_Scientific American_, Jan. 13, 1912, A Memorial Honor to a Pioneer Inventor. _The Smithsonian Institution 1846-1896. The History of its First Half-Century_, edited by G. B. Goode. A. F. Zahm, _The First Man-carrying Aeroplane capable of Sustained Free Flight_, Annual Report of the Smithsonian Institution, 1914 (illustrated). CHAPTER XVIII SCIENTIFIC HYPOTHESIS--RADIOACTIVE SUBSTANCES The untrained mind, reliant on so-called facts and distrustful of mere theory, inclines to think of truth as fixed rather than progressive, static rather than dynamic. It longs for certainty and repose, and has little patience for any authority that does not claim absolute infallibility. Many a man of the world is bewildered to find Newton's disciples building upon or refuting the teachings of the master, or to learn that Darwin's doctrine is itself subject to the universal law of change and development. Though in ethics and religion the older order changes yielding place to new, and the dispensation of an eye for an eye and a tooth for a tooth finds its fulfilment and culmination in a dispensation of forbearance and non-resistance of evil, still many look upon the overthrow of any scientific theory not as a sign of vitality and advance, but as a symptom of the early dissolution or at least of the bankruptcy of science. It is not surprising, therefore, that the public regard the scientific hypothesis with a kind of contempt; for a hypothesis (ὑπόθεσις, foundation, supposition) is necessarily ephemeral. When disproved, it is shown to have been a false supposition; when proved, it is no longer hypothetic. Yet a page from the history of science should indicate that hypotheses play a rôle in experimental science and lead to results that no devotee of facts and scorner of mere theory can well ignore.