The thermal coefficients of the indices of refraction of air, ${\mathrm{N}}_2$, S${\mathrm{O}}_2$, N${\mathrm{H}}_3$, and C${\mathrm{O}}_2$.—For light of the visible spectrum these coefficients were not found to differ from the thermal coefficients of density change in the range 0-300°C. At 0°C, 760 mm the values of ($n-1\mathrm{}$) ${10}^7$ for $\lambda 5852\mathrm{A}, 6143\mathrm{A}, \mathrm{and} 6678\mathrm{A}$ were found to be: Incidentally the rate of expansion of quartz with temperature was found to be constant within the temperature interval 25-300°C, and to have an average value of expansion coefficient of ${0.946\left(10\right)\mathrm{}}^{-6\mathrm{}}$. A kilowatt neon Geissler tube is described.

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