Antoine Lavoisier · Science
How Lavoisier overturned the reigning theory of combustion. Burning was thought to release a fire-stuff, <em>phlogiston</em>; but metals grow <em>heavier</em> when they burn, and Lavoisier’s balance showed that burning is not a loss but a gain - a combination with a part of the air he named oxygen.
For most of the eighteenth century, chemists explained fire with a single elegant idea: phlogiston. Proposed by Georg Stahl around 1700, the theory held that every combustible body contains a hidden fire-substance, and that burning is simply the escape of this phlogiston into the air. Wood is rich in phlogiston; when it burns, the phlogiston flees and only ash is left. A metal, too, was thought to be a compound of its calx (what we would call its rust or oxide) plus phlogiston; heat the metal and the phlogiston departs, leaving the earthy calx behind. The theory was beautifully unifying - it tied together burning, rusting, breathing, and the smelting of ores - and it reigned for decades. But it carried a quiet flaw that would eventually destroy it, a flaw that only a chemist obsessed with the balance would take seriously.
Lavoisier’s great instrument was not a furnace but a balance. Where earlier chemists watched colours and flames, he weighed the reactants and the products, and included the air itself in the account. In a famous demonstration he sealed tin in a closed vessel with a fixed quantity of air and heated it. The whole sealed vessel weighed exactly the same before and after - nothing had entered or escaped. But the tin had turned partly to calx and gained weight, and when he opened the vessel, air rushed in to replace what was missing, and the calx-plus-metal had gained precisely the weight of the air consumed. The conclusion was inescapable: the metal had absorbed part of the air. Burning adds air to a body; it does not release a phantom fire-stuff. The balance had spoken, and it contradicted a century of theory.
Priestley had made this gas first, in 1774, and had described it to Lavoisier in Paris. But Priestley, faithful to the old framework, called it ‘dephlogisticated air’ - air so emptied of phlogiston that it greedily absorbed more, which was why things burned so fiercely in it. Lavoisier rejected the whole framework and renamed the gas oxygène, from the Greek for ‘acid-former,’ because he believed (wrongly, as we will see) that oxygen was the principle of acidity. The naming mattered: to name the gas by what it does in his new theory was to declare the old theory dead. Combustion, calcination, rusting, and respiration were all, Lavoisier argued, one kind of process - the combination of a body with oxygen. Phlogiston, the fire-stuff that explained everything, was replaced by a real, weighable substance that could be captured in a jar.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned that the phlogiston theory explained burning as the escape of a fire-substance, but could not account for the weight metals gain when calcined. Lavoisier weighed everything and showed combustion is combination with oxygen, a part of the air. Explain why the weight gain was fatal to phlogiston, in your own…
Leads to Joseph Priestley.
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