Joseph studied chemistry in his father's shop and later went to Paris where he gained the appointment of apothecary in chief to the Salpêtrière.

But when Napoleon invaded Spain, they burned Proust's laboratory and forced him back to France.

[1] Proust studied copper carbonate, the two tin oxides, and the two iron sulfides to prove this law.

Proust published this paper in 1797, but the law was not accepted until 1812, when the Swedish chemist Jöns Jacob Berzelius gave him credit for it.

[citation needed] Proust was also interested in studying the sugars that are present in sweet vegetables and fruits.