The nuclear spin of the chlorine atom was determined by observing the line intensities in the band spectrum of the homonuclear diatomic molecule of that atom. The lines of even $J$ values show a different intensity from those of odd $J$ values, the ratio of intensities being given by $\frac{\mathrm{}(I+1\mathrm{})\mathrm{}}{I}$ where $I$ is the nuclear spin of the atom in question. In the band absorption spectrum of chlorine the intensity ratio of lines of the ${\mathrm{Cl}}^{35}$${\mathrm{Cl}}^{35}$ molecule has been measured and compared with the value found by Elliott. These values are in substantial agreement and yield a value of 5/2 for the nuclear spin of ${\mathrm{Cl}}^{35}$. In this spectrum no lines of the molecule ${\mathrm{Cl}}^{37}$${\mathrm{Cl}}^{37}$ were observed due to the low concentration of ${\mathrm{Cl}}^{37}$ in the gas. By the method of thermal diffusion 200 ${\mathrm{cm}}^3$, S.T.P., of chlorine gas was obtained containing 45 percent ${\mathrm{Cl}}^{37}$. The absorption spectrum of this gas showed lines of the ${\mathrm{Cl}}^{37}$${\mathrm{Cl}}^{37}$ molecule in the 1→12 band of the ${}_{}{}^1{}_{}{}^{}\Sigma \to {}_{}{}^{+}{}_{}{}^{}0$ band system of ${\mathrm{Cl}}_2$. For this band the alternating intensity ratio for the ${\mathrm{Cl}}^{37}$${\mathrm{Cl}}^{37}$ molecule (1.28) was the same, within the experimental error, as for the ${\mathrm{Cl}}^{35}$${\mathrm{Cl}}^{35}$ molecule (1.26), indicating that the nuclear spin of ${\mathrm{Cl}}^{37}$ is probably also 5/2. In interpreting the intensity ratios one must take into account the overlapping of lines and general background present.

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