Summarising the most novel facts and theories which were coming into prominence at the time, particularly those which had not yet been incorporated into standard textbooks, this important work was first published in 1921. The subjects treated cover a wide range of research that was being conducted into the atom, and include Quantum Theory, the Bohr Theory, the Sommerfield extension of Bohr’s work, the Octet Theory and Isotopes, as well as Ionisation Potentials and Solar Phenomena. Because much of the material of Atomic Theories lies on the boundary between experimentally verified fact and speculative theory, it indicates in a unique way how the future of physics was perceived at the time of writing. It thus throws into stark relief not only the immense advances made since the 1920s, but also, perhaps, highlights the importance of not rigidly adhering to a particular program of future discoveries.