Heat
. Heat (physics), in physics, transfer of energy from one part of a substance to another, or from one body to another by virtue of a difference in temperature. Heat is energy in transit; it always flows from a substance at a higher temperature to the substance at a lower temperature , raising the temperature of the latter and lowering that of the former substance, provided the volume of the bodies remains constant. Heat does not flow from a lower to a higher temperature unless another form of energy transfer, work, is also present. See also Power . Until the beginning of the 19th century, the effect of heat on the temperature of a body was explained by postulating the existence of an invisible substance or form of matter termed caloric. According to the caloric theory of heat, a body at a high temperature contains more caloric than one at a low temperature; the former body loses some caloric to the latter body on contact, increasing that body's temperature while lowering its own. ...