Throughout recorded history, people have realized that air is critical to life. Hundreds of years before the birth of Christ, Greek philosophers suggested that air was one of four basic elements, along with fire, water, and earth. For nearly 2000 years most people, even scientists, believed in this idea, or some variation of it. Then in the 1600s and 1700s, the modern science of chemistry was born, and early chemists began to learn that air is made up of different gases. But the process of discovering these gases and understanding the ways that they react with each other took several decades, and it was not until the late 1800s that all of the gases present in air had been isolated and identified.

The first clue that certain gases could be distinguished from ordinary air was discovered around 1630 by Jan van Helmont, who introduced the term gas. During...