Heat

Heat is a form of energy that is transferred between two bodies that are in thermal contact and have different temperatures. For instance, the bodies may be two vessels sharing a heat-conducting wall and containing a liquid of different temperature on either side of the wall. Heat flows spontaneously from the higher-temperature to the lower-temperature body. The effect of this transfer of energy usually, but not always, is an increase in the temperature of the colder body and a decrease in the temperature of the hotter body.

A vessel containing a fluid may lose or gain energy without a change in temperature when the fluid changes from one phase to another. For instance, a gas condensing to a liquid does this at a certain fixed temperature (the boiling point of the liquid) and releases condensation energy. When a vessel, containing a condensing gas, loses heat to a colder body, then, as long as there is vapor present in the hotter vessel, the temperature of the container remains constant at the boiling point of the liquid, even though it loses heat to the colder body. In a similar way, if the colder body is a vessel containing a melting solid, its temperature will remain constant while it is receiving heat from a hotter body, as long as not all solid has been molten. Only after all of the solid has been molten and the heat transport continues, the temperature of the colder body (containing only liquid) will rise. The important distinction between heat and temperature (heat being a form of energy and temperature a measure of the amount of that energy present in a body) was clarified by Count Rumford, James Prescott Joule, Julius Robert Mayer,  Rudolf Clausius and others during the 18th and 19th centuries. Also it became clear that heat is not an invisible fluid, named  caloric, but a form of motion. The molecules of the hotter body are (on the average) in more rapid motion than those of the colder body. The law of the conservation of energy, discovered in the 19th century, makes clear that heat is a transfer of kinetic energy between the molecules constituting the bodies.