How did oxygen affect a mouse in Joseph Priestley’s experiment?

In Joseph Priestley’s experiments, the mouse was able to thrive in the gas he had isolated by heating red mercuric oxide. Because the mouse remained healthy and active in this environment, Priestley concluded that the gas supported life and combustion, leading him to name it "dephlogisticated air".1

It is important to note that while Priestley discovered this gas, he interpreted his findings through the lens of the phlogiston theory rather than identifying it as oxygen. It was later the French chemist Antoine Lavoisier who conducted further investigations to recognize the gas as the "active" principle in the atmosphere and formally name it oxygen.12