Nicolas-Léonard Sadi Carnot first suggested that all energy will eventually break down into an unusable form. This idea was seized by William Thomson (Lord Kelvin), who reintroduced it several years after Carnot's death. Thomson explained that this unusable form is waste heat, and that it dissipates into the environment. While this was to be an important concept, it was not viewed as such until the German physicist Rudolf Clausius discussed it in his 1850 essay. It was not until 1865 that Clausius finally named the concept entropy, from the Greek word for "transformation."

Entropy, as described by Clausius, was the ratio of the amount of heat in a system to that system's absolute temperature. He assumed that the system was closed--that is, that no heat could escape to the outside world, nor could heat enter from without. In such a system, the internal energy could, and would, be...