<P> It states that the change in the internal energy ΔU of a closed system is equal to the amount of heat Q supplied to the system, minus the amount of work W done by the system on its surroundings . An equivalent statement is that perpetual motion machines of the first kind are impossible . </P> <P> Investigations into the nature of heat and work and their relationship began with the invention of the first engines used to extract water from mines . Improvements to such engines so as to increase their efficiency and power output came first from mechanics that worked with such machines but only slowly advanced the art . Deeper investigations that placed those on a mathematical and physics basis came later . </P> <P> The first law of thermodynamics was developed empirically over about half a century . The first full statements of the law came in 1850 from Rudolf Clausius and from William Rankine; Rankine's statement is considered less distinct relative to Clausius' . A main aspect of the struggle was to deal with the previously proposed caloric theory of heat . </P> <P> In 1840, Germain Hess stated a conservation law for the so - called' heat of reaction' for chemical reactions . His law was later recognized as a consequence of the first law of thermodynamics, but Hess's statement was not explicitly concerned with the relation between energy exchanges by heat and work . </P>

The first law of thermodynamics was developed by