He combined his studies at the Oratorian College with work in the family pharmacy, where he acquired his first knowledge of chemistry and herbalism, eventually participating in the creation of a botanical garden in the city.
At the age of 21, he obtained the position of Chief Pharmacist at the Hospital de la Salpétrière in Paris (France) by competitive examination. During his stay in this city, he became friends with the chemist Antoine-Laurent de Lavoisier.
Between 1778 and 1780, he taught Chemistry and installed the Chemistry and Metallurgy laboratories at the Royal Seminary of Vergara (Guipúzcoa, Spain) which was founded by the Royal Basque Society of Friends of the Country.
In 1786, the Spanish government, following a recommendation by Lavoisier and through an agreement between King Charles III and Louis XVI, hired Proust to teach chemistry in Madrid. After a brief stay, he took charge of the Chemistry and Metallurgy classes at the Royal College of Artillery of Segovia until 1799.
The laboratory of the Royal College was endowed with the best means of the time and in it, Proust carried out numerous experiments on the composition of substances that led him to enunciate and disseminate the Law of Definite Proportions: "Substances combine in constant proportions and concrete” between 1794 and 1799. This law led him to a public dispute with another French chemist Claude Louis Berthollet, who defended the variability in the composition of compounds depending on their method of synthesis.
In 1799, the government decided to merge the Chemistry laboratories of the Ministries of State and Finance into what would become the Royal Laboratory of Madrid, and Proust was appointed director. It was a very fruitful time in which he was able to combine research, teaching and in which he publishes several books.
Proust enjoys freedom of research and prestige in Spain, he even rejects a substantial offer from a French company in 1806, but at the end of that same year he must travel to France for family reasons and the political situation prevents his return to Madrid, because, between other issues, the abdication of Carlos IV of Spain in 1808 deprived the laboratory of funds.
He settled in Craon (France), where he continued to work and maintain his controversy with Berthollet.
In 1811, Proust got the prestigious Swedish chemist Jöns Jacob Berzelius to recognize his statement, which laid the foundations for the establishment of Dalton's atomic theory.
Part of Proust's legacy, such as the Artillery Academy's laboratory and Library, were lost during the French invasion.
If you want to know more about this scientist, click on the following link: Joseph Louis Proust