Rudolph Clausius – A pioneer of the modern theory of heat

Rudolph Clausius (1822–1888) played an important role in advancing the theory of heat during the 19th century. His contributions concerned the development of the two fundamental principles of heat as well as the microscopic approach of kinetic theory where he introduced the new concept of the mean free path. He always strictly separated these two fields. When Clausius took up his studies the idea that heat belonged to the so-called imponderables which were weightless and invisible had not yet disappeared. Carnot had still used that idea for his well-known cycle. Clausius was able to make the Carnot-cycle compatible with the concept of heat as a kind of motion.His research opened the way for thermodynamics later chiefly advocated by Planck as well as for modern statistical physics mainly connected with the names of Maxwell and Boltzmann. Scientific education and research of Clausius will be discussed here in the context of the development of the theory of heat. As he published most of his important papers on this subject already during the first two decades of his career we confine on this period. Clausius began his studies in Berlin in 1840, habilitated there in 1850 and was appointed at the newly founded Polytechnical School in Zürich in 1855. It will be shown that Clausius remained an outsider in the physics community of his time as he himself did not perform any research experiments.