How did Claude-Louis Berthollet’s chemical theory contribute to the law of mass action?

Claude-Louis Berthollet’s primary contribution to the law of mass action was his challenge to the prevailing belief that chemical affinities had absolute, fixed values.12

Through his observations of double decomposition—specifically noting that the formation of natron in nature differed from laboratory results using the same components—Berthollet concluded that the direction and outcome of a chemical reaction are significantly influenced by the concentration of the reagents. In his 1803 work, Essai de statique chimique, he argued that physical conditions, such as temperature, solubility, and the mass of the reactants, modify these affinities.23

By asserting that reactions seek a static equilibrium based on these variable physical factors rather than fixed elective affinities, Berthollet provided the conceptual foundation that eventually led to the more precise, mathematical formulation of the law of mass action in the mid-19th century. His work also sparked a significant debate with Joseph-Louis Proust regarding definite proportions, which proved instrumental in the development of atomic theory.23