Oxygen is the invisible architect of Earth's atmosphere, yet for most of human history, it remained a ghost in the machine of science. Before the 17th century, the very air that sustained life was misunderstood as a single, indivisible element. The first known experiment hinting at its existence was conducted by Philo of Byzantium in the 2nd century BCE, who observed water rising into an inverted vessel over a burning candle. He incorrectly concluded that air was converted into fire, a misconception that would persist for nearly two millennia. It was not until the 17th century that scientists like Robert Boyle and John Mayow began to suspect that air was a mixture, and that a specific portion was consumed during combustion and respiration. Mayow, writing in 1668, proposed that this portion, which he called 'nitro-aerial spirit,' was essential for both fire and life, but his insights were largely ignored by the scientific community dominated by the phlogiston theory. This theory, established by J. J. Becher and modified by Georg Ernst Stahl, posited that combustible materials contained a substance called phlogiston that was released during burning. The theory was so entrenched that even when scientists like Carl Wilhelm Scheele and Joseph Priestley isolated oxygen, they could not recognize it as a distinct element because it did not fit the phlogiston framework. Scheele, a Swedish pharmacist, produced oxygen by heating mercuric oxide as early as 1771, but he did not publish his findings until 1777, calling it 'fire air.' Priestley, an English clergyman, independently isolated the gas in 1774, noting that candles burned brighter and mice lived longer in it, yet he too described it as 'dephlogisticated air.' The true nature of oxygen remained hidden behind a veil of philosophical dogma until Antoine Lavoisier stepped in to dismantle the old order.
The Lavoisier Revolution
Antoine Lavoisier, a French chemist, is the figure who finally gave oxygen its name and its place in the chemical world, but his discovery was built on the shoulders of others and required a complete reimagining of how matter behaves. In 1774, Priestley visited Lavoisier and described his experiment with the new gas, while Scheele had also sent a letter to Lavoisier describing his discovery, though Lavoisier never acknowledged receiving it. Lavoisier conducted the first adequate quantitative experiments on oxidation, proving that air was not a single element but a mixture of two gases: 'vital air,' which supported combustion and respiration, and 'azote,' which did not. He demonstrated that when tin was heated in a closed container, the weight of the tin increased by exactly the amount that the air inside the container lost, proving that the gas was consumed and not released as phlogiston. This quantitative approach, documented in his 1777 book 'Treatise on Elementary Chemistry,' discredited the phlogiston theory and established the law of conservation of mass. Lavoisier renamed 'vital air' to 'oxygen' in 1777, deriving the name from the Greek roots 'oxys' meaning 'acid' and 'genes' meaning 'producer,' because he mistakenly believed that oxygen was a constituent of all acids. Although Sir Humphry Davy would later prove that acids like hydrochloric acid did not contain oxygen, the name had already taken root. The etymology of the word reflects the scientific arrogance of the era, as the name stuck despite being chemically incorrect. Lavoisier's work also revealed that the atmosphere was a dynamic system, with oxygen levels fluctuating over geological time, a concept that would later become central to understanding Earth's history. His experiments laid the foundation for modern chemistry, transforming oxygen from a philosophical curiosity into a measurable, quantifiable element with a specific role in the universe.