Caloric theory treated heat as a subtle, weightless substance stored in matter. A hot body contained more caloric, and cooling transferred some of the fluid to colder surroundings. The model explained many everyday observations and gave eighteenth-century physicists a useful language for heat flow.
Friction created a serious problem. In cannon-boring experiments, Count Rumford observed apparently inexhaustible heat generated by mechanical work. If heat were a fixed material substance already contained in the metal, continuous production by friction was difficult to explain.
James Prescott Joule turned that qualitative objection into measurement. Using electrical and mechanical experiments—including his famous paddle-wheel apparatus—he showed that a reproducible quantity of mechanical work produced a corresponding quantity of heat. Heat behaved as a form of energy transfer, not as a conserved fluid being released from matter.
The emerging conservation-of-energy framework and kinetic theory then connected temperature and internal energy to microscopic motion and interactions.
Caloric theory was not useless; some equations and intuitions developed under it survived into thermodynamics. What disappeared was the physical substance itself. Modern physics treats heat as energy transferred because of a temperature difference, not as material stored inside an object.