Krypton was hiding in plain sight

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Perfection can be boring, and so it is with the chemistries of the noble gases. Once argon was discovered 1894, the others were diligently sought and found hiding in liquid air in 1898. First came krypton--its name meaning hidden one--the other two, neon and xenon, were found a few weeks latter. Lord Rayleigh and William Ramsay won the 1904 Nobel Prizes in Physics and Chemistry for their discoveries.
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Helium, the first noble gas, was first found in the solar spectrum, not in thin air.