The understanding of air shifted from a philosophical concept to a measurable chemical mixture through centuries of discovery. This progression was a gradual realization that the atmosphere was not a simple, pure substance. The journey involved dismantling ancient theories, proving the physical reality of air, and finally isolating its gaseous components. The true nature of air as a complex mixture was established in the late 18th century, fundamentally changing the course of chemistry and biology.
Ancient Concepts of Air
For over two millennia, Western thought held that air was one of the four fundamental elements of nature, alongside earth, water, and fire. The philosopher Empedocles, in the 5th century BCE, first proposed these four “roots.” This framework was formalized by Aristotle, who taught that air was a primary substance characterized by being hot and wet.
Aristotelian cosmology placed air in a spherical region between the earth and the heavens, viewing it as a singular, pure, and uniform element. This philosophical view meant air was considered a basic, indivisible building block of the universe, not a compound. The concept of air as a simple element persisted well into the early modern era, limiting early attempts to explain phenomena like combustion.
Proving Air Is Physical Matter
The first major shift away from the philosophical view came in the 17th century when experiments demonstrated air possessed measurable physical properties. Italian physicist Evangelista Torricelli, in 1643, invented the mercury barometer, proving that air had weight and exerted pressure. He showed that the mercury column was held up by atmospheric pressure, not by a “horror of a vacuum.” Torricelli concluded that humans live submerged “at the bottom of an ocean of the element air.”
Robert Boyle, assisted by Robert Hooke, developed a sophisticated air pump in 1660 to conduct controlled experiments. Boyle demonstrated that air was elastic, possessing pressure that varied inversely with its volume (Boyle’s Law). By observing that a candle would not burn and animals would die in a vacuum, Boyle established that air was a physical substance required for life and combustion. These pneumatic experiments solidified air’s physical reality, setting the stage for chemical analysis.
Identifying the Invisible Components
The 18th century marked the true chemical breakthrough, revealing air as a mixture of distinct gases. Daniel Rutherford, a Scottish physician, isolated “noxious air” in 1772 by removing the components that supported life and combustion. After a candle went out and a mouse died in a sealed jar, he absorbed the carbon dioxide, leaving a residual gas that supported neither process. This gas, which makes up about 78% of the atmosphere, was later identified as nitrogen.
The other major component was independently isolated by two researchers in the same decade. Swedish pharmacist Carl Wilhelm Scheele isolated “fire air” before 1773, though his findings were published later. English scientist Joseph Priestley isolated the same substance in 1774 by focusing sunlight onto mercuric oxide. Priestley named this gas “dephlogisticated air,” noting it caused a candle to burn with extraordinary brightness and allowed a mouse to survive longer.
The French chemist Antoine Lavoisier synthesized these findings, leading to the definitive realization that air was a mixture. Lavoisier performed precise quantitative experiments, recognizing that Priestley’s gas combined with substances during combustion and respiration. In 1778, he named this reactive gas oxygen, meaning “acid former.” He named the inert gas isolated by Rutherford azote (nitrogen), meaning “without life.” Lavoisier’s work established that air was composed primarily of life-supporting oxygen and non-supportive nitrogen, proving it was a complex mixture.
Revolutionizing Scientific Understanding
The realization that air was a mixture of gases, particularly the identification of oxygen, triggered a revolution in scientific thought. Lavoisier used this new understanding to dismantle the prevailing phlogiston theory of combustion. He demonstrated that combustion was the rapid chemical combination of a substance with oxygen, documenting this using precise measurements. This focus on quantitative analysis, where mass was conserved, became the foundation of modern chemistry.
The discovery of oxygen also clarified the biological process of respiration. Lavoisier, working with Pierre-Simon Laplace, showed that animal respiration was a slow form of combustion. Inhaled oxygen reacted with organic material to produce heat and carbon dioxide. This established a direct link between the atmosphere and life processes, explaining why animals require air to survive. The understanding of air’s composition provided the tools to explore the atmosphere’s role in the cycle of life, including photosynthesis.