While common knowledge now, this was contrary to the caloric theory of heat popular at the time, which considered heat as a liquid. Recognizing the significance of James Prescott Joule's work on the conservation of energy, Rudolf Clausius was the first to formulate the second law in 1850, in this form: heat does not spontaneously flow from cold to hot bodies. He was the first to realize correctly that the efficiency of the process depends on the difference of temperature between the hot and cold bodies. The first theory on the conversion of heat into mechanical work is due to Nicolas LĂ©onard Sadi Carnot in 1824. The second law is often quoted as the reason that we cannot build perpetual motion machines. In other words, over time, we will have less useful energy in the same volume at the end of a work process. The second law states that energy becomes less dense over time and since we cannot move backwards in time, we cannot return to the prior energy density. The first law of thermodynamics states that one form of energy (such as kinetic, potential, electrical, thermal, etc,) is always converted into another without total loss of energy. A system operating in a cycle cannot produce a positive heat flow from a colder body to a hotter body ( Clausius).A system operating in contact with a thermal reservoir cannot produce positive work in its surroundings ( Kelvin).The entropy of a closed system never decreases (see Maxwell's demon).It is impossible to obtain a process such that the unique effect is the subtraction of a positive heat from a reservoir and the production of a positive work.Succinctly, the second law of thermodynamics states: The second law of thermodynamics has been stated in several ways. 8.1 Thermodynamics in the creation-evolution debate.3 Derivation of the Second Law from Time Reversible Mechanics.