Leipzig, S. Hirzel, 1921. Cont. hcalf. Titlelabel gilt on back. Small stamp on title. (4),35 pp.
First German edition and the first edition in book-form, as this Bakerian lecture was published in the Proceedings 1920. In this work Rutherford not only considered the neutron as a possibility, but he furthermore predicted its likely properties (""the idea of the possible existence of an atom of mass one, which has a zero nuclear charge"").