Heat Is a Fluid Substance Called Caloric

Early Britannica reflected the widespread caloric theory, which *explained* many thermal phenomena remarkably well: why objects equilibrate in temperature, why heat conducts through materials, why substances expand when heated. Caloric theory predicted that heat was conserved, if caloric flowed from one body to another, the total amount remained constant. The theory persisted from the 1600s through the early 1800s because it worked, at least phenomenologically. Count Rumford's cannon-boring experiments (1797) and later James Joule's precise measurements of mechanical work converting to heat (1840s–1850s) demonstrated that heat wasn't a conserved fluid but rather a form of motion at the molecular level. Heat was energy, transferable and convertible. By mid-century, Britannica's entries on thermodynamics began acknowledging this paradigm shift, though caloric language lingered in some popular discussions for decades longer.