The law of definite proportions says that a substance always contains the same proportions of each element of which it’s composed. For example, a molecule of water (H2O) always contains two hydrogen atoms for every oxygen atom. This is commonly understood among modern chemists, but it was an important step in working toward a microscopic understanding of the composition of matter. The first to make such claims, in the early 1800s, was the French chemist Joseph Proust. It was a controversial idea at that time, and other chemists believed that elements could be combined in any proportion.