  Just as the chemical elements from hydrogen (Z = 1) to oganesson (Z = 118)
once were discovered, so were the numerous isotopes. The histories of how the
isotopes were discovered are less well known, but in a few cases they are as
interesting and instructive as those of the elements figuring in the periodic
table. Following an overview of criteria usually associated with the concept of
discovery in general, this paper examines in detail the historical developments
that led to the discoveries of deuterium and tritium and also, as a by-product,
the helium-3 isotope. It also includes a brief section on the neutron, which in
the 1920s, when it was still a hypothetical particle, was sometimes discussed
together with the mass-2 and mass-3 hydrogen isotopes. The paper concludes with
a discussion of priority questions relating to suggestions of the two heavy
isotopes as well as to their actual discoveries.
