Joseph Priestley, an English chemist and theologian, made a significant discovery in the field of chemistry by identifying oxygen gas. While conducting experiments with a burning candle placed inside a closed container, he noticed that the flame continued to burn for a longer period than it should have, indicating that the air was somehow changed in the process. Upon further investigation, Priestley successfully isolated the gas produced when the candle was extinguished, which he referred to as 'dephlogisticated air'. This term reflected the prevailing 'phlogiston theory' at the time, which posited that combustion involved a substance known as phlogiston. Priestley’s experiments were conducted in Birmingham, England, bringing a new element to the study of gases and their properties.

The discovery took place within a scientific context already affected by the works of other chemists, particularly Carl Wilhelm Scheele, a German-Swedish chemist who had also isolated oxygen several years earlier. However, Scheele’s findings were not published until after Priestley’s experiments, leading to Priestley being credited with the discovery of oxygen in the English-speaking world. Scheele's work, published posthumously, was crucial in laying the groundwork for understanding the composition of air and the role oxygen plays in combustion and respiration. Priestley's findings would eventually contribute to a more comprehensive understanding of chemistry and the eventual debunking of the phlogiston theory.

Priestley’s identification and subsequent studies on oxygen gas would pave the way for a wave of scientific inquiry into the nature of gases. His methods of experimentation and documentation influenced notable figures like Antoine Lavoisier, who would later formulate the law of conservation of mass and develop modern chemistry principles. The collaboration of these early chemists and the debates surrounding combustion and respiration set the stage for a deeper understanding of chemical reactions, altering the trajectory of science in the years to follow.