How did Joseph-Louis Proust defend the law of definite proportions against Claude-Louis Berthollet?

Joseph-Louis Proust defended the law of definite proportions against Claude-Louis Berthollet by distinguishing between true chemical compounds and mixtures or solutions.1

While Berthollet argued that chemical combinations could occur in variable proportions depending on physical conditions like concentration and solubility, Proust maintained that elements combine in fixed, invariant proportions by weight to form specific compounds. To support his position, Proust conducted extensive analytical research, particularly on metallic oxides, sulfides, and sulfates. He argued that substances Berthollet identified as having variable proportions were actually mixtures or solutions rather than true binary compounds.12

The debate, which played out in French scientific journals, was eventually resolved in Proust’s favor through the emergence of John Dalton’s atomic theory. This theory provided a theoretical framework that explained why elements must combine in fixed, discrete ratios, a relationship later clarified by Jöns Jacob Berzelius.13