Caloric theory
The discredited eighteenth-century idea that heat is a weightless, conserved fluid called caloric that flows from hot bodies to cold.
Definition
The caloric theory held that heat is a material substance — a weightless, invisible, indestructible fluid named caloric — that flows from hotter bodies into colder ones until balance is reached. It successfully explained thermal equilibrium, thermal expansion (caloric crowding pushes particles apart), and the warming of compressed gases, and it dominated the science of heat for most of the eighteenth century.
Its fatal commitment was conservation: caloric could move but could not be created or destroyed. Count Rumford's 1798 observation that boring cannon produced heat without limit, and Humphry Davy's melting of ice by friction in a cold vacuum, showed that mechanical work could generate heat inexhaustibly — impossible for a finite, conserved fluid. Joule's later measurement of the mechanical equivalent of heat sealed the case.
The theory is a textbook example of a productive idea that was eventually falsified: it organised real phenomena and made predictions, but a single class of experiments — friction without limit — revealed that heat is energy, not substance.