Law of Definite Proportions

The law of definite proportions (also called the law of definite, or constant, composition) was established by the French chemist Joseph Proust, who compared two samples of cupric carbonate — one that occurred naturally, and one that had been made synthetically in the laboratory. He found that, despite their entirely different origins, both samples contained exactly the same percentage composition by mass of copper, oxygen and carbon (see the accompanying comparison table). This led Proust to state that a given chemical compound always contains exactly the same proportion of its constituent elements by weight, irrespective of the source of the sample or how it was prepared. This finding — that composition …