Greek philosopher Aristotle believed that air was one of four elements (along with water, earth, and fire) that combined to make everything in the world. And for 2,000 years, air -- a word used to describe any gas, including the very oxygen we breathe -- remained a mysterious and misunderstood state of matter. Even Galileo and Descartes were unaware of its true nature and behavior.

But beginning in the seventeenth century, the veil that obscured our understanding of air steadily began to lift. Isaac Beekman was the first to realize, in 1618, that air had physical properties. About 50 years later, John Mayow suggested that air was composed of two gases, one of which supported life and combustion, the other of which did not. Finally, three scientists working in the 1770s made the discovery of oxygen, which became known as "pure air": Carl Scheele was the first to synthesize oxygen; Joseph Priestley was the first to report its discovery; and Antoine Lavoisier was the first to truly understand the chemical nature of oxygen. Lavoisier used his newfound knowledge to mount a confident attack on the influential yet misguided phlogiston theory, which attempted to explain why air initially supported combustion but gradually lost its capacity to do so.

Throughout the late eighteenth and early nineteenth centuries, Lavoisier, Scheele, Priestley, and others identified many of the chemical elements that, in 1869, would be arranged in order of increasing atomic mass by Russian chemist Dmitri Mendeleev in a listing called the Periodic Table of the Elements.