On 1st of August 1774, Joseph Priestley heated red mercuric oxide using a large magnifying glass to focus sunlight, accidentally isolating a gas no ine had identified before. He tested it by burning candles, observing mice and even breathing it himself., finding that it supported life and combustion far better than ordinary air. Hi mistakely called it dephlogisticated air.
The discovery was harder to make that it seems. Swedish chemist Carl Wilhem Scheele had produced oxygen a few years earlier, but published his findings later, while Priestley was the first to report it publicly. A few years later, Antoine Lavoisier explained that the gas was a new chemical element essential for combustion and breathing.
Jospeh Priestley was an english clergyman, teacher, philosopher and scientist whose curiosity led him to became one of the most important experimental chemists of the 18th century. He carried out pioneering research on gases, isolating around 10 different ones durring his lifetime. He was also a strong supporter of religious freedom and political reform, views that made him controversial and eventually forced him to leave England for the United States, where he spent the rest of his life.