What did Claude-Louis Berthollet mean by static equilibrium in chemical reactions?

In his 1803 work Essai de statique chimique, Claude-Louis Berthollet used the term static equilibrium to describe the state toward which chemical reactions gravitate. Drawing an analogy from physics, he proposed that the direction and outcome of a reaction are not determined solely by absolute chemical affinities, but are instead modified by physical conditions such as the concentration of reagents, temperature, and solubility.1

Berthollet argued that these physical factors could offset the effects of affinity, allowing substances to combine in variable proportions depending on their environment. By focusing on the mass of reactants and forces like cohesion and elasticity, he sought to establish general laws for chemical behavior. While his specific challenge to the law of definite proportions was eventually contested by Joseph-Louis Proust, Berthollet’s emphasis on the role of concentration in determining reaction outcomes provided a foundational concept that contributed to the later formulation of the law of mass action.12