By taking N to be the Avogadro constant N A, and using the relation R = N A k B between the gas constant R and the Boltzmann constant k B, this provides an explanation for the Dulong–Petit law of specific heat capacities of solids, which stated that the specific heat capacity (per unit mass) of a solid element is inversely proportional to its atomic weight. A modern version is that the molar heat capacity of a solid is 3R ≈ 6 cal/(mol·K).