Discovery of Oxygen — The Gas That Ignites Life Itself

Year: 1774-1777 | Field: Chemistry | Impact: Overthrew ancient theories and launched modern chemistry

In August 1774, Joseph Priestley focused sunlight through a powerful lens onto a sample of red mercury oxide, watching as the compound decomposed and released a mysterious gas. When he tested this new air by inserting a glowing candle, the flame burst into brilliant intensity—far brighter than in ordinary air. A mouse placed in the gas became unusually lively and active. Priestley had discovered what he called "dephlogisticated air," but he couldn't grasp its true significance. Meanwhile, across the English Channel, Antoine Lavoisier was conducting his own meticulous experiments with combustion and breathing. The French chemist would soon realize that Priestley's gas was the key to understanding fire, rust, and life itself—the element that would demolish centuries of scientific dogma and birth the modern science of chemistry.

The Problem

For over a century, scientists had struggled to explain combustion using the phlogiston theory, which claimed that burning substances released an invisible essence called phlogiston into the air. This theory seemed logical—wood turned to ash, metals became oxides, and flames died in enclosed spaces when the air became "saturated" with phlogiston. But troubling contradictions plagued the theory: some metals actually gained weight when they burned, and no one could isolate or measure phlogiston itself. The mystery deepened with studies of respiration and plant growth, which seemed connected to combustion but defied easy explanation. Scientists needed to understand what actually happened during burning, breathing, and the formation of rust—processes that seemed fundamental to life and matter itself, yet remained stubbornly mysterious despite centuries of investigation.

The Breakthrough

Priestley's discovery began with his systematic study of "airs"—different gases that could be isolated and tested. Using his burning lens to heat mercury oxide, he collected the released gas over water and began testing its properties. The gas supported combustion far better than ordinary air, kept mice alive longer, and made flames burn with extraordinary brightness. Priestley found he could breathe the gas himself, reporting that it made him feel "light and easy" for some time afterward. However, trapped by phlogiston theory, Priestley interpreted his discovery as air that had been purged of phlogiston, making it hungry to absorb more from burning substances.

Meanwhile, Lavoisier was approaching the same phenomenon from a different angle, conducting precise quantitative experiments that measured the weights of substances before and after combustion. When Priestley visited Paris in 1774 and described his new gas, Lavoisier immediately grasped its significance. Through careful measurements, Lavoisier demonstrated that combustion involved the combination of this gas with other substances, not the release of phlogiston. He named the gas "oxygen," meaning "acid-former," believing it was essential to all acids.

The breakthrough crystallized when Lavoisier showed that respiration and combustion were essentially the same process—both consumed oxygen and produced carbon dioxide and water. This insight unified seemingly disparate phenomena under a single chemical principle, revealing that animals were essentially burning food slowly and controlled, using oxygen to extract energy just as fire consumed wood.

The Resistance

Priestley himself never accepted Lavoisier's oxygen theory, defending phlogiston until his death in 1804. As one of phlogiston theory's most prominent supporters, Priestley argued that his "dephlogisticated air" fit perfectly within the existing framework—it was simply air purified of phlogiston and therefore eager to absorb more. Many established chemists shared Priestley's resistance, having built their careers on phlogiston theory and finding it difficult to abandon familiar concepts.

The resistance was particularly strong in Britain and Germany, where phlogiston theory had deep roots. Scientists like Henry Cavendish and Carl Scheele (who had independently discovered oxygen around the same time as Priestley) struggled to reconcile their observations with traditional theory. The debate raged for decades, with some chemists proposing hybrid theories that tried to preserve elements of phlogiston while incorporating Lavoisier's insights. Only gradually, as younger chemists embraced Lavoisier's systematic approach and quantitative methods, did the oxygen theory gain widespread acceptance.

The Revolution

Lavoisier's oxygen theory demolished the phlogiston paradigm and established chemistry as a quantitative science based on conservation of mass. His systematic nomenclature replaced alchemical names with logical chemical terminology that described composition rather than mystical properties. The discovery enabled chemists to understand oxidation, reduction, and acid-base reactions as electron transfers and chemical combinations rather than mysterious essence exchanges. This foundation made possible the development of atomic theory, the periodic table, and eventually the entire edifice of modern chemistry.

The medical implications proved equally revolutionary. Understanding respiration as controlled combustion led to insights into metabolism, nutrition, and disease. Doctors began to understand how the body extracted energy from food and why fresh air was essential for health. The discovery also launched industrial chemistry—understanding combustion enabled more efficient engines, better metallurgy, and the controlled chemical processes that would drive the Industrial Revolution.

Today, oxygen remains central to fields from medicine to space exploration. Oxygen therapy treats respiratory diseases, hyperbaric chambers accelerate healing, and life support systems enable everything from deep-sea diving to space travel. Climate scientists study oxygen levels in ancient atmospheres to understand Earth's history, while astrobiologists search for oxygen signatures as potential indicators of life on distant worlds.

Key Figures

• Joseph Priestley: English clergyman and natural philosopher who first isolated oxygen in 1774, though he interpreted it through phlogiston theory and never accepted the modern explanation
• Antoine Lavoisier: French chemist who recognized oxygen's true role in combustion and respiration, establishing the foundation of modern chemistry through quantitative experiments
• Carl Wilhelm Scheele: Swedish pharmacist who independently discovered oxygen around 1772 but published his results later, calling it "fire air"
• Henry Cavendish: British scientist who studied "inflammable air" (hydrogen) and helped establish the composition of water as hydrogen and oxygen
• Marie-Anne Lavoisier: Antoine's wife and collaborator who translated English works, illustrated his experiments, and helped disseminate his theories throughout Europe

Timeline Milestones

• 1772: Carl Scheele discovers oxygen independently but doesn't publish immediately
• 1774: Joseph Priestley isolates oxygen and demonstrates its properties
• 1775: Lavoisier begins systematic experiments proving oxygen's role in combustion
• 1783: Lavoisier and Pierre-Simon Laplace demonstrate that respiration is controlled combustion
• 1789: Lavoisier publishes "Elementary Treatise on Chemistry," establishing modern chemical nomenclature
• 1840s: Medical oxygen therapy begins for respiratory ailments
• 1960s: Oxygen becomes crucial for space exploration and deep-sea diving

Part of the Discovery Chronicles collection