What was the main argument of Claude-Louis Berthollet’s Essai de statique chimique?

In his Essai de statique chimique (1803), Claude-Louis Berthollet aimed to establish general laws for chemical reactions and a systematic approach to physical chemistry.1

The main arguments of his work included:

• Variable Proportions: Berthollet challenged the law of definite proportions, arguing that chemical compounds could combine in variable proportions within certain limits, depending on physical conditions.21
• Influence of Physical Conditions: He asserted that chemical affinities do not have absolute values but are modified by physical factors, most notably the concentration of reagents, as well as temperature and solubility.1
• Chemical Equilibrium: Drawing an analogy from physics, he proposed that chemical reactions seek a static equilibrium, which explains why the physical environment dictates the direction and outcome of a reaction.1

Berthollet’s ideas were controversial and led to a famous debate with Joseph-Louis Proust, whose defense of definite proportions eventually prevailed with the rise of atomic theory. However, Berthollet’s focus on reagent concentration was a significant step toward the later formulation of the law of mass action.213