  Gallium (Ga) displays several metastable phases. Superconductivity is
strongly enhanced in the metastable $\mathrm{\beta}$-Ga with a critical
temperature $T_\mathrm{c}= 6.04(5)\,\mathrm{K}$, while stable
$\mathrm{\alpha}$-Ga has a much lower $T_\mathrm{c}<1.2\,\mathrm{K}$. Here we
use a membrane-based nanocalorimeter to initiate the transition from
$\mathrm{\alpha}$-Ga to $\mathrm{\beta}$-Ga on demand, as well as study the
specific heat of the two phases on one and the same sample. The in-situ
transformation is initiated by bringing the temperature to about
$10\,\mathrm{K}$ above the melting temperature of $\mathrm{\alpha}$-Ga. After
such treatment, the liquid supercools down to $232\,\mathrm{K}$, where
$\mathrm{\beta}$-Ga solidifies. We find that $\mathrm{\beta}$-Ga is a
strong-coupling type-I superconductor with $\Delta(0)/k_\mathrm{B}T_\mathrm{c}
=2.00(5)$ and a Sommerfeld coefficient $\gamma_\mathrm{n} =
1.53(4)\,\mathrm{mJ/molK^2}$, 2.55 times higher than that in the $\alpha$
phase. The results allow a detailed comparison of fundamental thermodynamic
properties between the two phases.
