In 1819, shortly after Dalton had introduced the concept of atomic weight in 1808, Dulong and Petit observed that if they measured the specific heat per unit mass of a variety of solids, and divided by the atomic weights of those solids, the resulting per-atom specific heat was essentially constant. This is the Dulong-Petit law, although we have since given a name to that constant, which is $3R$ or $3k_B$, depending on whether the relative atomic mass (atomic weight) or the absolute atomic mass is used. This law isn't precisely true, and isn't always true, and is never true at low temperatures. But it captures some physics that we will later call the equipartition theorem. We will write Dulong-Petit's law as: $$C_p=3Nk_B$$ where $N$ is the total number of atoms.