Carl Wilhelm Scheele, Joseph Priestley, and Antoine Lavoisier all contributed to research on oxygen. Producing the gas, publishing the results, and explaining combustion were separate achievements.
Lavoisier used quantitative research to change explanations of combustion. Oxygen's discovery was also tied to contemporary ideas about the composition of air and chemical reactions.
Phlogiston theory
Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes.
Established in 1667 by the German alchemist J. J. Becher and modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts. One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx.
Highly combustible materials that leave little residue, such as wood or coal, were thought to be made mostly of phlogiston, whereas non-combustible substances that corrode, such as iron, contained very little. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea; instead, it was based on observations of what happens when something burns, that most common objects appear to become lighter and seem to lose something in the process.
Scientific era
Swedish pharmacist Carl Wilhelm Scheele produced and described some properties of oxygen sometime around 1770–1775 but did not publish his work until a few years later because he was unable to interpret his work in the framework of the phlogiston theory. Scheele had produced oxygen gas by heating mercuric oxide (HgO) and various nitrates in 1771–1772. After reading about Priestley's work in 1775, Scheele published in 1777, calling the gas "fire air" because it was then the only known agent to support combustion.
In the meantime, on August 1, 1774, an experiment conducted by the British clergyman Joseph Priestley focused sunlight on mercuric oxide contained in a glass tube, which liberated a gas he named "dephlogisticated air". He noted that candles burned brighter in the gas and that a mouse was more active and lived longer while breathing it. After breathing the gas himself, Priestley wrote: "The feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards." Priestley published his findings in 1775 in a paper titled "An Account of Further Discoveries in Air", which was included in the second volume of his book titled Experiments and Observations on Different Kinds of Air.
The French chemist Antoine Lavoisier later claimed to have discovered the new substance independently. Priestley visited Lavoisier in October 1774 and told him about his experiment and how he liberated the new gas. Scheele had also dispatched a letter to Lavoisier on September 30, 1774, which described his discovery of the previously unknown substance, but Lavoisier never acknowledged receiving it (a copy of the letter was found in Scheele's belongings after his death).
Discrediting Phlogiston theory
Lavoisier conducted the first adequate quantitative experiments on oxidation and gave the first correct explanation of how combustion works. He used these and similar experiments, which began in 1774, to discredit the phlogiston theory and to prove that the substance discovered by Priestley and Scheele was a chemical element.
In one experiment, Lavoisier observed that there was no overall increase in weight when tin and air were heated in a closed container. He noted that air rushed in when he opened the container, which indicated that part of the trapped air had been consumed. He also noted that the tin had increased in weight and that the increase was the same as the weight of the air that rushed back in. This and other experiments on combustion were documented in his book "On General Combustion" (Sur la combustion en général), which was published in 1777. In that work, he proved that air is a mixture of two gases: 'vital air', which is essential to combustion and respiration, and azote (from a Greek word meaning 'lifeless'), which did not support either. Azote later became nitrogen in English, although it has kept the earlier name in French and several other European languages.