How did the mercury oxide experiment help Antoine-Laurent Lavoisier and Joseph Priestley study oxygen?

The experiment involving the heating of mercury oxide (HgO) was a pivotal moment in the history of chemistry, serving as a bridge between the phlogiston theory and the modern understanding of oxygen.1

In August 1774, Joseph Priestley discovered a new gas by heating red mercuric oxide. He observed that this gas supported combustion and life, though he interpreted his findings through the lens of phlogiston theory, calling the gas "dephlogisticated air".2

Later that year, Priestley shared his findings with Antoine-Laurent Lavoisier in Paris. Lavoisier, who had been struggling to explain the weight changes observed during calcination, repeated the experiment and realized that the gas released from the mercury oxide was the "active" component of the atmosphere. By isolating this gas, which he named oxygen, Lavoisier was able to demonstrate that combustion and calcination were reactions involving the combination of substances with oxygen, rather than the release of phlogiston. This discovery was essential to his development of the oxygen theory of combustion and the conservation of matter.3245