This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
For most of the eighteenth century, chemists explained burning with phlogiston, a fire-like substance they held every combustible material contained and released into the air as it burned. Johann Joachim Becher first sketched the idea in 1667 as terra pinguis, a fatty earth given off by burning matter, and Georg Ernst Stahl renamed and systematized it as phlogiston around 1703. The theory organized a great deal of real chemistry correctly: it explained why a candle goes out in a sealed jar, phlogiston released by the flame saturates the enclosed air until it can absorb no more, and why charcoal, mostly phlogiston by this account, burns down to almost nothing. What it could not explain was why metals gain weight when they burn, when a substance is released, the remainder should weigh less, not more, and by the mid-eighteenth century this was a well-known, unresolved problem sitting inside the theory's own logic.
The experiment that would end up deciding the question was not designed to test phlogiston theory at all. On 1 August 1774, the English clergyman and chemist Joseph Priestley heated red mercuric oxide with a burning lens and isolated a colorless gas that made a candle burn unusually brightly and let a mouse breathe far longer than ordinary air allowed. Working entirely within phlogiston theory himself, Priestley called it dephlogisticated air, air that had been stripped of its own phlogiston and could therefore absorb an unusually large amount more. That October he described the experiment to Antoine Lavoisier in Paris, who repeated it.
Lavoisier read the same result completely differently. Working with sealed vessels and a precise balance, weighing everything before and after combustion, he showed that the total mass in a sealed system never changed, and that a burning metal gained exactly the mass of the gas it had combined with, not lost some invisible substance. His 1783 paper, Reflections on Phlogiston, presented combustion as a chemical combination with this gas, which he named oxygen, and proposed it as a direct replacement for phlogiston theory, one that explained the weight gain the older theory never could.
The most striking part of the story is that Priestley, whose own experiment supplied the decisive fact, never accepted Lavoisier's reinterpretation of it. He defended phlogiston theory for the rest of his life, publishing arguments for it even as the wider chemical community moved to Lavoisier's account through the 1780s and 1790s. The data that ended phlogiston theory and the chemist who generated it belonged, to the end, to two different sides of the argument.