Ancient Chinese and Greek philosophers believed that fire was an element, along with air, earth, and water. This notion gave way to the idea that fire, and things that feel hot, contained a substance called caloric fluid that flowed invisibly from one body to another. In 1798 Count Rumford showed that heat is not a substance but a form of energy. James Joule (18181889) devoted his life to the study of heat and made many important discoveries.

In 1900 Max Planck introduced the quantum theory of energy transmission—an idea that explains, among other things, why substances still possess some energy near absolute zero. In the 1930's Enrico Fermi and others carried out radiation studies that led to controlled nuclear reactions.

Today physicists are exploring phenomena at the extremes of temperature: the behavior of substances at very low temperatures; and plasmas, ionized gases formed at temperatures of millions of degrees.