 A first step towards a theory of chemical combination was implicit in Lavoisier’s careful experimental work on water. In his Elements of Chemistry, Lavoisier established the mass proportions of hydrogen and oxygen obtained by the complete reduction of water to its elements. The fact that his results were based on multiple repetitions of this experiment suggests that he assumed compounds like water are always composed of the same elements in the same proportions. This widely shared view about the constant proportions of elements in compounds was first explicitly proclaimed as the law of constant proportions by Joseph Louis Proust (1754–1826) in the first years of the 19th century. Proust did so in response to Claude Louis Berthollet (1748–1822), one of Lavoisier’s colleagues and supporters, who argued that compounds could vary in their elemental composition.