What fraction of the volume of a glass flask must be filled with mercury so that the volume of the empty space may be the same at all temperatures?
$\left(\alpha_{\text {glass }}=9 \times 10^{-6} /{ }^{\circ} C , \gamma_{ Hg }=18.9 \times 10^{-5} /{ }^{\circ} C \right)$