A chemist heated a red powder and discovered a gas that made candles burn brighter

In 1774, Joseph Priestley focused sunlight through a lens onto mercuric oxide and collected the gas it released. Candles burned more vigorously in it, and a mouse he placed in a jar of it stayed lively far longer than in ordinary air. Priestley had isolated oxygen, though he misunderstood what he'd found, calling it 'dephlogisticated air' within the chemical theory of his time.

— Joseph Priestley, Experiments and Observations on Different Kinds of Air — 1774-1775