Caloric
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Caloric is the central concept of an obsolete theory of heat from the late 18th century.
In the history of thermodynamics, the initial explanations of heat were thoroughly confused with explanations of combustion. After J. J. Becher and Georg Ernst Stahl introduced the phlogiston theory of combustion in the 17th century, phlogiston was thought to be the "substance of heat".
Caloric was introduced by Antoine Lavoisier. Lavoisier had discovered oxygen and developed an explanation of combustion in terms of oxygen in the 1770s. In his book Reflexions sur le phlogistique in 1783, Lavoisier demonstrated that phlogiston theory was inconsistent, and proposed caloric as the "substance of heat". Caloric was supposed to be an imponderable fluid which flowed from warmer objects to cooler ones.
In 1798, Count Rumford published a report on his investigation of the heat produced while manufacturing cannons. He had found that boring a cannon repeatedly does not result in a loss of its ability to produce heat, and therefore no loss of caloric. This meant that caloric could not be a conserved "substance", and eventually led to the experiments of James Joule and the modern kinetic theory of heat.