A FEW days ago Dr. King and I announced the discovery of an isotope of carbon, mass 13, appearing in the λ4737 Swan band of neutral C2. Since then, have found this isotope in two additional sources, namely, in Hopfield's absorption spectrograms of carbon monoxide, and in King's furnace (emission) spectrogram of the λ3883 CN band.
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Raymond T. Birge (1929) studied this question.