Lavoisier
When something burns, is it losing or gaining?
That time, that place
Lavoisier was a member of the private company that collected France's taxes, and with that income he built one of the finest laboratories in Paris. His wife Marie-Anne recorded the experiments, drew the apparatus, and translated English papers for him.
Why this question
His weapon was not a new discovery but a balance.
Run the reaction in a sealed vessel and weigh precisely at both ends. If something escaped, the total should fall; if something joined, it should rise. And the total weight of the sealed vessel never changed at all.
What was found
He burned tin in a sealed vessel. The total weight held steady — yet the tin, taken out and weighed, had gained, and the air inside had diminished by just that much.
Nothing had escaped. Something in the air had attached itself.
He isolated that something, studied it, and named it oxygen. Phlogiston was no longer needed. A single measure — weight — had displaced a century-old theory.
He also rebuilt chemical naming so that a name tells you what a substance is made of. Chemistry stopped being a private language.
Burning was the escape of phlogiston
Total weight unchanged in a sealed vessel, while the metal inside grew heavier
In 1794 he was executed by the revolutionary tribunal. Lagrange said it took a moment to cut off that head, and a hundred years might not produce another like it