Second law of thermodynamics
The second law of thermodynamics may be expressed in many specific ways, the most prominent classical statements being the statement by Rudolf Clausius (1854), the statement by Lord Kelvin (1851), and the statement in axiomatic thermodynamics by Constantin Carathéodory (1909). These statements cast the law in general physical terms citing the impossibility of certain processes. The Clausius and the Kelvin statements have been shown to be equivalent.
Theory of heat
In the history of science, the theory of heat or mechanical theory of heat was a theory, introduced in 1798 by Sir Benjamin Thompson (better known as 'Count Rumford') and developed more thoroughly in 1824 by the French physicist Sadi Carnot, that heat and mechanical work are equivalent. It is related to the mechanical equivalent of heat. Over the next century, with the introduction of the second law of thermodynamics in 1850 by Rudolf Clausius, this theory evolved into the science of thermodynamics. In 1851, in his "On the Dynamical Theory of Heat", William Thomson outlined the view, as based on recent experiments by those such as James Joule, "heat is not a substance, but a dynamical form of mechanical effect, we perceive that there must be an equivalence between mechanical work and heat, as between cause and effect."