Original Petit-Dulong Paper (in Translation)

In “Research on some important points of the Theory of Heat” (Annales de Chimie et de Physique 10, 395-413 (1819); contemporary translation from Annals of Philosophy 14, 189-198 (1819)) by Alexis-Thérèse Petit (1791-1820) and Pierre-Louis Dulong (1785-1838), they famously conclude that:

The atoms of all simple bodies have exactly the same capacity for heat.

Their data:

	Specific heats	Relative weights of the atoms	Products of the weight of each atom by the corresponding capacity
Bismuth	0,0288	13,30	0,3830
Lead	0,0293	12,95	0,3794
Gold	0,0298	12,43	0,3704
Platinum	0,0314	11,16	0,3740
Tin	0,0514	7,35	0,3779
Silver	0,0557	6,75	0,3759
Zinc	0,0927	4,03	0,3736
Tellurium	0,0912	4,03	0,3675
Copper	0,0949	3,957	0,3755
Nickel	0,1035	3,69	0,3819
Iron	0,1100	3,392	0,3731
Cobalt	0,1498	2,46	0,3685
Sulphur	0,1880	2,011	0,3780

In “7.14: Heat Capacities of Solids- the Law of Dulong and Petit,” Paul Ellgen (Retired Teach (Chemistry) at Oklahoma School of Science Mathematics) writes:

Heat capacities of solids have been investigated over wide temperature ranges. For most solids, CP is approximately constant at room temperature and above. For any of the heavier elements, this constant has about the same value. This observation was first made in 1819. It is called the law of Dulong and Petit, in honor of the discoverers. It played an important role in the establishment of correct atomic weights for the elements. The value of the constant found by Dulong and Petit is about 3R. Remarkably, the law can be extended to polyatomic molecules containing only the heavier elements. Often the solid-state heat capacity of such molecules is about 3R per mole of atoms in the molecule. Correlations that are more detailed have been developed. These relate the heat capacity of a mole of a molecular solid to its molecular formula. In such correlations, the heat capacity per mole increases by a fixed increment for each atom of, say, carbon in the molecule; by a different fixed increment for each atom of nitrogen in the molecule; etc. For the lighter elements, the increments are less than 3R. For the heavier elements, the increment is approximately 3R, as observed by Dulong and Petit.