= Solution
(Bi) and $(\beta\mathrm{Sn})$ are Bi-rich and Sn-rich <terminal solid solutions>, not intermetallic compounds. At the eutectic composition, cooling through $138\,{}^\circ\mathrm C$ gives
$$
\boxed{L\longrightarrow(\mathrm{Bi})+(\beta\mathrm{Sn}).}
$$
The two solids grow together as a fine <eutectic microstructure>, commonly lamellar or rod-like. Below about $13\,{}^\circ\mathrm C$, the tin-rich component has an equilibrium tendency to transform from metallic tetragonal <white tin> to diamond-cubic <grey tin>, with a substantial volume increase. This is <tin pest>, which can disrupt the microstructure mechanically. The dashed transition does not transform the Bi-rich phase into <tin>. Actual transformation can be slow and depend strongly on impurities and <nucleation>; crossing the line does not guarantee immediate disintegration.
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