Dulong and Petit's law
Dulong and Petit's law is a physical rule stating that the product of specific heat capacity and relative atomic mass (r.a.m.) is approximately equal to 25 (when the specific heat is measured in J mol-1 K-1) for the heavier solid elements.
The specific heat capacity of a solid was one of the easier parameters to measure during the early days of chemistry, so this law was used to obtain an approximation of the relative atomic mass. The rule was formulated by Pierre Dulong and Alexis Petit.