410By the time he was a teenager, Scheele had learned the dominant theory of gases in the
1770s, the phlogiston theory. Phlogiston, classified as "matter of fire", was supposed to
be released from any burning material, and when it was exhausted, combustion would
stop. When Scheele discovered oxygen he called it "fire air" because it supported
combustion, but he explained oxygen using phlogistical terms because he did not believe
that his discovery disproved the phlogiston theory.
Before Scheele made his discovery of oxygen, he studied air. Air was thought to be an
element that made up the environment in which chemical reactions took place but did not
interfere with the reactions. Scheele's investigation of air enabled him to conclude that air
was a mixture of "fire air" and "foul air;" in other words, a mixture of two gases.
He performed numerous experiments in which he burned substances such as saltpeter
(potassium nitrate), manganese dioxide, heavy metal nitrates, silver carbonate and
mercuric oxide. In all of these experiments, he isolated gas with the same properties: his
"fire air," which he believed combined with phlogiston to be released during heat-releasing
reactions. However, his first publication, A Chemical Treatise on Air and Fire, was not
released until 1777, at which time both Joseph Priestley and Lavoisier had already
published their experimental data and conclusions concerning oxygen and the phlogiston
theory.