Vital air and lifeless air

Antoine Lavoisier explained how combustion uses oxygen with a very clever experiment. Later, he lost his head.

Nargeot [CC BY]

We know now that burning is a chemical reaction in which fuel and oxygen combine to produce light, heat, and by-products like smoke and ash. As a species, it took us a surprisingly long time to work this out – probably because it took us a surprisingly long time to discover oxygen.

Oxygen is a good example of simultaneous discovery. A Polish alchemist named Michael Sendivogius described oxygen as the “food of life” in air in 1604, but credit usually goes to Carl Wilhelm Scheele of Sweden and Joseph Priestley of Britain, who both discovered oxygen in the 1770s. Both Scheele and Priestly had told French chemist Antoine Laurent Lavoisier about their discoveries, and it was Lavoisier who worked out the role of oxygen in combustion.

Lavoisier’s experiment was simple and elegant: seal up a container containing a metal like lead or tin. Weigh it. Heat it. (Heating lead or tin leads to calcination, i.e. oxidation of the metal.) Weigh it again… no change. But when you open the container some air rushes in. What happened to the air that was already in the container?

This is the clever part: find out how much air has rushed into the container after the heating. Weigh that air. Pull out the metal and weigh that metal. Lavoisier discovered that the metal had increased in weight by exactly the same amount as the weight of air that had been consumed. He concluded, correctly, that a chemical reaction combined something in the air with the metal. He repeated the experiment with other materials and concluded that combustion must always involve this kind of combination.

Lavoisier named the part of air that did the combining “vital air” and the part of air that did not “lifeless air” – what we now know as oxygen and nitrogen. (The name oxygen, funnily enough, was made popular in Britain through a poem by Charles Darwin’s grandfather… but that’s another story.)

Lavoisier came to a bad end: he was guillotined as part of the French Revolution on trumped-up charges of tax fraud – but not before chairing the commission that instituted the metric system. His contributions to chemistry and science did not save him: the judge is said to have declared that “the Republic does not need scientists or chemists.” Okay, that specific pronouncement may be a fiction added to the story of Lavoisier later… but the sentiment, evidently, carried over regardless.

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