Dulong and Petit's law

For a solid element the product of the relative atomic mass and the specific heat capacity is a constant equal to about 25 J mol⁻¹ K⁻¹. Formulated in these terms in 1819 by the French scientists Pierre Dulong (1785–1838) and Alexis Petit (1791–1820), the law in modern terms states: the molar heat capacity of a solid element is approximately equal to 3R, where R is the gas constant. The law is only approximate but applies with fair accuracy at normal temperatures to elements with a simple crystal structure.

http://web.lemoyne.edu/~giunta/petit.html Original paper