It is shown that a simple bubble model for electrons in liquid hexane that has been suggested recently does not satisfy the necessary stability criteria if the macroscopic properties of liquid hexane are used. Stability can be achieved, however, by assuming a higher dielectric constant ϵ than the macroscopic value. For hexane and some other hydrocarbons we have varied ϵ until the stable bubble model yields the experimentally determined activation energies of the mobility. The resulting values of ϵ turn out to be nearly the same for all liquids investigated.