Lecture 24

The coming of age of chemistry.

Introduction

In Lecture 21 we defined revolution as "a fundamental departure from any previous historical pattern" and found that at the end of the 18th century science underwent a revolution that replaced qualitative ideas about nature by quantitative experiment. We described this revolution through the development of mechanics, which until then had understood gravity as a force that moves objects towards the centre of the universe and motion as a property of matter. The scientific revolution had changed mechanics in a fundamental way; it recognized gravity as a property of matter and motion as the result of acting forces.

In Lecture 19 we saw that this revolution was preceded by a period where two ideas, the Ptolemaic view of the universe and the Copernican system, competed with each other for a lengthy period of time because the available evidence was insufficient to eliminate one of the two. The contest was decided when it was discovered that the new concept of gravity and motion could also explain observations that were not part of the controversy itself (such as the ocean tides).

In our discussion of the intellectual climate during the late 17th and early 18th centuries (Lecture 20) we found that, as the evidence in support of the Copernican system increased, the proponents of the Ptolemaic system did not immediately concede defeat but proposed amendments to their system of ever increasing complexity. More epicycles were added to the planets' orbits, and Tycho Brahe tried to construct a compromise system in which Sun and Moon circle the Earth but the other planets revolve around the Sun.

When we now discuss the passage of chemistry from a philosophy of nature to a science we will find that chemistry went through a similar process: development of two alternative views, rearguard action of the defenders of the traditional view, accumulation of evidence in support of a scientific revolution, and finally acceptance of the new insight into the structure of matter. The major difference between the scientific revolution in physics and in chemistry is in its timing and in its duration:

It is seen that the revolution in chemistry originated a century after the revolution in physics, and its completion took half a century longer. As a consequence it developed under different social and political conditions, an aspect that will be part of the discussion of this lecture.

Earth, air, fire, water and phlogiston

Early philosophers always had two ideas about the consistency of matter. One school of thought postulated the existence of atoms and explained differences between substances through variations in the arrangement of the atoms. This point of view was particularly dominant in Indian philosophy (Lecture 14), which explained chemical changes observed during the heating process as changes in the molecular arrangement of the atoms and the transmission of light as an emission of particles.

Atomism had its supporters among the philosophers of ancient Greece as well, Democritus being regarded as its most prominent proponent. The majority of Greek philosopher-scientists adhered, however, to the alternative view that all matter was made from some basic substance or a combination of a very limited set of such substances. The "standard set" of such substances in Greek philosophy consisted of earth, air, fire and water. Some philosophers had their preferred substance: Anaximenes declared that all matter was made from air, Heracleitus took fire as the basic principle, Thales preferred water. Empedocles took the view that all four substances contribute to the make-up of matter. Aristotle concurred, and his view eventually prevailed and became the foundation of Arabic and European chemistry.

A similar view existed in Indian philosophy, which used an expanded basic set of fundamental substances consisting of earth, air, fire, water, ether, time, space, spirit, and mind (see Lecture 14). The expansion into "substances" of non-material character indicates that the basic substances were understood as qualities of matter and not necessarily as a statement on their material composition. This was also the attitude of the Greek philosopher-scientists and certainly the understanding of Arabic and European alchemists of the Middle Ages, who followed the Greek approach of earth, air, fire and water as the basic "substances" or "principles."

Medieval attempts to extract gold or silver from other materials soon showed that the four basic substances of ancient Greece were insufficient to explain all processes observed during the smelting process, and alchemists included other basic substances in the list. Thus, through the theories of Paracelsus sulfur became the basic substance that gives matter combustibility, mercury added volatility or fluidity and salt provided the quality of incombustibility.

A severe limitation of the alchemist's understanding of matter was the lack of differentiation between elements, compounds and mixtures. An element is a substance that cannot be decomposed into simpler substances through chemical processes, a compound is a substance composed of a combination of elements in fixed molecular arrangement, a mixture is a substance composed of several elements or compounds without a fixed relationship between its components. Thus, mercury and sulfur are elements, water is a compound of hydrogen (H) and oxygen (O) in the fixed composition of two hydrogen molecules to each oxygen molecule (H₂O), and air is a mixture of mainly nitrogen and oxygen at variable ratio and several other gases in minute quantities.

In contrast to physics, which before its revolution had concentrated on philosophical speculation rather than quantitative experiment, alchemy had some experience of the importance of accurately measuring the weights of substances. But this experience was of little use as long as alchemists envisaged all matter to be made up of a combination of elements, compounds and mixtures.

The first to recognize the difference between compounds and mixtures was Robert Boyle, an English chemist of the 17th century. Boyle was a contemporary of Newton, and the starting point for his experiments was in physics. Having constructed an air pump with the assistance of Robert Hooke, he began to study the role of air in various processes including combustion. He concluded that earth, air, fire and water could not be the building blocks of matter because they could not combine to form other substances or be extracted from substances. This led him to adopt the alternative approach of atomism and introduce "corpuscules" as the fundamental building blocks of nature.

Although Boyle received much appreciation for his work during his lifetime chemists did not follow his reasoning for another century. Instead, his contemporary Johann Joachim Becher introduced the idea of three kinds of earth, which he called vitrifiable, mercurial and combustible earth, and stated that combustible earth is liberated during the burning of substances.

A few decades later, around the beginning of the 18th century, Georg Ernst Stahl coined the name "phlogiston" (Greek for "burnt") for combustible earth and declared that the oxidation of metals (such as the formation of rust on iron) was another form of combustion and therefore liberated phlogiston in the process.

The phlogiston theory was, in essence, an adaptation of classical Greek nature philosophy to the world of experimentation. It became the established base for the teaching of chemistry in all universities. It offered only a qualitative explanation of observed chemical changes, but quantitative measurement, which had become the basis of physical experiments during the 17th century, did not concern chemists much until a century later. Alchemists had known for a long time that substances change weight when they burn or oxidize. The shortcomings of the phlogiston theory did not become obvious until the moment when chemists turned to quantitative determination of the weight change.

The breakthrough came with the experiments of Antoine-Laurent Lavoisier, the founder of modern chemistry. Lavoisier is sometimes called the Newton of chemistry because Newton's name is more widely known than his. Lavoisier's achievements are comparable to Newton's, and in the scientific revolution of the Enlightenment both names have to stand side by side. Lavoisier proved once and for all that air was not a basic substance but a mixture of different gases and that these gases react separately with others during combustion, oxidation and respiration. In 1783 he showed that water is a compound of two gases (hydrogen and oxygen) which can be extracted separately.

Lavoisier was not the first to describe these facts. oxygen had been discovered by Joseph Priestley and by the Swedish chemist Carl Wilhelm Scheele before him, and the composition of water by the English chemist Henry Cavendish. What made Lavoisier's contribution stand out from the scientific activity of others was his interpretation of the new discoveries. Lavoisier declared that there are many more basic substances than earth, air, fire and water and that these substances form the building blocks of matter by reacting chemically and forming compounds.

Lavoisier's book Traité élémentaire de chimie ("Elementary Treatise of Chemistry") was published in 1789. In particularly clear and simple words it set out Lavoisier's new nomenclature. Substances that could not be decomposed any further were called substances simples and were described as the elements from which other substances are made. Lavoisier based the process of combination and decomposition on the principle of conservation of matter: "We must lay it down as an incontestable axiom, that in all operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment. ... Upon this principle, the whole art of chemical experiments depends."

Phlogiston versus substances simples: Two views of nature

At the time of its inception Lavoisier's concept was just that, a concept designed to explain the world that required extensive verification before it could be accepted as superior to the concept of phlogiston. For a short period of time - less than half a century - both concepts existed and developed in parallel, and for a while it appeared as if the phlogiston theory could explain the new findings just as well as Lavoisier's idea of many elements. Take the example of the extraction of metal from metal ore through smelting. Two explanations were at hand:

The process had all the hallmarks of the "improvements" of the Ptolemaic system through additions of epicycles on other epicycles and similar additions, and it achieved the same: It enabled the adherents of the phlogiston theory to explain - at least in a qualitative sense - all known observations of their branch of science. Henry Cavendish wrote in the Philosophical Transactions of the Royal Society of 1784 about Lavoisier's new nomenclature:

"It seems therefore, from what has been said, as if the phenomena of nature might be explained very well on this [Lavoisier's] principle without the help of phlogiston; and indeed, as adding dephlogisticated air to a body comes to the same thing as depriving it of its phlogiston and adding water to it, and as there are, perhaps, no bodies entirely destitute of water, and as I know no way by which phlogiston can be transferred from one body to another, without leaving it uncertain whether water is not at the same time transferred, it will be very difficult to determine by experiment which of these opinions is truest; but as the commonly received principle of phlogiston explains all phenomena at least as well as Mr. Lavoisier's, I have adhered to that." (Conant, 1964)

But the real difficulties for the defendants of the phlogiston concept did not arise from the growing number of elements that were discovered, they arose from the quantification of experimental practice. When a burning substance is placed under a vessel full of air and the air is weighed before and again after the substance stops burning it is found that the air suffered a weight loss: Its oxygen reacted with the substance and is now contained in its ash. According to the phlogiston concept the substance added phlogiston to the air until the air was saturated with phlogiston. The weight of the air was reduced in the process, which could only mean that phlogiston is a substance with a negative weight.

Another problem occurred with mercury oxide 2HgO, a red powder that has the unusual property that it can be decomposed into mercury and oxygen by heating it to high temperature and that unlike other smelting processes the procedure does not require charcoal. It is therefore possible to obtain mercury oxide from mercury by heating it and return to the original state by heating the mercury oxide even more:

2Hg + O₂   ---(heated)--->    2HgO

2HgO   ---(heated very hot)--->   2Hg + O₂

In the phlogiston theory heating means the setting free of phlogiston; it cannot explain a reversible process based on adding more and more heat.

Priestley, one of the discoverers of oxygen, remained the staunchest supporter of the phlogiston concept. In his last book, published in 1796, he still did not admit defeat but conceded the revolutionary character of Lavoisier's system:

"There have been few, if any, revolutions in science so great, so sudden, and so general, as the prevalence of what is now usually termed the new system of chemistry, or that of the Antiphlogistons, over the doctrine of Stahl, which was at one time thought to have been the greatest discovery that had ever been made in the science. I remember hearing Mr. Peter Woulfe, whose knowledge of chemistry will not be questioned, say, that there had hardly been any thing that deserved to be called a discovery subsequent to it. Though there had been some who occasionally expressed doubts of the existence of such a principle as that of phlogiston, nothing had been advanced that could have laid the foundation of another system before the labours of Mr. Lavoisier and his friends, from whom this new system is often called that of the French." (Conant, 1964)

These words from an eminent chemist indicate the power of the chemical system introduced by Stahl. For nearly a century his phlogiston concept provided the only consistent interpretation of many experiments and was quickly adopted throughout Europe for that very reason. Lavoisier himself was so much under its influence that even after breaking with it he still included elements of it in his work where he could not explain things in his own terminology. Even his famous definition of "element" is still ambiguous about the question whether an element is a substance or a principle:

"Everything one can say about the number and the nature of the elements is no more, in my opinion, than metaphysical discussion; these are indeterminate problems susceptible of an infinity of solutions, probably not one of which is in accord with nature. I will therefore content myself with saying that if by the name of element we intend to designate the simple and indivisible molecules which make up the substances, it is probable that we do not know them: but if, on the contrary, we attach to the name of element, or of principle, the idea of the last term arrived at in analysis, all the substances which we have not yet succeeded in decomposing by any means are elements for us; not because these principles are never separated, or rather, because we have no means of separating them, they act as far as we are concerned like simple substances, and we should not suppose them to be compound except when experiment and observation have furnished us with proof." (Bensaude-Vincent, 1995)

Lavoisier's support for the principle of science that it is idle to speculate about the nature of matter, that all the scientist can do is draw conclusions and develop laws from observation and experiment, and that if the conclusions are correct they will enable the scientist to make correct predictions without the need to claim knowledge about the nature of matter itself - Lavoisier's adherence to this principle was the basis for his success. It also led to some erroneous inclusions in his list of substances simples: Lime, alumina and silica are very stable compounds, which he was unable to decompose; so he listed them as elements.

Two other members of his list deserve notice: light and heat (matière du feu). Lavoisier included heat because he was unsure how to account for the three states of aggregation solid, liquid, and gas. He assumed that they were modes of matter determined by the amount of heat with which the substances were associated - a clear influence of the classic Greek idea of principles associated with matter.

These uncertainties did not deter chemists from taking up the challenge to find the new substances simples. A rapid series of discoveries began. 12 elements were known before 1700; another 21 were added during the 18th century; by the middle of the 19th century the total number of known elements had reached 67. How many more could one expect? Could some kind of order be brought into the accumulation of new building blocks of nature? Lavoisier had ended the rule of earth, air, fire, water and phlogiston, but he had not delivered the instrument that would enable chemists to bring order into their science.

The Periodic Table

Lavoisier's new system achieved for chemistry what Newton's Laws had achieved for physics a century earlier: Just as the new physics had eliminated motion as a property of matter and placed it into the realm of forces, the new chemistry eliminated "principles" such as fire and phlogiston as properties of matter, allocated to them a place in the realm of forces and processes, and replaced them by elements.

In early civilizations a time difference of a century was not of much consequence. At the time of Newton and Lavoisier a century meant the difference between the Enlightenment of the 18th century and the industrial revolution of the 19th century. Only a few years after Lavoisier's premature death James Watt introduced his improvements of the steam engine, and the resulting boost of the mining industry created a demand for better chemistry. All over Europe chemists were involved in the search for new elements and the analysis of their properties.

Progress with the new science was severely hampered by a lack of a system of units. Physicists had agreed on the International System of Units introduced by the French Revolution (Lecture 21); in chemistry confusion reigned about the proper way to classify the many discovered elements. Atomic weight, atomic mass, molecular weight and a range of other measures were proposed as standards, and no one was sure whether an element named "x" by one scientist was or was not the same element called "y" by a scientist of another country.

To end the confusion and agree on a uniform nomenclature the First International Chemical Congress was convened in Karlsruhe, Germany, in 1860. It was attended by some 140 delegates. No common system of elements was adopted, but the presentation of the Italian chemist Stanislao Cannizzaro, in which he discussed Avogadro's Law (that at equal temperature and pressure the number of molecules in a given volume is the same for all gases) and outlined the principal difference between atomic weight and molecular weight, greatly impressed his audience.

One of the participants in the Congress was the Russian lecturer in chemistry Dmitry Mendeleyev. His desire to provide his students with a logically consistent introduction to chemistry led him to discover that if the elements are arranged in order of increasing atomic weight their chemical properties do not change randomly, nor do they evolve in one direction but show a cyclical behaviour. In his textbook Principles of Chemistry of 1868 - 1870 he introduced the concept of the Periodic Law or Periodic Table of Elements, which today forms the basis of explanations of the chemical behaviour of different elements.

Modern knowledge about the structure of the atom has greatly facilitated our understanding of the laws that form the basis for the Periodic Table. In today's concept an atom is composed of a nucleus surrounded by shells. The nucleus contains protons and may also contain neutrons. Each proton carries one positive electrical charge; neutrons do not carry an electrical charge. The shells are occupied by electrons that move around the nucleus at great speed; each electron carries one negative electrical charge. The number of electrons is close to the number of protons, so that the electric charges in an atom are usually balanced. (Atoms where the number of electrons differs from the number of protons, and which therefore carry a positive or negative charge, are called ions.)

The number of protons in an atom is called the atomic number and used in today's definition of an element: An element is a substance made up of atoms with the same atomic number. (Neutrons are of no concern in this respect; their number determines the possible isotopes of an element.)

The number of electrons that can occupy a shell is limited and increases from the first (inner) shell outward. If a system is imagined in which the elements are arranged in increasing order of the number of protons (the atomic number) the shells are successively filled and more shells added.

Chemical reactions involve the exchange of electrons. The chemical properties of elements therefore depend strongly on the availability of free places on the electron shells. Elements that have no free place on their shells are chemically particularly stable or "inert" and therefore known as the "noble" gases. They turn out to be the elements with atomic numbers 2, 10, 18, 36, 54 and 86.

Mendeleyev did not know about protons, neutrons, electrons and atomic numbers. When he attempted to get order into the multitude of elements he had to use their atomic weight as a guide. The atomic weight is the sum of all protons, neutrons and electrons of an atom; it can therefore only serve as a guide for the construction of a periodic table but not as its defining principle. The fact that the electron shells are not filled after a constant number of electrons are added but at intervals that - without insight into nuclear physics - appear arbitrary made it additionally difficult:

Several chemists attempted to arrange the known elements in some order on the assumption of a regular periodicity. Mendeleyev proceeded by listing all known elements in order of increasing atomic weight and allocated them their place in his periodic system by carefully comparing their chemical properties. In doing so he realized that to place elements with similar chemical properties next to each other he had to leave gaps in the table. He realized that these gaps indicated elements that were not yet discovered and even predicted their properties. Within a decade several of his predictions led to the isolation of the predicted elements in the laboratory.

Mendeleyev's Periodic Table completed the scientific revolution in chemistry. It showed that the new chemistry could not only explain chemical reactions that occur in oxidation, the burning of substances, respiration and other daily processes but could produce an ordered concept of the world. As we said in our very first lecture, science is based on the observation that nature follows rules. Lavoisier revolutionized the concept of nature in chemistry, Mendeleyev discovered its rules.

The scientific and the social revolution

The scientific revolution of physics happened in 17th century England; the scientific revolution of chemistry began in 18th century France and was completed in 19th century Russia. The development spanned centuries, but in all three situations it happened at a moment in history where the societies of the three countries had just overcome or were about to overcome feudalism. In all three countries the end of feudalism was achieved through a revolution. The scientists that pushed the scientific revolution to victory were also part of the social revolution, but in very different ways.

Newton was just 7 years old when England's Civil Wars ended and established a bourgeois republic. The year after he published his Philosophiae Naturalis Principia Mathematica the "glorious revolution" confirmed England's capitalist political structure. These were safe times for new scientific ideas; freedom of thought was guaranteed and Newton's position as professor at a university was never challenged.

Newton did involve himself in affairs of the state when it was safe and brought good rewards. At the age of 53 he became warden of the mint. Recognition of his scientific work continued at home and abroad, but Newton had resigned from science and spent his time bringing counterfeiters to court.

Lavoisier was 46 when the French Revolution began. His father had bought him a title of nobility and a country estate, and from an early age Lavoisier was an extremely wealthy member of the aristocracy. His administrative ability was noticed by the royal government, and at the age of 25 Lavoisier became a member of the Ferme Générale, the state tax collecting agency. This is not a popular job under the best of circumstances; during the dying days of French feudalism, under the exploitative regime of Louis XVI, it proved fatal.

Lavoisier was an aristocrat but a man with a sense of responsibility for the fate of the common people. During times of famine he gave interest-free loans to two cities to allow them to obtain emergency supplies. When the Revolution established its government he freely used his financial expertise to assist in the drawing up of the state budget. He served in commissions and was a member of the committee to establish uniform units and measures.

When the Revolution abolished the Ferme Générale, the turbulent times led to the senseless decision to take revenge at the old regime by executing the former members of the taxation agency. Thus, Lavoisier and 27 others ended their lives at the guillotine.

Lavoisier's death was one of the many wrong decisions made during the course of the Revolution. When Lagrange, the great mathematician and Senator of the Republic, heard about it he remarked: "It required only a moment to sever his head, and perhaps a century will not be sufficient to produce another like it." Whether having a gifted head alone should be a reason to be spared from death is debatable; in a fair world Lavoisier's proven good character as a man of compassion with the people should have rescued him.

But let us take a moment and reflect over Lagrange's statement, made as an expression of admiration for a colleague and grief over his death. Is it possible to arrest the development of science by killing a scientist of outstanding gift? Or does science develop because the time is ripe for certain insights, and do the people appear who bring these insights into the world when they are needed?

After Lavoisier's death science did not stand still; on the contrary, activity in the chemical sciences grew at a tremendous rate. Hundreds of chemists began the search for new elements and accumulated much empirical knowledge. Many chemists constructed periodic tables of doubtful quality. Among the 140 delegates to the Chemical Congress of 1860 was the gifted head that could bring order into the development, Mendeleyev. This was 71 years after Lavoisier's publication of his Traité élémentaire de chimie. Mendeleyev's Principles of Chemistry were published 80 years after Lavoisier's main work. In that sense Lagrange was correct: It took 80 years to produce another head like Lavoisier's.

But Mendeleyev needed Cannizzaro's work to discover the principle of the Periodic Law. In that sense the Periodic Table required more than a single head. Mendeleyev came from the vast regions of Siberia. How many heads like Mendeleyev's were there in Siberia that never had a chance to prove themselves? How many serfs could have excelled in science, had they not been tied to the land and its daily routine?

Much has changed since the 19th century, but millions and millions of people still do not have any hope to fulfil their potential and develop their gifts because their time is fully taken up by the daily routine of securing food and shelter. Human biology is able to produce another head like Lavoisier's every year or every month. But Lagrange was correct that under the conditions of 19th century society only one in a century could realize its potential to the full. There are still many countries today where the situation has not changed much.

Mendeleyev was born nearly a century after Lavoisier, yet he still lived his entire life under feudalism. During his travels to France and Germany he had seen how much Russia had fallen behind in its development, and at home he pioneered scientific methods for Russian agriculture and supported the development of industry. During a trip to the USA he criticized the American oil industry for concentrating on expansion and its lack of interest in energy efficiency. After his return to Russia he promoted the elimination of foreign interests from the Russian oil industry.

The fall of Russian feudalism did not occur until ten years after Mendeleyev's death, but the decades before the event were a period of intense struggle and controversy. Mendeleyev took the side of the radical students, and his support for their demands led to his removal from the university. He spent the last 17 years of his life in administrative positions, to which he was appointed by a government that wanted to use his scientific expertise but feared him as a teacher.

In England, France and Russia, over a period of two centuries, scientific and social revolution went hand in hand, and scientists who were instrumental for the progress of science had to make decisions how to contribute to the progress of society as well. Mendeleyev made conscious decisions and had to suffer for it. Lavoisier was guided by empathy for the people and his sense of social responsibility and became a victim of cruel times. Newton was in the comfortable position of not having to make hard choices and secured himself a life of comfort, leaving science to others.

We conclude with one final consideration. We said again and again in these lectures that science develops where there is a need for it, a statement that was easy to prove for the ancient civilizations. Verification will require increasingly more effort as we get closer to our own century. With respect to the revolution in chemistry the key event was the meeting between Priestley and Lavoisier in which Priestley described how he isolated "dephlogisticated air" (oxygen).

Priestley was a member of the Dissenting Churches of England and an independent thinker. England's universities were only open to members of the Anglican Church at the time, and Priestley could only teach in an institution set up by the Dissenting Churches for its members. Because non of its members could hope to enter government service or academic careers, Priestley emphasized the practical aspects of science that were useful for careers in industry and commerce. While many of his experiments have to be considered basic science, his motivation was always the possible application in industrial processes. Priestley himself contributed to the improvement of a number of industrial practices, for example in the soda industry.

Lavoisier immediately repeated Priestley's isolation of oxygen in his own laboratory. This experiment caused him to question the phlogiston concept; it was the starting point of the revolution in chemistry. In that sense the revolution in chemistry developed because there was a need for it: The development of industry required it.

Summary

• Until the 18th century chemistry was based on the idea that the building blocks of all matter are earth, air, fire and water.
• During the 18th century this idea was formalized into the theory of the phlogiston.
• At about the same time Antoine-Laurent Lavoisier developed the concept of elements (substances simples) as building blocks of matter and showed the existence of several elements.
• For about 50 years the phlogiston theory and the concept of elements competed with each other as equally valid descriptions of nature.
• The end of the phlogiston theory came when science turned from a qualitative description to quantitative experiment because it could not explain the changes in weight during chemical reactions.
• Mendeleyev completed the scientific revolution in chemistry in the 19th century by devising his Periodic Table.
• Throughout this process the scientific revolution went hand in hand with the social revolution, and scientists faced the challenge to take sides.
• The revolution in chemistry occurred because the development of industry required it.

References

Bensaude-Vincent, B. (1995) Lavoisier: A Scientific Revolution. In: M. Serres (editor): A History of Scientific Thought, Elements of a History of Science. Blackwell, Oxford, 455 - 482. (Translation of Éléments d'Histoire des Sciences, Bordas, Paris, 1989)

Conant, J. B. (1964) The Overthrow of the phlogiston Theory: The Chemical Revolution of 1775 - 1789. In: Conant, J. B. and L. K. Nash (editors.) Harvard Case Histories in Experimental Science, Harvard University Press, Cambridge, Massachusetts, vol. I, 65 - 115.