  A quantum phase transition that was recently observed in a high-mobility
silicon MOSFET is analyzed in terms of a scaling theory. The most striking
characteristic of the transition is a divergence of the thermopower, according
to an inverse linear law, as a critical value of the electron density is
approached. A scaling description of this transition yields predictions about
the critical behavior of other observables, e.g., the specific heat. We also
explore the possibility that this transition realizes a recently predicted
transition from a Fermi liquid to a non-Fermi-liquid state.
