This Day in History — Oxygen Is Discovered

The Off-Key Bard peers into a sealed glass vessel, where an invisible gas is about to overturn centuries of mistaken science and forever change our understanding of life itself…

"For thousands of years, air was treated as a single, simple element. On this day, a beam of focused sunlight proved it was far more complicated than anyone had imagined."

On this day in 1774, English minister and natural philosopher Joseph Priestley conducted an experiment at Bowood House — the country estate of his employer, Lord Shelburne — that would alter the course of chemistry forever. Using a large magnifying lens to focus intense sunlight onto a sample of red mercuric oxide sealed in a glass container inverted over a mercury trough, he collected a colorless gas that no one had characterized before.

A candle burned faster and brighter in it. A mouse thrived in it far longer than in ordinary air. And when Priestley inhaled it himself, he recorded that "the feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt particularly light and easy for some time afterwards."

He had isolated oxygen. He called it "dephlogisticated air."

Three Men, One Discovery

The story of oxygen's discovery is genuinely triangular, and history hasn't quite settled who deserves the most credit:

Carl Wilhelm Scheele, a Swedish-German pharmacist, had actually produced the same gas as early as 1772 by heating red mercuric oxide — two years before Priestley — and called it "fire air." He sent his findings to the printer in 1775, but they weren't published until 1777, after Priestley had already gone public with his results. Scheele also interpreted his findings through the phlogiston framework. Lavoisier later received a letter from Scheele describing the experiment, but never publicly acknowledged it.

Priestley conducted his experiment on August 1, 1774, and published first. That autumn he traveled to Paris and demonstrated his experiment for Antoine Lavoisier in person, showing the French chemist exactly what he had done and how.

Antoine Lavoisier took what Priestley showed him, repeated the experiments meticulously between 1775 and 1780, and understood it correctly. He recognized that Priestley had discovered not "dephlogisticated air" but a distinct component of the atmosphere — the active agent in combustion and respiration. He named it oxygène in 1778, from the Greek for "acid-former," and used it to demolish the phlogiston theory entirely, founding the framework of modern chemistry in its place.

The irony that defines the story: Priestley, who made the key observation, never accepted what it meant. He remained a committed phlogistonist for the rest of his life, even as the entire scientific community abandoned the theory around him. The French naturalist Georges Cuvier later wrote of him with characteristic precision: "the father of modern chemistry who never acknowledged his daughter."

Priestley's final years were no easier than his intellectual ones. His outspoken religious views — he was a Unitarian who publicly supported both the American and French Revolutions — made him deeply unpopular in England. In 1791 a mob burned his house, his chapel, and his laboratory to the ground. He emigrated to Pennsylvania, where he spent his last years and died in 1804, still insisting that phlogiston was real.

Today the gas he isolated comprises roughly 21 percent of Earth's atmosphere, drives cellular respiration in every living organism, and serves as an essential element of modern medicine and industrial manufacturing. The man who found it never quite understood what he'd found — and yet without him, Lavoisier might never have known where to look.

"No flash of gold, no distant star,
Yet truth emerged from where we are…
For sometimes wonders fill the air,
Long before we know they're there."

History reminds us: scientific breakthroughs are rarely clean. The same experiment can reveal a truth that one man refuses to accept and another uses to transform all of human knowledge. What matters, in the end, is that someone pointed the lens at the right substance on the right afternoon — and wrote down what they saw.