In very compressible fluids, such as fluids near their critical point, the bulk fluid is adiabatically thermalized by the expansion of a hot boundary layer. Thanks to this thermomechanical processthe so-called piston effectthe fluid velocity at the edge of the boundary layer can become very high when the heating power is concen-trated in a fissure. Spectacular jets are then observed in SF6and CO2. Data obtained under weightlessnessinorder to remove convectionand data obtained under earth gravity are compared and analyzed. They empha-size the key role of the boundary layer expansion for thermal phenomena in compressible fluids and thehydrodynamic nature of the piston effect