Persistence ofλ2537in Mercury at Low Pressures

The persistence of $\lambda 2537$ excited by electron impact in mercury vapor was measured for vapor pressures corresponding to the temperature range 78° to -19°C, using the alternating potential method previously described by one of the authors. Between 78° and 17°C this persistence was found to vary inversely as the first power of the pressure, and not inversely as the square of the pressure as predicted by theories of the diffusion of radiation by repeated absorption and re-emission. A modified theory of diffusion of radiation seems necessary to explain the persistence which is much too small to be explained as depending upon the life of the metastable atoms. As the pressure is lowered the persistence approaches a limiting value of the order of ${10}^{-7\mathrm{}}$ seconds.