Law of definite proportions

Careful analysis of copper carbonate demonstrated that, no matter how it was prepared or isolated, copper carbonate always contained five parts of copper to four parts of oxygen by weight. The resulting law "Proust's law" says chemical compounds always combine in definitive, fixed ratios of mass regardless of their method of preparation or source.
 Joseph-Louis Proust Spain 1799 Law of definite proportions

Careful analysis of copper carbonate demonstrated that, no matter how it was prepared or isolated, copper carbonate always contained five parts of copper to four parts of oxygen by weight. The resulting law "Proust's law" says chemical compounds always combine in definitive, fixed ratios of mass regardless of their method of preparation or source.