Dulong, Pierre Louis

[dülõ] (1785–1838) French chemist: co-discoverer of law of constant atomic heat.

Originally a physician, Dulong moved to chemistry as assistant to . In 1811 he discovered NCl3 , which cost him an eye and two fingers. He was an early supporter of the hydrogen theory of acids. From 1815 he worked with A T Petit (1791–1820) on thermometry; and in 1819 they published Dulong and Petit’s Law. This stated that the specific heat capacity of a solid element, when multiplied by its atomic weight, gives a constant which they called the atomic heat. The law is only approximately observed and is best followed at or above room temperature (eg C, B and Si only have specific heat capacities in accord with it at high temperature). It had some use, however, for easily giving rough atomic weights for new metals, at a time when this was valuable. In modern parlance we can express the law as:

relative atomic mass (ie atomic weight)

× specific heat capacity ˜ 25 J K -1 mol -1 = 3 R

where R is the gas constant.