  The $\gamma-\alpha$ transition in Ce$_{0.8}$La$_{0.1}$Th$_{0.1}$ is measured
as a function of applied magnetic field using both resistivity and
magnetization. The $\gamma - \alpha$ transition temperature decreases with
increasing magnetic field, reaching zero temperature at around 56 T. The
magnetic-field dependence of the transition temperature may be fitted using a
model that invokes the field and temperature dependence of the entropy of the
$4f$-electron moments of the $\gamma$ phase, suggesting that the volume
collapse in Ce and its alloys is primarily driven by entropic considerations.
