  The magnetic field and temperature dependencies of the magnetic moments of
superconducting crystals of ${\rm V_{3}Si}$ have been studied. In a constant
magnetic field and at temperatures somewhat below the superconducting
transition temperature, the moments are hysteretic in temperature. However the
magnetic moment-magnetic field isotherms are reversible and exhibit features
that formally resemble the pressure-volume isotherms of the liquid-gas
transition. This suggests the existence of a first-order phase transition, a
two-phase regime, and a critical point in the superconducting phase diagram.
The entropy change, determined from the data using the Clausius-Clapeyron
equation, is consistent with estimates based on the difference in the vortex
densities of the two phases.
