Petit, Alexis(-Thérèse) (1791–1820)

French physicist, co-discoverer with Pierre Dulong of Dulong and Petit's law, which states that, for a solid element, the product of relative atomic mass and specific heat capacity is approximately constant.

Petit was born in Vesoul, Haute-Saône, and studied in Paris at the Ecole Polytechnique, becoming professor there in 1815.

Petit's early research was conducted in collaboration with French scientist Dominique Arago. They examined the effect of temperature on the refractive index of gases. Their results led Petit to become an early supporter of the wave theory of light.

Petit and Dulong began their collaboration in 1815, and in 1819 announced their law of atomic heats. Chemists who at that time were having difficulty determining atomic weights (and distinguishing them from equivalent weights) now had a method of estimating the approximate weight merely by measuring the specific heat of a sample of the element concerned.