1799 – France
‘Chemical compounds contain elements in definite proportions by mass’
Proust’s law is now referred to as the law of constant composition or the law of definite proportions.
Claude Berthollet (1748-1822), then the recognised leader of science in France, rejected Proust’s law. Berthollet believed that the force of chemical affinity, like gravity, must be proportional to the masses of acting substances. He suggested that the composition of chemical compounds could vary widely. Proust showed that Berthollet’s experiments were not done on pure compounds, but rather on mixtures. Thus for the first time a clear distinction was made between mixtures and compounds.
When Dalton proposed his atomic theory, Proust’s law helped to confirm the hypothesis. According to Dalton, atoms would always combine in simple whole number ratios. For example, all water molecules are alike, consisting of two atoms of hydrogen and one atom of oxygen. Therefore, all water has the same composition.
Proust’s law has been confirmed by experiments. For example, water always contains 11.2 percent hydrogen and 88.8 percent oxygen.
In recent years chemists have discovered certain rare compounds in which elements do not combine in simple whole number ratios. These compounds are known as ‘berthollides’.
In contrast, compounds in which elements do combine in simple whole number ratios are sometimes referred to as ‘daltonides’.