Before the seventeenth century, people thought that air was the only gas that existed. In fact, air was called an "element," along with water, earth, and fire. First proposed by Greek philosophers in ancient times, this concept was accepted for nearly 2000 years. The discovery and study of gases during the seventeenth and eighteenth centuries mark the beginning of chemistry as we know it today.

The first scientist to distinguish specific gases from ordinary air was Johannes Baptista van Helmont, who coined the term gas around 1620. Helmont applied scientific techniques to study "gas sylvestre," which is now known to be carbon dioxide. Although he also described several other gases, Helmont lacked the experimental equipment needed to collect gases in containers and study their qualities. In the 1650s, Robert Boyle became the first scientist to collect a gas ( hydrogen), using an inverted, submerged flask. Stephen Hales...