Francis Aston and the mass spectrograph
The chemical determination of atomic weights gives the average weight for an aggregate of a large number of atoms. Although this is useful in many applications, the determination of the masses of individual atoms gives further important information, in particular the stability of the atoms or more precisely of their nuclei. The first accurate determination of the masses of individual atoms was made by Aston in 1919. His measurements demonstrated the existence of isotopes in non-radioactive elements and paved the way for our present picture of the nuclear atom.