By the 1840s, scientists realized that, in every energy transformation, a certain loss occurs. For example, when heat is applied to water at a constant volume, changing it to steam, a small amount of the heat energy goes into the changes in the molecular structure and is not used for the production of steam. That tiny amount of heat may be said to be lost. The internal absorption of heat became known as entropy. The concept of entropy figured prominently in the later work of the American physicist Gibbs.
Heat loss is one of the steam engine's greatest inefficiencies.
Scientists working with the laws of thermodynamics readily incorporated Kelvin's temperature scale in their calculations. All natural processes, said Gibbs, involve change. In a way, Gibbs expanded the definition of equilibrium to mean that the only thing which does not change is change. Gibbs recognized that, the higher a particle's velocity, the higher its energy. He proved that, the higher the temperature of a molecule, the higher its energy.