Onset of convection in a binary mixture near the plait point

Recent experiments on the onset of convection near the liquid-vapor critica l point of He-3 have shown the crossover from the Rayleigh criterion to the Schwarzschild criterion for the threshold of convection as the critical po int is approached. In contrast we show that for He-3-He-4 mixtures near the liquid-vapor critical point (plait point), the Rayleigh criterion would ho ld right through. Interestingly enough, this is a consequence of the proper boundary conditions. This prediction should be easy to rest experimentally .