Law of Definite Proportions

This law — which the book notes is sometimes also referred to as the Law of Definite Composition (equivalently, of constant composition) — states that a given chemical compound always contains its constituent elements in a fixed ratio by mass, regardless of its source or method of preparation. For instance, pure water ($H_2O$) from any source—rain, river, or laboratory synthesis—will always consist of hydrogen and oxygen in a mass ratio of $1:8$. This fundamental principle arises from the fact that atoms of each element have a fixed, characteristic mass, and they combine in definite whole-number ratios to form molecules.

Proust's own supporting evidence for this law came from cupric carbonate (copper carbonate). He analysed two samples -- one occurring naturally, the other prepared synthetically in the laboratory -- and found the percentage of copper, carbon, and oxygen was identical in both, regardless of the very different origins of the two samples. …