Word: xenon
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Dates: during 1941-1941
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...afterglow of the gases they caught in a spectroscope. They found the colors to be not only those of excited oxygen and nitrogen, the most plentiful components of the air, but also of helium, which makes up only .0005% of the atmosphere and is exceeded in scarcity only by xenon. Conclusion: there is probably lots of helium in the upper atmosphere...