In 1774, while experimenting in southern England, Priestley isolated a remarkable gas that made candles burn more brightly. Later, he placed two ounces (60 ml) of that gas in a glass along with a mouse. The mouse survived twice as long as it would have in a glass filled with regular air! Priestley himself inhaled the gas, and he said that he “felt peculiarly light and easy for some time afterwards.”

Joseph Priestley had discovered oxygen.a Yet, he called the gas dephlogisticated air, assuming that he had discovered ordinary air that lacked phlogiston, a hypothetical substance that was thought to hinder combustion. Priestley’s conclusion was wrong, but many still consider this discovery to be “the very pinnacle of his lifework.”