Rudolf Clausius

Rudolf Julius Emanuel Clausius was born in 1822 in Köslin, Pomerania (now Koszalin, Poland), one of many children of a pastor and schoolmaster. He studied at Berlin and Halle and went on to hold professorships at Zürich, Würzburg, and finally Bonn.

Clausius is the physicist who turned thermodynamics from a collection of engine lore into a rigorous mathematical science. In his landmark 1850 paper he reconciled Carnot's principle with the newly established conservation of energy, stating the second law in the form that heat cannot of itself pass from a colder to a hotter body. Over the following fifteen years he refined the subject, culminating in the introduction of entropy.

In 1865 he coined the word entropy from the Greek trope, transformation, deliberately modelling it on the word energy so that the two great quantities of physics would sound alike. He summarised the laws of thermodynamics in two sentences that have become famous: 'The energy of the universe is constant. The entropy of the universe tends to a maximum.'

He also did pioneering work in the kinetic theory of gases, introducing the concept of the mean free path. He was seriously wounded leading a student ambulance corps in the Franco-Prussian War of 1870, and suffered chronic pain for the rest of his life, which slowed his later research.