Onset of convection in a binary mixture near the plait point

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