Complex sentence: 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.
Split into simpler sentences: When Scheele discovered oxygen he called it "fire air" as it supported combustion. Scheele explained oxygen using phlogistical terms because he did not believe that his discovery disproved the phlogiston theory.