Why did Claude-Louis Berthollet accept that some reactions combine in fixed proportions?

Claude-Louis Berthollet initially challenged the law of definite proportions, arguing that chemical compounds could combine in variable proportions depending on physical circumstances like concentration, temperature, and solubility.12

However, Berthollet eventually accepted that some reactions combine in fixed proportions because he was impressed by the newly proposed atomic theory. Despite this acceptance, he remained cautious; in the preface to the 1809 French translation of Thomas Thomson’s System of Chemistry, he continued to object to the idea that this observation should be generalized into a universal law applicable to all chemical reactions.1