How did the law of definite proportions support John Dalton's atomic theory?

The law of definite proportions, which states that chemical compounds always contain the same elements in fixed proportions by mass, provided essential empirical evidence for John Dalton’s atomic theory.

Dalton reasoned that if elements are composed of discrete, indivisible atoms with unique, characteristic weights, then chemical combinations must occur through the aggregation of these atoms in specific, whole-number ratios. Because these atoms have fixed masses, their combination into compounds necessarily results in the constant mass ratios observed in the law of definite proportions.1234

Dalton utilized these regularities in stoichiometry—the study of combining weights—to justify his inferences about atomic weights. By examining the macroscopic reacting weights of elements, he was able to deduce the relative weights of atoms, thereby providing a physical, particle-based explanation for why chemical substances maintain consistent compositions.2