British chemist who discovered oxygen (1774) and 10 other gases, including hydrogen chloride, sulphur dioxide, and ammonia.
Biography Raised a strict Calvinist, Joseph Priestley originally hoped to become a minister, but his exposure to and interest in more liberal theological and philosophical issues ultimately led him to the calling of science. When Priestley met Benjamin Franklin in 1766, Franklin's enthusiasm for experimentation with electricity inspired Priestley to conduct his own experiments. One of Priestley's first discoveries was that graphite conducts electricity. Intrigued by the quality of the air emitted by fermentation at a nearby brewery, he later developed an improved technique for isolating and storing gases—at the time understood as varieties of air—in sealed glass vessels. Priestley also noted that the “damage” done to air by the respiration of animals, which slowly rendered it less and less life-sustaining for animals, was reversed by the respiration of plants. Using a magnifying glass to focus the Sun's rays on a piece of mercuric oxide and capturing the emitted gas, he discovered that this gas made a candle burn more brightly and could keep a mouse alive while all the other gases he tested extinguished the candle's flame and killed the mice. Priestley did not appreciate the full implications of his discovery, however. After he discussed his results with the French chemist Antoine Lavoisier, Lavoisier repeated Priestley's experiments, showing that combustion required the presence of Priestley's gas and implied that air was not an element but was made up of various parts. Lavoisier named the gas oxygen, and the modern theory of combustion was born.