The Dulong-Petit law, found in 1819 by Pierre Louis Dulong and Alexis Thérèse Petit, states the classical expression for the specific heat capacity of a crystal due to its lattice vibrations. The result is extremely simple: regardless of the nature of the crystal, the specific heat capacity is equal to 3R/M, where R is the gas constant and M is the molar mass. Despite its simplicity, Dulong-Petit law offers fairly good prediction for the specific heat capacity of solids with relatively simple crystal structure at high temperatures. It fails, however, in the low temperature region, where the quantum mechanical nature of the solid manifests itself. There, the Debye model works well.
See also: Stefan-Boltzmann law