This article was first published in the July/August 2024 issue of IQ, the exclusive magazine for Mensa members. Find out more about becoming a Mensa member here.

Words: Katie Cutforth

Fire. Described by Charles Darwin as “probably the greatest” discovery ever made by humankind, our ability to use and control fire is central to human life and evolution – altering our dietary habits, encouraging geographic expansion and even fostering social cohesion. Though we have been using fire to our advantage for as long as a million years, our grasp of its true nature emerged much more recently.

For millennia, the world’s great thinkers sought to answer the many burning questions about this most mysterious aspect of our world. Greek philosopher Empedocles believed fire to be one of four classical elements, along with earth, water and air. Later, Aristotle argued that all matter is composed of these four elements in varying proportions – for example, a burning log contains flames (fire), smoke (air), hissing steam (water), and ashes (earth). Fire was thought of as a substance; burning was considered to be a process of decomposition.

Belief in the classical elements persisted for nearly 2,000 years until 1667, when German scientist Johann Joachim Becher eliminated fire and air from the classical element model, replacing them with three forms of the earth: terra lapidea, terra fluida, and terra pinguis. The latter, Becher believed, was a substance that was released when certain substances were burned. This assertion was the basis for what would become phlogiston theory, as refined and named by one of Becher’s students, Georg Ernst Stahl.

Stahl believed that phlogiston was a substance found in combustible materials. Phlogiston was not fire itself, but the invisible ‘matter of fire’ – an entity contained within all combustible animal, vegetable and mineral bodies. Substances that burned easily were thought to be rich in phlogiston, emitting particles of it when they were set alight. Rusting iron, the theory believed, slowly released phlogiston, returning the iron to its elemental state.

The phlogiston theory was widely accepted for over a century; it seemed to neatly explain commonly observed processes like combustion and rusting. But in the 18th century, it was threatened by quantitative experiments, which revealed that certain metals gained weight when burned – a phenomenon that could not be easily explained by supporters of the phlogiston theory.

In 1774, Joseph Priestley – a defender of the theory who discovered several gases and invented the carbonated drink – isolated what he called ‘dephlogisticated air’, in which candles could burn and mice could live. Priestley believed this air supported combustion so well because it had no phlogiston in it, so could absorb the maximum amount during burning.

French chemist Antoine Lavoisier, a long-time phlogiston critic, had a different interpretation of Priestley’s findings. ​​Lavoisier demonstrated that combustion required the presence of a gas with a weight, and also linked the process of rusting to the presence of this gas.

​​Lavoisier argued in 1775 that “the dephlogisticated air of Mr Priestley is the true combustible body” and that “there is no longer need…of supposing that there exists an immense quantity of fixed fire in all bodies, which we call combustible.” This was the death knell for phlogiston theory; Lavoisier renamed dephlogisticated air ‘pure air’ and, later, ‘oxygen’.

However, despite being debunked, the theory did mark an important shift from alchemy towards an understanding of chemical elements and how they react – paving the way for modern-day chemistry.

Credit: French chemist Antoine Lavoisier. Shutterstock