ABSTRACT
All life responds to warmth, to heat generated in one place and absorbed in another. The relation between thermal absorption and emission was debated at length in the eighteenth and early nineteenth centuries, notably by Pierre Prévost (1751-1839) whose Theory of Exchanges 1 put forward rational, common sense ideas on the nature of thermal equilibrium. Prévost believed in the caloric, a substance that flowed in and out of materials rather like corpuscular light, but he also understood that the properties of a system at equilibrium were independent of models. He regarded equilibrium as dynamic, characterized by an equal flow or “exchange” of heat in both directions. It was a sensible, practical approach applying equally to equilibrium established by contact, convection or radiation.
