Until he was twenty, Joseph Proust followed dutifully in his father's footsteps, learning chemistry from him and eventually becoming his apprentice in a pharmacy in Angers, France. But in 1774 Proust left for Paris, against his family's wishes and apprenticed himself to another pharmacist. By 1776 he had won a position at a Paris hospital, where he worked as a chemist and pharmacist while lecturing at the Royal Palace.

In 1778 Proust went to Spain, having obtained the post of chemistry professor. In 1780 he returned to France and stayed there for five years; during this time he taught chemistry and experimented with the new scientific sport of ballooning. In 1784 Proust participated in a ballooning event at Versailles that was witnessed by French and Swedish royalty.

In 1785 Proust accepted a lucrative teaching position offered by the Spanish government. He spent the next twenty years in Spain at various posts in Madrid and...