Joseph Priestley heated mercuric oxide and collected the gas released, finding that a candle burned with unusual brightness in it and that mice thrived when placed inside a jar of it. He called the substance "dephlogisticated air," unaware he had isolated what Antoine Lavoisier would soon rename oxygen, a discovery that would overturn the reigning phlogiston theory of combustion.
Priestley was a Unitarian minister as much as a scientist, and he approached his laboratory work with the same restless curiosity he brought to theology. Working in a makeshift lab funded by his patron Lord Shelburne, he used a large magnifying lens to focus sunlight onto a sample of mercuric oxide inside a sealed glass vessel, watching a colorless gas bubble out.
He tested it the way any curious 18th-century chemist would: he stuck his nose in it, breathed it, and reported it felt "peculiarly light and easy" on his chest for some time afterward. He also famously kept mice alive far longer in jars of this gas than in ordinary air.
Ironiously, Priestley never abandoned phlogiston theory, the prevailing but incorrect idea that combustible materials contained a fire-releasing substance. It took Antoine Lavoisier in France, using Priestley's own findings, to correctly interpret the gas as a distinct element essential to combustion and respiration, naming it oxygène. Priestley's discovery instrument, ironically, dismantled the very theory he championed.
Key people: Joseph Priestley