Aléxis Petit entered the École Polytechnique as a student in 1807 and was in the same class as Poncelet. Petit was awarded a doctorate in 1811 for a thesis on capillary action. In 1818 he won the Academy Prize for work on the law of cooling and, in the same year, he published on the general principles of machine theory.
The following year he published on the theory of heat. Working with Pierre Louis Dulong he formulated, in 1819, an empirical law concerning the specific heat of elements. The Dulong-Petit law states that the specific heat of all elements is the same on a per atom basis. The law has exceptions and was not fully understood until quantum theory was used.