"In all combustion, pure air in which the combustion takes place is destroyed or decomposed and the burning body increases in weight exactly in proportion to the quantity of air destroyed or decomposed."
Antoine-Laurent Lavoisier, Memoir on Combustion in General, Memoirs of the Royal Academy of Science, 1777
In the pre-modern era, the terms alchemy and chemistry were nearly synonymous. They are both derived from the same root words. Alchemy is from Arabic, but its origins are still debated among linguists. It is generally believed that the root - chem- came from Egypt through the Greek language. In practice, there was little distinction between the two pursuits. Alchemy had two aspects that were connected in their methods and goals. The esoteric or spiritual side of alchemy is more familiar in the modern age. It concerned the transmutation of metals to turn base metal into gold or silver. It involved searching for the philosopher’s stone to facilitate this transmutation. In this pursuit, alchemists would develop their arcane language imbued with ancient philosophy into which one needed to be initiated. Nonetheless, many early practitioners of science were, themselves, devoted alchemists. Esoteric alchemy remained a highly respected discipline until the latter half of the 18th century. On the other hand, the exoteric side of alchemy did not have this same image. This was the practical side of alchemy, closer to what we consider the modern science of chemistry. This practical alchemy had its roots in metallurgy, medicine, dying, and distilling. Its interest was in the composition and properties of matter.
Central to the alchemical tradition was Aristotle’s belief that everything was composed of four essential elements: earth, air, fire, and water. This would change slightly in the mid-16th century with the teachings of Swiss medical reformer Paracelsus. Paracelsus proposed three metallic principles: sulfur, mercury, and salt. Sulfur represented flammability and combustion, mercury represented volatility and stability, and salts represented solidity. This was the tria prima, and all organic materials could be distilled into their volatile, inflammable, and saline parts. Paracelsus used the burning of wood to demonstrate this idea. Sulfur was the flames, and salt was the ash that was left. Mercury, the cohesive quality, left the wood as smoke; therefore, it fell apart as it burned. Paracelsus’s ideas would profoundly influence the study of physiology and pharmaceutical compounds well into the 17th century.
Older modes of thinking had been challenged in other areas of science, and paradigm shifts revolutionized the study of astronomy, physics, and anatomy. By the end of the 17th century, thinkers were starting to question alchemical traditions. The most notable of these was the English philosopher Robert Boyle. Boyle is known for identifying the inverse gas law (Boyle’s Law), which says that a gas’s volume is inversely related to its pressure. And he was one of the founding members of the Royal Academy of Science in London. Boyle was also an alchemist, and he saw the esoteric side of alchemy with the same esteem as astronomy and physics. He wished to elevate the exoteric, viewed as the more vulgar pursuit, to the same level. Earlier in the century, Sir Francis Bacon outlined what would emerge as the modern scientific method, and Boyle advocated using this method in alchemy. He argued for the use of rigorous experimentation to understand chemical reactions. He redefined the concept of elements, directly going against both Aristotle and Paracelsus. His definition of an element was closer to the modern notion articulated by John Dalton in the early 19th century. Though he had not intended it, Boyle had begun the movement to separate chemistry from alchemy.
In the early part of the 18th century, a theory arose to explain combustion much more straightforward than the current alchemical views. Even though he didn’t initially propose it, Georg Ernst Stahl became the leading proponent of phlogiston. All combustible materials had a universal component called phlogiston, from the Greek meaning inflammable. When a material burned, it lost weight, attributed to the loss of phlogiston to the air. The lesser the residue meant the more significant amount of phlogiston in the original substance. Wood, then, was a combination of ash and phlogiston. When metal compounds were heated with charcoal, it was observed that a pure form of the metal was formed. Therefore it was believed that the phlogiston combined with the metal. The phlogiston theory was used to describe many types of natural phenomena. In 1772, a Scot, Daniel Rutherford, discovered “noxious air” (later called nitrogen) and used phlogiston to explain its behavior. His mentor, Joseph Black, would describe “fixed air,” or air in which a candle could not burn (carbon dioxide). This was the residue of air left after burning. It was thought then that air could only hold so much phlogiston and would not burn once saturated. This type of substance would be termed “phlogisticated.”
In 1774, an Englishman, Joseph Priestly, conducted a pivotal experiment using a mercury calx. A calx is a substance formed when an ore or mineral has been heated. In this case, Priestly used a red mercury powder. He heated it with sunlight focused through a large magnifying glass. He captured the residual air and lit a candle. He observed that the candle burned longer and brighter than ordinary air. He felt this purer air could absorb more phlogiston resulting in the candle burning longer than expected. He called this “dephlogisticated air.” He performed the same experiment but used a mouse instead of a candle. The mouse lived longer than in a similar amount of ordinary air. A Swedish scientist, Carl Wilhelm Scheele, had come to the same conclusions the year before. (He would not be credited with the discovery because he failed to publish his results before Priestly.) Priestly would go on to tour Europe, replicating his experiment for other scientists. One of these was Antoine-Laurent Lavoisier in Paris.
Lavoisier was born in 1743 to a wealthy Parisian lawyer. As expected, he studied law and earned his law degree in 1763. But Lavoisier had an interest in natural science. While attending Mazarin College in Paris, he studied chemistry, botany, astronomy, and mathematics. He became acquainted with one of the leading French scholars, Etienne Condillac, who encouraged Lavoisier’s interest in chemistry. He made friends with the French geologist Jean-Etienne Guettard, and the two men undertook a geological survey of France in 1767. In 1764, Lavoisier read a paper regarding gypsum's chemical and physical properties to the French Academy of Sciences. He became a member of that prestigious body in 1768. That same year he bought shares in the Ferme Générale. This was the private tax-farming company used by the royal government. Despised by the general population, the Ferme Générale became symbolic of the tyranny of the Ancien Regime on the eve of the revolution. Lavoisier was instrumental in commissioning a wall around Paris so custom duties could easily be collected. In 1771, he married the daughter of one of the company’s senior members, Marie-Anne Pierette. She would play a vital role in Lavoisier’s scientific endeavors. She assisted him in his laboratory and translated English into French so he could keep up with what was happening in England.
Though he was intrigued by Priestly’s experiments, Lavoisier had always been skeptical of the phlogiston theory. He derided phlogiston as entirely imaginary and unsupported by empirical evidence. Alchemy, and, in turn, chemistry, had long been based on qualitative observation. Changes in heat, color, and volume were essential indicators. Priestly and others tested different types of air in terms of solubility, their ability to promote flames, whether they were breathable, or how they behaved with acids and alkalines. Lavoisier proposed a different approach. He believed that chemical reactions could be understood using the empirical methods of other sciences, and chemistry needed to be quantitative. Precise measurements were vital. In particular, he felt others had failed to take exact weight measurements. Two years prior, Lavoisier had conducted experiments with phosphorous and sulfur which easily burn. Lavoisier kept meticulous records and balance sheets of the weight measurements of the substances before and after the reaction. He showed that both substances gained weight by combining with air. In reverse, he used lead calx to capture a large amount of air given off by the reaction. With these experiments, he demonstrated what would become the law of conservation of mass - the total mass of a system remains the same. But Lavoisier also realized that combustion, and the opposite, calcination, required air.
For Lavoisier, this wasn’t enough to invalidate the phlogiston theory. He decided to replicate Priestly’s experiments to learn more about the nature of this “dephlogisticated air.” He conducted Priestly’s investigation and some of his own using mercury and other metals. From this, he concluded that what people perceive as standard air comprises two parts. One part combined with metals and supported breathing. The other was an asphyxiant that did not support combustion or breathing. In 1777, Lavoisier advanced his theory of combustion. Combustion was a reaction of a metal or organic material with that part of the air that Lavoisier described as “eminently breathable.” Further experiments showed that acids and metal calxes contained this breathable air. Lavoisier would go on to call this part of air oxygen, from the Greek words oxys and genes: acid producer. Calcination would become known as oxidation.
Meanwhile, back in 1766, another Englishman, Henry Cavendish, had isolated a gas that he termed “inflammable air” since it burned quickly. Priestly observed that when this gas was combined with standard air and ignited, dew formed within the vessel. Cavendish repeated this and discovered that this dew was actually water. Both men would explain this using phlogiston. They assumed that water was already present in both airs before being ignited. When Lavoisier concluded that combustion meant a reaction with oxygen, he explained inflammable air in terms of his theory. In June 1783, he reacted oxygen with inflammable air. He discovered he could obtain water in a very pure state. Like air, Lavoisier concluded, water was a compound made up of oxygen and inflammable air - hydrogen (“water producer”). He performed the reverse process and decomposed water into oxygen and hydrogen. Finally, he felt, the idea of phlogiston could be discarded.
Unwittingly, the phlogistonists had helped Lavoisier dismantle the old Aristotelean and the Paracelsus view of elements. Both air and water were shown to be composed of other substances and, therefore, not pure elements. Lavoisier and his French Academy of Science colleagues fully adopted Robert Boyle’s proposed definition of an element. Lavoisier wished to reform chemistry, and in doing so, he introduced a new system of thinking. He laid out this new system in the Traité élémentaire de Chimie (Elements of Chemistry), published in 1789. Here he thoroughly explained the role of heat in chemical reactions, the nature of gases, how acids and bases reacted to form salts, and the equipment needed to conduct proper chemical experimentation. He defined the conservation of mass and added a table of substances, the first listing of known elements. And he introduced a new naming system. Many elements retained their old names, but when elements are combined, the new name should reflect their unique composition. Thus, substances like mercury calx became mercury oxide. This forms the basis of naming compounds today.
Lavoisier has taken the old system and entirely separated chemistry from the esotericism of its sister, alchemy. Chemistry would now be considered a pure science. He initiated the paradigm shift that would revolutionize the science of chemistry and lay the foundation for modern chemistry, which would begin in the 19th century. But his association with the Ferme Générale would come back and haunt him. He was caught up in the wide net of the Reign of Terror. On May 8, 1794, he was convicted by a revolutionary tribunal and executed by guillotine that same day. His friend, the mathematician Joseph-Louis Lagrange, would mourn by saying, "it took them only an instant to cut off that head, and a hundred years may not produce another like it."
Further Reading
Antoine Lavoisier: Science, Administration and Revolution: Arthur Donovan