How did Joseph Priestley’s research show that atmospheric air was not a single substance?

Joseph Priestley’s research contributed significantly to the understanding that atmospheric air is not a single, homogeneous substance, though the initial breakthrough regarding air as a mixture is often credited to his contemporary, Joseph Black. Black discovered that "fixed air" (carbon dioxide) was a component of common air, providing the first clear evidence that the atmosphere was a mixture rather than a single element.12

Priestley expanded this field by discovering 10 new gases, which he referred to as different "kinds of air". His experimental success was largely due to his development of an improved pneumatic trough, which allowed him to isolate and study gases that were otherwise soluble in water. By heating red mercuric oxide, he isolated a gas that supported combustion and life more vigorously than ordinary air, which he termed "dephlogisticated air". Furthermore, he designed the eudiometer, an instrument used to measure the "purity" or oxygen content of atmospheric air, further demonstrating that the composition of air could vary and was composed of distinct gaseous substances.231