A law proposed by the French physicists Pierre Louis Dulong (1785–1838) and Alexis Thérèse Petit (1791–1820) for the molar heat capacity of a crystal. The law states that the molar heat capacity of a solid element is approximately three times the universal gas constant, which is about 25 J mol⁻¹ K⁻¹. They proposed the law from experimentation before the kinetic theory of gases had been established. The law is only approximate and applies to elements with simple crystal structure.