Pierre Louis Dulong

Pierre Louis Dulong was born in Rouen in 1785 and orphaned young. He trained at the École Polytechnique in Paris and worked initially in medicine before turning to chemistry and physics, becoming an assistant to the great chemist Claude Louis Berthollet.

Dulong's research was marked by both brilliance and danger. In 1812 he discovered nitrogen trichloride, a violently explosive compound, and the investigation cost him an eye and several fingers — a price he reportedly concealed at first so that others would be warned only after he had finished the work. He went on to study the thermal expansion of gases, the measurement of high temperatures, and the specific heats of substances.

With Alexis Petit he established, in 1819, the law that bears their names: the molar heat capacities of simple solid elements are nearly constant, close to what we now write as 3R. The regularity, soon used to estimate atomic weights, hinted at the equal sharing of energy among atomic vibrations that equipartition would later explain, and its low-temperature failure would help motivate quantum theory.

Dulong rose to lead the École Polytechnique and was elected to the Académie des sciences. He continued precise thermal and optical measurements until his death in Paris in 1838.