Final answer:
Joseph Proust is the chemist who formally stated the law of constant composition, also known as the law of definite proportions.
Step-by-step explanation:
The first chemist to formally state the law of constant composition was Joseph Proust. This fundamental principle, also known as the law of definite proportions, was supported by the observations and experiments of John Dalton who is renowned for enunciating the modern atomic theory. However, it was Proust who demonstrated that all samples of a pure compound contain the same elements in the same proportion by mass.