The age of confusion

Abstract The concept of kinetic energy was identified already in the 17th century but it was of limited use until the other forms of energy were identified. Although the view that heat reflects atomic motion is ancient, until the middle of the 19th century heat was thought to be a fluid, ‘caloric’. Carnot used this idea to determine the maximum efficiency of any heat engine and the success of his theory leant weight to the concept of caloric. In the middle of the 19th century heat was finally recognised to be a form of energy and Carnot’s arguments were reworked in terms of energy rather than caloric. Whereas Carnot’s theory started from the assertion that perpetual motion was impossible, the new theory was based on the impossibility of moving heat from a cold body to a hot one without effecting any other change.