1774

Oxygen

Henry Cavendish (1731–1810), Joseph Priestley (1733–1804), Antoine Lavoisier (1743–1794)

The history of oxygen and its chemistry is a tangled one, but its discovery demonstrated a very simple and very important point: air is not a single substance, but rather is divisible into different components. Chemists had produced several sorts of gases by this point—including carbon dioxide, hydrogen, hydrogen sulfide, and hydrogen cyanide—but their relevance to the air everyone breathed was an open question. For one thing, it was obvious that there couldn’t be much hydrogen sulfide around, since no one could have missed that one.

English polymath Joseph Priestley was the man who discovered oxygen. He conducted experiments with the various “airs” known to him and made a key observation: “Fixed air” (carbon dioxide) was a known poison to animals, but it did not kill green plants. In fact, in a sealed container of fixed air, the plants somehow managed to “detoxify” the air in the container. Since it was known that animals breathed out carbon dioxide, Priestley speculated that plants somehow balanced things out in the atmosphere by removing the gas and (in turn) giving off something else. In 1774, he conducted another experiment, in which mercuric oxide (HgO) heated with a magnifying glass and sunlight began to decompose and give off what he thought must be the same substance, because instead of killing mice, this gas kept them alive, and instead of putting flames out, it made them burn brighter and hotter. Breathing it, Priestley found, was pleasant.

Priestley believed that this new air had had its phlogiston removed from it (and that Henry Cavendish’s hydrogen might be phlogiston itself ), but Frenchman Antoine Lavoisier offered a more accurate theory of combustion in a 1777 paper, “Reflections on Phlogiston.” Priestley’s new gas—which Lavoisier dubbed oxygen in 1778—was what burned things, what combined with metals and other elements when they burned, and what animals needed in order to breathe. There was no such thing as “phlogiston.”

Quietly and invisibly, green plants around the world strip carbon from the air and release oxygen into it.