Chemistry is often accredited with fostering the development of civilizations. Early chemists were responsible for developing the fields of metallurgy, pharmacology and that obsolete brand of science, alchemy. In fact, a distinction between chemistry and alchemy was only established in the 17th century, when Robert Boyle described chemistry as applying scientific methods, while alchemy retained its focus on the esoteric. The Chemical Revolution, which took place between the 17th and 18th centuries, is frequently described as the turning point between ancient and modern chemistry and is often attributed to Antoine Lavoisier, the eponymous father of modern chemistry. But many of the fundamental theories of chemistry, such as the nature of elements, combustion reactions and the theory of heat, were grounded in flawed beliefs. Here, a brief and not-at-all comprehensive look at three superseded chemical theories.
The classical elements
The four classical elements of antiquity, as taught by the Greek philosopher Plato (428–348 BC), were earth, water, air and fire (a very easy to memorise periodic table).
Hundreds of years before the discovery of the atom, the Ancient Greeks’ understanding of the elements was tied up in their search for the prote hyle or first matter, a substance that underlies all the substances in the natural world.
There was great contention over which of the four fundamental elements were the most fundamental, so to speak. Thales (c. 620 — c. 555 BC) believed that the only fundamental substance which made up the physical world was water. Anaximander (c. 611–547 BC) disagreed with his predecessor, arguing that air was the primary element, while Heraclitus (d. 460 BC) believed that it was fire.
Empedocles (c. 490 — c. 430 BC) took a more democratic approach, describing natural substances as a blend of earth, water, air and fire. He famously set fire to a stick and ascribed the observed products to the remaining three elements (a dirty residue = earth, damp residue = water, smoke = air).
Ultimately though, the acceptance of the four elements did not negate the belief in the prote hyle. Plato’s most famous student, Aristotle (383–322 BC) concurred that there was a single primal substance, “but it was too remote…too unknowable…to serve as the basis for a philosophy of matter”. Leucippus (5th Century BC) probably introduced the concept of atoms, and his pupil Democritus (c. 460–370) adapted the concept to coexist with the classical elements by claiming that the atoms of each element were composed of the same matter, but had different properties due to their unique size, shape, mass, position and arrangement.
Elemental chemistry had a long way to go. Elements such as hydrogen and oxygen remained bound within their classical categories of water and air until well into the Chemical Revolution.
The theory that air was one of the basic elements persisted well into the 17th century. Dutch physician and naturalist Herman Boerhaave (1668–1738) suspected that there was some life-supporting ingredient in air, as did Anglo-Irish scientist Robert Boyle (1627–1691), who said it was probably related to the substance needed to maintain a flame. The discovery that both oxygen and hydrogen are elemental gases was delayed by the theory of phlogiston, an early attempt to explain combustion.
In 1697, the German scientist Georg Ernst Stahl published his theory that a substance called “phlogiston”, which disappeared from any material during the combustion process, imparted flammability. We now understand that combustion is a chemical reaction between a fuel and an oxidant (usually atmospheric oxygen). But Stahl mistakenly believed that combustion was the release of phlogiston from the burning material. By this logic, metals were not elemental.
This is rather counter-intuitive if you observe the increase in weight that occurs during the oxidation of a metal. But Stahl had a trick up his sleeve to explain this anomaly. He claimed that phlogiston was so light that it was repelled by the Earth. When phlogiston was removed, the material gained weight because it had lost a component that lightened it.
Phlogiston theory is not just an ill-conceived scientific theory; it is a cautionary tale of moulding natural laws to explain a theory, and not vice versa. One of Stahl’s biographers wrote: “He did not hesitate to exclude facts if they violated his ideas: unity of thought was his ultimate goal above all factuality.”
Phlogiston theory was eventually debunked by Lavoisier and his theories of oxidation. In the 1770s, Lavoisier showed that combustion, respiration and calcification are all forms of oxidation and that metals are elemental (although he did not believe oxygen was). He also released a fairly scathing play about Stahl. It seems that there’s a historical precedent for the harsh treatments of pseudo-science spouters.
The nature and source of heat were long pondered by scientists throughout history. We now understand heat to be a form of energy, but it wasn’t always this way. Phlogiston theory hints at why heat was once also believed to be a physical substance.
Before we tackle this notion, a quick primer on thermodynamics and heat. You probably recall that energy can never be destroyed, only transferred. However, it is not always possible to convert all available energy into work (the thermodynamic concept, not gainful employment. In thermo, it’s the energy transferred by a system to its environment). That unavailable energy is measured by a system’s entropy. You can think about entropy as the expression of order or chaos of a system, or alternatively a measure of how much energy is not available to do work.
Like energy, work can take different forms. Mechanical work involves the application of a force over a distance. The mechanical theory of heat, which was introduced in 1798 by Sir Jordan Nugent, defines heat as thermal energy. James Joule (1818–1889; who gave his name to the unit of energy) described heat as a “dynamical form of mechanical work” produced by the vibration of atoms (temperature is proportional to the kinetic energy of atoms). Thermal work, therefore, involves heat transfer.
A recap of the second law of thermodynamics: the total entropy of an isolated system can only increase with time, while the total entropy of a system and its surroundings remains constant in a reversible process. But for all natural processes, energy transfer is irreversible. Intuitively, we all know that heat transfer can only flow in one direction: from hot bodies to cold. The reverse can never occur spontaneously, because the entropy of the system would decrease. This theory was developed in depth by French physicist Nicholas Léonard Sadi Carnot (1796–1832; known as the father of thermodynamics), who mathematically formalized the second law of thermodynamics.
There we have the mechanical theory of heat. So how exactly was heat conceptualised prior to this?
In the 17th century, phlogiston was considered by some to be the source of heat. However, it was Lavoisier (no fan of phlogiston himself) who proposed that heat resulted from a “subtle fluid”, which he called “caloric”. Caloric was believed to be a weightless fluid that flowed from hotter bodies to colder bodies. It was said to be self-repellent, which explained why it flowed from hot bodies (which were meant to be dense in caloric) to cooler ones. The caloric theory set out that the quantity of caloric was constant throughout the universe, so conservation of heat was a central assumption.
As the mechanical theory of heat gained credence, the concept of heat as a physical substance faded. But despite being fundamentally incorrect, caloric theory was the basis for Carnot’s heat engine theory, essentially giving us modern thermodynamics. Experiments to measure heat changes during a chemical reaction in the heyday of caloric theory have given us a lasting legacy: they were measured in the familiar unit of energy known as the calorie.
These three superseded theories are some of the most famous examples of scientific misconceptions. While they may amuse and even arouse disdain now, they serve as an excellent reminder that “theories cannot claim to be indestructible”. This timeless observation on science comes from French chemist and Nobel prize-winner Paul Sabatier (1854–1941). Fellow Frenchman and chemist Jean-Baptiste Dumas (1800–1884) had the following insight to share: “In chemistry, our theories are crutches; to show that they are valid, they must be used to walk.”