How much mercury must be placed inside a glass flask, having an internal volume 300 cc so that the volume of the remaining space inside the flask remains constant at all temperatures ? $\left({\mathrm{\gamma }}_{\mathrm{Hg}}=1.8\times {{10}^{-4}}^0{\mathrm{C}}^{-1},{\mathrm{\alpha }}_{\mathrm{g}}=1\times {{10}^{-5}}^0{\mathrm{C}}^{-1}\right)$