What topic did Max Planck study for his doctoral dissertation?

Max Planck studied the second law of thermodynamics for his doctoral dissertation at the University of Munich, which he completed in July 1879.1

Throughout his career, Planck maintained a deep conviction that the entropy law was an absolute law of nature, a belief that served as the foundation for his early research and eventually led him to the discovery of the quantum of action, now known as Planck’s constant.1