What was the disagreement between Claude-Louis Berthollet and Joseph-Louis Proust?

The disagreement between Claude-Louis Berthollet and Joseph-Louis Proust centered on the nature of chemical combination and the validity of the law of definite proportions.12

Proust, an analytical chemist, argued that elements combine to form compounds in fixed, invariant proportions by weight. He maintained that substances form a limited number of distinct compounds, such as specific metallic oxides or sulfides.2

Conversely, Berthollet challenged this view, suggesting that chemical combination was not restricted to definite saturation proportions. Influenced by his work on chemical affinity and equilibrium, Berthollet proposed that compounds could combine in variable proportions depending on physical circumstances, such as the concentration of reagents, temperature, and solubility. He viewed solutions as chemical combinations and believed they represented a continuum of combining proportions.213

The debate, which took place in French scientific journals, was eventually settled in Proust’s favor by the emergence of John Dalton’s atomic theory, which provided a theoretical foundation for definite proportions. Today, nonstoichiometric compounds that do not conform to the law of definite proportions are known as berthollides in honor of Berthollet’s original hypothesis.234