A Historical Note about how the Property was Discovered that Hydrogenated Substances Increase the Radioactivity Induced by Neutrons

At the "Domus Galilaeana" in Pisa, many original documents and records are kept, which belong to the scientific activity carried out by Enrico Fermi until 1938. I compared those documentary sources with the supported evidences, the personal recollections, concerning the discovery that hydrogenated substances increase the radioactivity induced by neutrons: such a comparison leads to the conclusion that the discovery occurred on October 20th 1934, i.e., two days before the date that all the accounts that have been supported so far report. That suggests that any historical study to come, and regarding the experiments carried out by Fermi and his group on neutrons, cannot neglect to analyse carefully the accounts regarding those experiments and to compare them with the archive records.