The gesture was more than symbolic. Chemists needed to understand how and why substances changed when they burnt and the so-called phlogiston theory was the first real attempt to do so. It had ruled the roost for more than 100 years, but unfortunately it was wrong.

When you burn wood, it turns to ash. When you roast metals, they turn to powder. Scientists had decided this was because the fire was allowing something to escape. And what was that something? According to Johann Becher, a Swede working in Germany, it was terra pinguis. In 1669, Becher declared that all substances contained three essences or earths. Terra pinguis, or sulphur, was the essence of fire; mercury or terra mercuralis, was the essence of fluidity; and salt or terra lapida, was the essence of fixity and inertness.

Along came Georg Stahl, also a Swede working in Germany. He ditched the term terra pinguis in favour of “phlogiston”, from the Greek for burnt. Stahl also identified the presence of phlogiston in rusting, saying that it was a form of combustion which reduced the metal to an ash, its true “dephlogisticated” state.

Phlogiston’s supporters had not actually isolated the stuff. They had not poked, prodded and teased it in a laboratory. In fact, they were keen to stress that it was an “imponderable”, a fiery “principle” that was allowed contradictory properties. It had, insisted Stahl, no colour, smell or taste.

But one “imponderable” was eventually to reduce the theory to dust. Some metals get heavier when they burn - a fact hard to square with the loss of phlogiston. Fans tried to wriggle out of this by saying that their fiery principle could sometimes have a negative weight, but this didn’t wash. Scientists were slowly accepting that their ideas had to agree with what happened in the real world. Oxygen was discovered and Lavoisier came up with the theory of oxidation to explain what was really going on. The empiricists had struck back.