Deuterium

An isotope of hydrogen-1 was theorized to exist but not yet found. Reasoning that hydrogen-2 (aka deuterium or heavy hydrogen) would be far more massive and thus less easily evaporated than hydrogen-1, Urey evaporated liquid hydrogen and confirmed its discovery through spectroscopy.
 Harold Clayton Urey United States 1931 Deuterium

An isotope of hydrogen-1 was theorized to exist but not yet found. Reasoning that hydrogen-2 (aka deuterium or heavy hydrogen) would be far more massive and thus less easily evaporated than hydrogen-1, Urey evaporated liquid hydrogen and confirmed its discovery through spectroscopy.