This figure, taken from the dome of the NAS building’s Great Hall, represents the element Air. Along with Earth, Fire, and Water, Air was one of the four elements ancient Greek scientists thought were the basic building blocks of all things. The philosopher-scientist Empedocles (5th c. BCE), who introduced the theory of the four elements into Greek thought, proved that air was a body and not simply a vacuum by showing through experiment that a container filled with air could keep water out.