Before revision: Discovery of protons and neutrons Back in 1815, William Prout observed that the atomic weights of the elements were all multiples of hydrogen's atomic weight, which is true if one takes isotopes into account.
After revision: Discovery of protons and neutrons Back in 1815, William Prout observed that the atomic weights of many elements werefor all of them multiples of hydrogen's atomic weight, which is true if one takes isotopes into account.