The Dulong–Petit law, a thermodynamic law proposed  by French physicists Pierre Louis Dulong and Alexis Thérèse Petit, states that the classical expression for the molar specific heat capacity of certain chemical elements is constant for temperatures far from the absolute zero. 
In modern terms, Dulong and Petit found that the heat capacity of a mole of many solid elements is about 3R, where R is the universal gas constant. The modern theory of the heat capacity of solids states that it is due to lattice vibrations in the solid.