When a fluid initially stratified by a vertical variation of species concentration is heated from a sidewall, the classic thermal convection could be dramatically altered by the stable gradient of mass. The convection starts near the heated wall but it is then limited to convective cells separated by diffusive interfaces with sharp variation of density and temperature. The evolution of the multilayer system is characterised by the thickness of the cells, their lateral propagation and teir ability to merge witht he destruction of diffusive interface. Experiments with a linearly stratified medium allow to compare the results in liquid Freon with the previous results obtained in salt water. The experimental stability criterion, the initial thickness and the front propagation velocity agree satisfactorily with the salt water case. In order to understand the influence of the tank width, we present results in salt water demonstrating that the confinement effects take place only when the cells attain the last quarter part of the tank.
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