. This explained the loss of weight and bulk of a burning object. However, by careful measurement, it had been shown, that certain metals gained weight when they were roasted over high heat. Scientist had suggested that some phlogistons possessed a negative weight -- somewhat like Aristotle's concept of "lightness."
Lavoisier, member of the committee, re-installed an old Greek atomist assumption: Matter is Conserved. This meant that he rejected the idea of negative matter, and, thus, he rejected the phlogiston theory. In a series of carefully controlled experiments, he found out air consisted of two parts. One part makes a burning candle lit up, while the other extinguishes it. He also saw that the inflammable part reduced with a burning object in a closed environment, leaving the nonflammable part behind. Instead of introducing a mysterious particle that left the burning object, he found out that fire is nothing but a "chemical reaction" between the inflammable part, which he called "oxygen," and the material. This destroyed the phlogiston theory and opened up a way to study other "chemical reactions."
ad 3. What is the basis of chemical phenomena?
Rather than postulating the existence of the atom, Lavoisier's work necessitated its existence. The hypothetical approach was initiated by Newton, who had asked: "If atoms exist, how would they behave?" Although Lavoisier didn't "believe" in the existence of atoms, his theory enabled his successors to accomplish to prove it. Nonetheless, people were far away from the "right" atomic theory. That only came by a further investigation into the nature of other chemical phenomena.
In the 17th and 18th century slowly the concept of a chemical element developed. It was operationally defined as a substance which cannot be separated into different components by any known methods. It meant the shift from the question "What is an atom?" to a more pragmatic view that helped to bring about new developments. Even Dalton's postulate of the wrong, "static theory" gave falsifiable predictions that initiated new theories.
Dalton accepted the "static theory" of atoms, because he believed that Newton had actually demonstrated it to be right. Together with this theory he accepted the caloric view of heat. He thought of atoms as centers around which a layer of caloric lay. The edge of every caloric layer touched the adjacent caloric layer (Dalton didn't believe in the repulsive force between atoms working at a distance). Moreover, atoms were unchangeable, compounds are made of molecules, all atoms or molecules of a pure substance are identically alike, and in chemical reactions atoms are only rearranged, not created or destroyed. But the most important assumption he made was the following: in forming molecules during chemical reactions, the number of combining atoms of the different elements form simple, definite ratios. Even though the assumption is false, it sparked a lot of fruitful researched. Dalton set out, for instance, to determine the relative weight of the atoms, because he "knew" in what ratio the different atoms reacted. He assumed, for instance, that water was the product of the reaction of 1 Hydrogen and 1 Oxygen atom -- the simplest ratio. Of course, in the subsequent determination of the relative weights of many atoms, several inconsistencies came up, which led Berzelius to make corrections to Dalton's work.
It was finally, in 1811, an Italian, Avogadro, who saw the inherent restrictions and contradictions of the "static theory." The "static theory" could not explain how it was possible that two volumes of Hydrogen and 1 volume of Oxygen would become two volumes of water-vapor. Avogadro took two important steps to overcome the deadlock into which chemistry was about to get:
- He rejected the static theory and accepted a form of kinetic theory instead.
- He formulated a postulate, which today is known as Avogadro's Law, i.e., Equal volumes of all gases, whether elements or compounds, or even mixtures, contain equal number of elements.
This finally solved the problem how two volumes of Hydrogen and 1 volume of Oxygen could become two volumes of water-vapor, namely by assuming that water consisted of 2 Hydrogen and 1 Oxygen element [2 H2 + O2 --> 2 H2O]. Nonetheless, in the next 50 years Avogadro was forgotten and chemistry continued in Dalton's steps. Slight amendments were made, but always on the assumption that the atoms in a gas are contiguous. This led to constant contradictions, which, in the 1840s, almost led to the loss of faith in the entire atomic theory. At that point Avogadro's work was rediscovered, which necessitated the acceptance of the kinetic concept of the atom. Thus, in a century the foundations of chemistry had been layed out, by determining certain basic qualities of the atom. The concept of the atom has been expanded to accommodate quantum theory, and it has undergone some dramatic corrections in recent years, but this goes, unfortunately, beyond the scope of this class.