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a hypothetical substance (weightless fluid) representing heat by whose presence in bodies scientists of the 18th and early 19th centuries attempted to explain observed heat phenomena, such as the heating of bodies, heat exchange, thermal expansion, and thermal equilibrium. The caloric theory required that certain unusual properties be ascribed to this fluid, for example, weightlessness, an elasticity greater than that of all other substances, and an ability to penetrate and expand the tiniest pores of bodies. In the 18th century, scientists hypothesized the existence of weightless fluids besides caloric, among them phlogiston, to explain the physical and chemical properties of substances. Only at the beginning of the 19th century was it conclusively proven that heat phenomena are caused by the chaotic movement of atoms and molecules. An important role in refuting the caloric theory was played by the experiments of B. Rumford (1798) and H. Davy (1799) that proved that the heating of bodies can be brought about by mechanical work.