The historical caloric theory treated heat as a conserved substance. The modern first law of thermodynamics conserves energy, allowing net heat input to become work in a heat engine.
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Caloric theory is a historical scientific theory that was proposed in the 18th century to explain the nature of heat. According to this theory, heat was thought to be a fluid called "caloric" that flowed from hotter objects to colder ones. Caloric was considered a substance that was conserved in thermodynamic processes, meaning that it could not be created or destroyed but could only move from one body to another.