At the age of 10, he proved that he was already capable of taking the difficult entrance exam to France's most prestigious scientific school of the time, the École polytechnique of Paris.
Petit is now probably best known for the surprising Dulong–Petit law concerning the specific heat capacity of metals, which both men formulated together in 1819 and which Albert Einstein explained almost a century later.
He was credited with an instinctive scientific intuition, a power of premature invention, certain presages of an assured future that everyone foresaw and even desired, so great was the benevolence which he inspired.
In the absence of that vivacity of the mind which invents easily, but likes to rest, he had the sense of scientific exactness, the gusto for precision experiments, the talent of combining them, the patience of completing them, and the art, unknown before him, to carry them to the limits of accuracy[.
Petit had more mathematical tendency, Dulong was more experimental; the first carried in the work more brilliant easiness, the second more continuity; One represented imagination, the other reason, which moderates and contains it.