Rudolf Julius Emanuel Clausius (January 2, 1822 - August 24, 1888), was a German physicist and mathematician and one of the central founders of thermodynamics. His PhD thesis concerning the refraction of light proposed that we see a blue sky during the day, and various shades of red at sunrise and sunset due to reflection and refraction of light. His most famous statement of the second law of thermodynamics was published in German in 1854, and in English in 1856: "Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time." In 1865, Clausius gave the first mathematical version of the concept of entropy, and also gave it its name. The landmark 1865 paper in which he introduced the concept of entropy ends with the following summary of the first and second laws of thermodynamics: "The energy of the universe is constant, and the entropy of the universe tends to be a maximum." His wife died in childbirth in 1875, leaving him to raise their six children. He continued to teach, but had less time for research thereafter. He died in 1888 at the age of 66.