Priestley Isolates Oxygen Gas in Key Experiment

Context

In the decades before 1774, European natural philosophers were deeply engaged with questions about the composition of air and the processes of combustion and respiration. The prevailing phlogiston theory, advanced by Georg Ernst Stahl and others, held that combustible materials contained a fire-like principle called phlogiston that was released during burning, leaving behind a calx or ash. Experiments by figures such as Stephen Hales, Joseph Black, and Henry Cavendish had already identified several distinct “airs,” including fixed air (carbon dioxide) and inflammable air (hydrogen), but the nature of ordinary atmospheric air remained puzzling.

What Happened

Priestley, serving as librarian and tutor to Lord Shelburne at Bowood House in Wiltshire, maintained a well-equipped laboratory where he pursued systematic investigations of gases. On August 1, 1774, he placed a sample of red mercuric oxide, known then as mercurius calcinatus per se, inside an inverted glass vessel standing in a trough of mercury. Using a large convex lens twelve inches in diameter, he concentrated the sun’s rays onto the oxide, causing a colorless gas to evolve and displace the mercury.

Aftermath

Priestley immediately tested the new gas. A candle flame burned with exceptional brilliance in it, and a mouse remained active far longer than in an equal volume of common air. When he inhaled a small quantity himself, he reported a light and easy sensation in the chest. He shared preliminary findings with scientific colleagues and, during a European tour later that year, described the experiment to Antoine Lavoisier in Paris.

Legacy

Priestley’s 1775 paper in Philosophical Transactions publicized the discovery under the name “dephlogisticated air.” Lavoisier repeated and extended the work, recognizing the gas as an element essential to combustion and respiration, and in 1777 proposed the name “oxygen.” The episode helped dismantle phlogiston theory and established the modern framework of chemical elements and reactions. Although Carl Wilhelm Scheele had produced the same gas earlier, Priestley’s prompt publication and Lavoisier’s theoretical synthesis gave the discovery its lasting impact on science and industry.