On theoretical grounds it is expected that the decay time of imprisoned resonance radiation in the vapor of a single even isotope of mercury should be some six times larger than that observed with the natural samples of mixed isotopic constitution. To investigate this effect experimentally, decay measurements were carried out with samples of ${\mathrm{Hg}}^{198}$ (3 percent ${\mathrm{Hg}}^{199}$ and 0.1 percent ${\mathrm{Hg}}^{200}$). For vapor densities $N\lesssim \frac{{10}^{14}}{\mathrm{cc}}$, the predicted effect was verified. For $N>\mathrm{}\frac{{10}^{14}}{\mathrm{cc}}$, the observed decay time drops below that predicted theoretically for pure ${\mathrm{Hg}}^{198}$. This secondary effect is here attributed to the transfer of excitation from ${\mathrm{Hg}}^{198}$ to ${\mathrm{Hg}}^{199}$ and ${\mathrm{Hg}}^{200}$ by collisions of the second kind. Comparison of experimental data with an appropriately generalized theory permits estimation of the cross section for such collisions. The value so obtained is ${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^2$, some twenty times the gas-kinetic cross section, and in order-of-magnitude agreement with theoretical expectations.

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