medium, and $\tau$ the ratio of the cloud evaporation timescale to the remnant age. While $C$ is clearly a fixed value that characterizes the conditions in the ambient medium, WL concede that $\tau$ may not be. Since the temperature of the interior of a remnant evolves with time and the evaporation timescale would generally be expected to depend on the temperature and/or the pressure. In fact it can be shown, using the results of @Dalton+Balbus_1993, that in the limit of highly saturated evaporation, the evaporation timescale is $\tau_\mathrm{evap} \propto R_\mathrm{cl}^{7/6} P^{-5/6}$, with no explicit dependence on temperature. Since the pressure in the Sedov-Taylor solution drops as $t^{-6/5}$, if the medium surrounding the enveloped clouds is close to that for Sedov-Taylor expansion in a uniform medium, we expect that the evaporation timescale should be proportional to the remnant age and so $\tau$ should be constant. Deviations from these assumptions, for example because the evaporation is not highly saturated or because the cloud-cloud or cloud-shock interactions cause substantial deviations from a Sedov-Taylor evolution that lead to different evolution of the evaporation rate, would then be expected to cause variation of $\tau$ with remnant age. We discuss this further below.

Results
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In 