Dalton's table of elements, 1808.
© Science Museum / Science & Society Picture Library
This simple diagram represents one of the greatest single strides made in the history of chemistry. It was not until 1803 that the idea of an atom was used to explain a scientific theory linked with exact measurement. John Dalton, a Quaker teacher, suggested that each element is made of characteristic atoms and that the weight ratio of the atoms in the products will be the same as the ratio for the reactants.