In the first half of his 1920 Bakerian Lecture on the Nuclear Constitution of Atoms, Rutherford described in detail his experiments in which he fired high-speed \(\alpha \)-particles across a chamber filled with dry nitrogen gas. On the opposite side of the chamber occasional scintillations from a zinc-sulphide detector screen signalled the arrival of tiny particles. Surprisingly, these particles did not have the known characteristics of \(\alpha \)-particles. Rather, they had the charge and mass of hydrogen ions, despite the fact that every effort had been made to rid the gas of any hydrogen contamination. From this Rutherford concluded that “some of the nitrogen atoms are disintegrated by their collision with swift \(\alpha \)-particles and that swift atoms of positively charged hydrogen are expelled.” He also went on to say “this is the first time that evidence has been obtained that hydrogen is one of the components of the nitrogen nucleus.” Now in the following reading selection—the second half of the same Bakerian Lecture—Rutherford continues to explore the artificial disintegration of atomic nuclei with high-speed \(\alpha \)-particles. What new isotope does he discover? How does this discovery help to shape his theory of the internal structure of the atomic nucleus? And what new elementary particle does Rutherford postulate in the course of his analysis?